Hitachi 3d7 MKII noise

[IVH]

Ouch, i should have seen those resistors in parallel earlier ! Your clear schematics really helped !
I'm using a small wurth digital multimeter, the advertised precision is 1.2% for potential measurement.
I forgot to mention I cleaned all sliders and switches thoroughly with contact cleaner when I received it, and moved the rec and tuner/tape switches ~50 times (as said by you in another thread).

I don't know why I forgot to measure the transistors, but here are all related to the tape audio part:
Q401L: E = 0v B = 0.698v C = 1.9v
Q401R: E = 0v B = 0.704v C = 2.28v
Q801L: E = 1.123v B = 0.496v C = 4.27v
Q801R: E = 1.175v B = 0.55v C = 3.65v
Q502: E = 6.76v B = 0.653v C = 9.04v
Q402: E = 13.71v B = 13.4v C = 13.80v ???
Q801L: E = 13.25v B = 12.34v C = 13.9v " " "

I didn't mention the output of the AC/DC circuit is pretty high: between 14 and 15.5v (it might not be made for france's 250v supply...) according to the service manual, it can be rewired for 250v. But in another thread I read you saying that the voltage from acdc's is often higher while not under load.

I'll have to remember the formula you used in your 2nd post, it's very useful !

Wow ! Those 'quick rules' are good to know !! could be a sticky if you ask me. Tomorrow I'll recheck the whole circuit, but with those rules applied, especially the last one.

I don't know if I'm making your day, but what I know is that you"re making mine

 

[Superduper]

I'll offer the following notes:

• When doing voltage checks, it is important to note that the values indicated in schematic diagrams are based upon a constant prescribed source voltage. Generally speaking, one should hook up the system to a lab adjustable power supply to ensure that the system is getting the voltage that is indicated. In this case, 12V. It is not 100% requirement to use a regulated power supply and if you do not and use the AC transformer instead for a power source, then some values will change and you'll need to account for that.
• You have Q801L listed twice with 2 different sets of voltage measurements. This is probably a mistake but regardless, Q801 L/R are aft of IC401 and probably not where we should be focusing at this time, if for no other reason than that we know that part of the circuit should be capable of operating normally when a radio circuit is applied. So, for now, you should probably ignore that portion, and focus on the other part of the circuit.
• You are getting different collector voltage for Q401L and Q401R. They should be the same. You should probably check R404R for possibly higher than normal resistance and R405L for possibly higher than 470k, .
• B-Q502 seems to be low compared to schematic. Are you sure that is a correct measurement and does your super-bass woofer appear to be working normally?
• Q402 These measurements are off, which I presume you already noted. You may want to check R424/426 (possibly too low?), R427(possibly too low?). and R428 (maybe open).With these measurements, I presume that you are not able to record to cassettes? If these measurements were accurate, and the resistor failures are true, this might well be the source of your problems.

Your DMM is not very accurate so you need to understand that your measurements aren't 100% accurate. The 1.2% you mentioned is probably the best case scenario. Add probes, and other factors and the deviation could be very noticeable. By comparison, the Fluke meters I use have lower than 0.1% accuracy.

 

[IVH]

I'll post a longer reply this afternoon when I am free. From what I read, there is some hope for a simple fix.
I forgot to replace the last Q801 with Q501, the ripple filter transistor. Doesn't sound too illogical if there is something bad too.
When I measured the whole circuit a few days ago, I marked R428, 426 and 424 as being a bit off. I'll remove those from the circuit this afternoon. There's still some hope left !

 

[Superduper]

You may want to recheck Q501 (readings previously indicated for Q801L correct?). The emitter voltage should not be higher than the base, although I suspect that you have those two figures reversed. If not, you may want to check the transistor out of circuit to see if it is working properly. A cheapo eBay transistor checker will help simplify your diagnostics.

[ebay]172320564326[/ebay]

One similar to the one above will serve you just fine, although I don't personally recommend any particular one, but there is a whole bunch of different versions. I think that they are mostly all based on a custom arduino platform. Actually for all you guys, I recommend you get one. For less than the cost of acquiring an arduino and the associated parts, you can get one completely custom programmed for this purpose. It's not going to replace a $5000 specialty lab instrument, but for the crude testing we do on boomboxes, you can't beat it for the money.

Anyhow, worse case is that Q501 is not working properly, which means that it's not filtering and supplying excess voltage to your tuner (bad, maybe future issues). Currently, it is clearly passing current otherwise your tuner won't be working. But when operating properly, the emitter should be controlled by the base, and with the emitter greater than the base, that does not seem to be the case based on your measurements. Not sure this will have any affect on your issue at hand, still it's something worth noting since your measurements indicate that this is a potential issue and as you already have the boombox open......

================

Unrelated question: IVH, you are a french-person eh? You seem to have an excellent grasp of the english language, even my occasional typos (happens when I type fast to try and save time) don't seem to cause you to miss a beat. Does your country teach english to the general population or how is it that you come to understand english so well?

 

[IVH]

Maybe I should connect it to a 12.6v lipo, that's a way smaller difference than the transformer's output.

Here are the values of all the resistors you asked for:
R424: 269Ohm, 270 indicated
R426: 390Ohm, 390 indicated ! this one was measured off circuit as parallel resistors were affecting the readings.
R427: 56.2kOhm, 56k indicated
R428: 7.1 Ohm, 6.8 indicated. This one has been measured between one leg of Q402 and a leg of C417 like you said.
R405R: 464kOhm, 470k indicated
R405L: 456kOhm
R404R: 38.2Ohm, 39 indicated
R404L: 38.6Ohm

The woofer seems to work properly, the difference between 3d on and 3d off is clearly noticable and bass is good. It's not performing very well at very low frequencies, but that might be normal for a woofer in a non airtight enclosure/box.

You are right, I just noticed i reversed B and E loads of times... sorry for that. I've remeasured all those just to be sure.
Q502: E = 6.44v B = 6.69v C = 8.95v
Q501: E = 12.26v B = 13.10v C = 13.68v
Q801L: E = 0.490v B = 1.117v C = 4.22v
Q801R: E = 0.545v B = 1.173v C = 3.64v
Q402: E = 13.37 B = 13.7v C = 13.81v

I understand and will take the accuracy into account. My father is electrician and got some professional multimeters, I think I can use them if necessary



About the transistor tester: I'll look into it, as I have an arduino laying around programmed as a low voltage cutoff for the lipo that i'm going to build into the hitachi.
Even though the transistors are is possibly good now after having remeasured everything properly. It's always handy to have a transistor tester laying around, especially with all those vintage amps laying around



Answer related to an unrelated question: Thank you for the nice compliment! It's a bit complicated as I'm dutch and moved to france at the age of 9. Dutch have the reputation to be quite good at english. Unfortunately, english teaching is pretty bad in france, pupils (and most grown men) think english isn't important and refuse to learn it. This slows down lessons for the ones willing to learn it. I learned your language thanks to the internet ! Without it, my english would've been deplorable.

 

[Superduper]

You even know the word deplorable? I would say even many people in USA probably can't fully articulate the word deplorable, much less the phrase basket of deplorables.

Ok, it's getting too hard for me to flip back and forth now from your readings and this reply text box, so I'm going by memory here and making some presumptions. Firstly, I believe your readings are all now (with some amount of deviation), relatively close to the specs, right? So with that, nothing that I'm seeing in your readings suggest they would be the source of your problems except Q402 readings which are totally off but that is because you made measurements in PB mode, and those readings (in parenthesis) should be taken in Record mode. I suspect that if you retake the measurements in Recording mode, that those readings will come in line, approximately with specs.

The L to R imbalance in your readings disturbs me since they should be identical but even with the deviations, "probably" isn't the source of your problems. There is a reason for the Q401 L/R imbalance and I don't know, I might even take a resistance measurement between R407 and the C/B of both Q401's. Perhaps there is some additional resistance in the circuit path that is the cause. If not, then it may be due to a leaky (C406 L/R) capacitor. I would also check the emitters of Q401's to ground to see if they are equal.

At this point, you've nearly exhausted the V/R test readings which are the measurements I use to be able to give you an analysis and opinion from thousands of miles away. You haven't checked all resistors yet and they might be important (see the thread below for an example). After that, then next up is capacitors which are harder to check since they can't really be checked in circuit reliably. Additionally, better and more capable equipment is also required such as an LCR meter but that cheapo ebay tester I mentioned earlier might work in a pinch. I always do a quick test by checking all caps and see if any are shorted. If yes, they should immediately be considered bad. This simple test can be done in circuit. Since it sounds like you only have a DMM, you'll have to see if your meter can test caps too. If so, then you can test them by lifting one leg.

Other tips. Bad solder joints is the source of many problems that are not obvious at first glance. It does not cost anything except a bit of time to redo those connections. Also, any caps that has one leg tied directly to ground should be checked more carefully. Many are ceramic and not necessarily electrolytic. Hum issues can be a pain in the butt to figure out, especially with ground loops but on verified circuits (not in prototype or new designs) that worked fine before, ground loops would probably only be introduced by poor connections compromising a good ground circuit.

If you have time, you can review this thread by Joe (BluFuz) where he had a very hard time fixing a hum issue. You can skip to the fix by going directly to approximately post #255 +/-.
https://boomboxery.com/forum/index.php/topic/19914-fixing-a-legend/

 

[IVH]

Don't flatter me too much ! But indeed, many americans are quite bad at english, and tend to not even know the basics of their counties history. That surprises me alot.

You are right, the hiss is not any different between channels. But volume may be, I don't know, from what I understand it would be best to fix the imbalance too.

I checked almost every resistor on the tape part (didn't post everything), but I marked loads as being off because of my lack of understanding parallel resistors. I will soon recheck all with the new service manual and do the necessary math.

As usual, it's getting late, I'll reply to and do the things you said in the 2nd to 5th paragraph tomorrow. Thanks again for all the tips, I could never have thought those by myself.

I did read page 3 of that topic a few days ago, it was an interesting read. I immediately started looking for those stupid printed resistors, I couldn't find one luckily, hopefully there really aren't. But yes, I might learn alot from that thread, so reading the whole thing might not be that bad of an idea

 

[Superduper]

It's good that there's no printed resistors as that would add another dimension of craziness to troubleshoot. Much of the problem with Joe's boombox was not only that the printed resistors were all drifting or open, but also just the printed traces were going bad too. Since they could flip up/down the board through vias many times while traversing the board, it could get tedious trying to track them all down. Some even go under components making it very hard to follow.

I just noticed that your boombox does not have a true line-in. The diagram didn't make sense to me since I was sure you mentioned listening to music via line-in. This must be while monitoring the tape during record, right? I don't have and never had one of these so I'm not sure. Also, you didn't test the mixer circuit. That is on the small headphone jack and includes Q403. Highly suggest you investigate that as a bad ground wire or emitter resistor could certainly introduce hum. In fact, R437 on the mixer board was exactly the equivalent resistor on Joes boombox that caused the hum. More things for you to check, LOL.

 

[IVH]

It's complicated enough like this I can't imagine how it would be with those printed resistors.

I don't exactly know how it works, but line in works without the tape deck connected to the board, you just have to ''press a slider'' that is normally pressed by the record button. This can be seen in the video.

I wanted to rule out that the issue is related to the mixer board. So I connected the board to my father's MFB through line out without the speakers and mixing board connected. The hiss is still there but it's not noticable at low volume. (maybe related to noise cancellation in my father's amp)
Line in seems to not work through line out, as the rec switch just cuts line out off, even while in tuner mode.
There will be 2 videos demonstrating this.
This should rule out the mixer board. I'm now going to do all the testing you mentionned in your second last post !
The more things to check, the more possible fixes to be found

https://www.youtube.com/watch?v=4NOY0DC0aS0

https://www.youtube.com/watch?v=-5GtRSgZg34&feature=youtu.be

 

[IVH]

Today I found some time to do all the measurements you asked for. Unfortunately I found nothing too staggering...
I did also reflow the part before IC401 (if by that you mean remelting the existing tin).

The resistance between Q401L (E) and ground is 38.9Ohm, a mere 0.4 Ohm less than the one between the emitter of Q401R and ground.

I did measure the resistance between R407 and both the collector and base of Q401R and L
The resistance between both the collectors is the same: 7.68k Ohm
Between Q401L's base and R407 the resistance is 463k, the R side gives 472k. That doesn't sound too off to me.?

I finished measuring all resistors beginning with 'R4' I could not find anything off.

The same goes for the capacitors, except for a minuscule ceramic one (C426) that appears to be open. It's resistance is ∞ from the beginning, it doesn't slowly nor rapidly rise. Is it broken or does it charge up instantly because of it's small rating (68PF) ?

Like you said, there isn't much left to measure. Is it a good idea to just replace all electrolitic caps ? Someone once told me, when ordering a recap set, that ceramic ones don't go bad . I was thinking of buying a bunch of nichicon FG caps, The 3D7 might even sound better than when it was fresh from the factory with those soldered in

 

[Superduper]

1) Like I said, your boombox does not have a conventional line-in feature. If it did,you would have that option available in your function selector and you would be able to play that just like as if you selected "tuner." That you need to depress the record button indicates that you are monitoring the line-in during a recording session, unless you tell me you have a "special" record button lever that allows you to operate the record button without the deck working or without the need to simultaneously record-pause. In any event, it is irrelevant to the issue at hand. The only thing that is important is that you have this issue and it appears related to your tape section. Or... is it possible that it is also happening during tuner mode too but due to a higher signal to noise condition, you are not hearing it? Please tune your radio to a quiet location on the bandwidth and see if is still making the noise?

2) Reflow does indeed mean to remelt the solder/tin. However, you need to observe and recognize if the connection is a good solder joint or not. This takes some experience to recognize whether the parts to be joined are wetting or balling the solder. If the solder is balling, then I would desolder the entire connection by removing old solder and doing it over to ensure that the solder is actually sticking. A bunch of solder balled around a lead is not what you want to see, although I suspect from your posts here that you have sufficient experience to know the difference.

3) Quick test is exactly that.... it's quick but since the results aren't always conclusive, what you are looking for are numbers that make no sense and obviously wrong. For example, you never want to see capacitors or resistors shorted or open although resistors usually open rather than short. You never want to see resistors that tests higher in-circuit than the printed value since that is almost a guarantee that there is an issue. Through experience, you'll learn to recognize that low value resistor readings can generally be trusted more than high value resistor measurements and when they are off, usually a quick examination of the circuit diagram tells you whether that off-spec reading makes sense or not. You don't really need to "compute" the actual expected value since you can usually see at a glance whether a reading is about right based on the circuit diagram. The "quick" test is something we do simply to see if there is something obvious.

4) How "off" is "off." There is almost always some amount of deviation, not only in the circuit itself but also in the equipment you use to do the testing. The multimeter itself has some inherent inaccuracies and that's before you add the probes and the surface oxidation of the parts tested. Resistor drifting really isn't critical. In fact, if you have a 1k ohm signal resistor in the auido circuit and it has now changed to 10k, your boombox will likely still operate sort of normal although the audio might be a bit attenuated. However, an 100R resistor that has changed to 130R in a bias circuit could cause enough problems to become noticeable or problematic. V/R (voltage/resistance) measurement tests are useful to see if something in a circuit is amiss since it gives us a good picture of the overall health of a circuit, but the V readings posted usually is only accurate when the power supply is at spec. So if the schematic shows 12V at the power supply and it is currently powered by AC or batteries, then the voltage could be different and the voltages in any non-regulated section will all be globally off.

5) A 68pf capacitor will charge almost instantaneously. Any small capacitor will charge fast enough that, depending upon the voltage applied to the circuit under test by the particular meter, be too fast to see. A resistance test on capacitors isn't a very good way to test them anyhow unless they are either shorted or open, and with small capacitors, even that is usually inconclusive. This is where a higher end DMM is useful since they usually have a cap testing function, or an LCR meter. Some of the cheap transistor testers I mentioned earlier can test small caps accurately. Out of curiosity, I just now stuck a brand new 82pf ceramic cap into one of those testers I got off eBay and after about 3 seconds, the reading came back at 82pf.

6) Replacing all electrolytic caps is certainly an option. Ideally, I'd like to see where the problem is first, but lacking some more sophisticated instruments than a cheaper DMM, then it's certainly a service that one could try. Some folks don't like the shotgun capacitor approach but if you ever want to recap a boombox someday, might as well be now since a partial replacement today means that in the future, you will end up replacing the same ones again? Would I recap a boombox like this? Absolutely without a second thought. The service manual only shows 12 transistors total for this boombox so it's pretty simple and doesn't have that many components. By comparison, the Marantz CRS-4000 boombox, about similar size to this one which I recapped about 10 years ago had around 100 electrolytic caps, if I recall correctly.

7) I don't agree that ceramic caps do not go bad. I have definitely seen them go bad. The difference between electrolytics and non-electrolytics is that electrolytic ones have a more predictable failure rate based on age and usage and heat, and stress whereas non-electrolytic ones fail randomly and therefore are not replaced unless they go bad. That's why you test them if you suspect them.

8) By the way, this is not a very sophisticated boombox. One look at that PCB shows this. Unfortunately, I find that simple designs like this often aren't always as immune to noise as they can/should be.

 

[IVH]

1) It's exactly the way you describe it, line in is part of the tape function. I didn't understand a phrase you wrote a few post ago about 'monitoring the tape' that explains my misunderstanding (I think).
Not in any way against you, as this thread is getting really long and messy But somewhere near the beginning of this thread I mentionned that the hiss is not appearing on a (near) clear frequency. It's possible that I'm completely wrong, but to me the hiss is not the same in tuner mode than in tape.
On tuner mode the hiss appears to come from the antenna picking up some noise.
It's difficult to discribe, but the hiss in tape mode sounds more 'excited' and it takes a sec to appear/get loud after switching from tuner to tape, it's like the noise has to 'charge up'.
What i'm saying here is surely bull****, but that might be because the caps are charging up. The tuner does not do this, indicating the noise is coming from the outside.
Edit: The same happens when the unit is powered up, the hiss takes a second to get loud.
Here is a video:
The down position/ the position near the little blue switch is tape mode. the center posistion is tuner mode.


https://www.youtube.com/watch?v=S9aV5RH2DX0


2) There were indeed a few of those bad solders where the solder formed a ball instead of something resembling a tipi tent. Those have been fixed.


3) I understand a quick test isn't the best thing. Although I cannot see if something is correct in a glance, I do now know, thanks to this website, that when 2 resistors are in parallel. The total resistance is, depending on the difference of resistance between R1 and R1, between a half of and a little less than the lowest R.
It's a beginning


4) Hmm, interesting. Maybe I should do some more serious testing of those important resistors. There were some just past the 5% tolerance.


5) That's a pity, I was hoping this was the source of the noise. Those cheap testers are surprisingly accurate to measure that 82pf cap correctly !


6) It's true that it would be good to know where the problem is coming from, maybe to help someone else someday with the same issue, or for yourself if the same issue reappears years later. (yes, I plan to keep this one for a long long time ). You're right, I've till now never done a partial recap, I like it better to do all at once. They aren't that expensive and once you have the thing open, why not do everything ?
It does me good that you too you'd do the recap on this boombox, it might be low end, it certainly wasn't expensive at the time, nor it is now. Still I really like it's design and isolator42's adoration for the 3d7 made me love it's sound without ever hearing it myself. Anyways, thanks for that little motivation !
That must've been a big job, and if ever something goes amiss, a lot to troubleshoot. Why does it have so much more components ? Is that because the quality is way better (which is obviously true. I still want the Philips/Marantz FA-930 (MK2). Which combines my countries best brand and a pioneer) or because it has more functions ?

7) Wow, that's interesting ! learned something today.

8) What a pity, it would have saved us loads of time if hitachi did a better job in the 80's ! You should start a company producing simple, quasi fail proof boomboxes with all the modern stuff, but still based on old designs. I would buy one for sure


Just a question, is/are there any cap(s) that is/are most likely to cause such a problem ? I have a couple laying around for my fathers speakers, I could solder those in and buy him new ones. If that fixes it the move to buy all caps necesary a whole lot easier !

 

[Superduper]

Quality is dictated by design, not by the number of components, however, some designs are purposely designed to reject noise, and provide cleaner sound. The power of an amplifier isn't due to the number of components. In fact, the output section typically has very little components compared to the preamp section. But an amplifier is exactly that, it amplifies and it doesn't care what it is. So if you feed it garbage in, you will get BIG garbage out. If you look at a high fidelity device, you will see that it typically has a lot of components. All are there for a reason and I assure you that if there was no purpose, they wouldn't waste money to put it in. In a circuit, you might omit bypass caps. It may work fine, or the circuit might become unstable and oscillate. Or they can pick up hum or RF or other externally introduced signals which gets amplified by the amplifiers. Adding bypass caps into strategic locations ensures a stable operating circuit. As well, you will find that in may quality devices, that strategically placed stiffening caps exist. If you remove them, the circuit might still appear to operate fine, but under the right conditions, performance will suffer. And on some amplfiiers, you'll see output capacitors that are placed in series with the speakers. These are often large, 1000uf to 2200uf caps. They are not cheap. IF you remove them and jumper them, you'll likely notice NO degradation of sound. That is, until you crank the system and it begins to distort. Under certain conditions, the amplifier could output DC in which case, you will blow your woofers. The tweeters might remain OK and this is because the tweeters are protected by the high-pass caps. What I'm saying is that manufacturers don't just complicate a design just to earn the higher price tag of the device they are selling, those extra components are there for a reason. Some for protection of certain components, some protect against catastrophic avalanche failures that occur when a single component fails, some to clean up sound, and etc. etc. etc.

Forget investigating those resistors for the moment, as the deviation isn't significant enough to make me think they are problematic. If it were a bias issue, it would be more distortion or other issue instead.

As for the 68pf capacitor, I'm not saying it's bad or not bad, you would have to either test it or replace it, or tack another one behind it to see. You simply can't test it with a ohm meter since a properly working one charges up so fast it will behave like O/C and even if you can see it charging, what value would you assign to a capacitor that is capable of charging? Would you guess based on how long it takes to charge up? Also, what if it were leaky -- it may still charge. Or how about if the ESR is unacceptably high?

As for using your Dad's speaker caps to do this job, don't bother. Just order a proper set and be done with it. Firstly, home speaker caps are usually higher voltage and very large compared to the signal caps you will likely be using. Speakers also don't use large values, they are almost all low value caps. You only need about 3-dozen caps anyhow which really isn't that many for a boombox. In fact, I could go into my inventory right now and collect the whole lot for you, but the shipping costs alone to France probably exceeds what it will cost you just to buy them (caps & tax & shipping) from France. Probably a lot faster too especially if it gets caught up in Customs. Anyhow, since your issue is global (not limited to L/R), I would suspect more a cap that is tethered to ground rather than a coupling capacitor (series). The purpose of some caps is to shunt hf noise to ground before it gets amplified.

I asked you to recheck the hiss sound and the reason I said to make sure that it's not also coming from the tuner is because if it is, then the issue won't be isolated to only the tape preamp section. You certainly can expect it to have a slightly different sound when you switch modes because the circuitry is taking a different path. I'm more concerned with the level of the hiss and at least when I listen to your video on my smartphone, it sounds like the hiss is about the same level in either mode. That could suggest that the issue is not isolated to only the tape section fore of the IC401 preamp chip. Based upon what I'm hearing, I'm not convinced that this is the case. You say you don't hear it on a clear radio station -- I don't know but that could simply be because of the higher SN ratio from a strong station overwhelms the hiss so you don't hear it. You are the best judge of that since you can listen to it live. I almost have to listen to it in front of me to decide since audio recorded in a video recording is often adjusted up/down automatically by the recording device's ALC.

I'd admit that I really didn't listen to your videos previously since my office computer does not have speakers. I'd always thought that you had more of a hum issue than a hiss issue. This really sounds more like a grounding or poor shielding issue to me, but could also be due to noisy semiconductors. Do you have access to an oscilloscope or an audio signal generator, or audio signal tracer? They could help you narrow it down. Also, you might be able to strategically isolate some sections one at a time to see if you can rule out certain circuits. For example, if you lift one leg of R501 R/L, you will cut out the audio signal to the full range amplifier and hopefully, your speakers will be silent. Or if you isolate C523, you'll hopefully remove the SW circuit from the equation in case it's somehow injecting some hiss. These are just some techniques that you could employ to try to narrow down or rule out certain circuits as potential contributors. You can also jumper the entire EQ section just in case that section is somehow overamplifying the high frequencies in the frequency of the hiss.

 

[jimmyjimmy19702010]

This thread is very interesting - anyone who has had a 3D7 apart will know they are a very basic box but the stuff they did include works well for its intended market - heck, the deck doesn't even have auto stop but it does sound really good!? All the money went into the 3 amps, the centre speaker and the sensitive tuner. My 3D7 needed everything serviced to make it work right but it ended up one of my most reliable boxes to date - the deck & tuner have worked perfectly since the major overhaul.

I can confirm that the deck should have very little hiss even at maximum volume with play depressed with no tape inserted. The tuner should also have no hum when off station and lock on solidly with a very quick FM stereo lock on.

The highs and mids are super crisp on my example. The bass from the BTL 3rd amp is fantastic too - not as bassy as the bigger 3D8 but you can crank it without the centre 8 ohm driver distorting (unlike the 3D8).

I hope you get your example working right - it will be worth it as they really are a super loud and solid performing early eighties boomer.

 

[IVH]

Another really interesting paragraph. I believe it would be so cool to know what component does what in a circuit, like you do. Good to know my vintage philips amp has some 'signs of quality', I found a few of those huge output caps in there, the biggest being 9*4cm (!) Clipping is my biggest fear when I bring it to a party, most people won't believe when I say that turning the volume too loud might kill the speakers.

ok, I will forget about those resistors !

Can't argue against that

As for the caps, I'll follow your advise, it will save me some hassle too. I'll make a list and order asap. Is it a good idea to replace the power supply one too ? if so, would a higher voltage mean a longer lifespan?

Ouch, sorry if I did not describe the issue well! About the hiss sounding different because it takes a different pad, It sounds logical now, but I didn't think of it :/
And thanks to jimmy, we know it shouldn't make the noise it does when off station in tuner mode. So I agree it surely is the same noise. But there is some good news to come !

I did the tests you asked for, and I think I got some good news !
After unsoldering one feet of R501L, the left channel was DEAD silent !
If I read the schematics well, this won't exclude the equaliser circuit, you mentionned bypassing the equilisers, but I'm a bit scared of doing something wrong.
Is soldering a wire between R421R and C810R the correct and a safe way to bypass the equalisers for the right channel ?
Unsoldering C523 unfortunately didn't change anything to the hiss.

The hum/hiss confusion is because in the beginning I thought that a hum is a hiss. Well I was wrong.. I hope all the troubleshooting we did is applicable for a hiss issue too.

 

[IVH]

This thread is very interesting - anyone who has had a 3D7 apart will know they are a very basic box but the stuff they did include works well for its intended market - heck, the deck doesn't even have auto stop but it does sound really good!? All the money went into the 3 amps, the centre speaker and the sensitive tuner. My 3D7 needed everything serviced to make it work right but it ended up one of my most reliable boxes to date - the deck & tuner have worked perfectly since the major overhaul.

I can confirm that the deck should have very little hiss even at maximum volume with play depressed with no tape inserted. The tuner should also have no hum when off station and lock on solidly with a very quick FM stereo lock on.

The highs and mids are super crisp on my example. The bass from the BTL 3rd amp is fantastic too - not as bassy as the bigger 3D8 but you can crank it without the centre 8 ohm driver distorting (unlike the 3D8).

I hope you get your example working right - it will be worth it as they really are a super loud and solid performing early eighties boomer.

Thank superduper for the interesting part of this thread

I'm so glad to hear your positive feedback about your 3D7 ! I do really hope we will get this thing working as it should, I'm looking forward to it. Especially now I heard how great it can be from yet another person !

I confirm the good tuner, It was the first thing I noticed. It's really easy to lock it on a station compared to some other tuners that are laying around here.

 

[IVH]

I tried counting the caps, but there were a few surprises.
There are some differences between the schematics and the board itself. The mk2 version isn't possibly only a visual upgrade. But also a technical upgrade.
As an example, on the mk2 board, I count 3 3.3uF caps. According to the schematics, there should be 5 (excluding C903, which is part of a seperate board for the led level indicator).

The opposite goes for the 22uF ones; There are 3 on the board, and a single one in the schematics.

C515 has a capacitance of 1000uF, the schematics show a 220uF one. The voltage remains the same though (16v). This one might have been replaced with a different one though, as it's color is slightly different than the others.

In addition to this, there are loads of additional components in the tuner section: C163, C164, C159, C158, C157 and L156, just to name a few.


Unfortunately, it appears someone has been messing with this box, luckily only the tuner section appears to be affected though.
There are many components missing, those include C160, R155, C165, C166, C211 and a 'bridge' named j001. C160 has been ripped off, as the remainings of its 'feet' are still soldered on. I will not be able to replace some of those, as the mk1 service manual does not include all of the missing parts...
Someone must have had a reason for doing this, but it remains unclear to me.

 

[Superduper]

If you are going to recap the whole box, then you should probably just order and replace whatever is in there. I'm not sure about the 1000uf one though. On the diagram, it shows that it is a stiffening cap for the cassette motor. That sounds awfully large, almost like it is there to compensate for some issue. 220uf sounds about right for a motor of that purpose.

As for the tuner component deviations, if your tuner is working right, probably shouldn't worry about those too much. The cap that is ripped off is definitely concerning though. Maybe it got damaged by a novice and they just ripped it off so it wasn't hanging and didn't know what to do? However, as it is in the tuner, and not likely the cause of your immediate concern, perhaps just ignore them for now.

As for missing components, that might not necessarily true. That is, if I understand you correctly. Are you saying that there are vacant places on your PWB without components? That is not unusual. Many boards are designed with more than one configuration. That might've been intentional and for additional features such as SW etc. that may be included in other regions. The bridge you speak of sounds like a jumper.

I'm got my hands busy right now with cooking dinner as we have guests and some household chores but when I finish and get a chance, I'll give you some tips on how to isolate or jump certain parts of the circuits so you can test and see whether there is any improvements, which can help us narrow down where to focus.

 

[jimmyjimmy19702010]

The 'missing parts' on the tuner board may only be used when it's configured as a 4 band tuner as this model was also fitted with a 3 band tuner - notice the right hand switch with one less switch option:View attachment 30232

The much more common 4 band tuner:
View attachment 30228View attachment 30229

I was concerned that the 7620 service manual may not be entirely correct for the 3D7 Mark 2 that was made several years after the original 1984 TRK-7620.

 

[IVH]

@jimmy It's cool to see the differences between both versions. Your version seems to lack long wave, and mine has only a single short wave. Could that be related to the region where it was sold or just because you got the mk1 and I the mk2 version ?





@superduper Ohh, didn't see it is for the tape motor, in that case I'll not replace it as I'm not planning to use it anyway.
That might be the explanation for the 'missing' components ! Here is a photo to respond to your question:



I would really like to replace the ripped of cap, but I'll have to leave it as it's not present in the schematics, but as you say, as long as it works fine...