Hitachi 3d7 MKII noise

[Superduper]

If I had to take a guess, I am going to say that the cap was probably installed in error at the factory. When the assembler realized this, he simply cut the un-needed cap off. Now that I know that 3 and 4 band versions exist, I'm pretty certain now that it's like I said... the extra components are due to changes in the circuitry for the optional tuner configuration. I doubt that installing a cap where it isn't needed is going to have any effect, so if you really want, you can go ahead and install them. The value is probably same or similar to the adjacent ones.

 

[crazygamer]

I´m owner of 2 of those 3D7s- both are TRK-7620E´s, one is the earlier version without 3D7 logo, second one is with 3D7 logo. Both of them are 4 band radios, FM/AM/SW/LW, without fine tuning knob. Only minor differences inside and outside. These look fairly simple from inside, i´ve never gone into those depths of it, but i got to know about them a lot more.

 

[Superduper]

So what is 3D7? Is that not a model number?

 

[jimmyjimmy19702010]

So what is 3D7? Is that not a model number?

The 7620 was the first of the 3rd speaker (3D) boomers. After the huge success of this model, they started making bigger versions and needed to give the 1st edition its unique model number whilst identifying it as part of the expanding 3D series.

 

[IVH]

That's some interesting history lessons Jimmy
Here is a small update about the repair, norm helped me by-passing components like the equaliser, the pre amp,..
From that we concluded that the pre amp might be faulty, I've ordered one, it costed me 5€ plus shipping (!). He also recommended me to buy one of those 16 pin dip sockets, the IC will snap right in.
I'll report back to you as soon as it's soldered in, can't wait

 

[isolator42]

AFAIK, Hitachi first tried the extra, separately amplified bass speaker in a boombox thing with the TRK7620 (Can't say for sure if it was the first as the Sanyo BigBen may well pre-date it). It was a success (particularly with me), leading to Hitachi's 3D series in the mid 80s, which included a twin-deck (3D8), a 3pc (3D9), a sort-of mini (3D2) & even a CD boomer (disappointingly named CX-W800). The original Hitachi 3D (TRK7620) stayed in this first-gen 3D line-up, becoming the 3D7 somewhere along the way.
If anyone can adequately explain the numbering of the 3Ds (beyond the higher the number, the bigger it is), I'd be very interested.

I've always felt that the success of the Hitachi 3Ds was mainly that previously, a decent amount of bass was only possible with the more expensive boomers. This concept brought the more impressive sound into the mainstream budget, so people like me as a teenager could afford one.
Pretty much every boombox manufacturer very quickly had their own version available - Sony's Megabass, Sharp's X-Bass, JVC's Hyper-Bass, Panasonic's XBS, etc. More bass became a fashionable feature in the late 80s, which, at the budget end of the market, led to some rubbish 'extra-bass' buttons added. Without any extra amplification or decent woofers these just muddied the sound at lower volumes & distorted the sound at high volumes.

 

[IVH]

Fortunately the 3d7 does not have a button like that What's the point of such button anyway, playing with the equilisers achieves the same thing, doesn't it ?

And you must be happy cause another 3D7 has been 'saved' !!
Thanks to norm, the hiss is gone !

He did loads of troubleshooting, it ended up being 2 pages long. Way too long to post here, but here is a sum up:
- We started with bypassing components and 'cutting the circuit' starting with the power amp and going up to the line in RCA jacks. This resulted in the pre amp and anything before it being suspected.
- With a brand new pre amp installed, the hiss persisted.
- We checked if the voltages measured at the different legs of the pre amp were as indicated, that seemed to be OK.
- Then norm came with the (according to me) genius idea to bypass the tape preamp transistors, and installing smaller value resistors on the left and right channels. The old one (680k) was heavily attenuating the signal. A smaller one would make the tape pre amps unnecessary. The 680k resistors (R412 R and L) have been replaced with 288k ones.

This did the trick !! The sound is even louder now, and it sounds way cleaner to me too.
I'm impressed with this boombox (and norm's knowledge), it is much louder and sounds much nicer than I ever thought !

I am sooo grateful to norm, without his help I would've given up long ago.

 

[IVH]

Ps: here is a photo of the fix:

 

[isolator42]

You have certainly put in alot of effort, good for you.
I made my 3D7 look the part again - restoring back to original from a sticker-tastic 80s, but when I found the tape motor was clapped out after many years of use, I went with a Digisette & never regretted it:
https://www.amazon.co.uk/Digisette-AR496-Duo-MP3-Player/dp/B0000A1O6K
http://www.stereo2go.com/topic/the-digisetterevisited
I swear the 3D7 never sounded so good as it did with the Digisette. With a following wind, it really did make a decent, rich sound, despite it's lowly status compared to the top flight boomers.


...these days, of course the likes of the Bose SoundLink Mini have tempted me with their outstanding sound to convenience ratio, but let's not dwell on that

 

[Superduper]

We had concluded that the noise was being introduced at the tape preamp stage. With nothing more than a voltmeter, there was no way for the OP to narrow it down further except to do parts swap. While the plan had always been to restore the tape preamp section, when IVH clarified that there were no plans to use the tape deck, ever, and that it was to be modified for other purposes, then that is when we decided that it was best to bypass the tape preamp section entirely.

There are 2 reasons for this: 1 ) this boombox does not have a true line-in feature. Instead the line-in jacks are intended for those signals to be recorded to tape. Therefore, the line-in signals are highly attenuated to suit the recording circuitry, and then re-amplified via the tape preamp for monitoring. For obvious reasons, this is not nearly as desirable as having the line-in signal treated as a line-level source. Without a working tape deck, there was no point in restoring that faulty circuitry because it would never be used. Additionally, some of the circuit could be disabled to reduce idle current consumption since they really are just going along for the ride without being used. 2) Additionally, since the OP's priority was for listening to line-in programs, bypassing the recording circuitry will allow the line-in program to be played without the need to put the boombox into "record" mode, especially since that would involve the signal path to traverse several switches including the dreaded oxidation prone record-bar, and also require the use of the record button to be locked down, either by modifying the tape deck mechanics, or by inserting an appropriate tape into the chamber. And of course, that would mean that both the tape and erase heads be energized with AC bias current, and that the tape motor be operating (unless of course that were to be cut out of circuit).

So with careful bypassing of the audio signal, and by disabling the (now) parasitic current draw of the 1) tape preamp, 2) mic mixer, 3) tape bias oscillator and 4) if desired the internal mics, the boombox now can playback line-in signals by simply putting the unit into tape function. All of this was done without any parts needed except replacement of the line-in attenuating resistors with a more suitable value. Based on IVH's observation of the results, it sounds much better than before now so I suppose this is a good modification for anyone that has one of these guys and wants a line-in feature, without need of future use of the tape deck.

I will note that for those with a good working tape deck that wants to have a (true) line-in/aux feature without having the circuit traverse the tape circuitry, this model is a good candidate for another method which I documented in a line-in mod on a Wards Airline. It looks like it could be done the same way with this model.

 

[Superduper]

AFAIK, Hitachi first tried the extra, separately amplified bass speaker in a boombox thing with the TRK7620 (Can't say for sure if it was the first as the Sanyo BigBen may well pre-date it). It was a success (particularly with me), leading to Hitachi's 3D series in the mid 80s, which included a twin-deck (3D8), a 3pc (3D9), a sort-of mini (3D2) & even a CD boomer (disappointingly named CX-W800). The original Hitachi 3D (TRK7620) stayed in this first-gen 3D line-up, becoming the 3D7 somewhere along the way.
If anyone can adequately explain the numbering of the 3Ds (beyond the higher the number, the bigger it is), I'd be very interested.

After flipping through some of my service manuals, I found one for a 3D2, 3D5, the 7620, the 3D8 and even a 3D30. Of interest is that the 3D8 appears to not only include a double deck (compared to the single deck of the 7620) but studying the diagram, it appears that it also has a real line-in feature too.

 

[Helmar]

I also have one of the TRK 7620 retrofitted with a bluetooth receiver.
I added a switch in the battery compartment, which enables the receiver and disables the tapedeck motor (and vice versa).
In order to upgrade the power supply like in in old days of analog audio I added a bigger capacitor (decoupling Capacitor 1,69€, 6800uf/35V www.conrad.de).

Now I read this interesting post and I would like to know if the gurus could add a more detailed instruction in order to achieve better audio quality (which is not bad in the original state of the boombox like it is now) with regard to this:

QUOTE
So with careful bypassing of the audio signal, and by disabling the (now) parasitic current draw of the 1) tape preamp, 2) mic mixer, 3) tape bias oscillator and 4) if desired the internal mics, the boombox now can playback line-in signals by simply putting the unit into tape function
UNQUOTE.

That would be an interesting upgrade.

A detailed report of the restoration and some more photos will follow once I find the time ...

Best regards
Helmar

 

[IVH]

Hey helmar,
Good to see another 3d7 fan in europe! Yours seems to be in great condition.
You mention you made a switch to switch between the tape motor and the module, does this mean you're still using it to play tapes ?
You won't be able to after this mod.

If you don't have one, I recommend buying the 3d7's service manual from analogalley, it makes the job much easier. You can also use the service manual posted here, but it can sometimes be a bit difficult to read.

To bypass the tape preamp, you'll have to lift a leg or remove R412R/L, C405R/L and R417R/L. The last one is not a must, but I had completed the bypassing when superduper told me so, so I left it like this. As for removing them or lifting a leg, I recommend just lifting a leg so it can be restored at any time.
Be careful to lift the correct leg, as you'll need some 'holes' to solder the bypass wires.
After this, you'll have to solder bypass wires between the former R412 hole (the one that shows zero resistance to the RCA socket) and the hole from C405's desoldered minus pole.
R412 does, as learned from superduper, an attenuation resistor, without it, the signal the preamp receives will be too high and it will distort all the time, even at low volume. But, as we bypassed the tape preamp, you can replace it with a lower value. The value depends on your bluetooth module's volume.

I replaced it with a 288k one, that suited if I connected my phone through the rca ports. But my bluetooth module has a much higher line out signal, so I had to add another 160k on the bluetooth module's audio cable.

I recommend starting with a resistor about the same value as mine, turning the bluetooth volume to max and test it out. If it distorts, add 100k, if the volume is too low, remove 100k, then repeat.

Here are some instructions from superduper regarding the disabling of the other components:

I took a look at the schematic and I think you should be safe to remove the following components (or lift a leg). But if you do, you should do them in stages and check for proper operation between each stage. Otherwise, if you do them all and then there is a problem, which do you restore?

Mic Mix: R434 R/L, R435
Tape Bias Oscillator: R424, R425, R426
Tape Preamp: R407
Internal Microphone: R430

Hope this helps

 

[Helmar]

Thanks for the instructions - good to know.
Currently I will leave the 7620 as much as possible in its original and genuine state. I also have a 3D88e waiting for its restoration, which I prefer over the 3D8 and 3D80. However the build quality of the 3d7 is the best and was thinking about changing the loudspeaker chassis to better ones and making a closed box resp. bass reflex box out of it. But this approach is only an idea which might consume more time than available for that.

Helmar

 

[IVH]

I finally put it together a few days ago, it played fine for a few hours. And the lipo battery lasts for hours !!
But then the left speaker started making a 'farting' noise when there is some bass. I thought the speaker was dead, but I was hoping for some bad wiring. After getting the top off, and reconnecting some connectors, the noise was gone !
Being happy the speaker is fine, i put it back together only for the farting noise to be there again ??!!
It had to be related to the top cover pushing some wire, right ?
Well, after some time I noticed it only occurs when the VU led board is plugged in:



I don't quite understand how this affects the sound, and even less only the left speaker's.
The only explanation I see is that the vu meter is only monitoring the left channel. But that isn't logic.

 

[Superduper]

The problem could be that the preamp module (BA343) is unable to supply enough current during heavy bass transients. This could be due to either the current dropping resistor values have changed in the signal path, or that the LED chip is drawing too much signal current, or that the preamp is not an OEM original part, rather it might be a counterfeit or a poor performing generic replacement.

 

[IVH]

Thanks for the information, I removed the vu leds for now. I'll look into it some day but first I'll enjoy it!

The first possibility sounds right, as I noticed that all other frequencies are mute when there is some bass, this happens even when the low frequency equaliser is at 0 db

 

[isolator42]

After flipping through some of my service manuals, I found one for a 3D2, 3D5, the 7620, the 3D8 and even a 3D30. Of interest is that the 3D8 appears to not only include a double deck (compared to the single deck of the 7620) but studying the diagram, it appears that it also has a real line-in feature too.

Yup the 3D8 does have a 'proper' line in & a very similar sound to the 3D7 too. The downside is the EQ on the 3D8 is only 3 band...

 

[IVH]

The problem could be that the preamp module (BA343) is unable to supply enough current during heavy bass transients. This could be due to either the current dropping resistor values have changed in the signal path, or that the LED chip is drawing too much signal current, or that the preamp is not an OEM original part, rather it might be a counterfeit or a poor performing generic replacement.

Hey superduper,
I just wanted to let you know that my 3d7 is still working great!
I am still extremely with it and your help fixing the noise issue.
Once I get a better camera, I'll post a video on YouTube of it in action.
Hope you are doing well!

 

[trk7620]

Ps: here is a photo of the fix:

20161016_190358.jpg

Nice to see so many Hitachi TRK fans - I bought mine in the mid 80-ies brand new and it was a good bang for the bucks.

Everything but the tape was working when i looked at it last year, it had then been put away for 15 years.



Many thanks for the picture of the pre-amp fix.

Last year I decided to update my TRK-7620E with a blutooth mod. I always suspected the hissing sound on the line in originated from the tape pre-amp. Not knowing how o fix this i put the bluetooth mod in hibernation and put the machine away.

I removed the tape head but got sound through the line in

I've tried to use the power feed to the tape engine to run the BT device that runs on 5V 1,0 A. The power feed to the tape engine have a high variation on the voltage though. I ended up buying a USB port with an buildt in power regulator to fix this and made an USB-A to 4x1,7 mm barrel power jack. This did not solve my problem in the long run even though I was able to listen on BT streamed music for 5-10 minutes yesterday. Pressing differnt tape controller keys gives me different voltage on the power feed to the (former tape enginge)/usb poert with voltage regulator.

I might have to find a way to run the usb and BT some other way but it was a nice feature to power up BT when pressing record and play

The hissing sound was there since i had not done the pre amp bypass.

I'll get some pictures and update here when i dig deeper - I have a BT device that draws less amps, that might fix it.