New owner of RX5150. Left channel very weak.

[robgmn]

Hi all.
Proud new owner of a pretty clean 5150 that I bought untested, but was advertised as "worked when plugged in". (I probably shouldn't say what I paid for it

I gave it a wipedown and plugged it in, and it made a horrendous screeching noise in Tape and Radio mode (not line-in) even with volume all the way down.
I plugged in a set of 'phones and it still screeched, so I started giving all controls a workout. The screeching subsided and music started to play, but the left channel is VERY weak, through the speakers and the headphones, when listening to radio or line-in (I'm afraid to try the tape deck).

Since it's both speakers and 'phones, is it more likely a dirty pot, or a bad amp? For what it's worth, the controls are all very quiet when I move them around, and I cycled the record bar (via the front control button) about 10 times with no change.

I can open it up and clean stuff if needed, but would rather have a specific goal than just go fishing and screw up my new find (I've read some of the nightmare stories of broken tuner strings, etc).

Rob

 

[Superduper]

I bet the record bar needs deep cleaning with assistance of solvent, try Deoxit.

 

[robgmn]

I bet the record bar needs deep cleaning with assistance of solvent, try Deoxit.

Wow! Fast reply!
How tough is it to get to? Can I just pop the back cover off, or is this one of those things that's built like a Jenga puzzle with every piece held in pace by another piece, and all kept together by the shell?

 

[Superduper]

The front cover comes off. You'll have to remove the cassette door in order to do it. Everything else is populated onto the back cover.

 

[Fatdog]

Yep, screeching / squealing is usually caused by a dirty record bar. Here is a link to what you are looking for (note the following link is a different boombox, but the record bar will still be similar):

https://boomboxery.com/forum/index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=35118

Spray some contact cleaner in there and work the mechanism by hand a few times. It should help clear things up.

 

[robgmn]

Update: I left it unplugged last night (I don't trust new high voltage stuff until it's proved itself in my presence).
When I plugged it in this morning, major squeal for 30 seconds, then it stopped an played music.
I unplugged and plugged right back in and got a momentary squeal, then music (with the almost-dead left channel).

I'll be working on attacking that record bar tonight or tomorrow night and will report back.
Thank you!

 

[robgmn]

Update #2.

I opened up the unit and sprayed the record bar internals with Deoxit D5, then I manually operated the lever for about 50 cycles.
I let the unit sit for an hour, and did it again. I also did all of the switches and pots, and gave the headphone jack a shot and plugged and unplugged a headphone plug about 15 times.

When I plugged the unit in, I got that horrendous screech from the left speaker (quite loud, so i KNOW in some way it can drive the speaker to a high volume). Once that calmed down, I could get it to play radio, but the left was still very quiet, with a hum
I swapped the speaker connection to make sure it's not a bad speaker. (It's not.)

I did a visual and don't see any burn marks on the board, no bulging capacitors, nothing that would immediately say "broken".
I'm good with electronics as far as being able to replace parts at board level if I know they're bad, but I cannot diagnose bad circuits.

Does it sound like it should go to a pro at this point?

 

[Superduper]

If after all this it still does the screeching, then might be time to recap the box. Mine did the same thing, weird gremlins and I didn't bother to mess around, just did a full recap of the unit. You can remove, test and replace just the individual bad caps, but I'm of the opinion that if a set is 35 years old and you have one (or more) failing caps, the rest aren't far behind. Furthermore, if you remove a 35 year old cap for testing, do you put it back in? Small caps are extremely cheap. Only the large ones cost anything significant. If you are good at this, then you can either do this or send it in for repairs -- up to you. For me, I have more time than money. If you have more money than time, than a pro might be a good option but be prepared to pay.

 

[robgmn]

I guess I'm going to have to flip a coin. I don't have a lot of spare money, I don't have a lot of spare time.

What if I replace every cap, and it still doesn't work? A lot of time, plus a bit of money lost.
Then again, I'm sure it won't be terribly cheap to have a pro diagnose and repair only the bad part/s either (gone are the days of Radio Shack repairing a dropped Sony camcorder for less than $20).

Is that screeching/squealing likely a cap "re-forming" until it actually does some of its job?

 

[Superduper]

Is that screeching/squealing likely a cap "re-forming" until it actually does some of its job?

Probably a weak leaky cap trying to charge up. Screeching, motor boating, etc type of sounds are a symptom of oscillation in the circuits. I don't know about $20 but I can't imagine anyone even willing to look at it for less than $60 to simply give a diagnosis or estimate. Most shops will require you to pay to get it back even if you don't do any work, or apply the diagnostic fee to the actual repair. And yes, it's possible that you can replace every cap and still might not fix it since you are shotgunning and haven't truly diagnosed and concluded that it's a cap. You said yourself that you aren't good at diagnosing so that's the option to you based on your limitation. As caps are the usual culprit, that's the suggestion since they are components with an age-related mode of failure. Resistors don't fail with any pattern or regularity unless they are fusible types which I don't believe this unit has. Semiconductors simply aren't replaced unless they fail and unless you can troubleshoot (you said you can't), then those should be left to professionals to fix.

A final note and caution: If you do a recap, and you inadvertently reverse the polarity on the caps (I'm only referring to electrolytic caps or certain tantalum caps for replacement, not other types), you may complicate the troubleshooting even further for the tech. Nobody likes to clean up someone else's mess and it could extend the diagnostic time well beyond what it might've been.

 

[robgmn]

I have the svc. manual, and there are (!) 167 capacitors between the ceramic, mica, polyester, styrol, semi-condcuctor, and electrolytic type.

Am I correct in assuming that it would be only the 60 electrolytic caps that i would be replacing if I were just doing a full recap, or does that get narrowed down even further to just doing the ones in the amplifier section (if such a possibility exists)?

 

[HRmeteohub]

Well, there is no simple answer besides yes, you could start from amplifier and then go backwards. However, there are many strategies. As Superduper said, if one electrolyte is bad, usually more of them are going that way.

Just start from somewhere, go slow, do not turn the power on until you are finished and positive that you did it all the right way. Be patient and take pictures before & after, so if something goes wrong, you can have some clue what you did wrong.
Come on, this is not heart surgery, only a boombox and opportunity to learn. Even if it bursts in flames, you should consider that as a lesson and therefore money well spent. Don't give up before you have made it! We have all started that way and we are still learning.

 

[Superduper]

In answer to your question, yes, only replace the electrolytic ones. As you can guess by the name, they have an electrolytic fluid in them that over time, tends to dry out causing problems. 35 years is a long time, and if they haven't failed, they would be living on borrowed time anyhow. The other components don't have this predictable mode of failure so those components are replaced on an as needed basis. Generally speaking, their failure usually isn't like an on/off switch. More like gradual performance deterioration which can lead to oscillation, open or short circuit etc. causing weird problems. It would usually be at this point that service is then contemplated. As for picking and choosing where to replace -- it's more likely in the mainboard. The tuner has a few and probably don't need changing but it's only a handful and why not change them at the same time?

The low operating voltages of boomboxes and relatively lower operating temperatures have been kind(er) to capacitors than other audio gear (such as home audio) which has a far higher rate of capacitor degradation and failure. Most all will need capacitor replacement to some degree for proper operation now.

 

[robgmn]

Well, it looks like I'm going down recap road. I really don't want this to become a "learning experience" that results in a dead box, so wish me luck.
Any recommendations on a source for caps (DigiKey? Newark? etc.)?

Also, on a slight tangent: Has anyone ever modded the inside of the box in any way to improve the speaker sound? I was thinking maybe a pad of rock wool behind each "woofer" to make a better approximation of an actual speaker cabinet (I noticed that the box is vented in the back, which makes me think it would act liek an open baffle and reduce the bass a bit).

I tried searching for any ideas on this, but most of my searches of this site come up with "no results found", and then I get told to wait a while before trying another search.
thank you for the help in getting me this far in the "project"!

EDIT to add: A weird thought just crossed my mind - What are the odds that I could tap into the audio signal at line-level before it goes into the amp circuit (but after volume control), and just feed that to a little class D amp that I could stuff into the cabinet?
(Can you tell I'm afraid of doing the recap? )

 

[Superduper]

Mouser, digikey are good sources. I've never had a problem with Mouser, always has in stock what it claims with no funny business when you get the bill. I had nothing but trouble with another company, not sure if that was Newark or not.

These speakers are intended to be operated with the back vented. If you seal them up, you will also seal up any ventilation to the interior which the system might need for heat expulsion. Also there is no internal isolation between the left and right speakers. If you seal up both speakers together into a single cabinet, you will turn your system into a weird mixed mono system. However, with the cassette chamber, tape counter reset hole, all the plugs and ports and knob openings... good luck trying to do an effective seal job. However, on mine, I noticed that the tweeters (piezo) were not emitting any sound. I put true cone tweeters in there and rigged them up through a high pass cap filter. Not only did this improve the treble but I swear that the bass improved as well. On the other hand, I did recap the whole box so what to attribute this to is not 100% clear.

If you stuff your D amp in there, and stay with stock speakers, you will probably blow those speakers. Also, if your left channel is dead due to a failed cap in the system, chances are if you install a D amp at where it connects to the old amp, you will have a brand spanking new amp with a "still" dead left channel. That you get a loud screeching noise out of the left channel on occasion already suggests to me that your amp has no trouble making output. If you have a signal tracer and follow the signal path from line-in to amp, you can find the bad component.

 

[robgmn]

You gave me a moment of clarity.
Very loud screech means amp is able to drive a speaker. If amp is able to drive a speaker, it's likely not a bad amp, but whatever is in the chain feeding that amp. +
Makes total sense (same goes for the cabinet "mod").
OK, I'll be putting together my parts order and crossing my fingers.

Thank you very much for helping me out with this.
Updates as I soon as I have more to share!

 

[Superduper]

On the Panasonic RX-5150, both L and R amps are embedded within the same amp IC. Although it's not unheard of to have only 1 channel in a module fail, that's not the first thing I would look at.

Just a little advice. When you order the caps, a lot of them might have the same uf value but have different voltage specs. You can save yourself a lot of time and confusion during the recap by ordering all the caps the same (higher) voltage. For example, if you have caps with 10uf needing 10v, 16v, and 25v, you can just order them all in 25 (or higher voltage) and be done with it. The reasons for going with lower voltage caps has to do with physical size and cost. Since modern replacements are typically smaller than vintage caps anyway, size isn't an issue so it only has to do with cost. For a manufacturer that is ordering millions of caps, saving a few pennies here and there adds up. But for you, the difference amounts to nothing and the higher voltage caps will have greater safety margin. Furthermore, it's much simpler when you need a 10uf to just grab a 10uf and not have to worry about going through the 10's to find the proper voltage too. Been doing this for decades and never had a problem going with higher voltage caps in place of low voltage ones except where higher voltage ones won't fit.

 

[Fatdog]

That's a neat little tip regarding the capacitor voltages, Norm. Thanks!!

 

[robgmn]

Caps are on order. All Nichicon except for a few Panasonics.
Shipping is cheaper with Mouser, but it's easier to build the order on Digikey (though still clunky and time consuming).
Looking forward to delivery so I can get started!

Am I correct in thinking the whole board is one big slide-in/slide-out module? (i.e. Unless I really screw up, I don't have to undo the tuner string or mess with it in any way)

 

[robgmn]

And, the caps showed up and I replaced all but seven of them. The seven unreplaced ones didn't have a match in the pile that I ordered. I'll be going over the board again on Saturday as the cap values for servicing may be different than the original build (?).

But with over 50 caps replaced, the only change is that it doesn't squeal/screech from the left speaker on startup. The left channel is still playing at much lower volume than the right. I already did a DeoxIT treatment on everything, and have now almost recapped the entire thing. Is my next step to replace the amplifier IC (if such a beast is available)? Again, keep in mind that when it squealed on startup, it was very loud and was coming from the left channel.