RC-M70JW with permanent low volume issues

[lazol]

Still seems like you didn't properly check the function switch, which would be first on my list.
An improper wire connection, as docs said, would be the second.
Try using a compressed air can upside down on the boards and on the components, little areas at the time, while the radio is on. This will create iced spots which may engage a lost connection somewhere, and may pop up the sound for a moment. This might help figuring out where the issue is coming from.

 

[hopey]

could even be the volume potentiometer as they have contacts that move they may have failed after use.

 

[lberstein]

Hello again to everyone, I'm very sorry for taking so long.
I managed to get hold of a 12V DC power supply and retested the suggested spots. Power amps still seem to be ok, this time (with DC power) I get 14ish V on the 15V pin, and 6.8V on the 7.5V pins. Considering that with AC power I got slightly overvolted results, I think this are well in the expected range.

Transistor X305 however shows 0V on all 3 pins. Obviously there's something wrong there, but the fact that all 3 are 0V makes me think the problem is somewhere before that, maybe? Local suppliers don't carry that transistor (it's 2sd636), and before sourcing it from a very expensive foreing provider I'd like to keep testing to make sure that is the one (and only?) cause of the problem. Any suggestions what I should test prior to X305?

I'm not sure how to check the function switch properly. I have cleaned it to the best of my ability and have been swinging it back and for in all manners. Whatever I do to any switch or slider it doesn't produce any change (except the expected, i.e. turning the radio off when I switch to line in or phono). There is no variation in volume level, no crackling, no weird noises, nothing.

 

[docs]

The thing is, when trying to find a fault like this, it is logical to think it could be a single component which is tied to both channels. Because, for example, what are the chances that both x305 for left and x405 for right have both failed?
A very basic signal injector is needed here in my opinion, it will make it easier to find where you can here a tone from in the circuit and where you can’t. They are pretty cheap and you might be able to use the leads from your dmm.
Did you say you tested all the sliders on the volume board?

 

[lberstein]

I'm looking for a good signal injector I can buy locally, I'll try to get one soon.
I agree that anything that is channel-related is very unlikely to be the cause of the issue.

 

[lberstein]

I did test all the sliders, cleaned them and moved them all around. But then again, what is the chance of both volume sliders failing simultaneously and in the exact same way?

 

[lazol]

I keep thinking about a switch, because you said that radio has low volume, tape has low volume and line-in has NO volume. All of this both channels equally.
The only places those 3 signals meet together are the switches (loudness, function etc.).
Personally i would carefully inspect the board around the switches and around the connectors for cold joints and broken paths.

 

[lberstein]

Hello everyone. Sorry for the long delay. I finally got my hand on a can of contact cleaner. I have been spraying the unit vigorously up and down, all sliders and switches, moving them a lot. Issues continue in exactly same way.
Also at some point, not sure when, deck player stopped producing any sound. It works mechanically, but no sound when playing a tape. So now I have low voume on radio and no volume on deck player.

I ordered a signal tracer, not really sure how to apply it to this, what should I test with it?

I'm starting to lose hope I'll get this unit working again

 

[caution]

Didn't you say the volume sliders aren't doing anything? How about bass and treble, do they affect the sound?
I'd verify the volume sliders change resistance as expected, make absolutely sure the slider board and its connector are not the culprit.

 

[Radio raheem]

Personally if you seriously want this box to work i would send her to someone on here who knows what there doing (long since i messed with boomboxes myself) good luck i admire you're dedication sunshine

 

[lberstein]

Didn't you say the volume sliders aren't doing anything? How about bass and treble, do they affect the sound?
I'd verify the volume sliders change resistance as expected, make absolutely sure the slider board and its connector are not the culprit.

Volume sliders do work. If I slide them down volume goes even lower (down to zero). If I slide them up to max volume gradually raises but even at max the unit produces low volume. What I said is that fiddling with the sliders doesn't produce any change in the sense of volume spikes, crackling, nor electric noise of any kind. Same goes for the other sliders: treble, bass, loudness, etc. They all work as expected, but it's as if the whole volume level of the unit is capped.

 

[hopey]

It's sounding like a pre amp cap problem. On a different box I did a full tear down on all pots Volume Balance etc. I had some crackling going on. When I powered it on it worked perfectly then a cap blew with a puff of smoke. I think what happened was the Pots with high resistance were protecting the cap once I cleaned the pots and applied full voltage to the cap it let go. Its a domino effect.

 

[caution]

Yeah, that's kind of what I'm thinking too, hopey. The louder-than-max-volume spikes from turning it on are coming from the main amp, and the preamp has issues letting any audio source get to the amp inputs at full volume. Could be a bad cap or bad resistor I suppose?

 

[hopey]

Yeah, that's kind of what I'm thinking too, hopey. The louder-than-max-volume spikes from turning it on are coming from the main amp, and the preamp has issues letting any audio source get to the amp inputs at full volume. Could be a bad cap or bad resistor I suppose?

Yes due to the dirty Pot sending full volume voltage spikes due to short circuit on the inside of the pot has damaged the Cap. This can appear as an intermittent fault until the cap eventually lets go. If you did a trace you could pickup that faulty cap straight away.

 

[lazol]

Personally if you seriously want this box to work i would send her to someone on here who knows what there doing (long since i messed with boomboxes myself) good luck i admire you're dedication sunshine

+1
Distance troubleshooting can only go to a certain point. After all the basics are covered, you need to have the box in your hands to find out what's going on... Unless you want to learn the job, in that case take your time and you'll get to the solution at some point!