Need some help fixing vintage RCA amp

[Pityocamptes]

Its an RCA VTT301L turntable with 2 channel amp.

Instead of retyping everything and what I've done if you could please read it here:

https://www.edaboard.com/threads/i-n...ier.413493/

I will upload the schematic here if that helps. Thx

[eccerr0r]

well looks like most of the work is done isn't it?

Did you try swapping C416 and C417 or swapping the suspect bad capacitor with a spare or even a smaller µF if you don't have an exact one?

I think you can ignore R416/R426/C420 and R417/R425/C419 for now, they shoudn't fail in a way that would cause no audio output. They will pull a weak capacitor to 0V as they are a resistor network after all.

So you say the speakers are good, so C416, swap it and see... A dried up, broken C416 will behave the way you seem to observe.

[Pityocamptes]

Thanks! Speakers have been removed from amp output terminals, just testing power at amp out. I did swap speakers with known good channel and both speakers worked perfect. I also swapped c416 and c417 and it made no difference. I'm at a loss. Right up to the 1000uf + caps pads every voltage is identical or within 5%. After the cap, is where the one failed channel has 0 output. You can still hear audio its just not amplified. So probably just raw input?

[Pityocamptes]

I have an oscope but it's in storage, been a while since used. Would something simple like this be better to diagnose over a VM?

3 in 1 Digital Oscilloscope Set Handheld 500khz Bandwidth LED Display Oscilloscope Transistor Tester Signal Generator for Office Home https://a.co/d/79F0veQ

Thanks.

[R_J]

Do you know if the signal is being lost in the preamp circuit or in the power amp circuit? If you do not know, try connecting a jumper wire between A and C on that plug and see if you get audio from both speakers, if you do then the fault is not with the power amp section.

Are you using the internal speakers or external speakers? if internal make sure both external speaker connections jumper straps are connected to the center screw and all three screws are secure.
Most problems like this on old stereos is caused by oxidized switch contacts or controls.

[redwire]

The power amp uses output coupling capacitors, it's not DC-coupled. If the problem is the power amp, measure some voltages. The schematic shows many of them. The voltage at C416, C417 will indicate overall health of the amp and 15.43V is expected on (+) side and 0V on the (-) side.
It's not a capacitor problem for something vintage 1974 to have a dead channel.

[eccerr0r]

Technically in steady state/quiescent/0V input, the voltage at the input of the capacitor should be about half the rail. and the output should be near 0 V because the speaker is pretty much a short to ground at DC. If it's not 0V there's an open somewhere after the capacitor...

if there's no open after the capacitor and all the voltages prior to the capacitor matches the other channel, then you'll need the oscilloscope to find where the signal is lost. Severe loss in volume is a rare failure mode in circuitry unless it's a bad pot/connectors/loose ground or something like R402 got really high resistance but not go completely open somehow...

You did test both channels independently right, i.e. not using a stereo source and seeing if one channel is working or not?

[redwire]

Back in the day, it was a major operation to get these console stereos (or CRT console TV's) repaired. Hauling them to a shop was like moving furniture, a sofa just really big and heavy in a 1/2 ton. If you fixed it and it didn't last or work and customer had to haul it back, you were in trouble...
Once boss did house calls but you never had a schematic or all the right parts in the van to fix things. It was not easy.

RCA was king of transistors in this era, they had good power transistors competing with the best and even some neat IC's. So this is actually good tech for 1974.
For the search engines RCA VST300, VST310, VST320, VST330 with RC-1245 and RS252 chassis inside. The turntable is a RP234C.

One mod OP can consider if he gets this fixed is adding a Bluetooth audio module inside so you can use the amp with modern source material. Just need to add a 5V power supply and a switch for the audio input.

[Pityocamptes]

Thank you for your help! I really appreciate it. At the advice of another forum and due to the system being 50 yo, i decided to replace all the caps on the amp. I went down to the electronics store and picked up the caps, they looked at them via pics and the schematic I took and gave me the correct ones. Now, some were at most 3uf off, the guy that was helping me said that the parts they carried were standard and some of the cap values I assume are no longer considered standard? Anyways, cleaned up everything, double and triple checked everything and soldered. Well the sad part is now the channel that was working no longer works... this is so frustrating. I did some measuring and here is a pic of what I measured. Please let me know how to proceed, I really would like to continue using this system. Thank you.

[Pityocamptes]

Technically in steady state/quiescent/0V input, the voltage at the input of the capacitor should be about half the rail. and the output should be near 0 V because the speaker is pretty much a short to ground at DC. If it's not 0V there's an open somewhere after the capacitor...

if there's no open after the capacitor and all the voltages prior to the capacitor matches the other channel, then you'll need the oscilloscope to find where the signal is lost. Severe loss in volume is a rare failure mode in circuitry unless it's a bad pot/connectors/loose ground or something like R402 got really high resistance but not go completely open somehow...

You did test both channels independently right, i.e. not using a stereo source and seeing if one channel is working or not?

Yes, I swapped the inputs and the channel that worked continued to work, and the one that did not, still did not. I also swapped speaker wires to eliminate a blown speaker, both sides worked on the functioning channel (which after the cap replacement nothing works... ) Thx

[Pityocamptes]

well looks like most of the work is done isn't it?

Did you try swapping C416 and C417 or swapping the suspect bad capacitor with a spare or even a smaller µF if you don't have an exact one?

I think you can ignore R416/R426/C420 and R417/R425/C419 for now, they shoudn't fail in a way that would cause no audio output. They will pull a weak capacitor to 0V as they are a resistor network after all.

So you say the speakers are good, so C416, swap it and see... A dried up, broken C416 will behave the way you seem to observe.

Yes prior to it not working now, I did swap the caps and no change. I did replace the resistors for the r416, 417, r426, r425 while at the parts store. No difference.

[Pityocamptes]

Here are some pics of the new cap components, hopefully it looks normal?

[Pityocamptes]

Previous images before cap replacement

[eccerr0r]

Reading your other followups in the other forum it looks like you failed to follow an important mantra when working on something you don't understand:

>>> Change one thing at a time.

Your first priority is to fix your previously working channel and you definitely did something to break it. Check all your connections carefully.

Also I disagree with the person who claims low frequency scopes are of limited use -- pretty much any scope, due to the low frequency of audio, is usable. In fact a bandwidth of 100KHz is sufficient for most debug and a 200KSPS DSO is plenty for audio work, unless you're trying to debug some really high treble issue -- but a totally silent amp is not this.

Another useful tool is a signal injector or just something that you can inject audio signal into the signal path to see if it propagates to the output. The finger induced 50/60Hz hum (from line frequency noise) works too but you have to be careful to not touch dangerous spots and it doesn't have very low impedance, which will get swamped out by transistor drivers. Also depending on the bandwidth of the amplifier and how big your speakers are, 50Hz can be hard to hear.

[Pityocamptes]

Yes, I failed at that, thought that exact thing but it was to late. I swapped out everything from mid way to the outputs and the one channel still worked. After swapping out the other half, starting at the red line in pic it stopped working. So would the 50 dollar oscope on amazon be ok to use? I believe it had a sig gen and operated at 500khz. So at this point should i be measuring voltage with a VM or go the route of the oscope? Been a long time since Ive used one, would need some help. Thanks.

[Pityocamptes]

Would something like this help me? https://www.amazon.com/Oscilloscope-...8&sr=1-10&th=1

[eccerr0r]

As said, any scope will work as long as you know how to use it.

Did you only disturb electrolytic capacitors or did you also replace ceramic/polyester caps? I don't think any capacitor really needed to be changed but for the "J"/Left channel:
capacitors removed/broken that would cause no sound: C403 C417
The other ones would cause sound quality issues.
If C403 and C417 are intact, then did you check all the voltages against the voltages on the schematic and find a point where the voltages don't mismatch?

[eccerr0r]

Oh it did look like you changed the polyester and ceramic disc caps. Nope, it's on you to figure out what was disturbed.

[Pityocamptes]

So I have been going through the power circuit, waiting to get an oscope tomorrow, using my DVM at the moment. Anyways on the attached image I isolated the power supply and I do have approx 32 volts at pad 1. I tested the circuit to pad 2 and before R42 resistor I have 32v, after the 680k resistor I have approx. 14 volts. It appears that this also feeds the other channel circuit as well. Is this an issue, am I doing something wrong? Thanks

[eccerr0r]

it appears that power is used solely for biasing the bases of Q41a and Q41b. What are the voltages there?
This is a high impedance power supply so it will be heavily dependent on these loads.