I need assistance tracking down failed component on old RCA amplifier

[pityocamptes]

The right channel is being amplified, the left is not. I tested the transformer voltage on both outputs and trans is working at around 23v per leg. At the output im getting around 4 volts or right, 0 on left. You can still hear unamplified signal through left channel though. Does anyone based on the pic have an idea where I can start taking readings? I have VM. Any help appreciated. Thx

 

[betwixt]

Capacitors should pass audio but not DC so it is quite normal fro there to be zero volts on the negative sides of both C416 and C417, in fact if there was a voltage it would indicate the capacitor was leaking current which is bad.

Try this test, you will need to turn the volume up with some sound expected, for example from a radio station:
Put your testmeter on a low AC voltage range, ideally less than 2V,
You should be able to hear sound through the working loudspeaker,
Connect the meter probe between chassis and the positive (+) side of C416 and then the positive side of C415.
Repeat with the meter probe between chassis and the negative sides of the capacitors. You should now have four measurements.

You are measuring the signal voltage that should reach the loudspeaker in both cases. Because the left channel is working, you should measure a voltage on C417, if you also see a voltage on C416 it means the main part of the amplifier is passing signal. Note that the voltage will vary with the volume of sound so expect it to go up and down. The voltage will also depend on the volume control setting, turning it up should result in higher voltages.

Let us know what you measure, it will help us diagnose where along the signal route the break lies.

Brian.

 

[pityocamptes]

I can't seem to find C415 on schematic. Thx

--- Updated Jan 1, 2025 ---

So far i have:

C416 chassis to cap + = 34v
C416 chassis to cap - = .2v

C417 chassis to cap + = 34v
C417 chassis to cap - = 7.5v

--- Updated Jan 1, 2025 ---

So here is a pic. Pos leg of C416 is 34v but neg is .2v

--- Updated Jan 1, 2025 ---

Here is what I wrote down on other trans and caps. They pretty much mirrored each side:

 

[betwixt]

Sorry, my bad typing (or thinking!), I meant C417 not C415 but you took the correct measurement anyway.

From the measurements you took I'm beginning to think the amplifiers are actually working correctly and the 'blockage' is between their outputs and the loudspeakers. The prime culprit would be C416, it seems to have signal on its positive side but nothing arriving at its negative (loudspeaker) side. However, another test is called for, this one with the power turned off completely.

Set your testmeter to the Ohms range, I can't tell what kind of meter you have but I'm expecting a resistance of between about 10 Ohms and 2K Ohms so you want to select a range to cover that.

1. Measure the resistance between chassis and the negative ends of C416 and C417
2. Measure the resistance between chassis and each end of R451
3. Measure the resistance between chassis and each end of R452

That's six measurements in total. Let us know what you find. Note that you might hear a small 'pop' from the loudspeakers when you connect the probes, this is quite normal and expected.

Sorry for all the questions but I suspect we are on the opposite sides of the World so hands on diagnosis isn't possible.

Brian.

 

[pityocamptes]

Ok so while I've been testing from the beginning speaker terms from J, K , L outputs have not been connected. Just trying to test amp. I did swap left and right side speaker line as initial diagnoses and the working channel J powered the left and the right side when speaker lines were swapped between J.

From Chassis to C416 is 1.32kohm
From Chassis to C417 is 1.08kohm

I pulled c420 and lifted neg ends of r426 and r416. R426 is 1300ohm
R416 is 12ohm.

I tested voltage and neg pad of C416. I now have voltage on neg pad, same as pos value.

I dont have a cap tester on my VM could, C420 be bad in the rc circuit? If so anyway to determine? Thanks

 

[pityocamptes]

I dont understand this. Everything identical up to junction 154. Both sides have exact voltage at pos caps, but the minute r416, r426, c420 are connected it basically gets pulled to zero.

So in ac volts with resistors removed from going ground and cap C420 pulled i get 5.4v on cap C416 from neg leg and other prob on grd. However it doesnt fluctuate like the working side (goes from 2v, to 4v, up and down etc., which is the signal oscillation?)

It appears on the non working side that once the resistor r416 goes to ground voltage goes to zero.

 

[pityocamptes]

As mentioned my oscope is in storage. Been a while since I've used one. Would something like this be better and more helpful for diagnosing?

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

 

[betwixt]

An oscilloscope is the ideal tool for tracing signals but the fact that the voltages are almost identical on both channels makes me think the problem is an open circuit capacitor. Usually, although not conclusively, a dead channel would show some significant DC differences so my guess is the problem lies with a component that doesn't conduct DC at all and hence isn't upsetting the voltages you measure. The component meeting that criteria is a capacitor, I'm still thinking the output capacitor is at fault. Regardless of whether that is the culprit, after all those years it will have deteriorated considerably and for the low cost of replacements it would be a good idea to replace it, and all the other electrolytic capacitors with new ones.

Try another test please, select a quiet input, record deck without a record for example and using a screwdriver as a probe, place your finger on the screwdriver shaft and touch its tip on points 137 then 149. Hopefully you will hear a hum from the working channel but not from the faulty one. The methodology is that your body will be picking up electrical signals from nearby sources, primarily 50Hz or 60Hz ones and you are using the screwdriver as a signal injector to feed it into the amplifier inputs. It is safe but just to be sure, don't touch anything else at the same time. Let us know if you get a big fat hum from one or both loudspeakers please.

Incidentally, that scope from Amazon is of limited use because of its low bandwidth. It will just about do audio tracing but useless for RF or digital signals.

Brian.

 

[pityocamptes]

Thank you. I really appreciate the help. With regards to the caps would you also replace the large brown poly caps or just the blue electrolytic caps? I'll take the values and see what the parts store has. I'll also do that final test before getting parts and post back. Thx again.

 

[betwixt]

I would replace them all, the same degradation will occur whatever the value, rated voltage or manufacturer. You might see some capacitors rated for a specific usage time, for example 5000 hours which is a figure the manufacturer quotes it will stay within specification but they degrade anyway even in storage before being fitted. 50 years is a long time for an electrolytic capacitor even if it isn't used.

In general, the value isn't critical, if you can't find a matching value, use the next higher value you can find. The working voltage IS important, never fit one with a lower rated voltage but again higher voltage rating is acceptable. For example you could replace a 1000uF 50V capacitor with a 1000uF 63V one without problems.

Brian.

 

[pityocamptes]

I changed out all the caps and now the working channel does not work either. This is frustrating. I have put together an excel sheet on voltages any help appreciated. Thanks.

--- Updated Jan 4, 2025 ---

Here is pics of the new caps. Does it look normal? Parts store said they were the same though some were a few uf lower as they were not standard, I guess things have changed over 50 years.

 

[betwixt]

There's something very wrong with most of those voltages. Can you confirm where you had placed the negative meter probe when taking the measurements please. It would be useful to have voltages measured directly between the base and emitters of each of the transistors but be careful not to short anything out, they are close together!

In general, the voltage between the base and emitter pins of a normal transistor will not exceed about 0.7V but some of your readings are indicating more than 30V. In particular, the base of Q43b looks WAY out and the collector can't be more than 0.7V because the base-emitter junction of Q402 should hold it down.

I'm treating the schematic with some suspicion, it shows pad 1 as having different voltages in different places, including some higher than the power supply produces.

Brian.

 

[FvM]

The brown parts are film caps and don't degrade so fast, at least if the coating has no fissures.

 

[betwixt]

Another odd thing on the schematic - the diode setting quiescent current (CR41a/b) has 1.73V across it according to the figures shown. The type however is listed as a 200V/250mA silicon switching diode and equivalent to a 1N914. It certainly looks like an ordinary small glass diode in the photographs and it should be forward conducting all the time so where does the extra volt or so come from? For this design the voltage looks reasonable, it has to overcome Vbe of Q43,Q44 and Q403 so what really is the diode type. Given the vintage I originally thought it might be a selenium diode but that not what the parts list states.

Brian.

 

[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

 

[betwixt]

14V seems a bit low but I doubt that is the cause of one channel not working. To be honest, some of the voltages on the schematic don't make sense at all so they are either mislabeled or taken from a model slightly different to yours.

R42a and R42b are in parallel so their effective value is half of 680K (340K) so it is possible one or both have gone high in value and that is causing the extra voltage drop but it could equally be that more current is being drawn from that node and the voltage is being pulled down a little.

It is quite common practice to use an output from the power supply to feed both channels, just as the main supply line does. There is a capacitor C408 that ensures it only has DC on it so there is no chance of one channel 'contaminating' the audio on the other through that route.

Trying to diagnose a problem like this is frustrating, it would be so much easier if I could get my hands on it. I imagine it is just as frustrating for you, especially because of the time delay in us communicating. I guess from the time you post and my local time and it being RCA equipment that you are mid-west or west USA.

Brian.

 

[pityocamptes]

I appreciate the help, time delay is workable.. So I have my oscope, can you let me know what I should do first? Thx

 

[betwixt]

The technique with a scope is to start following the signal from input stage towards the loudspeakers. Somewhere along the chain the signal will disappear and that will give a big clue as to where the problem lies. Remember an oscilloscope just shows a graph of voltage (height) against time (across the screen) so you must first have something to produce that voltage. A radio station would be a good choice, particularly one playing music as it tends to have long periods of signal compared to one with dialog. Turn the volume control up or nothing would reach the amplifier anyway!

Use AC coupling on the scope input for now. Accepting that there will be some differences between left and right channels because it is stereo, the signals at the same points on both channels should be very similar. Using the right channel on the schematic, try probing these points and let me know what you see:

1. point 147
2. point 149
3. collector of Q41b / base of Q42b (they are the same connection)
4. collector of Q42b
5. base of Q404
6. base of Q402
7. positive side of C416
8. negative side of C416

Brian.

 

[pityocamptes]

Thx so I had the oscope set on ac. Since changing all caps last week both channels are not working now. I should have done one at a time, in the event this were to happen. Anyways, I measured both channels:

147 - sine wave 1.31v
135 - sine wave 1.8v
149 - half sine .12v
137 - half since .24v

coll Q41a - .01v no apparent wave
coll Q41b - .05v no apparent wave

coll Q42a - .07v no apparent wave
base Q42a - .05v no apparent wave

coll Q42b - .05v no apparent wave
base Q42b - .05v no apparent wave

base Q404 - .01v
base Q402 - .05v

pos C416 - .03v
neg C416 - .04v

pos C417 - .03v
neg C417 - .02 v

 

[betwixt]

OK, progress is being made....

It appears the signals are arriving at the amplifier correctly but the first stage (Q41) has wrong bias conditions. What you should have seen is 135 & 137 having similar levels and 147 & 149 having similar levels, maybe a little smaller on the 137 and 149 than at the inputs. I presume by 'half sine' you mean a sine wave going in one direction but clipped off on the other half, this indicates Q41 is not running in linear mode but acting more as a diode.

This might be tricky but using your testmeter again, can you try to measure the voltages between the base and emitter pins of all the transistors please. Ideally use the same probe on the base each time so I can check the voltage polarity.

Trying to explain simply, each stage of the amplifier adds a little amplification but in order for the output to be able to equally rise and fall in voltage so it can drive the loudspeaker, it has to start off at half supply voltage (positive ends of C416 & C417). This gives it most headroom to go up and down and hence produce most sound volume. Unfortunately all transistors and other components are slightly different so if the voltages at the first stage were set, by the time any errors were amplified they could be significant at the output. To combat the error, some of that half voltage at the output is fed back to the input stage through R406 & R407. If the voltage is too high, it reduces the bias on the first stage to bring it back until equilibrium is reached. We call it 'negative feedback'. What we don't want to do it feed the audio back as well because it would also try to hold the signal voltage at the output steady and that means no sound! That is where R411/C411 and R412/C412 come into play, they reduce the signal in the negative feedback path but as capacitors don't conduct DC, they still allow the steady voltages to stabilize. In fact those components set the overall gain of the whole amplifier.

Back to the fault, something is upsetting the DC conditions and causing the wrong feedback voltage to go back to the input stage. It makes a vicious circle, the fault is preventing the DC conditions from stabilizing. If you can let me know the B to E voltage on each transistor it should be possible to find where the break is in the feedback loop.

Brian.

 

[pityocamptes]

Here are the pics of the Q41 - Q44 both sides for B to E

--- Updated Jan 10, 2025 ---

Here is B to E on these