... if that plan doesn't seem to work, see if you can find R225 and R226.
The input signal should be present on one end or the other of both these resistors.
You should be able to hear a clear sound coming from the audio source, such as the tuner.

 

... if that plan doesn't seem to work, see if you can find R225 and R226.
The input signal should be present on one end or the other of both these resistors.
You should be able to hear a clear sound coming from the audio source, such as the tuner.

OK, I reworked my cable to add a capacitor and tried both the board and the two resistors. Unfortunately I wasn't able to get any sound coming from any of the test locations. (I also tested my cable on phono output from an old phone to confirm it's working) Could the unit be stuck in 'protect mode'? Thanks again for all your help on this. I really appreciate it.

 

If the protection mode is active, then there will be a flashing red LED.
So, if no flashing LED, then something else is wrong, maybe someplace in the input signal area.
... It is necessary to find some point at which a good signal can be picked up with the test cable.

 

Can you locate a 6 pin header labeled CN6A?
Pins 1 and 2 of CN6A come directly from the tuner section.
Try testing those two pins, they are the two leftmost pins in the header, looking at the schematic
One pin is left channel and the other is right channel..

 

Can you locate a 6 pin header labeled CN6A?
Pins 1 and 2 of CN6A come directly from the tuner section.
Try testing those two pins, they are the two leftmost pins in the header, looking at the schematic
One pin is left channel and the other is right channel..

Confirmed. Clear audio from both pins.

 

... good.
The next checkpoint following the CN6A header is a chip that seems to have several internal switches that are digitally activated somehow. The switches kind of allow one or another of the various input signals, such as the tuner, to connect to the speaker amplifier section, which then goes to the speakers.
... The first thing to check is to verify that IC 201 has power going to it. There will be a kind of notch in IC201 at one end of the chip, on the top surface. The first pins, that is, the first one on either side of the chip, at the notch end, are the power connections. One of the power pins should have +15 volts DC, with respect to chassis ground, and the other pin should have -15 volts, with respect to ground.

 

... good.
The next checkpoint following the CN6A header is a chip that seems to have several internal switches that are digitally activated somehow. The switches kind of allow one or another of the various input signals, such as the tuner, to connect to the speaker amplifier section, which then goes to the speakers.
... The first thing to check is to verify that IC 201 has power going to it. There will be a kind of notch in IC201 at one end of the chip, on the top surface. The first pins, that is, the first one on either side of the chip, at the notch end, are the power connections. One of the power pins should have +15 volts DC, with respect to chassis ground, and the other pin should have -15 volts, with respect to ground.

Looks like one of these is off. The pin to the right of the notch measures +15.8 volts but the pin on the left measures +5.8 volts.

 

... See if you can locate capacitor C406. On the schematic, it is not too far from IC201.
It may be possible to check C406 to see if it has shorted out and is causing that -15 volt pin to go positive.
First, turn off the unit power for 5 minutes or so.
Next set your meter to the ohm scale. ... Maybe 200k or something like that.
Then place the test leads across C406.
If you see a very low ohm reading immediately, that indicates a faulty capacitor.
If you read something like 100k ohms, then it is probably good.
... This test is kind of a first guess. The more confident test is to remove the capacitor and test it out of the circuit.

 

... See if you can locate capacitor C406. On the schematic, it is not too far from IC201.
It may be possible to check C406 to see if it has shorted out and is causing that -15 volt pin to go positive.
First, turn off the unit power for 5 minutes or so.
Next set your meter to the ohm scale. ... Maybe 200k or something like that.
Then place the test leads across C406.
If you see a very low ohm reading immediately, that indicates a faulty capacitor.
If you read something like 100k ohms, then it is probably good.
... This test is kind of a first guess. The more confident test is to remove the capacitor and test it out of the circuit.

OK. With the meter set at 200k I took two rounds of tests and got the following:

Initial reading at around 29k with it then briefly fluctuating up to around 75k and then a final reading of about 59k. I took several measurements and got this result. Subsequent measurements then began ending at 100k. Unit had been off for at least two hours before these readings.

...Turned the unit on and then back off and waited five more minutes and got the following:

Initial reading at around 50k with it fluctuating up to and ending at 100k. Identical subsequent measurements.

 

...just to be sure, can you take another voltage measurement of the negative power pin on IC201?
The reason is that electrolytic capacitors like C406 are sometimes subject to ageing, overeating, voltage stress, and can exhibit varying behavior. If the unit has not been powered for several years, that may also affect electrolytic capacitors sometimes.

 

...just to be sure, can you take another voltage measurement of the negative power pin on IC201?
The reason is that electrolytic capacitors like C406 are sometimes subject to ageing, overeating, voltage stress, and can exhibit varying behavior. If the unit has not been powered for several years, that may also affect electrolytic capacitors sometimes.

Sure. Voltage readings were essentially the same: +5.79 and +15.79 respectively for the left and right pins (looking down at the top of the IC with the notch on the top).

Regarding the unit's history, as noted, I'm the only owner but it did sit unused for a number of years. When first powered up after this period of disuse it exhibited trouble starting up. Specifically, the display would not turn on and the relay you had me check earlier wouldn't close. At first I could let it 'warm up' for a bit and it would eventually turn on but the issue got progressively worse and ultimately the unit would fail to fully turn on at all (indicator source lights would glow but unit was otherwise unresponsive). This prompted me to replace C645 which resolved the startup issue but introduced the speaker issue.

 

... ok ... good to know.
Just a point to confirm ... When you replaced C645, did you make sure to note the minus sign on the capacitor label, and place it in the same position as the original capacitor?

 

... ok ... good to know.
Just a point to confirm ... When you replaced C645, did you make sure to note the minus sign on the capacitor label, and place it in the same position as the original capacitor?

Yes, I believe so. This capacitor has a stripe down the side that I believe is the negative, correct?

Looking down, the positive side of the capacitor is facing the interior of the board and the negative (striped side) toward the outside, which I believe is what the schematic indicates. There is also a circle with a line through it on the negative side and this symbol appears alongside similar capacitors across the board and corresponds to their negative leads.

 

The minus sign on the capacitor should be going to circuit ground, if you able to see that from the component side, and the other capacitor terminal would be going towards the voltage regulator input.

 

... The voltage measured at the voltage regulator input pin should be about 18.3 volts DC.
That might be something to check.

 

The minus sign on the capacitor should be going to circuit ground, if you able to see that from the component side, and the other capacitor terminal would be going towards the voltage regulator input.

Just double-checked and yes, I believe this to be the case.

... The voltage measured at the voltage regulator input pin should be about 18.3 volts DC.
That might be something to check.

Voltage measured at input is 18.48 volts.

 

... Another capacitor to check for shorting.
See if you can find C408.
It is kind of near the D509 rectifier bridge.... Someplace around there.
... Same as previously, power off for a few minutes.
Meter set to ohms scale.
Watch for immediate low ohms reading.

 

... Another capacitor to check for shorting.
See if you can find C408.
It is kind of near the D509 rectifier bridge.... Someplace around there.
... Same as previously, power off for a few minutes.
Meter set to ohms scale.
Watch for immediate low ohms reading.

Is it C508? C408 appears to be located down near IC201...

 

... Definitely C408.
It is right next to transistor TR417, if that is any help.

 

... Definitely C408.
It is right next to transistor TR417, if that is any help.

Got it. OK C408: Several measurements averaged about 34k. Measured C407 too which measured roughly the same.