Help troubleshooting problematic Pioneer SA-608 amplifier

[marini]

Hi all,

So I found this amplifier with a reported issue of "crackling, sometimes muted, left channel" and decided to grab it and see what I could do about the issue.
First thing I opened it and noticed it had a few parts in the dust - yes, it was so dirt I could barely make sense of what's what inside it.
I gave the whole thing a good bunch of air blasts, some brushing around and applied Servisol Super 10 (deoxit) on the switches and pots several times.
That is as far as I could go considering my pretty much zero experience in this field (but hey! I'm very curious and eager to learn!).
I gave it a quick test by connecting it to a pair of speakers and a CD player.
The left channel is definitely problematic. It didn't get muted anytime, but it has some "noise" mixed with the sound. I didn't notice any more crackling when operating the volume control or other switches.
The right channel sounds just fine.
The fluoroscan meter is shiny and bright and has everything working apart from the left channel output bar (coincidence?).
I have the service manual and some info I collected from similar posts around here, so I tried at least two tests I could run.

DC offset (speaker terminals):

Left = -28.6mV
Right = -43.1mV

Idle current:

Left (TP1 - TP2) = 20.1mV
Right (TP3 - TP4) = 301mV??? (pretty scary)

It's all switched off now and I would really appreciate some help trying to figure out what's wrong with it, and if I would be able to repair it.

Once again a big thanks for all the help you guys have provided everytime I asked for it.

 

[marini]

 

[marini]

 

[zebulon1]

Build a DBT aka Dim Bulb Tester.
With the set plugged into the DBT, see if you can adjust the idle. (If there is an adjustment) haven’t looked yet.
If it can be adjusted see if the trimmer is in the same relative position as the good channel trimmer. Monitor the amp on the DBT. Note: you can’t use the amp on the DBT. Volume at minimum setting. No inputs. Watch the Idle voltage making sure it’s stable.
Watch and build some confidence in the set.
If it can’t be adjusted or be trusted, you’ll have to check and test the components on the bum amp channel.
Symptoms indicate a bad transistor in the driver stage.
Disclaimer:
I have no previous experience on this model.

 

[marini]

Build a DBT aka Dim Bulb Tester.
With the set plugged into the DBT, see if you can adjust the idle. (If there is an adjustment) haven’t looked yet.
If it can be adjusted see if trimmer is in the same relative position as the good channel trimmer.
If it can’t be adjusted, you’ll have to check and test the components on the bum amp channel.
Symptoms indicate a bad transistor in the driver stage.
Disclaimer:
I have no previous experience on this model.

It doesn't look like it has this option when the voltage goes too high: