Scott 636S power amp

[Nyqusit5]

Hi,

I am currently repairing a Scott 636S. On top of having some hum at the output (main filter cap), the left channel doesn't work.
Compared to the right channel, voltages are all wrong. The single +60V rail is there but the intermediate voltage is very low (about 2V) at point 15. Also, the voltage on the base of Q621 is a lot lower than what it should be (about 11V). I have replaced C621, Q621 and checked C623. They are not to blame. I would tend to think that Q622 is faulty but it doesn't seem to be when tested (still inside the amp) and I can't find any available equivalents for it (BD419 maybe?). Any idea?

Thanks!

[Jazz P Bass]

You need to supply more voltage measurements.

[Leo_Gnardo]

In an amp that age - you already suspect the main filter cap is failing - and likely all other electrolytic caps are failing or soon will too. Before suspecting any solid state parts, I'd replace ALL EL caps, maybe other caps too, measure all R's & replace any that have drifted. If things still aren't right, then it's time to chase down transistor problems.

Guild in your avatar - very nice axe indeed!

[Nyqusit5]

Hi Leo,

Thanks for your input. The main filter cap has been changed and I have changed some of the left channel's (the right channel works fine on the same V+ rail). I'll check more of them.

Yes, I am a big fan of Guild's S series. Great guitars, great pickups (Guild's own or DiMarzio) and great craftsmanship all round!

[Nyqusit5]

I have changed all electrolytics for that channel and there is no change. In my investigations, I have also changed Q621, Q622, Q624, D621, D623. All resistors, remaing diodes and transistors check ok.
Here are the measurements I get with the power transistors out of the circui (or in for that matter): 12.7V at Q621 base (even with C623 out), 0.61V at Q624 emitter, 0V at Q622 collector and 0.6V at its base (even with new transistor), 1.8V at point 16, 0.86V at point 15, 0.6V at point 13, 59.2V at point 18 (rail), 30.6V at R628 and R629 junction, and 2.4V at point 17.
So something is shorting but I can't find what...

[Enzo]

Power off and discharge. From P18 to P17 the circuit goes through the two 2.2k resistors. Measure resistance from 18 to 17. Do you get about 4.4k? Or is it a lot higher? measure from the points themselves, NOT on the resistor leads. And drop to the junction between the resistors, do you get about 2.2k back to either 17 or 18?

You replaced Q622? With what?

[Nyqusit5]

Thanks for your input Enzo.
Measuring at P17 and P18, I get 4.57K which seems fine. Both resistors check ok individually. I have changed R629 just in case it shorts under stress.
Q622 was replaced with a BD237 (different pinout) and I tried a BD237 (different pinout) for Q624 and a 2N5086 for Q621.
Nothing changes unfortunately!