Acoustic 370 Repair Low Output

[J M Fahey]

Ok, the GROSS signal loss is across R301

It *might* be open , not too likely but to rule that out for good measure resistance across it, with amp off .
Use the 200k scale. I bet itīs fine (around 27k) but letīs check anyway.

After that, between it and C301, we have a couple grounded elements, which might be dumping signal voltage to ground.

Main suspect is Q315 Mute FET ; notice it is a P Channel Fet (most common ones are N Channel) , which needs positive voltage on its gate to open, schematic specifies almost +10V ; I suspect it has 0V or at least insufficient gate voltage, in which case it becomes a 100 to 300 ohm resistor shunting signal to ground..

So letīs start by measuring DC voltage referred to ground at point 15 .
Points 16 and 17 should also be around +10V and point 18 is the source voltage feeding them, some 80/90V (actually +V).



I suspect C315 is shorted or lossy , which coupled to verybhigh 6M8 resistior feeding it, amounts to about same in practice.

If significantly less than +10V is found where it should be, junk C317 and replace it by a 10uF x 25V or 35V , whichever you have available.
I bet when you recover proper Fet bias, youīll recover full signal into power amp

[Steelwitch]

Ok, the GROSS signal loss is across R301

It *might* be open , not too likely but to rule that out for good measure resistance across it, with amp off .
Use the 200k scale. I bet itīs fine (around 27k) but letīs check anyway.

After that, between it and C301, we have a couple grounded elements, which might be dumping signal voltage to ground.

Main suspect is Q315 Mute FET ; notice it is a P Channel Fet (most common ones are N Channel) , which needs positive voltage on its gate to open, schematic specifies almost +10V ; I suspect it has 0V or at least insufficient gate voltage, in which case it becomes a 100 to 300 ohm resistor shunting signal to ground..

So letīs start by measuring DC voltage referred to ground at point 15 .
Points 16 and 17 should also be around +10V and point 18 is the source voltage feeding them, some 80/90V (actually +V).

[ATTACH=CONFIG]52446[/ATTACH]

I suspect C315 is shorted or lossy , which coupled to verybhigh 6M8 resistior feeding it, amounts to about same in practice.

If significantly less than +10V is found where it should be, junk C317 and replace it by a 10uF x 25V or 35V , whichever you have available.
I bet when you recover proper Fet bias, youīll recover full signal into power amp

It was open! we now have 80v supplying, and 23v going to the gate of Q315, but no voltage on the drain or source. Let me know if that is an indication that it should be replaced, maybe I'm missing something. C315 has almost 10v on the positive leg, and 0v on the negative leg. points 16 and 17 also have 10v

[Enzo]

The bottom end of C315 is grounded, teh top end is wired to three resistors, and hte note says 9.8v there. Isn't 10v close enough? I'd sure think so.


But you didn't report if that restored the sound...

Well???

TP16 and 17 are wired together, so we would expect them to measure the same. The schematic says 10.4v there. 10v sounds close enough to me.

[Steelwitch]

The bottom end of C315 is grounded, teh top end is wired to three resistors, and hte note says 9.8v there. Isn't 10v close enough? I'd sure think so.


But you didn't report if that restored the sound...

Well???

TP16 and 17 are wired together, so we would expect them to measure the same. The schematic says 10.4v there. 10v sounds close enough to me.

It unforunately did not.. but we have 1.2v on the speaker out which is a step in the right direction. one thing I did notice is that the line out has 1.2v now as well

[J M Fahey]

Yes, it is a step in the right direction: now signal finally *reaches* the power amp , goes through it, and appears at speaker out.

Remaining problem is that power amp now has "gain of 1" or "unity gain" meaning: 1.2V in ... 1.2V out .

No voltage amplification

While it should show, as written on diagram: 20VAC (at speaker)/0.25VAC (at input)=40X

I suspect C306 (which is part of the Negative Feedback Network which determines gain) is dry/open so plain replace it.

Not very critical value, if you donīt have the exact one you may use up to 4.7uF or even 10uF there, by 35V or 50V , which you might already have in your parts box.

I am quite certain youīll recover proper gain and if not yet, we are very very close.

Ok, go and try it.