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Messages - Gonflable

I tested Q4 as a FET switch.  Drain and source were always open, even after charging the gate.  But I would imagine it was partially working in circuit as you stated.
Yes the octave circuit is working great.  Nice feature on this amp.

Thank you
I removed Q4 from the circuit.  Amp works fine now and I do not care to fix the limiter feature.

I bench tested Q4 and found it to be open (not turning on).

My positive rail is now at 12.5 V, still below spec but the amp seems to work fine and sounds great.

Thank you for all the help. 

Trying to understand the circuit and figure out where I could run a jumper to bypass the "limiter" feature without losing the "octave" feature.

I am thinking in the Q4 area?

Thoughts?  Thank you
Quote from: g1 on June 08, 2023, 06:45:42 PMMaybe it is Q4 that is responsible for Q5 source drop-out.
Looks like a limiter circuit consisting of Q4,Q5, U8 and U12.
Also, U8 and U12 are the only things running off 15VX (via R82), so maybe one of those IC's could also be responsible for the +15V rail issue.  Do either feel hot?
(edit: that would be the noise gate circuit that tassieviking mentioned, not a limiter)

Q4,Q5, U8 and U12 are all running cool.  65 F
Play with the clean channel gain when using the distortion channel and see if it affects the cut out level.

Yes it does effect cut out level.  As I raise the clean channel gain, it takes longer to cut out.  Even with clean channel gain dimed, it still cuts out, but takes longer to happen.

Also, I realize now that I can cause a note cut out by lowering the clean channel gain knob, and make the note cut back in by raising the gain knob.

And, it could be my imagination but, it seems more likely to cut out on higher freq. notes versus lower notes.

As you have both indicated, it looks like the issue is in this circuit.  I feel that we are getting closer!
Quote from: g1 on June 09, 2023, 10:24:56 PMIs there much difference in voltage across R45 when ch.2 is selected compared to ch.1?
What is the voltage across R86 (to tuner) ?

across R86 1 mV.  note that the tuner is not connected at this time.

across R45 V dist. ch. selected = 1.36V
across R45 V clean ch. selected = 1.2V

across R42 V dist. ch. selected = 8.9V
across R42 V clean ch. selected = 8.4V

TP17 and TP19 measured on either side of R42
TP18 and TP20 measured on either side of R43
What does the X designate in "15VX"?
The tuner is not in circuit during bench testing.

Zeners arrived and swapped out.  No changes.  Updated readings...

Same temp readings with D18 hotter than D19
D18  100 Faren
D19  85 Faren

Negative Rail...

TP18  -20.5 V
TP20  -15.4 V
R43  5 V drop
R85  126 mV drop
R46  0.78 V drop

Positive Rail...

TP17  20.5 V
TP19  12 V
R42  8.8  V drop
R82  0.53  V drop
R45  1.37  V drop

What does the X designate in "15VX"?
The tuner is not in circuit during bench testing.

TP17  20.7
TP18  -20.6
TP19  11.7
TP20  -14.8

I see that D18 and R42 are operating much hotter than D19 and R43

D18  100F
D19  85F

Is 100F too hot?

I have new zeners arriving tonight.

Thank you
Quote from: g1 on June 08, 2023, 12:31:58 PMDid you verify the resistance of R42 and check it's solder joints?

Yes, all resistors in that area were checked and are in spec.  Solder joints look good.  I added pics of the underside at

I had replaced the input jack that was broken, cleaned all pots, and reflowed the 3 distortion channel pots. 
Voltage drops

R82  0.55 V drop,  47 ohms  = .011 A
R45  1.3 V drop,   47 ohms  = .027 A
R42  8.6 V drop,  220 ohms  = .039 A

So R82 and R45 add up to .038 A or 38 mA?

Trying to figure out how to attach pics.

Thank you

 I have a weird issue going on with this amp. 

1.  The clean channel plays fine, the distortion channel cuts in/out.  If I hit a note it comes back, and then at some point it cuts out, instead of fading out.

2.   The 15V rail is low, only 11V.  This may or may not be related to the first issue.

Issue 1 seems to be happening around Q5.  OK before Q5 but cuts in/out at the Q5 Source.  Q5 Gate is around 0.5V and then at 8.5V when distortion channel is switched on.

Issue 2, I have the proper 21V at TP17, but only 11V at TP19 (should be 14V).  Something is pulling it down.  Maybe C47 is leaking, or D18 zener is bad.  All op-amps should have 14V at pin 8 but are actually 10.5V.

I would like to focus on the rail issue first, and once corrected see if issue 1 is resolved also.

Suggestions please!!!

Schematic link is

I am not sure how to attach a PDF to this thread.

Also, anyone know if Music Electronics Forum is ever coming back online?

Thank you, Gonflable