Fender Hot Rod Deville - Octave distortion effect on More Drive

[g1]

And when you short out C44 you still have "more drive" gain condition?

[stereomonostereo]

Yep, More Gain still functions, just without the ripple/octave

[stereomonostereo]

And it also goes away when I test DC voltage on the legs of Q1 and Q2 where they connect to R106. Nowhere else on the board though.

[52 Bill]

And it also goes away when I test DC voltage on the legs of Q1 and Q2 where they connect to R106. Nowhere else on the board though.

Any disruption to the gate voltages can cause changes to the effective parallel resistance to R23 and R24. I wouldn't short out C44, as you could damage the switching system opamp.

Seeing that somebody was in there before, are the bypass caps C8 and C9 the correct values? Do you have any J111 fets on hand to swap out Q1 and Q2? Double check all of the previous board repairs and make sure that there are no loose wires etc., causing oscillations.

[stereomonostereo]

I don't have any J111's here. The bypass caps are the correct values. I noticed at Test Point 36, where I should be getting +16 with 'more drive' engaged, that I'm getting -3vdc. I've replaced the U3-A 4560 and am still getting the same reading.

[52 Bill]

I don't have any J111's here. The bypass caps are the correct values. I noticed at Test Point 36, where I should be getting +16 with 'more drive' engaged, that I'm getting -3vdc. I've replaced the U3-A 4560 and am still getting the same reading.

TP36 should swing from +16 to -16, depending upon what mode is selected. What voltages are you getting at TP34 and TP35?

[dwmorrin]

I noticed at Test Point 36, where I should be getting +16 with 'more drive' engaged, that I'm getting -3vdc. I've replaced the U3-A 4560 and am still getting the same reading.

Ok, so what are pins 3 and 2 of U3-A doing? The voltages there should control the output.

And it also goes away when I test DC voltage on the legs of Q1 and Q2 where they connect to R106. Nowhere else on the board though.

You mean you're still getting "More Drive" but no more octave problem? Define "it" in the above statement.

The input impedance of your meter is probably in the MΩ region, so it will load that node when you go to measure it. By load, I mean it will pull whatever voltage is there towards ground if your black lead is at ground.

I don't know if that indicates that the 10M is too high or has a bad connection, but those things come to mind. Take resistance checks.

[stereomonostereo]

TP34 I'm getting 4.9 and 4.7. TP35 is showing -.28 and -3

[dwmorrin]

TP36 should be going either full on or full off, as shown in the expected measurements. How that could be -3V makes me first question what the power supply to the op amps is at.

Can you verify the voltages at pins 4 and 8 of the op amps?

[stereomonostereo]

I've got -16 at 4 and +16 at 8

[52 Bill]

Good call, have you checked for unsoldered R78 and R79?

[dwmorrin]

At TP34, anything positive is strange. Are you sure you didn't mean -4V?

TP35 checks out.

[stereomonostereo]

I did mean negative voltage, sorry. On U3-A, with 'more drive' engaged, I'm reading -3.1 at pin 2 and -4.7 at pin 3. Pin 1, TP36, is still showing -2.1.

I'm getting the 'More Drive' effect when testing R106 but without the octave effect. R106 checks out at 10m and the connections have good continuity. (If I ground the leg of R106 that connects to Q1 and Q2, the octave problem goes away.)

It's definitely a strange one.

[stereomonostereo]

They're both soldered well with good continuity

[52 Bill]

I did mean negative voltage, sorry. On U3-A, with 'more drive' engaged, I'm reading -3.1 at pin 2 and -4.7 at pin 3. Pin 1, TP36, is still showing -2.1.

I'm getting the 'More Drive' effect when testing R106 but without the octave effect. R106 checks out at 10m and the connections have good continuity. (If I ground the leg of R106 that connects to Q1 and Q2, the octave problem goes away.)

It's definitely a strange one.

It seems like the real problem is in the switching system. The control voltage going to the gates of the fets isn't strong enough to fully turn them on or off. I think if you solve the TP36 voltage swing it will cure the problem.

Check all of the more drive switching components like D26, D24, D25, etc.

Try and find out why you can't get the -10 volts at TP35. Clean the footswitch socket.