Carvin Legacy 100w repair

[xtian]

Legacy combo on bench, bad, blatty sound when turned up more than just a crack. Sine wave looks good all the way to the PI. At the PI, sine looks good on input grid (top triode in schematic) but sine is very small nearly undetectable on second grid. Output of PI looks like poo.

Already tried a couple of tubes.

So, assuming the lower grid should be seeing the same input signal as the top grid, is R47 or R45 likely to blame?

[xtian]

I'm reading 270v on top plate of PI and 470 on bottom plate (schematic says to expect 300 each). Hmm. When amp is powered off, both plate resistors read 97K ohms. Could one still be bad?

Connections from PI cathodes to ground via R46,44,43 seem ok.

[Firestorm]

What does the signal look like at both plates? I don't think the grids would look alike because the "bottom" triode is fed from the cathode. Check c40 to make sure it hasn't opened up.

[xtian]

Signal trace below.

[vibratoking]

The inputs should not be the same. The PI is a differential amp. It amplifies only the difference between the two inputs.

You checked the plate resistors and they measure correctly. The high voltage at the bottom plate indicates little to no bias current flow through the bottom tube. Could be a short around the lower plate? I'd double check all the connections in the bias circuit and also the values, but first I'd swap in another the tube, if you haven't already.

[JazzGuitarGimp]

Disconnect one end of C40 and see if the idle voltage on the bottom plate comes in closer to 300V. If that gets you back to 300-ish, then replace C40.

[Randall]

I'm looking at C40.

[martin manning]

I'm looking at C40.

I think you and JGG should be looking for your cut of the fee ;^)

[Firestorm]

I'm looking at C40.

I think you and JGG should be looking for your cut of the fee ;^)

I said it first and I'm not sure I'm willing to split the fee.

[martin manning]

I'm looking at C40.

I think you and JGG should be looking for your cut of the fee ;^)

I said it first and I'm not sure I'm willing to split the fee.

Ahh yes, but I think it might be leaking, so maybe Jazz is the only guy that should be writing it up.

[xtian]

Did as suggested, oh wise (ass) ones, with no change in voltages. Pin 1 is 273vdc, pin 6 is 437vdc.

[xtian]

Also did some chopstick-fu on the board and components. Couldn't get voltages to react to pressure or percussion.

[Firestorm]

Did as suggested, oh wise (ass) ones, with no change in voltages. Pin 1 is 273vdc, pin 6 is 437vdc.

Did you replace the cap or just try disconnecting it? I once rebuilt a BF Fender and accidentally left that cap out. Sounded just like that. That second grid has to able to see AC ground (through the tail resistor).

[xtian]

I had just lifted one leg of C40 as shown in photo.

Now I have replaced it with a new cap of same value. No change in voltages.

[xtian]

So let's look the other direction--if a coupling cap from PI plate to output tubes was bad, that might cause the same trouble with plate voltages?

However, I observe the bias voltage on pins 5 of all four power tubes is identical (-54.5v).