Peavey Session/LTD 400 help

[Randall]

This post makes me miss Enzo.

I have a ratty sounding LTD 400 on the bench that sounds like a blown sleaker, but it's not. I put a sine wave thru it, and it is missing the top half of the wave. Looking at the power section, I have good signal coming in at upper left IN. Signal looks good at the base of the 761 transistor on the left, but reduced and asymetrical on the base on the right side transistor. The joined emitters mirror the signal of the right side base, reduced and bulbous on the top half of the wave. Both collectors look the same, amplified with both top and bottom halves of the wave, slightly distorted.

I could use some help with this. These old solid state amps are not my strong suit.

Peavey-LTD-Session-400-Schematics (1).pdf

[Helmholtz]

Usual starter questions:

- Do you have positive and negative supply voltages?
- Any DCV at the speaker terminals?

The right side base of the differential pair is the NFB input. Signal there should be a (voltage divided) copy of the output.
As it's a differential amplifier, the collector signals should be equal but out-of-phase and corresponding to the difference of the base signals.

[Randall]

I've got the +/- 52v rails and -0.3vdc on the output.

[The Dude]

-.3VDC is high for a speaker output. I'd measure transistor voltages and look for something off. If you can label voltages on the schematic, we might spot something.

[nosaj]

-.3VDC is high for a speaker output. I'd measure transistor voltages and look for something off. If you can label voltages on the schematic, we might spot something.

With Load or no load on the speaker outputs?
nosaj

[The Dude]

Either. More than 50mv is usually considered unacceptable, although there are exceptions.

[TomCarlos]

-.3VDC is high for a speaker output. I'd measure transistor voltages and look for something off. If you can label voltages on the schematic, we might spot something.

Just an idea - You might also check the resistors on the output devices, the two 1 ohm and four .33 ohm, 10 watt resistors. Check for value, make sure they are soldered into the board.

[Randall]

Turns out it was an open, or nearly open at 10M, 1K 5W resistor coming off the +50v rail at upper left in the schematic. Now I have 28mV on the loaded output and a nice looking sine wave. Thanks for herding me in the right direction, Nothing on the boards is labeled, which I hate, so it really helped to have some extra eyes on the schematic. Saved me some time for sure.

As always, I very much appreciate this place and all the assistance I have received over the years. And I still miss Enzo. He would have known.