Another M80 Bass Amp Question

[nradina]

Hello again everyone,

I have a Fender M80 Bass Amp w/ Chorus on the bench.

Symtoms are a distorted and sagging volume output.

Line output is clean UNTIL a load on the speaker output.
Which leads me to an issue in the power amp section.

All voltages check out fine.

I'm new at this, obviously.
Wondering first if the symptoms are a tell tale sign of a obvious problem.

Thank you in advance for any help and guidance.

nicholas

[52 Bill]

Monitor the power supply voltages as you send a signal through the amp. Do they stay up when the amp distorts or do they drop off?

[Enzo]

Scoping the output at the speaker leads. Output waveform is good with no load, but collapses under load? What does the distortion look like? CLipping? One side? Both sides? Missing half of waveform?

Have we tried more than one speaker? Have we tried more than one speaker cord?

[nradina]

Thank you Enzo and 52 Bill for your help,

Worked a bit more on this unit today.

Voltages +16 and +50 seem fine.

Lifted emitter of each pair of output trans., one pair at a time, and no change.
Checked CR34 + CR35 protection diodes, all is well.

Noticed today some oscillation when the volume is low.

Yes, output looks fine through the Line Out until load is placed via speaker outputs.
Both outputs show distortion.
Preamp SEND stays fine.

Tried multiple speakers.

Can't honestly remember accurately the result of scoping some of the outputs today, but what I do remember is is looking clipped yet with a rounded top. (if that makes sense).

Thanks again for the help!

nicholas

[Enzo]

Output that collapses under load is usually the final stage being unable to source the current the load demands. The drive curcuits can usually make the output look nice unloaded but cannot drive the load impedance.

[g1]

How about your -16 and -50 volt supplies? You have a waveform that is round on top but clipped on the bottom, could be the negative supply bad.