Fender Bassman 1200 output problem

[Nyqusit5]

Hi all,
I am repairing this Bassman 1200 (same amp as the Sunn 1200S) with a faulty ouptu stage.
Without a load, the amp has a clean signal on its output (no apparent distortion and with proper swing), but with a load connected to it, there is almost no output (0.1v peak to peak max). The output is at 0V in both cases so it seems that the O/P transistors are ok. The dual rail supplies read fine too. I can't see any blown active part and I have resoldered all of the transistors located on the smaller heatsink (because that is often a source of problems on this amp)

Any ideas? (schematic attached)

[teemuk]

Driver failure...?
Short circuit protection "stuck on"...?
Commutation failure..?

It could be practically anything. Failed parts may not always show apparent signs of failure.

[J M Fahey]

Clearly you don't have a *current* capability.

You *might* have blown all emitter ballast resistors, so poor drivers alone are trying to drive the output ( no no ) or you might have lost output jacks ground connection or output inductor L1, in paralled with R224 might have wiggled open , then 1200W of power tried to pass through poor R224 and opened it ... or cooked it up to, say, 1ok or something.
Or you might have blown a trace or have poor contact at connector CP12/CP13 .

In sum, check for good connectivity between main speaker output rail and output jacks and good ground at them.

10000 ohms in series anywhre on the circuit will drive handsomely a meter or scope (hint ... 1M impedance) but not speakers.

[Nyqusit5]

Hi, thanks for your help.
Your third scenario was the right one. L1 was broken and R224 was cooked.
Again, thank you!