Solid State Guitar Amp Forum | DIY Guitar Amplifiers

scratching my head a bit with this one -

got it in the shop with a transistor missing from the back (TO-3 sized hole on the chassis...) -

replaced that and replaced one shorted power transistor in the power amp -

with no load plugged in i get a good looking waveform on the output but once i plug in a load not so good - flattened out and clearly there's a problem -

some voltages are not quite as they should be - none of the transistors (power or driver or otherwise) in the power amp seem to be forward biased - even though they are all measuring fine diode-test wise - in and out of circuit -

the +/- rails are good, but the bases and emitters are all stuck at 0V or thereabouts - the symmetry of this amp is confusing me so i am having a hard time trying to isolate the problem without just shotgunning the whole thing -

any suggestions?

Output looks good unloaded, but loaded, some part of the waveform collapses?  Like maybe one polarity can't drive the load?

There are three outputs and one driver for each polarity - total eight - on the rear panel/heatsink.  What transistors are in there now, what did you replace any with?

The two drivers shar three 1 ohm 10w resistors, any opened?  Then each output has a 0.33 ohm 10w ballast resistor, any opened?  Plus ther are a couple of 1k resistors in between those, opens?

ANy time we replace a power transistor, we need to check ALL relevant resistors.

If one of the replaced or missing transistors was in a driver position (drivers and outputs are same type), there is a good possibility the predriver might be damaged.

Look at the output circuit, see the inductor with a 22  ohm resistor in parallel?  Measure resistance across that resistor.  Or measure at the coil if that is easier.  If the resistor measures 22 ohms, then the coil is open - usually cracked free at one end, or the wire broke right at the board.  If the coil is intact, then the resistor reads as shorted.  Should read zero ohms. 

An open coil means we have a 22 ohm resistor in series with our speaker.

good call -

replaced drivers and pre-drivers and we are all good!

thanks, Enzo!