Studiomaster 708 powered mixer amp fault

[blindboybenton]

Hello

I have been given a powered mixer as a gift. My band is in need of one. Ive spend some time sorting out all the little faults and now ive turned my attentions to the amps

One has blown up somebody has been in there and done some work before me. I have decided to work on one at a time
This mono amp is working but the audio is distorted. If i play a cd directly into the power amp in. I hear the audio and its got volume but distorted... Its the same if i go in the channels on the mixer. Signal out of the pre amp out is clear as day. .

About a month ago i did some work changing some transistors and zener diodes, but im not 100% sure what ones i changed now
studiomaster powerhouse vision 908 & 912.pdf page one This is the correct schematic ... It shows a biasing pot. and a few resistors . NF = Not fitted

All voltages are correct and present
The output stages are not getting hot at all MJ15004 are cold and MJ15003 are slightly warm ( wouldnt melt snow )
Ive checked all semi conductors and all resistors

I have been trying to make sense of this with a signal gen and scope

On the bases of TR1 and TR22 should i have the same signal at both ?
using the scope Should i have the same signal at Collector TR14 - TR15, if so , I dont have.. At collector of TR15 its almost a flat with some spikes. at TR14 pretty much the same as the input signal

If anyone can shed some light on this for me. I play around with amps for fun, and please i don't need a lesson on how they are dangerous etc
Please forgive my ways of explaining it.......... I don't know any better

Bassman1965

[Enzo]

Doesn't matter what you did or did not do a month ago then, just approach the amp as it sits.

You have good 63v rails, AND good 76v rails, all four? And the output centers on zero OK? The amp is symmetrical - the positive side and the negative side look the same other than polarity. SO in DC terms the base of TR1 and Tr22 ought to be the same. Without a nice balanced DC situation, your signal chain won;t be right.

What are we trying to fix? " Distorted" could mean a million things. Scope the output, what does the distortion look like? Clipping? One side or both? Missing half of waveform? Is it missing without a load or does it look OK with no load and loses part under load?

TR14,15 are limiters and not the place to seek waveforms. In fact on more hairy cases I remove them or lift D12,13 to get them out of circuit until the rest is repaired. If removing or disabling them makes the amp work right, then one was faulty. Where you want to see the same waveform is the base of TR1,22.

[nsubulysses]

Is it missing without a load or does it look OK with no load and loses part under load?.

Enzo I was actually trying and having no luck finding some old post where you talked about this phenomenon. Would you mind explaining what would make a SS amp appear fine unloaded but then lose signal under load. In this scenario would the sine wave be visible at the speaker jack of the amp, but lesser/distorted at the speaker jack of the cab?? Or would the location of scope probe make no difference?

[Enzo]

Think about it, the speaker cord is a wire. Unless there is something wrong with the cord, whatever is on one end will be on the other. If it is more convenient to scope at the speaker, then go ahead. But I assumed we had the amp in front of us.

What makes my scenario happen? The output cannot provide the current the load needs. The good waveform unloaded is pretty much just the drivers pushing things, but under load the outputs have to provide the current, and for some reason they can't. Usually an open something - resistor, transistor, copper trace, solder joint.

[blindboybenton]

Many thanks Enzo it was staring at me right in the face. I had checked the 64+/64- but i didnt check the 76v the 76v- was missing blown supply resistor
well at least i have one amp working..

many thanks again

Bassman