Ampeg SVT 4 Problem

[Street 760]

Hi there, I have been asked to repair an SVT4 Pro for a friend, i have fairly good knowledge of electronics and spent a while working for an amp manufacturer. I'm not an expert but i have a fairly good idea what i'm looking at!!!

I have found that resistors 119 have burnt out and 118 is looking a bit scorched but still working.

Also, resistors 218 and 246 have also gone as well, although that channel hasn't worked for a while.

It was sounding fine with channel A and then started crackling and a burning smell came from it.

I'm just kinda puzzled as to what has caused the same resistors in each channel to fail.
The pre amp side seems ok, the clip light by the gain control works so i guess there is no problem with the power supply or the rectifier.

Anyone have any ideas?

[Jazz P Bass]

Not sure if we are on the same page here.
I have attached the SVT4 power amp schematic.
R118 & R 119 are the biasing resistors for Q102. for channel A drive circuit.
R 218 is the base biasing resistor for Q202 channel B drive circuit.
If it is not a misprint, then we jump to R246 on the Ch B output circuit.
That is a ballast resistor, 0.47 ohm, 5 watt, for the Q212 output transistor.
Both channels failed on the Hi Side of the drive circuit.
That you have found so far.
It may be that you have the real trouble on the output circuit.
Something demanded a boatload of current.
Upon repairing each drive circuit, at the least, all of the output mosfets & there associated resistors will have to be checked for shorts or opens (for the resistors).

[Street 760]

Brilliant!! Thanks for such a good response!!!

Do you think this could have been caused by overloading? A sansamp was plugged into the 'power amp in' at the time.

Thanks again for your help.

[Jazz P Bass]

The Power Amp In jack is expecting a low level signal from another amps Preamp Out jack.
The key word here is Low Level.
Low voltage & low current.
If a speaker output (ie: high voltage & current) was connected to the power amp in, that is not good.

[Street 760]

No, it was just a pre-amp signal running into the power amp section, just wondered if it could have been overloading the input a little.
Gonna check the mosfets and resistors as you said and hopefully get it going!!!!

Thanks again.