Trace elliot gp12 series 6

[Off-Beat]

Im working on this trace elliot gp12 bass amp, http://www.talkbass.com/attachments/...ic-pdf.798828/

There is about 15vdc on the speaker output and a hum coming from the amp, i have disconnected the speaker to protect it, of course i will use a dummy load but i have only been working with the amp off for now.

I disconnected the preamp board and the amp remains. So the problem has to be on the output/power amp which is on the same pcb.
Any suggestions?
I have gone through most of the compenents checking if out of tolerence etc. R19 which is a 1k resistor was missing, i stuck a 1.2k in for now as its all i have, didnt solve the problem tho.

[52 Bill]

I have gone through most of the compenents checking if out of tolerence etc. R19 which is a 1k resistor was missing, i stuck a 1.2k in for now as its all i have, didnt solve the problem tho.

A bit of background on this amp. Was it working normally right before you got it? Has someone else been working on it before you?

Sometimes components are missing due to circuit revisions by the factory. If the amp was working fine before, without the resistor, then I would not expect installing it to fix a problem.

If there is a voltage on the speaker output, then typically there is a balance problem with the output stage. The top half of the circuit is not balancing with the bottom half of the circuit. This could be caused by any number of components in the power amp circuit.

This amp uses a set of MOSFET power transistors that are no longer made. Are the outputs original parts or have they been replaced with substitutes?

[Off-Beat]

Sorry i should have mentioned, a guy left it in for repair, he had bought it online and of course by the time it got to him it had been damaged.
He heard some rattling inside and opened it up to find 2 resistors lying loose. I have looked at the pcbs quite abit and can only see one spot, R19 where a resistor once was. However the 2 resistors are .22 ohm and R19 on the schematic is 1k.

According to the seller, the amp was fine before he shipped it.
I will have to check the mosfets again, i know 2 of them are deff the originals, need to double check the other 2.

[52 Bill]

However the 2 resistors are .22 ohm and R19 on the schematic is 1k.

Resistors of that low of a value would probably only be used as ballasts for the output transistors. Are they high wattage types?

[Off-Beat]

Yeah they are around 2 or 3 watt i believe

[52 Bill]

Then look to see if there are any open connections from the output transistor drain or source terminals. They aren't shown on the schematic so they may have been added at an earlier repair.

[Enzo]

And PLEASE DO NOT connect a load, dummy or otherwise, to the amp until we are sure there is no DC coming out of it.

[Off-Beat]

I will have a look next chance i get, there are deff no obvious signs of any mods or anything, but i am an amateur at this myself so who am i to say!

[J M Fahey]

No .22 ohms shown anywhere, but somebody *might* have added them as Source (~"emitter") ballast resistors, but you would need to cut traces (unless transistors are wired to PVB) and 4 of them.

Start by building a bulb lamp limiter , plug the suspect amp there, all controls to 0 , no signal input, no speaker or any load, and measure Vbe and Vgs of all transistors, also all Vce and Vds as applicable.
Post a nice readable table such as:
Tr1 ... Vce -50V ... Vbe 0.68V (write what you actually measure)
and so on , up to Tr6 Tr7 which are top 2SK135 and Tr8 Tr9 which are bottom 2SJ50 , counting top to bottom and left to right

You might also add a couple sharp readable well illuminated top and bottom board pictures, also showing heatsinks if transistors are mounted outside PCB .

If you answer clearly these questions we'll advance troubleshooting a lot.