repairing c-audio st600

Hi

just got an amp off a mate. it puts out a signal but on opening it up 6 of the 18 mosfets are visibly blown. just ordered some replacements for these (2SJ162 + 2SK1058). anyone have any tips on other faults to check for with these amps?

I'm going to be asking for some money back from my mate - I've just spent £33 on parts + a few hours time. how much is a faulty ST600 worth?

Phil

thanks Mik. I've looked through the mods list -

General mods - someone has done 1) to 5)

I'm ordering the bits for 6 and 7

8) - there were four extra capacitors which I removed

9) had to replace two of the capacitors I had just removed I assume they mean TR7 rather than TR6 as this is what is shown on the circuit diagram and the picture of the board in figure 1

issue specific changes

the circuit board says issue 4 p1022 so procedure 1 and 2 definitely aren't applicable

at R55,R56,R19 and R20 there is a resistor and diode mounted in series off the circuit board. by the wording of the mods document procedure 3 isn't applicable in this case- can this be right or should I carry out procedure 3? if so there are two unlabelled components in figure 6 - I'm guessing they are 1N4004 diodes

would steps 5-7 of procedure 3 be a good idea in any case?

i'm going to replace the six resistors in procedure 4 because they do look like they have got hot.

sorry for all the questions

thanks
Phil

Hi nearly finished working on this but not sure of one of the issue specific mods. unfortunately although the working martin M1200 i have  is a re-badged ST600 it is an issue 8 and substantially different to the broken ST600 so I can't compare them.

mods document says:

1) Issue 6 PCBs: Go to Procedure 1
2) Issue 4 & 5 PCBs fitted with P1040 & P1041 protection daughter boards: Go to Procedure 2
3) Issue 4 & 5 PCBs fitted with Resistor-Resistor-Diode-Capacitor networks in position R55 , R56, R19, R20: Go to procedure 3
4) Previous Issues Go to Procedure 4

my PCB is Issue 4 with no daughter board. I'm not sure whether to do procedure 3. there isn't a resistor-resistor-diode-capacitor network in positions R55,R56,R19,R20 but this could be due to a previous owner not completing the modifications (they had done some but not all the general mods specified). does anyone know if issue 4 PCBs were all either fitted with a daughter board OR a "resistor-resistor-diode-capacitor"? or could mine be a third type not explicitly mentioned in the mods doc with just a resistor and capacitor in positions R55,R56,R19,R20?

also there are two different types of pads between the N and P Mosfets and the heatsink. should it be okay to remove all these and use thermal paste instead?




Edited by snowflake - 23 November 2011 at 1:44pm

Hi Mik
I was wondering if you could pass on any service info you have on the ST400 / ST600 amps.
Have a 400 in state of disrepair.... also wondering if it can be upgraded to a 600, or whether the transformer is different.
Thanks very much, Jim

Thanks Mik

more problems with my ST-600.

channel A works fine on its own. channel B sounds okay at low volume but as you approach moderate volume each peak in the signal produces a crackly popping sound. after a few seconds the clip light comes on channel B and the output is muted. if the gain pot isn't turned down things start getting hot inside after a few seconds. on examination I found that the fault on channel B was causing the Zobel resistors on channel A to get hot even though channel A had no input or load connected to it! can anyone suggest what might be going on here?

cheers
Phil