Marshall 3540 Solid State 400W Bass amp

[Mars Amp Repair]

Hello folks,
Had an interesting few rounds with one of these Marshall SS base amps. The power output boards had previously been repaired evidenced by many resistors replaced and the darkened board where those resistors were. Some driver transistors had been replaced.
The A channel had sound & about 1Vdc offset. The B channel had sound and about 2.5Vdc offset. There is no bias or dc offset adjustments on this amp.
I measured all the small wattage resistors & tested all diodes fwd & reverse (in normal ohm scale for leakage, then also diode checked as well as checked the driver transistors on my component checker that you connect to the scope in the X-Y mode and all tested good with no leakage evident. I know that's not the end all test for transistors but all I have at my disposal.
Nothing came up suspect. So from my experience with HIFI audio from years ago I suspected some kind of imbalance in the diff amp. So, I replaced the diff amp transistors TR1 & TR2 in both channels both MPSA06. The pair in the A channel had been replaced previously but the B channel still had the originals.
Viola, offset remedied. I dug out a Cricket xistor checking and although it's also not the definitive tester, it tested all four of the transistors identically with respect to gain also with no leakage.
So basically this post for anyone who might also have similar issues with this amp. I really didn't expect that would solve this issue.

[Jazz P Bass]

Good find.
Been there a number of times.
When all else looks good, change out the differential pair.

[Mars Amp Repair]

Thanx for the response P.
Not sure how you feel about it, but I could have easily just replaced all of the drive transistors in these channels as there aren't that many and solved the issue much quicker. However, I always want to learn something in the process of taking the troubleshooting that one step further to get to the exact issue.
I will say & I'm sure you would agree that there becomes a point where you have to balance the cost in time vs the learning experience esp after feeling like you've exhausted all avenues. Then, you just do the selective shot-gun approach. I never feel like I've gotten a 'win' when I have to do that except for getting whatever it is out of my face at that point! Thanx for response, glen

[Mick Bailey]

You used a Cricket! I have a Noisy Cricket I sometimes use (mainly to check Ge transistors for leakage) even though I have a microprocessor component analyser. I thought most of the Crickets would have been discarded by now, or nobody used them these days.

[Mars Amp Repair]

Yeah, I just acquire this Cricket xistor tester, I believe it's one of the older versions as it doesn't have the 6 push buttons as the fancier one did. Just a meter movement with a rotaing function switch. All it really indicates is leakage & HFE. Not very useful but got for $10.00 so what the hell.