Marshall MG100dfx Weirdness

[jp3zone]

Ok, so I get this mg100dfx for repair with distortion problems on the clean ch, as well as a weird octave sound coming from the crunch, as well as the two OD chls with certain notes. Turns out the trace between ZD1 and ZD2 was broken, causing the clean channel to distort. Fixed that and clean ch is now perfect, but the octave effect remains on the crunch and od1+2. I already replaced all the preamp opamps cuz the preamp rails are running +9.1v/ -15v, so I thought one of them was pull in the voltage down, but no change. Was thinking of changing DG212 switcher next. So before I go in too deep, I'd just like to get some other opinions.

The power section is perfect, put signal into cd-in. Also, injecting the Send signal into another amp, you still get the octave distortion.

[Enzo]

Fix the low rail before going any further.

It could be loaded down, or it could just be low.

Flip your meter to AC volts or get out the scope. Is that 9vDC smooth or is it unfilterd DC?

The two 15v rails are born of three-leg regulator ICs, so look at that 7815. 9v coming o0ut? OK, what is going in? If you have 25v going in, then perhaps the 7815 is bad. But if the input voltage is 12v or something, the IC can;t possibly put out 15v.

And look at how that raw voltage is made, instead of a plain old bridge rectifier, ther are a couple voltage doublers. A bad cap or diode there and your voltage is no longer doubled, and then you have low voltage for the V-reg IC.


Don't just throw chips at it hoping one of them was the problem.

[jp3zone]

Thanks Enzo, I was going to go the supply regs. and rectifiers next, but just needed some external input. I was just bummed that I didnt go for the power supply first when I knew I should have:/, and went through all the trouble changing the ic's. Well I have to say, in my defence, I had a repair on a crate BT220 last week where I suspected the power supply zeners for the unbalanced rails V's, and you suggested checking the opamps first and you were right, so thats probly why I went for the opamps in this case.

Will check the pwr supply and let you know.
JP

[Enzo]

I don't recall that thread, but if the previous repair involved two zeners having the same problem, i would have suggested looking elsewhere, probably because it was more likely in that case than two zeners coming down with the same disease simultaneously. But at this point I just don;t remember.

[jp3zone]

Exactly, the two zeners were both coming down...I would KILL for your brain

http://music-electronics-forum.com/t22496/

Back to the Mg100dfx, went over the PS and I found another broken trace, right on the edge of the solder joint of the filter cap (c49). Wiggled the cap and voila, 15v instead of 9.1v. You advised me to check if the voltage dropped before or after the regulator, so when I checked, I got a 12v reading before the reg, so that told me where to look.

She's running sweetly now, no more octave effect, except for one thing: I found that the clean channel tone controls have no effect on the signal, so any ideas?

Edit: Just removed the treble pot, and one of the legs were broken off, so disregard the previous question

Enzo, thank you so much for your time and sharing your knowledge, you rock man!

[Enzo]

Well, about the brain thing, quit watching those zombie movies.