Marshall MG250HDFX No Output

[stereomonostereo]

I'm working on a Marshall MG250 with no output. Here's what i have so far:

-The drive channel light is on and the channels won't switch

-The CD input and effects loop return both produce a signal

-The red and green clip LEDs for the drive channel both light up when a signal is applied

-I've replaced IC2 (DG212CJ)

Can anyone steer me towards any likely culprit?

Thanks!

[CharlieP]

Did you clean the EFX loop jacks?

[Enzo]

Do the headphones work?

Look on the power supply schematic page. Bottom center. Ther is a transistors, a zener, and a cap in parallel making the control signal STBY. Are any of those parts shorted? Is STBY shorted to ground?

Loop return makes a signal, then I guess the power amps are working.

On my schematic the DG212 is IC9.

So apply a steady signal to the amp input. trace it. is it at the volume control pot for each channel. IC9 switches the signal, so is signal present at pins 3 and 6? How about after the IC9? How about after C22? is there continuity from C22 to the FX send jack?


And is signal even getting into the preamp? Is ther signal leaving IC1, either half? Is C3 broken off th board? Are D1 or D2 shorted?

[stereomonostereo]

Thanks for the replies. Here's what i've found:

The headphones don't work.

The transistor/zener/cap network seems to check out fine.

Channel one is getting a distorted signal at the voulme pot, channel two is getting a full signal. IC9 is getting signal at 3 and 6, but i'm not getting anything at R72, R73 or C22. I'm not getting continuity fron C22 to the FX send either.

The signal is getting into the preamp. LEDs 3 and 4 are both lighting.

[stereomonostereo]

And this is probaly a dumb question, but are DG212BDJ and DG212CJ interchangable?

[Enzo]

OK, so signal is at pins 3 and 6 of the 212 - the inoput side - but apparently nothing at pins 2 and 7? That is the output side, putting signal across R72 and into C22.

Pins 1 and 8 are the controls for those two switches in the 212. Are those control gates being turned on? I mean, if both sides remain off, then no signal passes.

[stereomonostereo]

There's no signal at 2 and 7.

I'm not getting any voltage or signal to 1 and 8, should i be?

Pin 4 has the correct -15 and 13 has the correct +15.

[Enzo]

Look at the schematic. Pins 1 and 8 are the control inputs to those 212 switching sections. If you put no control voltage on the pin, then the section never turns on. When one of those is at ground voltage, the other ought to be at something like +5.

Look in the switching circuit. I am betting at this point you might have a bad TR1. A 2N7000.

[stereomonostereo]

That was the problem, bad 2n7000. Thanks a ton!