Marshall mg100dfx resistor value r8 & r20

[wavetekr]

Was fixing a mg100dfx with failed tda7293 chip, mute zener and transistor. But noticed r8 & r2o were changed as it's different in size! Now it sound not the usual loudness even with volume cranked up. Suspect these resistor values were changed but without the schematic, I'm looking for help here.

[Enzo]

Before it blew up, did the amp have its normal loudness? If so, it did this WITH those resistors, so it would seem they are not the problem.

Anyway 2.2k and 3.3k respectively.

[wavetekr]

Hi Enzo,

You saved my day! Thankyou for looking into my request. I decided to ask for help after my friend sent it for repair and blew again, could see the power module smoked! I notice both resistors were 330ohm and 440ohm(220 in series) and they were 1/4 watt carbon resistors unlike the rest which were much smaller. Do you know if that is the reaon that it sounded soft as I notice r20 is in the feedback loop?

I'll change both and see if that is all left to return the amp alive agai. BTW was D14 a 6.2v zener as I read somewhere regarding the mute circuit?

[Enzo]

The schematic is included in the Enzo gallery at www.ampix.org.

[wavetekr]

Had it fixed! Suspect r8 & 20 was modified by repairman to increase the chip gain since reverted back to dsigned value, the sound wan even softer. Traced to the input socket signal pin very high contact resistance. Cleaned & solved. Zener reverted to 6.8v.


Im really grateful to Enzo for helping me, now amp is alive and kicking.

[Joust]

The schematic is included in the Enzo gallery at www.ampix.org.

do you have the rest of the pages of this schematic? there are only three there.

[Enzo]

That is all there are, what do you think is missing?

[Joust]

DFX board to name one.
if you look at this PDF there are two pages labeled pg-2 and one is pg-4.
so there is definitely some missing.

[Enzo]

Page 1 of the file is mostly power supply, along with the footswitch circuits and the power amp. MArked "sheet 1 of 1."

Page 2 of the file is the overall amp schematic, and is also marked "sheet 1 of 1."

Page 3 of the file is the jack board and the power amp module board. Again marked "sheet 1 of 1."

The actual little DFX board itself is not included in the schematic set, and we generaly replace it as a component rather than attempt to repair it.

I just looked on AMpix and the files posted there look the same. I just rechecked at the dealer site and it looks the same. In the MG100HDFX version, the same three pages are in a different order.

Is this not what you are receiving?

[Joust]

woops, you are right. what I was looking at was a REV I should have blown it up more.
my apologies.

[Joust]

do you have the MG50dfx?
is it the same? I just got that amp from teh music store because they did not have any MG100dfx in stock. I'm wondering how difficult it would be to add the two missing switches to the 50 to make it the same as the 100.

[Enzo]

Look at your circuit board. I imagine there will be a number of parts that are not installed but the board is silk screened with their part numbers. Considering the similarity I'd imagine it is the same board and those extra parts would have to be stuffed.

For the doubled output power I'd be thinking there was a different and larger power transformer. Yep...

TRANSUD0013 for MG100 list $37.59
TRANSUD0012 for MG50 list $24.86

List price from the parts list and may not be up to date

[Joust]

good point. I havn't opened it up but they most likely use the same board inside to save cost.
I'm not looking for more power. so the transformer is not a problem.

[Joust]

I opened the MG50dfx. the board is definately not the same as in the mg100fdx.
but everything is there to make is stereo. so its the same board as the MG250dfx.

[Enzo]

Works for me.