Tuesday 23 October 2018

About guitar amplifiers and fixing my old Marshall MG15DFX guitar amp (effects not working)

I do have a very nice Vox AC30 CC1 valve amp (no longer made). Although this sounds great when cranked up, it is not very good at practice volumes, and being full of valves, it does make funny noises, chirps, farts, etc. It's also very heavy to move around.

VOX AC30CC1 valve amplifier

I had recently heard good things about the new Boss Katana guitar amplifiers, so I decided to buy a Boss Katana 50...
and I must say I am really delighted by it! Digital modelling seems to have come a long way in just 10 years or so. For the price, this amp. with 12" speaker, effects and  great sound even at low volumes, is incredible!

BOSS Katana 50

Playing around with the Katana 50 reminded me about an old Marshall MG15DFX guitar practice amp I bought many years ago. I never really liked it because the overdrive channel was way too 'heavy-metal' for me - the distortion/clipping affected me in the same way as when my teacher used to drag his finger nails down the blackboard just to upset us!

So I mainly used the MG15DFX on the clean channel and connected an Ibanez TubeScreamer pedal when I wanted overdrive! P.S. I have a very high opinion of Ibanez guitars too.

I remembered that my old MG15DFX had been sitting under the settle in my conservatory for about 7 years because it had developed a fault. So I decided to dust off the cobwebs and spiders and try it out again to hear how it sounded - sure enough the digital effects controls did not work - no reverb, no echo, no chorus and no flange at all! Everything else worked - just not the effects.

Marshall MG15DFX
I decided to have a prod around (technical term!) to see what I could find.

I could see the 15V-0-15V transformer, regulator and output amp and a Digital Effects card (B) plugged into a single multi-pin connector on the main board.

Inside the MG15DFX - the Effects Card is outlined in red.
Using a multimeter, I quickly found that the +/-15V supply was running at about +/- 12.5V, but apart from that it seemed OK. Since the main amp worked on both clean and overdrive, the only bit that was not working seemed to be the Effects Card. I could remove it and the rest of the amp still worked as before.

I was about to get out my oscilloscope, when I realised that this Effects Card had it's own special 5V supply rail. So I checked the row of connector pins on the Effects Card with my multimeter but could not find a 5V reading on any of the pins... hmmm.

So then I had to remove the whole main circuit board to get to the underside (this involved undoing about 12 nuts to release it from the front panel!).

Using my multimeter, I found that there was no 5V!

Using the multimeter on the underside of the board, it looked like there must be an invisible break in the track leading from C47 ('A' in previous picture) to the IC4 5V regulator. When I put my finger on C47, to my surprise, the pin of C47 that was soldered under the PCB actually wobbled! It seems that the join between the through-hole of C47 and the trace had broken!

X Arrow point marks the spot where the break was

The white arrow shows where I scraped away the green protective solder-resist layer so that I could solder a small wire between the pin of C47 and the 5V regulator as well as reinforce the through-hole with more solder.

After that it all worked just fine and after a bit of strumming  with my guitar plugged into the Marshall MG15DFX, I quickly concluded that my memory had not been impaired after all; this amp was still pretty dire and the Katana 50 was soooooo much better!

Anyone want a small Marshall practice amp?

P.S. If you are interested in the circuit design of these Marshall amps see here for an in-depth description of the MG10. The MG15 and MG30 old Marshalls were of a similar design.

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