MARSHALL MG50FX Accident - Help please!!!

[pickups_cba]

Hello there!! I have a Marshall MG50FX, Iīve changed some pots (which are this ampsīculprit) and while reassembling it, I accidentally shorted two pins of the TDA7293 mini board that is attached to the heatsink, and a spark flashed...so I turned on the amp and...you guessed it! All hum, no sound from the input and around -28VDC at the speaker output. The amp was obviously unplugged from the mains while reassembling, but it seems that some stored electricity on the filter caps was enough to ruin my day. The amp turns on OK and I have sound through the headphones out though. Iīm guessing that at the very least the TDA7293 is fried. The problem is that there are no schematics to be found for this model particular (MG50FX, the one just four pcbs: one for pre/FX, one for power supply/outputs jacks and a little one with the TDA7293 and some electrolytics attached to the heatsink. All pots are 12mm B10k and attached to the front panel PCB) Can anyone help me get this amp repaired or has access to schematics? The TDA7293 is hard to find and kind of expensive where I live, Iīve managed to track down a store that carries it, and my initial idea was to change the TDA7293 and all 5 electrolytics on the little board, but I donīt want to smoke it if thereīs still a problem somewhere else on the amp. Where/what should I check before plugging the TDA board back in? Thanks!!!

[The Dude]

Most likely just shorted the output IC. There isn't a location in your profile, so we don't know where you are. If you're in the US, the IC is available from several sources. Here's one:

https://www.digikey.com/product-deta...70-5-ND/599502

I wouldn't change the caps unless they are shorted or visibly bulged. Check them visually and with an ohmmeter to see. The traces on those little boards lift easily and you don't want to do any more soldering than necessary. Discharge the supply before installing the IC or doing any soldering to prevent further damage.

On a side note, you may want to add a location to your profile so we know where you are.

[pickups_cba]

Dude, thanks for your answer, I am in Argentina, I added that to my profile now. Tomorrow I will be picking up the part, I phoned the store and checked for availability, everything is ok. Since is not a cheap part I wanted to make sure that a bad 10c. electrolytic would not pop the TDA again, not to mention the task of carefully desoldering/soldering 15 pins on such a crappy board. To test the caps Iīll have to carefully desolder them and risk to lift the pcb trckas/pads. I prefer to clip the legs (both TDA and caps) easily and replace everything while heating the board as little as possible to avoid lifting. It is a shame that a schematic canīt be found anywhere...to discharge the PS should I use a resistor to drain the caps? filter cap +leg to ground?

[The Dude]

If you want to change the caps, fine, but you can check them for shorts without removing them from the board. According to the datasheet for the TDA, +&- supply is on pins 13 & 15 of the IC. You can check voltages there to see if you are discharged and could also use those points to discharge with a resistor, as you say, if getting to the filter caps is difficult.

This may help you some.

http://www.st.com/content/ccc/resour...CD00001887.pdf

[g1]

I would think the power section of the DFX should be quite close, as they both use the TDA7293. Looks like only 3 electrolytics on the power amp board though.

[Mick Bailey]

It's best to run amps that use these ICs on a variac and bring up the supply slowly, monitoring for DC. Just like Mode 4 Marshalls - bring the supply rails up to no more than 2v and look for any DC offset on the output. If there is any significant DC, stop and investigate. This way you can intercept any unusual conditions before the IC fries. I snip off the pins before desoldering the ICs to make things easier.

[J M Fahey]

No matter the preamp or effects variations, the power amp must basically be the same.
Marshall offers those little plug in boards with the TDA already soldered in so users can easily replace them by themselves, in different models, but carefully replacing the IC should work.
Just be **certain** filter caps are discharged, both of them, before plugging the new miniboard in, even residual voltage can blow those TDA.
Variacs are not popular in Argentina, I suggest you build a series lamp with a small 25W incandescent lamp in it.

[Mick Bailey]

There are some variations in design of the power amp according to model. At least one of the boards has a different socket, and later ones have revisions that reflect the updated application notes for the IC.

The ICs do not tolerate an imbalance in the supply rails, and the recommendation to discharge the filter caps should be taken as mandatory. If the amp's caps are charged then it's likely that one rail would connect momentarily before the other, destroying the IC. Even just a few volts is enough. If you wanted to be super-cautious you could desolder the amp chip and re-insert the board. Power up the amp and then check on the IC solder pads for DC on the inputs and check the supply rails are correct. Then power off the amp and discharge the caps. The reason I suggest this is if you have DC on either of the inputs it will swing the output to a DC level.

[Mick Bailey]

Just another thought - when these ICs short they often take out the standby circuit and the amp still produces no sound, even with a good power module. The effect of a defective standby circuit is that pin 9 of the amp is permanently grounded. Usually caused by a shorted switching transistor and/or associated components - in the case of the 50DFX schematic this would be TR2, C129, D19.

[pickups_cba]

Thank you all for the suggestions, I will have them in mind. I clipped off the TDA, and realized that the circuit is slightly different that the one on the MG50DFX pdf posted by g1. I have traced the tiny TDA board, and edited the MG50DFX circuit to show the differences, please check it out, so if you find any mistakes I can correct them.

[Randall]

Not to be a dick, but if you shorted and fried something while assembling, was it powered up?

[pickups_cba]

Not to be a dick, but if you shorted and fried something while assembling, was it powered up?

no, Iīve said that it was off and unplugged from mains in the first post

[Mick Bailey]

Out of interest, what did you use to draw up the schematic?

[pickups_cba]

Out of interest, what did you use to draw up the schematic?

Corel Draw, Iīve just edited the MG50DFX pdf that g1 posted

[pickups_cba]

Luckily it was only the TDA! After checking for correct supply voltages, that the standby circuit was OK and that the caps were discharged (those suckers holded around 30V) I soldered the new TDA and it worked perfect. Iīm ataching pics of the TDA board (without the TDA) for future reference. Thanks to everyone that contributed to solving this issue!!!