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I have a Marshall AVT50 that blew a while ago and I haven't had the time nor the reason to go poking around inside it until now. I have found a use for it now though, so I would like to fix it if possible. For those who are not familiar with the amplifier, it is a hybrid amplifier which has a single preamp tube and the rest is solid state. I am fairly positive that the preamp tube is not biased properly to be used as a means for amplification, but it is there nonetheless.

A while ago I was jamming on it just normally and smelled something burning. Then all of a sudden the sound cut off and I saw smoke rising from inside it. I immediately turned it off and haven't looked at it since, I have other amplifiers so it wasn't really a big deal. Today I opened it up to see if I could fix whatever blew, and nothing seems to be jumping out at me. Nothing looks visibly burnt or indicative of failure... What should I do? I am a bit hesitant to hook it up to the power line and try it again based on what happened last time I used it.

I just tested the fuse and it appears to be blown. The chassis says that it is a 1.6 amp fuse, but what other specifications should I look for in a replacement fuse? (Mouser)

Hope that helps...

JakeAC5253 wrote:I just tested the fuse and it appears to be blown. The chassis says that it is a 1.6 amp fuse, but what other specifications should I look for in a replacement fuse? (Mouser)

It could very well be a slow-blow type, if that's any help, what I would do is, after replacing the fuse, connect a variac between the amplifier and mains supply through an isolating transformer, then slowly turn up the setting on the variac while watching for any signs of trouble,use a multimeter to monitor voltages too if you see smoke starting to come from any of the parts,make a mental note and switch the power off,where the smoke came from is where the fault is likely to be...

Hope that helps...

The schematic tags it as T1.6A, which is a slow blow fuse, probably a 20mm x 5 mm (M205) size.

If the fuse blows quickly, disconnect the mains power and test the bridge rectifier BR1 with a DMM set to diode test, looking for shorts.

The switching noise suppressor capacitors C49 through C52 are in parallel with the diodes in the bridge rectifier, so if you read a short on any pair of legs, best to remove the capacitor and test it seperately.

Then test power supply filter capacitors C63 and C94 with the DMM set to a high resistance range, looking for shorts, followed by C59 (in parallel with ZD5) and C30 (in parallel with ZD6).

Look carefully at the TDA7293 power amp IC on the small circuit board at the rear of the chassis. If it looks burnt / carbonised / cracked, disconnect the wiring loom and see if the amp still blows the mains fuse when you turn it on.

blackbunny is on the money - that TDA7293 is most probably fried - the fan was probably making weird noises because they are shit fans - when they pack up the TDA output chip overheats and smokes - time to fit a new chip - if you don't know what you are doing take it to your nearest Marshall service guy (he will know what to do )
cheers
bajaman

Wow, I never expected to receive so much good advice on the topic

I suppose my next question is along the lines of answering the question posed in the last post of whether or not I know what I am doing. I really don't know honestly, I guess I'll have to have you guys answer that. I have been soldering and modifying (and lets face it, debugging as well) guitars and pedals for the last few years now, and I have picked up some good soldering techniques during that time. I have replaced opamps before without problems, I have even designed circuits from the ground up without problems as well. As for amps, the only thing that I have done besides tube swaps is solder a 10pF silver mica cap directly to the chassis connections of a 12ax7 socket as a high end shunt. Would this hybrid amp hold the same kind of danger to work on as a tube amp would? Should I have to discharge the filter caps before I begin working on it? I can't imagine that this simple solid state amp would be much different than working under the hood of a tubescreamer pedal would be, a bit more intricate, but then again I've never done it so I wouldn't know.

Caution should be used when testing or repairing any mains-powered device. Although the power supply voltages and currents are fairly low in this amp, there is a high-voltage supply for the 12AX7 valve.

I would recommend discharging all the power supply electro capacitors before doing any testing or repairs.

Some folks simply short out the terminals of the electro with a screwdriver, but I discharge electro capacitors using a handy tool I made with a 10 watt resistor (any value between 10 and 47 ohms) with insulated leads and alligator clips soldered to the leads and heatshrinked.

Check the TDA7293 first; if that's fried you will have to find a replacement, and get a better fan too - as Bajaman says, the stock fans are crappy. Check the power supply after that, just to be thorough.

JakeAC5253 wrote:Wow, I never expected to receive so much good advice on the topic

I suppose my next question is along the lines of answering the question posed in the last post of whether or not I know what I am doing. I really don't know honestly, I guess I'll have to have you guys answer that. I have been soldering and modifying (and lets face it, debugging as well) guitars and pedals for the last few years now, and I have picked up some good soldering techniques during that time. I have replaced opamps before without problems, I have even designed circuits from the ground up without problems as well. As for amps, the only thing that I have done besides tube swaps is solder a 10pF silver mica cap directly to the chassis connections of a 12ax7 socket as a high end shunt. Would this hybrid amp hold the same kind of danger to work on as a tube amp would? Should I have to discharge the filter caps before I begin working on it? I can't imagine that this simple solid state amp would be much different than working under the hood of a tubescreamer pedal would be, a bit more intricate, but then again I've never done it so I wouldn't know.

I concur with Blackbunny, with any piece of equipment that connects to the mains, it is always prudent to make sure there are no dangerous voltages present when you're working on it, first thing to do is to pull the mains cord completely out of the wall socket after switching off the power at the wall socket, then discharge any capacitors that are in the power supply circuit, check to make sure that the caps are really discharged with a multimeter, if they are, then the equipment is safe to work on, only reconnect the power when you've finished the work and are ready to test for correct operation, remember, it only takes about 50mA or so of AC mains power to go across your chest from one hand to the other to cause your heart to go into cardiac arrest and cause death, so please be careful....

I found that the best way to learn about repairing/servicing electronic equipment is to have a practical hands-on approach, eventually, you'll get to the stage where you can "read" the fault symptoms and have a good idea where to look, so the fault symptoms are actually a very handy diagnosis tool...