Another MARSHALL Valvestate 2000 AVT150 Blowing fuses......

[52 Bill]

Man oh man what an epic thread!! Good job!

Enzo could have had this amp done in 20 minutes tops.

What I like about this guy, is that he was willing to listen, sort facts, research and then do the work. He made a few mistakes along the way, but these were more learning experiences than anything else. I wish more of the newbies were as willing to learn as he is. Too many come here unwilling to listen to a different point of view.

Congrats on getting this fixed!

[blearyeyes]

No reversed caps.

I think what happened was a Power IC gave up the ghost and took out a capacitor in the power supply. I replaced the cap. I also replaced the IC chips and out of inexperience blew them up by forgetting to discharge the caps before plugging in the power supply. When I powered up the amp I watched the top of the new cap bulge. I replaced the cap again but I inadvertently broke a via that coupled the top and bottom traces on the board. I didn't know about vias. So the negative side of the power supply feeding the power ICs was down. Finally got the power supply working properly. G-one hung in there through all of that confusion. Lol.

I started to replace the power ICs again and got hung up on all the caps on the power modules giving different readings. At that point I realized I didn't know squat about capacitors so I studied online. With practical advice from you guys and seeing what it took to really test a cap I just pulled the Power ICs and re-tested all the caps and they behaved as they should have as far as I could tell. Enzo's statement that he had never seen a bad cap on one of these boards is the kind of advice you can't find In a book. It's experience.

So armed with a magnifying headband I could actually see the damage I had done the first time I removed and installed the Power ICs. I found a small piece of trace missing next to a pin. I ran a bridge from the IC pin to the plug pin to fix it.

Most of my second pass at repairing the amp was fixing the damage I had done the first time. I learned a ton and am grateful for all you guys chipping in.

[The Dude]

Glad to hear you got it fixed! I have to say, your willingness to stick with it is admirable. I think many would have given up a long time ago.

My POV on these types of repairs from someone who does this for a living is this: Every once in a while, you have to lose a little money on a repair to learn something. You'll make it back in spades from the knowledge you gain working on the tough repairs. It seems to me that many techs give up too early and just write up a "discouraging" estimate, or bail on a repair in some other way. The problem with this logic is that, while they may get out of a jam on that particular repair, they never learn anything. Their ability to solve difficult problems will never change and the next time they encounter a similar problem, they will be back in the same boat. So, again,......kudos to you for sticking with it!

[Jazz P Bass]

I will have to agree with that!
Oh so many times, as I followed this post, I said to myself "this amp is never gonna get fixed".
I apologize for the lack of faith.

[dansego]

I know it's been 2 years since this thread, but I just wanted to express my thanks that you spent all that time delving into this issue.

My AVT is coming up to 10 years old and last week in just stopped turning on. This was the first issue I had ever had with the amp so I really didn't know where to start. A shed load of reading later and I came across your discussion, 5 days later, even with having to wait on the mail order of parts and it's up and running again.

Excatly the same symptoms, blowing fuses when the 32V output off the transformer was connected. For me it turned out that a discarded staple had got into one of the cooling fans which had resulted in the fan burning out, the TDA7293 power chips overheating, and the C171 capacitor to the +ve 32v power rail burning out (diagnosed through the lack of resistance across R181).

The hasty construction of a bulb limiter, delivery of a new fan, two new TDA7293V chips, and a new 1000uF capacitor, some frustration at trying to desolder the old components without damaging the boards (one track did lift on one of the TDAs), and a laughable snapping of the lead to the internal speaker due to lifting the head with it still attached following my first successful post repair/pre reassembly test run, and everything is sounding as good as ever.

So, you have my thanks, hopefully the skills I rediscovered will stay with me long enough to help someone else in return.

Dan

[Enzo]

Hey, great, and welcome here.

Your experience is exactly why I cringe whenever someone expects his amp problem will have the exact same solution as someone else's. What are the odds the next guy's blown amp will be caused by a staple?

[gui_tarzan]

One thing I've learned here is these guys have a TON of patience and unlike many other fields, are willing to teach others how to repair their amps - or tell them NOT to if they don't think the person is capable of following safety rules.

On a side note, I found a screw wedged under a resistor on an amp that was shorting out. It looked like an extra from the factory that someone missed.

[The Dude]

I was working on a big screen TV back in the day. It had a 1/4" nut driver rolling around inside. It was a new TV and the markings on the tool were written in Japanese, so I'm pretty sure it was "installed" at the factory.

[Jazz P Bass]

Sh*t Happens.

[blearyeyes]

I'm glad this thread helped you! The guys here in this forum are awesome. Without them I obviously couldn't have done it. This started me on a journey into electronics, guitar pedal building and maybe some day designing something fun. Glad you got your amp fixed.

Daniel S.