Marshall AVT150

[insp166]

This is my first post here I build tube amps but someone asked to look at this one

This looks like an open a shut case I found a blown fuse inside.

Why don't they put the fuse in a holder and have it on the outside?






After I replace this fuse I hope for no smoke but a board may be damaged
this is what happened .
The good Doctor was playing thru an extra speaker all was going well.
He said he change to another cable and that is when it all went dead.
I think the cable must have had a short in it


I know it is the two TDA7293 CHIPS I ordered four at $7.43 each

I hope thy work any thing else I should look for

I think the fuse is a 4 amp slow blow?

MY TUBE AMP SITE

THANKS IN ADVANCE

[izzybell2810]

Hi there,
The reason the fuse is on the inside is prob due to the usual cost cutting excersises of manufacturers. The amp is fitted with an emf filter euro socket in place of the usual euro socket fuse combo. adding a chasis mounted fuse would be a quid of there profit gone.

If the cable you mentioned was shorted then I would bet a pound to a penny the 7293 outputs are fried, looking at the fuse in the pic would seem to confirm that something is shorted, as it looks very black. Is the fuse wire splattered up aginst the glass in more or less a straight line?

Dave.

[insp166]

I did a test removed the black power wires from the chips and the new fuse held.
I turned off the amp and replaced the wires on only one chip turned on the amp saw a pritty blue flame at the bottom of the chip blew fuse.
I did the same to the other on same thing

Could there be any more damage

Thanks

[izzybell2810]

I did a test removed the black power wires from the chips and the new fuse held.
I turned off the amp and replaced the wires on only one chip turned on the amp saw a pritty blue flame at the bottom of the chip blew fuse.
I did the same to the other on same thing

Could there be any more damage

Thanks

pull the chips from the board replace the fuse, power up, does it hold it's fuse now? If so fit the new chips and power up with a light bulb limiter or variac.

[52 Bill]

Could there be any more damage

Of course there could be, but you've already proven to yourself that the output chips are bad and need to be replaced. Re-testing this will cause more damage. as you saw with the arcing of the chip. Stop before you do any more damage to the boards and possibly the rest of the amp.

Just do as Izzybell suggested and replace the chips.

[Enzo]

ALWAYS replace BOTH chips when one blows.

NEVER connect or disconnect speakers while the amp is powered. Pulling a cable plug out of a jack often shorts the tip to sleeve on the plug.

DO NOT power up the amp with just one IC connected.

Why is the fuse inside? Why are there fish? Your fuse blew. What happened next? You found that simply replacing that fuse was not enough to get the amp runing again. That will usually be the case. SO what purpose is served by mounting THAT fuse on the panel? The fuse usually blows for a reason, and that reason requires the chassis to be out to repair.

[insp166]

ALWAYS replace BOTH chips when one blows.

NEVER connect or disconnect speakers while the amp is powered. Pulling a cable plug out of a jack often shorts the tip to sleeve on the plug.

DO NOT power up the amp with just one IC connected.

Why is the fuse inside? Why are there fish? Your fuse blew. What happened next? You found that simply replacing that fuse was not enough to get the amp runing again. That will usually be the case. SO what purpose is served by mounting THAT fuse on the panel? The fuse usually blows for a reason, and that reason requires the chassis to be out to repair.

That is what I told my customer he is also my Dentist

The blown fuse means that there is more damage he told me to try it any way.

I am a ham Radio operator age 61 and I have built many Heath Kits in my day
As you can see on my web site I can build a tube amp I know how things work in them.

The Doctor told me it was a tube amp, when I opened it up and saw only one tube I said I can't fix this I would not know what to look for.
In a tube amp I can see and smell damaged parts and check each section separately until I find the short.

I quit working on computers 5 years ago for people, when you fix one thing and another part fails a week later it is always your fault.
I was a grease monkey in my younger days yep if you changed a tire and their battery went dead you did it.

Now I can see why the fuse is in the inside. Tube amps you play to loud a fuse will blow easy to fix on stage.

With the help of other people on other threads and I found some here I think I can make this puppy bark again

Thanks to all

[bnwitt]

If you can fix a tube amp, then with a little practice, some study and the guidance of those in this forum you can start fixing solid state amps too. In my opinion there are some fellows in here who should be teaching at their local community college. Lots of brain power in here for sure. I've been driving them crazy with my questions for a couple of months now and I have learned a lot! Enzo told me to think of a transistor as a tube with the base being the grid and that helped a lot. Solid state seems foreign, but it follows the same Ohms and Kirchhoff's laws as hollow state. Don't look at this amp as an aggravation, look at it as an opportunity to take a new journey.

[insp166]

If Don't look at this amp as an aggravation, look at it as an opportunity to take a new journey.

Thank you for the advice I did fool with transistors in my younger days with heath kits build a small ham radio .
Transistor testers are cheaper than tube testers.

A tube can be in between and still work. A transistor is either good or bad
If I can get a schematic and the manual and know how the circuits flow I can fix it

When I look at a tube amp I can figure how the plumbing works most of the time.

This internet is the best thing since sliced bread. Back in the old days you had a problem with your radio some friends would stop by the house and pitch in and we get it fixed.

I would take a couple of weeks every body would ask some one else what did they think like we do on the web now in seconds.
Back then we had the Pony Express

You are right about the adventure
I am going where no man has gone before
But when I fix this one I will be an expert at Marshall AVT 150

You mentioned about people teaching in schools.
I think today the schools would get sued if a student got bite by an amp

I also Got this advice from another Threah HOFFMAN amps

If you're taking that chip out. It will be easier to remove if you first cut the leads from the chip to the pcb.
The chip won't act as a heatsink for your soldering iron.

[insp166]

[izzybell2810]

that would be a 0 ohm link. either a bare link wire or something resembling a resistor with a black band.

[insp166]

that would be a 0 ohm link. either a bare link wire or something resembling a resistor with a black band.

yes I saw the jumper wire but what does

DO NOT FIT
MUST FIT

In the box mean with the Caps and the resistors

Thanks

[bnwitt]

I have to assume that your schematic serves several models of AVT amplifier. The notation box is specifically for the AVT150. Must fit means the component is used in that version, do not fit means it is not used.

[Enzo]

That is exactly what it means. The same board is used for more than one model amp. In this one for example the two speaker jacks are linked together. IN another they may be separate with a power amp for each. Hence the wire link or not.

In the AVT150, it says do not fit - meaning install - certain parts because they only are used in the AVT275.

[buddybriol]

avt 275 and avt150

hi everyone,im a newbie to this forum.is there anyone who can help me with an AVT 275 or AVT 150 schematic?I desperately need one since i need to repair my dads amp as my bday for him.I would appreciate any help i can get.my email ad is: buddy@soundballoon.com

thanks...thanks....thanks