Marshall AVT50 Valvestate 200 Combo blowing main fuse

[music.man]

I have an AVT50 tValvestate 2000 that blows the main 4 amp fuse only when either of the power amp CB (CN1) is plugged into it...I started off by marking all the connections carefully, uplugging everything from the power amp boards and the "mother" board, then began from the Xfmr, and began plugging connections in one at a time then turning the power switch on. Everything was fine until I plugged in the CN1 connector that goes to one of the power amp boards (the board mounted to the heat sink, cooled by fans)...then the fuse blew. I replaced the fuse, unplugged the first Power amp board, plugged in the second power amp board...and it blew a fuse again. I measure +48V -48V and 0 V across pins 1,2 and 3 respectively...is this correct? I also poked around the "mother" board and found an unusual reading from the 7915 (- 15v reg chip)...it measured 80 ohms across the pins 2 and 3 (in both +/- directions) ...measured across the 22uf Cap right next to it and read 80 ohms again (in both +/- directions). The cap is designated C123 but I can't find it in the schematic. I little nervous now about installing new TDA7293's because I don't want to blow them up again if something else is causing them to blow (12 bucks a pop!...and I mean "POP"!)...any suggestions? Thanks!
P.S.: Enzo...thanks again for the schematic!!!

[Enzo]

TDA7293V is in stock at Mouser for $8.95 each. That price is up recently. If you buy 10, the price drops to 6.28.

Always replace both modules even if one seems OK. The three pin connector is the power connector, so yes, there is voltage there. DO NOT pull those connectors off or put them on while the power is on.

As to the Vreg, just disconnect the power modules and power up the amp. Does that regulator make -15VDC? If it does, then don;t worry about the circuit resistance.

[music.man]

Enzo: You sent me a schematic of the avt 150 valvestate 2000, but it doesn't look correct to me...maybe this is for the avt 150 head, which powers a separate cabinet...i needed the avt 150 valvestate 2000 schematic for the combo amp with 1 12" speaker...sorry to bother, my mistake!

[Enzo]

The circuit should be the same head or combo. There is the AVT150 which you have received. There is the AVT150X which as far as I can tell differes only in the power amp - they got rid of the 7293 modules and made a MOSFET power amp.

What exactly does it say on the serial number label?

For that matter, what is different on the schematic?

On the board in question, what is the board number?

[music.man]

numbers to confirm correct schematic

Enzo: Sorry for any confusion...the schematic I have is for an AVT50...I have an AVT150...is there a difference? the main board is silkscreened AVT150-61-00 ISS (also has Z00-40)...the power amp boards are silkscreened AVT50-62-00 (also has Z00-26).

The serial number of the amp is M-2001-03-1054-B

I think the confusion came in when I read the numbers from the power amp boards as AVT50, when in reality the amp is an AVT150. Hope this helps in finding if I have the correct schematic...Thanks again!!!

[Enzo]

I instantly delete all emails after reply, I no longer have yours, so send me another with exactly the model you need.

Yes they are very different.

[music.man]

Marshall AVT150 Valvestate 2000 Combo blowing main fuse

Enzo: Sorry for any confusion...the schematic I have is for an AVT50...I have an AVT150...is there a difference? the main board is silkscreened AVT150-61-00 ISS (also stamped Z00-40)...the power amp boards are silkscreened AVT50-62-00 (also stamped Z00-26).

The serial number of the amp is M-2001-03-1054-B

I think the confusion came in when I read the numbers from the TDA7293 power-amp boards as AVT50, when in reality the model amp is an AVT150. Hope this helps in finding the correct schematic...Thanks again!!!

[music.man]

Marshall AVT150 Valvestate 2000 Combo blowing main fuse

Enzo: hope you are well!...I've poked around in this amp now for the last week or so and still haven't come up with anything definite. The only discrepancy I've found is a possibility that the outputs to the Power amps A and B (seen on the "Back PCB" schematic as CN102 and CN103 may have been switched...could that cause any problems? I'll wait to hear back before I continue...thanks!

[xjrfast]

Schematics

Hi would it be possible to send me the schematics for the avt150h. Its all working fine now but would be useful if it dies on me. Thanks

[music.man]

Marshall avt150 schematic

Marshall AVT 150 Schematic.pdf

[xjrfast]

Thanks

Many thanks

Phil

[dizzyspiral]

problem with avt 150 its burning the dta7293

hjel pls, i changed both and burned again, i dont know what can i do

[unparalleled]

hjel pls, i changed both and burned again, i dont know what can i do

I had the same instability problem with AVT50.
Marshall seem to have built a lemon in this model. They used minimum supporting components for the power amp chip. I tried everything I could think of to no avail.
There does not seem to be many of this type of amp around so perhaps Marshall stopped producing them because of the instability?
The amp I worked on was only just out of warranty and had lots of problems from new.
I eventually gave up in disgust and fitted a kit amp module with a decent heatsink for good measure.
Schematic:

[loudnproud]

Parts Purchase

Where can a person purchase the boards and other parts for the avt150? I've got a lot of noise and there is no base. The tube has already been replaced. The cause was the fans failed.

Thanks for any help.

LnP

[Enzo]

Any MArshall dealer or from Marshallparts, aka PArts is PArts.

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