Marshall AVT 150 Valvestate

[Disco Dave]

My Valvestate AVT 150 has no output whatsoever. The red LED lights up and the transformer buzzes. I have checked and there is no evidence of burnt out components. The 2 TDA chips on the output board are not burnt out as far as I can tell though there is lots of corrosion on the PCB for one of the TDa's.
I read on this forum that this problem can be related to the mute circuit. Where is it in the main board? Or is it more likely to be the TDA chips have failed.
If anyone has a schematic then please email.
With thanks. Happy to be the newest member.

[Wakculloch]

If it's the same layout as the AVT275, then all channels also go through the valve stage, which would be my first suspect.

[Disco Dave]

Cheers for your tip. Since posting I have also done some homework and this amp is particularly prone to problems with the output amp. There are two TDA chips and I may replace these and see if it cures the problem.

Disco Dave

[KCblues]

I've repaired many of these and it was almost always the TDA devices. Both needed to be replaced at the same time to avoid another failure.

However, the symptom was usually a fuse blowing which you do not seem to have. I would disconnect both output amp boards, feed a signal to the input and see if you have a signal from the preamp at the input to the amps.

KC

[quazcha]

try a plugging a guitar cable into the effects send and return
Do you get sound thru headphones?
Do you get emulated line out sound

[quazcha]

I can send schem but need email

[kingrick]

I'm also currently repairing one of these amplifiers. So far I have diagnosed a failed fusible resistor and a blown TDA7293 output chip.

quazcha, if it's not too much trouble then I'd greatly appreciate a copy of the schematic. I can send you my email address by PM if that's OK with you?

Many thanks

[kingrick]

Much obliged quazcha, for your swift, efficient and friendly response. A true gent!

Best regards...

[mgreczek]

I just got avt275 from my brother in law. It has the same symptoms as Disco Dave described.
I tried quazcha's suggestion but I'm not getting any sound from headphones or emulated line out.
There is no burn signs anywhere.

Does it mean it might be tube?
I also read some posts about checking mute circuit but I don't know where it is.
quazcha could I have copy of the schematics?

Thanks

[quazcha]

The tube is a 12ax7, very common audio preamp tube (is there an orange glow in the tube? is it warm- the filament could be shot), check any "old-TV" shop if you can find the tube locally or maybe even borrow one from a friend's amp it is used in marshall, fender, mesa etc - even a music store will have a new or used tube you could buy for under $10. I sent the schematics- keep me posted on your progress and good luck!

[mgreczek]

quazcha, thank you for schematic I just got email.

Yes tube has an orange glow and it is warm.
I'll stop by few stores today to see if I could pick it up locally if not I'm sure I could get it online. In either case I want to get spare.

I would be working on that tonight. I'll keep you posted on the progress.

[mgreczek]

Just a quick update.

I did'nt have too much time to mess with things recently but I got my preamp working.
Negative 15v voltage regulator was bad. I replaced L7915CV and it fixed my preamp problems. Now I have nice clear sound thru headphones and emulated line out.

I still dont have main sound sound but I traced feed to the imput of each amp. Hopefully replacing both TDA7293 is going to fix that problem.

Back to work......

[quazcha]

is the fuse or output Ic's blown? with the connectors unplugged check voltages from the two signal and two power conns to the output Ic's as compared to schematic. I had a pc trace open up going from pin 1 of one signal connector to the other, unfortunately didnt find until my 3rd pair of TDA's got smoked:mad: keep us posted...and good luck

[quazcha]

If the output Ic's are not smoked and the fuse does not blow with all connectors connected check the voltage going to the mute pin on the power amp boards

[amptech]

Hello there,

I had a laney valve state with low o/p and poor preamp drive fault and after a few voltage checks I found dried up filter caps, dry joints.
the pre drive 12ax7 was passed it's best so I uesd ecc83 by jj and once all corrections were finished it worked great... job done.
wire up a 60w bulb in mains lead plug in and if it stays on your amp is pulling current! a working unit the bulb should flash and stay out if all's well.

amptech.

[Tubehead]

I've replaced IC 8,9,and 10 trying to eliminate the problem. Only the accoustic channel works. Without a schematic, I'm just guessing. Could you send a schematic for AVT 150? My email: lundyevo@yahoo.com
Any help would be greatly appreciated.

[amptech]

Hi there,

You say you have changed some ic's on the preamp board and the fault is still there, did you use the correct replacements for the old ic's 5201a?.
Ic 2120d is recommened by marshall to replace that and check replace tl071 at the tone stack aswell, did you replace the predrive valve 12ax7 use ecc83 in it's place and check resistors around that part of the circuit plus the cap's 33uf 450v/10uf 350v. sometimes when there's weak or no overdrive channel replace the old valve and see if things improve.


Amptech

[Tubehead]

I think my model is a later rev. AVT 150X. All the op amp chips are NJM072BL's which I replaced with NJM072BD's. This rev. also Does not have TDA 7293's for output. Looks like Marshall upgraded to SAP 16 NY 3117 and PY 2N28, which work fine. 12AX7 is good. Using a probe, the signal will go through the amp from ground points (like the pot chassis) and the legs of some of the channel switches, which makes us think there is a shorted cap maybe. All of the work has been done on the front pcb(the one with all the knobs). We haven't done any work to the preamp board, just made sure the 12AX7 is good. My tech thinks the problem might also be with the switching. However, the leds on the switches lit up and turn off when pressed. Also, the acc. channel works when pressed. Another curious thing is a gtr signal in the effects return will go thruogh the clean channel. THese schematics will help a great deal. You guys are a godsend! Again, any help is greatly appreciated.

[Enzo]

OK, so you don't have an AVT150 then. The AVT150x is a different model, no matter they are both similar. SO the schematic set is different.

What exactly are we trying to fix? Welcome to the place, by the way. I might suggest to all involved that it is always better to start a new thread for your amp than to tack it on to the end of another amp's thread. This AVT150x is now the fourth AVT in this 18 post thread. The first amp had no output at all, now this one apparently has output on some channels but not others. Well that isn't the same problem, and it is a different amp model.

SO what exactly are we fixing? Loss of some preamp channels only?

LEDs working has no bearing on whether the switching ICs are working.


If the two OD channels don't work, then look at IC1 at the input. You already rplaced IC8. The DG212 at IC2 could be bad.

If the clean channel also works, then this is the point they join together.

If yuo still don't have the schematic, I have it.

[Tubehead]

Thanks for the reply. I have the schematic thanks to Amptech. You're awesome! This is what I'm fixing: The acc. channel is the only one that works. OD1, OD2, and Clean do not. The Clean channel will work if I plug into the efx rtn in back. That tells me something, I don't know what.THe 12AX7 is good. We're going to replace IC1 next. We have 1 072 chip left. The others we replaced before we had the schematic, hoping we'd get lucky. Come to think of it, our schematic is different. You're right. Could you send me one please? lundyevo@yahoo.com. You guys are so much help I don't know how to thank you.

[Tubehead]

I screwed up in my last post. The schematic I have is wrong. AVT 150H not X. Could you send me the 150X if you have it? THanks: lundyevo@yahoo.com

[moulos]

Mate i needs the AVT-150 Valvestate 2000 schematics

Can you send them pls?
My email is : ioan_balab@yahoo.gr

Cheers

[mrgreengenes84]

Someone can send me the AVT150 schematics.I've your same problem...I've burn a lot of TDA7293,help me please!

Thanks!

[mrgreengenes84]

I've the same problem,I've burn a lot of TDA & can't fix it!
Quazcha have you solved?

[quazcha]

The TDA7293 is used in AVT150 and MG100HDFX. Check out post #13 under this topic I had an open PC trace...I will send schems

[Enzo]

If either one of the TDA chips is blown, replace them both. Also make sure the 0.22 ohm 10w R111 is not open.

[mrgreengenes84]

Thank you Quacha,today I'll try to fix this amp!!!

[mrgreengenes84]

Someone have fixed AVT150 Power Amp?I need some advice.It's necessary to replace ONLY TDA's (both) to run this amp?Or some replacement?

[thedetoure]

hi just wondering if you have a schematic for a avt150 thanks. thedetoure@hotmail.com