Monster Marshall amp problems. Help!!!

[Boater Sam]

Need some fast advice with this amp, I have 4 days to sort it out before I go off cruising for summer.

OK, Marshall AVT275, complex amp, never seen one before.
Full circuit diagram is on Electrotanya.

As presented, no output at all, various push buttons on the front light up when pressed, fans run, no visible component burn ups or damage. Has a token ECC83, all the rest is semiconductor/integrated chips. discrete components.

One speaker found to be disconnected, reconnected, nothing, no hum or on off clicks.

Pulled main board out with difficulty, I have +&- 50v, +&- 15v, 22v on standby terminal.
Ecc83 has 96v and 83v on anodes with 0.6v and 0.4v on cathodes, don't understand what it is for.

Don't understand the components C170,R182,D129,D128 in the middle of the power drawing AVT150-61-02 on the chassis to ground connection.

Gut feeling is that it is in some sort of standby, I know nothing about this system or how to get it out of standby if that is the case. How does it work?

So come on guys, brains in gear, what's to be done with it? Help please.

Sam.

[Silicon]

R182, C170, D128, D129 is a simple circuit used between the amplifier ground and mains earth to prevent earth loop hum while also provide protective earth safety.

The circuit seems incomplete.

The ECC83 seems to feed one triode into the other triode to provide gain and then feed the 2nd triode into a MOSFET source follower to reduce the output impedance.

[Boater Sam]

Have tested and passed the following components, D143, T104,ZD100,C217
Have +5v on regulator REG102, nothing gets hot, the silence is crushing.
Has anyone got a users guide, I am wandering in a dark space without a dog.
Sam.

[Silicon]

I would start at the output stage.

It uses TDA7293 100W DMOS amplifier chips. They have a mute & standby pins.

[Boater Sam]

Thanks Silicon, sussed that but why? What is it for? The circuit seems not to connect to much, it seems to take an input from CNB2-6 from the very complex front PCB, then does what back into the power module?

[Boater Sam]

Ok, mute and or standby sounds good, are these active high or low?

[Silicon]

The datasheet says it requires at least 3.5V positive on the mute and standby pins (10 & 9) with respect to pin 1 to turn OFF the mute or standby circuits.

To activate mute and standby requires a voltage below 1.5V i.e. grounded.

[Craig Sawyers]

The ECC83 is between sheet 1 and sheet 2. Having a single valve in that sort of duty is pretty common in pro amps, and is intended to be operated in soft clipping, with a control usually labelled "Drive level" or some such to adjust the level of cip, and a valve stage bypass switch which does what it says on the tin.

Trace Elliot do the same sort of thing, and Ampeg etc.

Craig

[Silicon]

CNB2 pin 6 is the input signal to the ECC83 and pin 4 is the output.

The standby circuitry seems incomplete on the schematic. It connects pin 9 on one power IC to pin 9 on the other IC.

Pin 10 (Mute) seems to be connected permanently to the +Vs.

I would temporarily connect pin 9 to the +Vs via a 10K resistor and see what happens.

[Boater Sam]

That gives me a place to start tomorrow, thanks.
I'll be back!

[Boater Sam]

I see what you mean Silicon, both pin 9s connected to nothing, is it feasible that they should connect to the "stby" on sheet ATV150-61-02, the power sheet? But there is 22v on that connection! Seems very high for what is only a logic connection.
But I can see nowhere else that it connects to!
If I have +22v on the pin 9 already, will that mean that the power chips are toast? The silence is total which I find odd.
Sam.

[Silicon]

The circuitry around T104 on drawing ATV150-61-02 sheet 1 of 4 looks like some kind of time delay cct. which eventually gives a positive output voltage across R219.

The power IC standby pin can tolerate a voltage up to +120V with respect to -Vs.

R220 and R218 should limit the voltage across R219 to something like +6V.

The drawings don't show how that standby voltage gets to the power IC.

Do you get continuity between R219 and the standby pin (with power off)?

If you disconnect one end of R218 the standby line voltage should drop to zero.

If it stays high it may indicate a fault in the power IC or its circuit board.

[Top Cap]

Might be just a duff output chip. I had this before and due to the musician un-plugging the speaker while the head amp was switched on. Always switch off the head amp first, wait 30 seconds then unplug the speaker. Likewise ensure speakers are fully connected before switching on amp.
The chip is available here :-
https://www.cricklewoodelectronics.c...keep_https=yes

Les

[Boater Sam]

Got plenty to check tomorrow, thanks guys.

I considered that Les when I found the lead had been pulled off one speaker, but this is a stereo unit with 2 power output chips. So perhaps the lead has been tugged after the amp failed.

I am working out the mute and standby, mute is just the switch on the input jack, standby is a logic gate in the output chips controlled by T104 on sheet 4 but the reason for it is defeating me so far.

I am thinking of trying the headphone socket to see if there is a signal there. Busy day tomorrow.
Sam

[Boater Sam]

No progress yet.

Realised that both speaker and input jack have to have plugs in to open the switches. Else we have mute
Tried pull up 10K resistor onto pin 9 of the output ICs, nothing.

Back to sheet 1, voltage at STBY, R218/R219 junction is zero.
Base of T104 is at 0.6v, collector is 0v so I deduce it is turned hard on, why?
+ rail off bridge L101 is 22.7v

Looking at the TDA7293s, the only pin with any volts on is B/S, pin 16, @14.9v.
Pins 1,3,10,4 are ground, so why C6 was added later I cannot imagine.

Sam.

[dryjoint]

Hi Boater Sam
I hope that by the time you read this you will have got the Marshall well under control.
Suggestions:
(a) check that both speakers are OK and not o/c. Reasoning is if both TDAs have failed (albeit at different times) this might lead to d.c. on outputs and goodbye speakers. These chips have a reputation of failing in these Marshalls. Despite fan cooling they tend to overheat and fail.
(b) Any d.c. volts on the o/put of those TDAs?
(c) Is the 2R2 resistor R223 (power schematic) OK and are the fans spinning? I know you said you have the -15 volts, but I had an HH amp once with one diode in the bridge failed and that still worked, so if R223 has gone high this might still give you -15v and mess-up the s/by volts.
(d) What standby volts do you get at the zener end of R218 on the power schematic? Either lift one end of R218 or unplug the power modules from the main board and see if that 22v disappears and you get a more "sensible value".
I think Top Cap is right with his diagnosis, but I think it would be sensible too to check that the speakers haven't suffered unduly.
Good luck!

[Boater Sam]

One speaker had its wires pulled off, I have not checked them but I have connected my shop speakers with the chassis on the bench as there is no room for the massive cabinet!

Not checked the outputs for volts, will do.

I'll check R223, the fans are on.

Zero volts on the hot end of the zener. Which is why I think T104 is hard on, having 22.7v on the unregulated + rail the other side of R220.

When you say unplug the TDAs, is that the 3 pin output connector, the 8 pin input connector, or both?

If I can establish that the input conditions for the TDAs are correct, I can understand that they could both be duff and then its an easy fix. But if the fault is also somewhere before the TDAs then I may well blow another pair.

I'm confused and not happy about that STBY output, I can't see what the circuitry around T104 does.

[Top Cap]

User handbook on my site, hope it downloads ok, Les
http://g4cnh.com/public/AVT150%20handbook.pdf

[dryjoint]

I was wondering if the 22v was coming from the power module board(s) under a fault condition and not from the s/by circuit. Whatever the volts on s/by and pin 9 are at the moment they should not be "low". If you're only getting volts on pin 16 of the TDAs, the three pin connector on the power module should have the +/-power supply to the chip - are they not there?
I know this is getting confusing, but back to the proverbial square 1, if the power modules are not getting the supplies,no volts=nothing. So are they getting what they need to make them work you did say something about having +/- 55 but is that on the module board? If so is there any indication (as far as reasonable) that they are faulty like d.c. on output? The number of connectors in this amplifier does not help either as they too can be a source of trouble.

[Top Cap]

Have a good look around the output chips, look for any cracks in the body etc.
The attached photo shows a burnt Pin 8 which was a failure of the driver inside the TDA. The speakers you can check with a single 1.5v cell, make sure they are not short circuit. My earlier post was about the problem where the speakers are unplugged at the speaker! If the jack plug is withdrawn from the speaker first, the socket on the speaker will impose a short circuit on the jack of the connecting cable. It should be safe if you disconnect the speaker cables from the amp first, it is doing it the other way round that can cause problems.
The only mods I have found on this amp is the input protection from static voltages using a pair of zener diodes. I will attach if I can find it.
Les