Marshall AVT275 - No Sound From Main Speakers

[Johnny Creeper]

Just wondering if anyone has any experience of Marshall Valvestate AVT275's....?

I'm looking at one for a mate at the moment, as he'd said it 'stopped working'.

As far as I can tell, preamp section appears to be ok, as the emulated line out and headphone jacks all work as I'd expect them to.

The problem is that there is no sound at all from the main speakers - it's as if it's got itself locked into a mute / headphone mode. Speakers and leads appear to be ok, as they work fine (but obviously at low level) when plugged into the emulated line out.

I've checked voltages on the pair of TDA7293's, and get 49v, or thereabouts, so it looks like they're at least operational.

I have heard that the jack sockets on these amps are meant to be suspect, so I've cleaned them with contact cleaner and checked the entire main board for dry joints and physically damaged components, but nothing has shown up so far.

Schematics seem to be impossible to find, apparently it's the same / similar as the AVT150 - but can't find that one either....!!

Thanks in advance to anyone with any ideas on this....

Johnny Creeper

[techineer]

Just wondering if anyone has any experience of Marshall Valvestate AVT275's....?

As far as I can tell, preamp section appears to be ok, as the emulated line out and headphone jacks all work as I'd expect them to.

The problem is that there is no sound at all from the main speakers - it's as if it's got itself locked into a mute / headphone mode. Speakers and leads appear to be ok, as they work fine (but obviously at low level) when plugged into the emulated line out.

I've checked voltages on the pair of TDA7293's, and get 49v, or thereabouts, so it looks like they're at least operational.

Johnny Creeper

The headphone/emulated line output is practically the last stage before the power amp.

IIRC, the output ICs on that amp are in modules. Each module has a long connector and a 3-pin connector. The 3-pin connector should have a large positive voltage on pin 1, an equally large negative voltage on pin 2, and zero volts on pin 3 with no signal. Pin 3 is the IC output pin. If that is off-zero, the device is most likely blown. Common failure mode.

One other possiblity is the Standby pin of the output ICs. Pin 9 on the chip, pin 4 of the long connector. IIRC, it must be HIGH to enable the ICs. It is controlled by transistor T104 near the power supply circuit. Check for a possibly shorted T104 or zener ZD100, a BZX558 15V zener diode.

Good luck....

[Johnny Creeper]

Techineer,

Thanks for the reply and the good info, sounds promising....

Will check these out and let you know....

Cheers,

Johnny Creeper

[Gtr_tech]

Was the headphone jack used frequently on this? Some designs pass main spkr output return thru that headphone jack....which is bad mojo if you ask me. But its a corner cutting way to make it work. Put a signal on it with the spkr hooked up and push down on the gnd leaf on the headphone jack. Got output now? Replace the jack, or add a dedicated spkr cutout switch on the rear panel.

[techineer]

Was the headphone jack used frequently on this? Some designs pass main spkr output return thru that headphone jack....which is bad mojo if you ask me. But its a corner cutting way to make it work. Put a signal on it with the spkr hooked up and push down on the gnd leaf on the headphone jack. Got output now? Replace the jack, or add a dedicated spkr cutout switch on the rear panel.

The headphone output is independent and not connected with the speaker. This model uses a separate amplifier for the headphone output as part of the emulated line output. The signal is split off after IC12 but before the master volume control, with the headphone signal being produced by IC102 and IC104. It should be possible with this model to have both the headphone and speaker going at the same time. If you want to mute the speaker, just turn down the master volume.

[Enzo]

I am pretty sure we have discussed the AVT150/275 before, so a search of this forum should reveal links to the drawings, but here they are

[Johnny Creeper]

Enzo,

Thanks for the full schematic - will certainly make life easier....

Having the schematic confirms one fault I've found so far, that resistors R202 and R223 at the PSU end of the board should be 2.2ohms - one is 2.9ohms, but the other is reading 20k+.

Will try replacing these first, then see what else shows up further down the line.

Thanks to everyone for the info so far - will let you know what happens....

[Johnny Creeper]

OK, following on from replacing R223 to get it back to 2.2ohms, heres the voltages I'm getting in relation to the TSA7293's;

Amp1 Amp2
Pin1 1.9mv 1.8mv
Pin2 1.9mv 1.8mv
Pin3 u/s u/s
Pin4 48.5v 48.4v
Pin5 1.4mv 1.9mv
Pin6 u/s u/s
Pin7 47.8v 48.6v
Pin8 -48.4v -48.6v

All reading taken directly from the chip pins. u/s meaning unstable voltage reading, randomly changing between 10's and 100's of mv's.

Amp1 to CON102
Pin1 48.6v
Pin2 -48.2v
Pin3 1.5mv

Amp2 to CON103
Pin1 48.6v
Pin2 -48.3v
Pin3 1.7mv

As before, line out, headphone jacks etc all work fine, preamp seems ok, up to power amp stage.

Thanks again,

JC

[techineer]

The TDA7293s have 15 pins, and pins 9 and 10 are the critical ones for your situation.

The readings you list for the 3-pin connectors sound normal. Output IC's are probably OK.

I've attached the data sheet for the 7293. Note the pin connection diagram on page 2. I suspect you are only looking at the odd numbered pins, as there are 8 of them. With that in mind, I've translated your readings to:
Amp1 Amp2

Pin1 1.9mv 1.8mv gnd
Pin3 1.9mv 1.8mv NI in
Pin5 u/s u/s clip/ss det
Pin7 48.5v 48.4v +Vs
Pin9 1.4mv 1.9mv Standby (THIS SHOULD NOT BE LOW)
Pin11 u/s u/s buffer driver
Pin13 47.8v 48.6v +VS Power
Pin15 -48.4v -48.6v -VS power

Look for and measure the voltages on that transistor and zener diode I mentioned earlier....

[Johnny Creeper]

Techineer,

Thanks yet again for the helpful info, had a feeling I was missing something when you mention Pin9 in an earlier reply....!! This now makes a whole lot more sense....

Will check these out, and that transistor and zener, and let you know what I find....

Cheers,

JC

[Johnny Creeper]

Well, have finally got this amp back up and running fine....

After checking the transistor and zener suggested by Techineer, and not getting the results I expected, I worked back through the schematic.

Turned out that, yet again, R223 had gone high, from 2.2ohms to 10m+. Correct me if I'm wrong, but that left me with the impression that from this point onwards I had effectively an open circuit....? That the -15v rail was dead, or as good as....?

Anyhoo, with help from a sparkie at work, who's electronics fault finding knowledge is far freater than mine, regulator REG100, a 7915, was replaced, along with another R223, and the amp was brought up gradually on a variac, with the DMM monitoring R223. As the juice went up on the variac, I was more than pleased to hear the speakers crackle to life....!

I've since replaced both R223 and R202 with new 2.2ohm fusible resistors, as apparently thats what they're meant to be, and it's working quite happily - and reliably.

Not sure if this is ever likely to be of use to anyone else, but you never know, it may save someone a bit of time somewhere down the line....

Thanks yet again to everyone for the helpful info along the way - it's good to keep on learning....

Johnny Creeper

[techineer]

Glad you got it going again. We have seen several of these units with those resistors open and haven't found a verifiable cause. Those resistors (R202 and R223) are meant to act as slow-blow fuses. We replaced them, verified that the amp was operating properly, and sent it out. They haven't come back.

However, if you had already replaced them once and they went out again, I suspect that you'll be working on it some more, because there's a high probability that something is drawing too much current and taking them out.

[Johnny Creeper]

Techineer,

Thanks again, looks like you've had a lot more experience of these than I have (and maybe I'm likley to have some more....!)

Yeah, I'd assumed the resistors were going high due to drawing too much current. Any chance the failed 7915 REG100 could cause that....?

Also, the schematic states 2.2ohm 1w, but the ones I removed look very small to be rated at 1w, looked more like 1/4w by their body diameter. Not knowing for certain that they were 1/4w, and knowing the schematic states 1w, I've replaced them with 1/5w.

Now wondering what the mean time between failures could be....!!

Thanks,

Johnny Creeper

[Enzo]

Wait. You say they looked like 1/4 watt but you used even smaller ones? And the drawing specified 1W? if it says 1W, I wouldn't use smaller ones. SOme 1W film resistors can look pretty small.

[techineer]

Yes, they do appear small, but I have seen similar resistors that were rated for much higher power dissipation than their size would indicate. It's a matter of at what temperature the resistive element breaks down and fails. With carbon film, that is much lower than with some metal films. It also depends on the substrate under the element and how well it can dissipate heat.

You replaced them with 1/5 watt? If true, that would be less dissipation than the 1/4 watt resistors. Probably won't last long at all.

I would suggest that you go with the diagram... get some 1 watt metal oxide film such as Mouser's part number 261-2.2-RC and stick 'em in there.
261-2.2-RC .

Yes, the regulator could cause the resistor(s) to open, if it failed short.