Marshall AVT150 Problem

[brian53]

Hi I hope somebody can help with this one. I have 2 avt150 mainboards

No avt150-61-00. One of the boards is marked Iss 5 and the other Iss 6,7,8,9,10


My problem is the schematics that I have are for the AVT-61-02.ISS 3.

On the transformer centre taps both pin w105 and w102 are shown linked together on the schematic as a common ground. On 1 of the boards I have (ISS 5) there does not seem to be a connection between the 2 points but on the other main board (ISS 7 8 9 10 ) the 2 points are electrically connected as per the schematic.

I have already blown a couple of TDA7293 amp chips and not to keen on blowing anymore.

I have done voltage checks without the power amps connected and on the ISS 5 board I am getting very strange supply voltages at the 3 pin connector to the power amp boards and also incorrect voltages when measuring the + - 15 Volt rails. The negative rail is measuring approx 137 v
I assume that the voltage are incorrect as there is no connection from the centre tap to ground on the one board and as such no proper reference to measure from.

There is a little circuit consisting of C170 R 182 D128 and D129 which is shown to go to Chassis 2. Could anyone advise what is meant by chassis 2 as this is not electrically linked to W102 and W105 on the suspect main board.

Sorry this is a long post but I am struggling a bit with this one. Any help would be greatly apprediated many thanks Brian

[Jazz P Bass]

I would measure the + - 15V supply by using the ground connection of the regulators.
There are smoothing caps for these rails that you can clip on the ground side.
137 volts sounds screwy.
Each transformer circuit has it's own ground reference. Center tap.
All of the grounds do not have to be tied together for the individual circuits to function.
Safety, hum reduction, etc. is a different matter.

[brian53]

Thanks for your help Jazz much appreciated.

I did as you suggested and the rails are in fact OK. I was originally checking the rails at one of the headers on the main board and embarasingly I have to admit that I was reading the header the wrong way round. I was measuring at what I thought was pin 1 when it was actually pin 8.

However when I was happy with all the rails I fitted 2 new TDA7293 chips and yes, you've guessed it they cooked. I am now going through the board again to see what went wrong this time.

[Jazz P Bass]

One thing you have to do when repairing these TDAxxxx amps is drain the power supply before you attach the power connectors.
If there is a high voltage on the + & - voltage pins, plug it in & nine chances out of ten, poof go the ic"s.

[brian53]

I went through the complete board and didn't find anything wrong so I got brave and fitted 2 more TDA's and I did drain the supplies as you suggested and this time all is well.

The only difference is that the 2 chips that failed were bought from China and the 2 I fitted today I had sent from USA. I live in UK and the suppliers over here get theirs from China. I have discovered that I can get them cheaper from USA (7 days delivery) and I don't have to wait nearly a month for them.

Cheers again.

[Jazz P Bass]

Glad to hear you have it working.