Marshall avt275 noise problems !!HELP PLEASE!!

[MARSHALL BUSTER]

Hello all.
I have a Marshall avt275 that has a very loud noise from the speakers when turned on. It is fine with through headphones with speakers unpluged. I have fitted two new output amps from Marshall and there is no change still the same problem. I have also changed cans c216,c191,c192 as i was told to try them, but as i had already thought, it made no change. Even with the vol turned right down it still makes this very loud buzz and cracking noise. have you guys got any idea what could be going on and where do i look next??
Thanks i am new to your site and it seems to be a very good source of much needed information to a novice like myself.
Keep up the good work.
p.s i hope you can help i'v been serching the net and i seem to be the only one with this kind of problem. I also have an avt150 and that was blowing fuses so i changed the amps and the problem was solved, but this time im realy stuck. Please please try to help.
Thanks

I need to know what readings to expect to find at C189,230 and cn102,103. Thanks. Mark

[MARSHALL BUSTER]

I'v found that the speaker outputs are giveing out 43v there abouts (that explanes the loud noise), i'm starting to think it could be a voltage regulator maybe? what do you think.
Thanks.
Mark

[Jazz P Bass]

Marshall AVT

You may have blown the new output chips.

On these amps you must drain the Vdc on the main capacitors before you plug in the output boards.

[MARSHALL BUSTER]

Thanks Jazz
Do you think i would blow them even by turning it on for a split second? caus thats all it takes to hear the noise. If i was clever enough i'd know how to drain the caps, is the old bulb trick enough for this job??. I realy appreciate your takeing the time to get back to me. Im determined to get this amp up and running again and educate myself abit more along the way.
Thanks.
Mark

[Jazz P Bass]

Draining power supply caps

I usually use a 1K ohm 2 watt resistor to drain the power supply.
First thing is hook up your DVM to one of the power rails. Amp off.
One DVM lead to + VDC & one to chassis ground.
With the amplifier OFF, you will read a voltage.
Now, again with the amp OFF & unplugged from the mains supply, attach the 1K resistor to the DVM leads.(that are attached to the poewer rail)
This will drain the voltage.
Now do the - Vdc supply.
On the Marshall you should find these voltage points at the 3 pin connector that goes to the output board.
IF, when attaching this connector to the repaired board, there is a residual voltage on the pins, you have a real good chance of killing the new board.

[MARSHALL BUSTER]

Thanks for the detailed instructions, thats all sorted now, i drained all voltage. Then i decided to take my AVT150 head, remove the fans and amps out in one part, i drained any power in caps and refitted the fans and boards from the AVT275 (slight diference shape but it fitted). Powered up the 150 with 275 parts and it worked as normal. Now i know that my TDA7293's are both ok, thats nice to know. I have now reversed everything back to where it was before and i still have the same problem with 275. I keep thinking a voltage regulator or resistor? i can't understand why somuch power is comeing out of the speaker jacks (43v). One other thing that i have been unsure about 'and i must come clean' is that caps C191,C192,C216 were replaced, not by me, although i said earleir that it was me, sorry to blabb on, and the ones that were fitted have a voltage rateing of 22uf 63v when the originals should be 22uf 50v do you think this could be causing an overload of some sort? I have printed off page 8 of the schematic and have been traceing the curcuts back with a meter and so far all seems to be in order as per schematic, i'm not finnished yet so i'l get on to that next. I guess i'l also change the 3 cap's to get them out of the equation. Now i know the TDA's work i suddenly feel closer to the truth.

Thanks again.

[Ronsonic]

The voltage ratings of the caps is not a problem. If they are installed correctly they're fine.

You say there's 43V on the speaker out!??! Is that DC? AC?

If DC disconnect the power modules and find and measure the DC voltage supplies to the outputs. There will be a positive and negative supply. Do they exist and match? If one supply rail is missing .... the output will not be balanced to 0VDC between them.

[MARSHALL BUSTER]

Marshall AVT275

Hello there.
The voltage is 48v DC at the speaker jacks not 43 my mistake, the voltage on the supply (CN102,Cn103 on schematic) is, both no.2 pins are at -49.2vdc and both no.1 pins are +49.2vdc. The no.3 pins are showing approx 2.2vdc.
Thanks for your help and joining my first thread.
Mark

[Ronsonic]

Is this a stereo amp?

This is a full rail DC offset. One of two things is happening, your outputs are shorted to one side and that's what's going to the speaker out. OR. there is a voltage offset going in to the amplifiers and they are amplifying this DC voltage. What voltage do you see on the shielded signal line going to the output PCB? Follow that back to the blown IC or whatever that is causing this.

[MARSHALL BUSTER]

Yes the amp is stereo.
Ok the best i can do with my knowledge is mesure all the terminals as follows, pin1= 0v, pin2= .8v, pin3= 0v, pin4= 5.7v, pin's 5 6 7 8 all 0volts. I hope thats what your asking for.
Mark

[Jazz P Bass]

Marshall AVT

I am attaching the schematic.
What 8 pin connector are you measuring?
The connector on the output IC board is on the last page of the schematic.
I would like you to measure this connectors voltages with respect to ground.
They are all important.
If I had to pick a critical list & would say.
(1) pins 1 & 2 (+ & - Vdc) & pin 5 (ground)
(2) pin #4 (standby) this pin will slowly rise to 6.8 Vdc at amp turn on.

[MARSHALL BUSTER]

Marshall AVT275

HI Jazz
Conectors C189 and c230 on page 8 and the results are as i said before.
Mark

[MARSHALL BUSTER]

Should i be measuring with everything conected up to the amp boards or not? so far the results have been done while C189,C230 disconected!

[Jazz P Bass]

Marshall AVt

You might want to make sure the ground pins on the 8 pin connectors do indeed go to ground.
I have been thinking about the board swap that you did.
That may or may not be a fully valid test.
The AVT 150 runs the 7293's in slave mode. Therefore Link 105 is installed in the main board. This provides -Vs as a ground at Conn 189/239 pin #3.
The AVT 275, being a stereo output, omits LK 105 & has LK 104 installed. This is the true ground connection for Conn 189/239 pin #3.
Make sure this is true.
I cannot come up with anything else that would make the output have Vdc on it.
You also might want to check that the output jacks ground terminal is at chassis ground.

[MARSHALL BUSTER]

Hi again

Sorry i'v beed over to Ireland seeing my mum for a few days, I will be getting on with the amp later as i have a few new parts now. So i'l check back with you soon ;-)