Marshall AVT150H Repair help needed.

[DMOS]

Hi all,
I am going to attempt to fix my son's Marshall amp and need some tips/hints.

The problem apparently started a few minutes after his band practice started. The top head chassis vent was blocked and I'm not sure about anything thing else concerning proper cords etc.
Looking at the heat sink rail, the cooling fans need cleaning. The sink compound used between the amp device and the sink looked discolored/yellow. Both TDA7293's are popped/burn't on the right side. Looking at the rest of the circuitry looks undamaged.

Here is what I plan to do.

Since his band is a metal band and the amp is at 3/4 to full power, I want to install and higher cfm fan to help cool the amps.

Both amps will have to be replaced, I need advice here as both sides of the PCB have tracks. What de-soldering techniques and size iron/solder do you suggest.

To be sure of the cord/speaker load double check the speaker ohms/and cord length and design.

Should I check anything else?? Please be specific as to what and where if you add help here. I don't have a scope, however I do have a digital V/O meter.

Schematics/Manual I'm using:
Manual: http://www.marshallamps.com/download..._275%20hbk.pdf

Scheme: http://music-electronics-forum.com/a...avt150-275.pdf

Many Thanks
Dmos

[Gtr_tech]

I don't think you need higher airslow fans. Actually the little tiny fans they use on those I don't think are even available in a higher CFM version. They're basically old style CPU fans. But....they tend to be of the low quality sleeve bearing type. So once they get some miles on them the bearings get worn and the fan rotor chatters....slowing down the fan. I'd just replace them with better quality *ball bearing* fans to insure they run at rated speed/CFM.

As far as removing the output amp chips, I normally just cut the leads off, remove the IC itself and then remove the pins from the pcb one at a time. Its quick and reduces the chances for a novice to tear up traces/pads. And.....never plug in/unplug the connector for the output boards with pwr on. The ICs don't like that....

Once its back up and running make sure the vents are not blocked. Those chip amps need all the help they can get to dissapate heat.

[Jazz P Bass]

I believe Saint Enzo brought up this tidbit.
Make sure the power supply is drained before you plug in the output IC connectors.
Now there is a thought. How do we go about getting Enzo declared a Saint.

[Gtr_tech]

Normally after an output section meltdown the caps are drained by the failure. Doesn't hurt to check tho just to be sure tho....

[DMOS]

Originally Posted by Gtr_tech

Normally after an output section meltdown the caps are drained by the failure. Doesn't hurt to check tho just to be sure tho....

Never hurts to ask. I have a degree in electronics but it's been awhile. Appreciate the help from you fellows..

[Enzo]

Actually, while the amp is normally totally discharged when you first get it, if it has been powered up at any point in the service process, we then need to be diligent to discharge any of that recent power application.



And thanks, but I am a sinner like the rest of us.