Avt 150H blowing fuse.....

[theholyweed666]

Hi everyone, I have a problem with an avt150h I was wondering if anyone could help me with. The amp powers up fine without the 2 power boards (with the TDA7293's) connected but when these are connected either the 2a fuse blows or the tracing on the top of the pcb from one of the filter caps (c221) glows (this happened the 2nd time I replaced the 2a fuse but the fuse never blew this time) I obviously havent tried again as i dont want to totally knacker the trace on the pcb (I have run a continuity test on that particular trace and it seems fine..if not a little scorched) the weird thing is I know for certain that the power boards & Chips work fine as I have several avt150's here and have triple checked the boards in the other amps.....

any help pointing me in the right direction would be great

[g1]

There are issues with how the heatsinks are isolated from the chassis. I think it has to do with insulating bushings for the fan mounting?

[theholyweed666]

Hey g-one, could you elaborate on that please? it doesnt seem that the heatsinks are isolated from the chassis on all 3 of our avts at the studio...

[g1]

I'm not sure about that exact model. Some of them have insulators between the output IC and the heatsink, some of them insulate the heatsink from the chassis. When it's installed in the way that blows the fuse, check resistance from the metal tab of the 7293 to ground and to chassis. The tab is connected to the -V supply, so you don't want it to connect to ground.

[theholyweed666]

according to my continuity test the metal tab of 7293 is connected to chassis, but not to ground, its exactly the same as the working avt....

[theholyweed666]

AH FUCK me dude, your right, just noticed on the 2 knackered avts the thin plastic insulation between the powerchip and the heatsink is missing, a gentle wiggle with my dmm probes eventually got a reading, replaced the insulation and regreased and hooray, nothing blowing up anymore..only problem now is that there a loud hum and nothing else coming from cabinet, more than likely with the damage thats been done with the original short.. no dc on output tho....hmm any suggestions?

[52 Bill]

...nothing blowing up anymore..only problem now is that there a loud hum and nothing else coming from cabinet, more than likely with the damage thats been done with the original short.. no dc on output tho....hmm any suggestions?

What I got from reading the earlier post was that there is some damage to the pc board, caused by the shorted module connections. If there was no damage done, I think that the fuse would have blown like the first time.

Check the power supply lines and see if both sides are there and getting everyplace they need to be.

[theholyweed]

everything seems to be, it starts of like some kinda mad distorted oscillation coming out cab then settles down to a very loud hum with the master volume acting almost like a tremolo! iv swapped out the zobel resistors (r110/r112) as they were pretty burnt (but still within tolerance) do you think it could be the the filter caps? as they were on the trace that looked like it took the worst of the short....

[theholyweed]

iv pretty much got a donor avt sitting here too, but I dont just wanna swap out random components until its fixed, would far rather track down the problem...

[g1]

If you've been disconnecting & reconnecting the amp modules without discharging their supplies that can cause problems.
Here's some more info: http://music-electronics-forum.com/t31030/#post324354