I wonder why this Amp goes into Protect Mode?

[dicky]

Found this corker of a fault in a QTX QA2400 Amp

Fault was amp immediately goes into protect mode on the Right channel when powered up

God only knows how this PCB got bent like that!!! :p

Actually a half reasonable amp from QTX, though it is not nearly as powerful as the 2400 monica would suggest, it has got a bit of kick.

[yourdj]

Nice. Thank god technology allows protect mode. I had an amp that caught fire once. it was my fault as it was full of dust. My first sound system prior to being a mobile DJ. I would never allow kit to get into that condition now.

Would heat have caused that do you think? I assume it would not have been direct impact, unless something was inside the amp pushing against it? 80% Of the QTX stuff i have got has worked well but has been pretty poor construction (like the battery speakers), hence the cheap price. :)

[dicky]

Actually Toby, I had a real good look at it and it was impact that caused the damage


The amp casing was a bit knocked about but not badly damaged, however when I examined it futher it is obvious that it has been dropped from some height onto one side, hitting the protruding edge of the 19" front panel first, and because

a: the amplifier chassis is made of thin guage mild steel

and

b: it is quite heavy

and

c: there is a huge great transformer solidly mounted in the middle of the amp with hardly any clearance to the amp heatsinks/PCBs either side of it


The result was that whole thing buckled enough to cause this damage



I've dismantled it all now and straightened the metal work in a vice then reassembled - and all is now good

The build quality is not brilliant (in fact it is rather shoddy and cheap to be honest) but for once, this QTX amp actually manages to kick out a fair bit of power :)

Rich

[ckpr2]

I treat myself to a 1980's Maplin 50 watt amplifier off ebay.
It was faulty and had components missing.
I replaced all the missing components and it worked but barely with almost no volume.
I tested every component on the pcb and they all checked out ok.
I narrowed down the fault to one transistor.
I took it out and tested it then noticed it was npn transistor instead a pnp transistor.
I replaced it with a new transistor and the amp burst into life.

Its hard enough fixing amps with the right components.

[dicky]

I treat myself to a 1980's Maplin 50 watt amplifier off ebay.
It was faulty and had components missing.
I replaced all the missing components and it worked but barely with almost no volume.
I tested every component on the pcb and they all checked out ok.
I narrowed down the fault to one transistor.
I took it out and tested it then noticed it was npn transistor instead a pnp transistor.
I replaced it with a new transistor and the amp burst into life.

Its hard enough fixing amps with the right components.

I've read that word for word somewhere before :confused:

[ckpr2]

I've read that word for word somewhere before :confused:

I have maybe mentioned it before on here.
It has also been mentioned on a few audio groups.

[Nakatomi]

Rich, now that's what I call some board warpage :lol:

[dicky]

as an addon to thiis - I was intending to put this on ebay so just to be sure it was all OK to sell I ran it up tonight and gave it a fair amount of stick.... and the damn thing blew up on me!!!

Well maybe 'blew up' is an over statement but it did go bang (note that is in lower case) and blew the mains fuse in the power lead which just so happened to be a 5 amp one by chance, therefore probably preventing an even louder BANG (in capitals lol)

Running it up again with a current limiter just to prevent any more damage or bangs/BANGS in either case scaring the crap out of me - literally! - it turned out the current limiting light bulb glowed brightly and on futher examination I now had short circuits on both positive and negative power rails on the right hand channel PCB that had been badly bent.

This turned out one short circuit PNP output transistor and one short circuit NPN output transistor on that channel.

Now I am not so sure if this is has something do with physical stress to the output devices due to the bent legs I had to straighten - or more to do with the fact I hadn't bolted one straightend output device back on to the heatsink properly :zip::rolleyes::zip: but I have now ordered a couple more output transistors and will give it the benifit of the doubt, or the benifit of my own stupidity :muppet:

But if the thing has the audacity to go BANG again, with or without capital letters, it is gonna end up as one big pile of spare parts :p

Rich