I have a QSC USA 1300 amplifier here of a friend. The B channel speaker relay is not engaging. At first I thought the channel is dead but after poking around with the oscilloscope I confirmed that its not the case.
Looking at the schematic it could be caused by a few things.
Anybody got some pointers for me whilst I am poking around some more?
Thanx in advance!
Looking at the schematic it could be caused by a few things.
Anybody got some pointers for me whilst I am poking around some more?
Thanx in advance!
I have a small pile of non working 1300s that I picked up on the cheap...
They do blow outputs.
The caps are notorious for blowing and leaking. The bank of PS caps.
They usually need replacement - not cheap! The caps they used were on-the-edge to begin with.
The relay/DC/protection circuit seems also to like to become unhappy.
To test mine, I jumper the relay or pop the plastic case open (inspect the contacts, btw, they can get crudded up from arcing) and use a rubber band to close them, then used the variac/ammeter method to bring it up slow...
I still have no clue what is wrong with all but one channel of the lot and that one is not happy either... but I haven't had the time to really go through them.
I'd also worry about the 5532 opamp at the input. Probably I will put in a socket so that that can be replaced easily...
_-_-bear
They do blow outputs.
The caps are notorious for blowing and leaking. The bank of PS caps.
They usually need replacement - not cheap! The caps they used were on-the-edge to begin with.
The relay/DC/protection circuit seems also to like to become unhappy.
To test mine, I jumper the relay or pop the plastic case open (inspect the contacts, btw, they can get crudded up from arcing) and use a rubber band to close them, then used the variac/ammeter method to bring it up slow...
I still have no clue what is wrong with all but one channel of the lot and that one is not happy either... but I haven't had the time to really go through them.
I'd also worry about the 5532 opamp at the input. Probably I will put in a socket so that that can be replaced easily...
_-_-bear
Thanx for the reply...
Like I stated, both channels work. Its just that channel B (top PCB) speaker relay doesnt engage...
It turns out it is poping fuses. Red light on power up then pops fuse. I believe I've got some bad capacitors( not completly flat) and getting very hot then pops fuse. I put an order in today for some 100v 2200uf caps. There are three or 4 that look questionable. If you have any ideas let me know. I don't have a scope. Thanks for responding.
Yes, the caps are the weakest link in this design - suggest replacing all at one time, and with higher voltage/temp units... they typically either short and blow or else get very leaky...
_-_-bear
You can try to run the unit with the presumed "bad" caps removed from the board... of course this limits the LF response to some extent and adds a little ripple and may unbalance the ripple, but it will show if the amp works...
_-_-bear
You can try to run the unit with the presumed "bad" caps removed from the board... of course this limits the LF response to some extent and adds a little ripple and may unbalance the ripple, but it will show if the amp works...
Iirc, the factory information shows the board layout... QSC's site...
It should be shown on the board itself... the little dimple on the end of the IC is the marker...
... and always mark ur stuff before you pull both channels!
I'm not near the amps in my lab, so I can't look for you - not near the lab!
_-_-bear
It should be shown on the board itself... the little dimple on the end of the IC is the marker...
... and always mark ur stuff before you pull both channels!
I'm not near the amps in my lab, so I can't look for you - not near the lab!
_-_-bear
Same idea as before, the amp will run with only one pair of outputs in each rail - of course it won't do FULL POWER into a LOAD, but it will run... You can run it into say a 30 ohm load or so and see if it actually works, but don't run it to clipping... or not for more than a second or two... but you need known good parts... and the protection circuit has to be either defeated or permit the amp to operate... and the drivers need to be good, no fried resistors, bad caps and the small transistors need to be good too...
I found them to be a royal PIA to service - in fact I have not fully gone through the ones I have here (no time) since they did not want to function quickly, like yours, either.
The shotgun approach (replace everything) tends to work... but again the PS caps need to be GOOD, not leaky...
_-_-bear
I found them to be a royal PIA to service - in fact I have not fully gone through the ones I have here (no time) since they did not want to function quickly, like yours, either.
The shotgun approach (replace everything) tends to work... but again the PS caps need to be GOOD, not leaky...
_-_-bear
In my experience servicing this amps (qsc), if I found that 3 or more outputs were blown I replace the whole bank and the driver transistors, saves me from the headache of back jobs from my costumers, I was wondering Frank why are yours are fitted with an MJ15022/23 as an output device (200Vce)? for a supply of +/-95VDC, I thought these amp comes with MJ21193/94 as an output. (250Vce with a larger SOA than 15022/24)
to audio machines: Maybe there lies the problem with QSC? This amp has all 15022 and 15023 in it. Could you give me location of drivers on board so I can replace. Also any other suspect parts to replace. Thanks
http://www.qscaudio.com/support/library/schems/Discontinued/USA%20Series/usa1300.pdf
http://www.qscaudio.com/support/library/schems/Discontinued/USA%20Series/usa1300.pdf
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