HELP: QSC USA 1300 Relay not engaging

[deltavektor]

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!

[bear]

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

[deltavektor]

Originally Posted by bear

I have a small pile of non working 1300s that I picked up on the cheap...

Thanx for the reply...

Like I stated, both channels work. Its just that channel B (top PCB) speaker relay doesnt engage...

[bear]

well then either the protection circuit is being activated or else you have a bad component or relay in that circuit... or so it would seem.

_-_-bear

PS. be very wary of the PS caps, if you see any bulge on the top - anything but dead flat, then they all need to be replaced.

[Frank C]

I have a usa 1300 also. It was blowing fuses so I Replaced a bad 15022 output transistor and this took care of that problem. Now the Amp runs but no sound. Each side starts seperately.Could this be the relays BEAR is talking about? FC

[bear]

The amp runs?
You see signal with a scope, or you mean it does not pop fuses?
It is easy for it to not pop fuses, harder to make sound.
The schematic is available from QSC online.
You'll see the protection circus and the relay. DC offset will not permit a turn on...

_-_-bear

[Frank C]

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.

[bear]

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...

[Frank C]

Help! I got the caps and installed them. I was swapping out the ic1's from good board to bad board. I need to know which direction the band goes on the chip. Facing the ic1 type on board? Thanks FC

[bear]

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