SWR SM 900 -15v rail trouble.

[Emmons]

Hello all,
Hope somebody can give me some pointers. I've got an SWR SM900 (with the green 'SWR power2000' output modules) that's got a problem on the -15v rail. Something's loading it down to -1v, but when I take any 4 tl072s out, it goes back up (down) to -15 in 2-4v increments with every op amp pulled. Ive tried replacing op amps, checked the zeners at the supply, checked the big 1k1 resistors, all are fine. I'd include the schems in this post, but I'm writing this on an ipad (although a quick search of the site will yield the correct, pre-97 ones).
Any help would be greatly appreciated.

Cheers.

[nevetslab]

I went thru my service notes on all the different renditions of the SM900 amps I've worked on, and had one with similar problems as you described, though it was a later version, having the 7815T/7915T IC Regulators sitting in the middle of the power amp boards. Problem on it was +15V supply was down or intermittent, and found to be the solder joints of the regulator. De-solder, re-solder restored order. Had similar problems on one of the fan circuits, also a TO-220 in the middle of the board, underneath, of course, out of view.

My schematics for the one you describe only has 3 pages....covering the main preamp board, the upper deck EQ board and the power supply....missing the amp module, fan circuit, AC mains wiring.

[Enzo]

SWR liked to modularize. The greem 2000 power a,p is in the SM500 file. But since the PA is not the issue...

Mine are 1999 drawings.

But your power supply seems the same. 1k1 resistors and paired zeners. So it collapses under load.

You have the series resistors, so use them. The input to that string is 75v on my print, so about 30v dropped across each resistor. I will assume there is about that unloaded, since you get the 15v, but verify anyway, and compare to the +15 which holds up. Now stick in some ICs to crash the supply, now what is read at each resistor end? The resistors may read OK, but that doesn't mean there is no open or high resistance junction between them somewhere. This will tell us if the thing is totally loaded for some reason, or if there is an issue along the chain.

For that matter, your -15 drops to -1. Did you measure that at the IC? Does it also measure that way right at the zeners? And don't forget that invisible 1N4004, under load are the two ends a mere junction drop apart, or is ther a large drop across it? So assuming the -75 is not falling, we need to determine where in the string the voltage drop occurs. I cannot imagine all the op amps went short, and when they are out, th voltage restores. So I am left with the working notion that there is a high resistance in series with the load. Like a bad 1N4004 or a bad solder joint on the wiring between supply and load.

[Emmons]

I feel pretty sheepish now, especially after you fellas have been so amazing. But I was using a lamp limiter and didn't realise the minus supply would pull that much more current than the positive. When I bypassed the lamp, the minus was still a little under (about a volt or so) but not enough to send any dc offset through to the PA(which it had been doing). I'd already replaced the o/p transistors in one of the power amps and was scratching my head at this.
Sorry for wasting your time and thanks again for your help.