SA-7100 diagnostic

kvining

AK Subscriber
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Hello all, I've picked up a SA-7100 off the bay for chump change. I've always loved the x100 series and would love to add one to my collection. This one is non-working but in beautiful cosmetic shape with hardly any sort of noticeable scratches on the wood case or face. Just wondering if anyone has run into this on this model who could give me a few hints on what to look for. When you power up the unit, immediately the protection light comes on for a few seconds, the relay does not click, then the protection light goes out. I've pulled the power transistors and they all test good doing a diode mode test using the DMM. All fuses are good. I went over the boards with a magnifier and cannot see anything obvious. I haven't done a dim bulb test on it yet, and I admit, I screwed up there and should have done it when I first powered up.

I have a complete SA-7100 power supply assembly and a complete power amp assembly (without the power trannies) on the back shelf I can use, and I can strip what I know is a good protection board and relay from an SX-727 parts machine I have that are the same for the SA-7100. If anyone with a hunch can tell me which one of those boards I should cut out for testing first or if there is a most likely cause elsewhere, I would appreciate the insight. Thanks!
 
DON'T disconnect anything to troubleshoot!

The protection light coming immediately on is intended in the design. Until the relay connects the speakers ("clicks") the protection light should STAY on just like the power light right next to it.
The protect light is SO hardwired that while you are not hearing a click, that is not indicative of the relay operating. The protect light going OFF means the relay DID CONNECT THE SPEAKERS.
The light coming on is a solid confirmation that the unit's power transformer is energized and that the 1 amp lighting circuit fuse is intact.
The light going OFF after a few seconds is a solid confirmation that the protect circuit was happy enough about conditions that it allowed the amplifier to connect to the speakers.

MUCH more is going on RIGHT in the unit than you give credit for.

In the spirit of assuming nothing, please don't be insulted - we ALL have blind spots:

You do know that the amp NEEDS jumper (bars) between the pre out and power in jacks in the pre/power amp four jack cluster?
That the rotary knob above the power switch selects between no speakers "OFF" , "A" speaker live, "B" speaker live, and both A and B speakers live "A+B", wired in parallel.
That the speaker connections are vertical in the back, with the right speaker's plus connected to the RED terminal above the R on the terminal strip and the minus connected to the BLACK terminal BELOW the R on the terminal strip.

I'm pretty sure you have the service manual.

ASSUMING that you have all of that covered, there's a lot that can go wrong that can keep the amp silent yet allow the protection to engage the speakers.

The protection runs on the REGULATED +35v, so with the protect light turning off, we KNOW that regulated voltage is OK.

The funny thing is that the power amps make their own regulated voltages, and supply the -13v to the protect circuit!!
They only run off the unregulated DC from the main capacitors.

So let's see if the amps are passing signal, wire in a small test speaker that isn't valuable, plug in an RCA cable to an amp in jack, and touch the center conductor to introduce hum.

Then if you are feeling feisty you can post the DC voltages found on pins 6, 7, 11, 12, & 13 of the AWR-026 power supply assembly, to look for low probability problems with power being sent to the preamp circuits.

The amps themselves are chock full of troublemaking transistors, the 2sa726's and the 2sc1451's. The 1451's can damage the output transistors. The troublemakers will eventually have to be replaced.

When poking around internally BE VERY CAREFUL OF THE STV-4H DIODES connected to the amplifier board yet mounted to the heat sinks of the output transistors. Their wires are very fragile and LOVE to break off flush with the diode given the slightest provocation. When moving them do not bend or flex the last 1/2 inch of wire by the diode's body!!!
 
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Thank you for the detailed response. I'm no expert but I've been a hobbyist for a while, I'm pretty savvy about checking the obvious, I can follow a schematic and I've done a lot of hobbyist style repairs, broken switches, posts and pots, that sort of thing, but this is my first attempt at recapping a board.

I didn't do any sort of sound tests, but I'll put it back together and do as you suggested. I was careful with the diodes because I read two excellent recent threads on here on SA-7100 rebuilds before I opened the case, and the same advice was posted there. It looks like I wrongly assumed that because I did not hear the relay click it was probably dead and went a little further than I should have. What I intended to do was to try to determine which board assembly has the fault and hopefully it would be one of my spares and I would do my first recap job on it first. Since I posted this I did find a bulging cap, CEA 471P-35 on the power amp board
.thumbnail_IMG_1414 - Copy.jpg

I also discovered that the STV-4 diodes differ on my spare board from the one in the case, but I doubt that matters muchthumbnail_IMG_1418.jpg thumbnail_IMG_1419.jpg

Also, one power transistor is flaking off material from the collector case, which apparently separated when I pulled it from the insulator.
.thumbnail_IMG_1417.jpg

Will report back after doing what you suggested unless you or someone else who is knowledgeable tells me to do otherwise based on the photo of the bulging cap.
 
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