In the late ’80s, almost certainly 1988, I used some of the money from my summer job to order an SAE (Scientific Audio Electronics; no longer in business) A502 amplifier from the DAK (Drew Alan Kaplan; no longer in business) mail-order catalog. The A502 is a two-channel solid-state audio amp rated for 200W per channel — but it’s as heavy as a tank (40-50 lbs) and actually able to live up to the promised wattage, unlike many smaller amps and modern receivers. At a comfortable listening level, the volume meter LEDs don’t even flicker on.
I’ve kept the amp and used it on and off since then, using it continuously for casual listening and home theater movie-watching for the last thirteen years. It’s never given me any trouble, and it performs admirably.
So admirably, in fact, that I’ve been shopping for more on eBay. In late December 2005, I bought two advertised as partially working; and this May, I finally repaired the first of them.
Replacing the SAE A502 Speaker Relay
The first of the two amps was advertised as having both A and B speakers work on the right channel, but only the B speaker work on the left channel. When I received it, I hooked up speakers to confirm the symptoms, then opened the amp to look around. The problem was immediately apparent:
Transient currents can generate popping and other unpleasant noises when a power amplifier is turned on. SAE designed around that problem by inserting relays between the output transistors and the speaker jacks, with delayed activation. Power on the amp, 2, 3, CLICK, and the speakers are engaged. Plenty of the time for the transients to disappear. And it works — my speakers are dead silent when turning the amp on and off.
Well, it works until one of the relays gets a little, um, melty.
I have no idea what happened to vaporize the contact, but I bet it was impressive. [Ron Tozier, local TV and electronics repair wizard, suggests that the contact was probably slightly oxidized, and the extra resistance led to heat buildup and eventual catastrophic breakdown.]
Now, although the relay is DPST, the amplifier circuit uses both contacts in parallel. So in theory, I could have cleaned the slag off the melted contact to make sure it stayed out of the way, polished the remaining contact, and resumed operation.
I really wasn’t comfortable with that idea, though. SAE put DPST relays in there for a reason, and I wasn’t really interested in changing their design. So I looked for a replacement relay, with no success. Google found me nothing to match the Matsushita part number, Digi-Key had no matches, and searching online surplus catalogs didn’t get me anything that matched the coil and current specifications with even close to the same form factor. So I set the amp aside until I could find a replacement relay, and kept searching occasionally.
At Last, a Relay
This April, after over a year of fruitless searching, I checked Digi-Key again, and they had the relay listed! Unfortunately, they had a minimum order of something like 108 pieces, and I didn’t need to fix 108 amps. Fortunately, one of their techs was able to cross-reference the relay, and in short order I had a bagful of perfect replacements. (I know, I don’t need to fix a bag of amps — but being prepared to replace another relay at some point in the future is worth a few extra bucks not to have to track them down again.)
The board is a pain to take out because it’s bolted down to greased heatsinks, it has huge power-filtering capacitors mounted on the back side, and its cables are bundled such that they’re too short to pull the board out. Bleah! But after dealing with the mechanical issues, it was only a few minutes work to remove the old relay and solder in the replacement.
The new relay (installed as RLY201, the right one) has a matte finish whereas the original was shiny, and its body sits a little lower with respect to its contacts, but those are insignificant issues. It fits, and that’s what counts.
The silk-screened circle to the left of the relays, BTW, is the footprint of the filter capacitor on the other side of the board.
Test Driving the New Relay
I threaded the board back into position in the chassis, bolted it back into place, and made sure the wires were all routed appropriately; but I didn’t close the case yet, as I wanted to test the new relay first. I plugged the amp into a power strip that was switched off, switched on the power strip, pushed the “On” button on the front panel, and POP! There was a moderately loud bang and a moderately bright flash, and I shut off the amp and the power strip quickly to have a look.
Strangely, the flash came from the right rear of the chassis — the opposite side from where I had replaced the relay. More strangely, I couldn’t find anything scorched or missing. I figured surely I’d see an electrolytic cap with its top blown off, or a resistor or diode vaporized, but nothing appeared wrong. I unmounted the board and checked the bottom side for brown scorch marks, but nothing wrong was visible there either.
My guess was that after the amp had been off for a year, some component got “tired” and the inrush current destroyed it; but even with repeated checking, I couldn’t find anything amiss. I found a copy of the schematic online and studied it in detail, but didn’t gain any new insight. I was reluctant to power up the amp and start poking around with a scope, for fear that the (hypothetical) broken component might lead to cascading, more expensive failures.
Testing the Repaired, Exploding Amp
Finally, I took the amp to visit Ron and ask his advice. I found his method of testing rather ingenious. This amp has 8A slow-blow fuses between the power supply and the power amplification section, not just a fuse on the line cord. We pulled all the 8A fuses and replaced them a pair at a time with 1A fuses, so we were powering up only one channel at a time. Anything seriously wrong would blow the 1A fuses pretty quickly, hopefully without doing further damage to the rest of the circuit.
After doing a quick check of the major capacitors and power transistors, Ron put an oscilloscope on the speaker outputs and I powered up the left channel — the one whose relay I had replaced. The scope stayed flat as it should — no unwanted DC component. Ron touched the line-level input, and we saw a low-amplitude 60Hz signal on the scope. All as expected.
Next, I pulled the fuses from the left channel and moved them to the right, where I had seen something blow up. Ron moved the scope and we powered up. The left channel output was flat as well — good so far. Ron touched the input, and we saw the same amplitude 60Hz signal on the scope.
Hey . . . what?! It works???
Well, yeah, it does. We couldn’t find anything wrong. I put the 8A fuses back in, took it home, reassembled the case and put it into my stereo, and it’s been running my surround speakers for a couple of weeks. Nary a problem since that day.
So what were the pop and the flash when I powered it back on? My best guess is that some piece of crud (dust bunny, cat fur, etc.) got shorted across a couple of high-voltage pads and lit up when the voltage said hello. Cort says he saw that happen a lot when he was repairing dusty arcade games back in the day.
One down, some more to go!