I have a Hafler DH-110 preamp and a Hafler DH-200 power amp. The top of the preamp was warmer than usual and the side was very hot. There was the smell of a component overheating. The preamp sounded normal when the switch was overheating. After unplugging it, the power switch was frozen. The preamp does have a row of outlets in the back. I had the power amp plugged into one of them, a turntable, FM tuner, cassette tape deck.

As can be seen in the attached picture, the switch melted and the wiring to the switch melted and fuse did not blow. Up until now, I have never repaired a solid state component. I restore tube radios and consoles. I'm stumped as to what caused the switch failure. All the electrolytics are original. The preamp is used every day for several hours since it was new.

To me all components except the switch do not appear to have overheated. Where do I start to troubleshoot? My first hunch is the possibility that the power amp might have started to draw too much AC current.

Since the fuse didn't blow your preamp ok
Either the switch is overloaded or it was going bad
And the load finished it.

Power switch failures like this are extremely common on audio equipment, so the overwhelming likelihood is that it started arcing due to the extreme inrush load, once that starts, it's just a matter of time.

I think it is a *very bad idea* to attempt to switch the power amp using the preamp power switched outlets. That puts an extreme load on an already notoriously failure-prone component. It's not just Hafler - almost all of them will have a similar problems at some point. The preamp itself is probably OK, but there is remarkable inrush current to the power amp, and the way this is being done, all of it goes through the preamp power switch.

I can't recall if the DH-110 has an arc-suppression cap across the power switch, but I bet the power amp does.

It's possible that there is something else wrong, too, but having seen nearly identical failures many times, I would bet that it is just that you overloaded the power switch, it arced, got pitted, that made it arc more and eventually it just fails completely. My very strong suggestion is to replace the switch, plug the power amp into the wall and use its own switch, and do a check on the current draw of all the components to make sure there is nothing else wrong.

Brett

+1 to all that. The power switch on high power amps such as your Hafler have enough to do with just switching the amp. If you want to switch that much "other stuff" with the amp, I'd use a relay and power strip to do it. There's probably something commercially available. You'd want to have arc suppression on the relay coil as well.

There should have been an arc snubber across the power switch, although I suppose some models did not include that. Your posted schematic does not show one. I've had many similar switches that arc on startup, making mysterious noises come out of the speakers for a few seconds. Eventually the switch will just burn out.

Replace both the switch, and the snubber, or add a snubber, although I'd bet your overload was due to the stuff you had plugged into the AC outlets on the amp. Don't use those.

Turns out it was the switch. I unsoldered the two wires on the switch and connected them together as a temporary fix. No signs of overheated wires or other parts. Will order a new switch. I will need to replace the two damaged wires. What gauge should I use? I don't think 18 gauge wire will fit through the connection hole on PC board.

Try to use the same gauge that is there now. If you have to use #18, just trim off a few strands till it fits through the hole on the PCB. Of course if it's solid wire, you can't do that. Either solder it to the other side of the PCB, drill out the hole slightly, or get the right size wire.

The wire only melted because the switch got so hot; it wasn't the wire's fault.

EDIT: Looking at the picture, you may just be able to top solder that wire to the pad on the PCB. That assumes that it's a plated through hole, and there's a connection from top to bottom of the board at the trace. Alternately you could leave the wire soldered to the PCB as it is, cut it back to where it's not melted, and splice a very short piece of bare wire from there to the switch.

Could also be that it was a crappy switch to begin with and a heavy load finished it off. High end gear now uses modular plugs with three terminals and I am unaware of gear with switched three terminal outlets on the back, that is something seen on vintage gear. I once had a DH 101 and a DH 220. I upgraded to a DH Iris preamp with the remote. It had a round knob for volume versus the typical up down buttons. The 220 was a kit, I think it cost me 350$ at the time. FROM the DH 110 specs page- AC Convenience outlets- 3 switched, 5 amps continious, 72 Amps surge. 1 unswitched, 5 amps continious. I would not ever hook a power amp to those outlets again.

Part of the upgrade I did on a DH-220 was to replace the power switch. It was a known weak spot.

That is true, but it is certainly not just Hafler - this has been a problem since transistor power amps have existed. They have so darn much inrush current, even the relatively small ones, that nearly nothing will stand up to it indefinitely.

One of the workarounds, unfortunately, is far worse - leave it on all time! You would be amazed at how many people do that, and usually get away with it. Occasionally, however, it fails catastrophically when no one is around, and fries absolutely everything.

Brett

I plug them in to a power strip and use its On/Off switch. Same with my tube HiFi.

That's probably OK, at least you can replace the power strip when it fails. Note that it is a much different situation with a tube amp, as long as it has a tube rectifier, the inrush is very low and usually overdamped.

In a transistor amp with a bridge rectifier you are putting a step function across your (extremely large) power supply caps. And I forget what a HD-220 has, but it was not at all uncommon in that era for the power supply caps to be "upgraded" to absurd levels - like the infamous Dynaco 416 modification that had 1/2 farad (in a separate chassis because it couldn't possibly fit). That's like throwing a crowbar across a car battery.

Brett