This is a view of the HK 775 mono block power amplifiers, and a Rotel RC-870 preamplifier. Only within the last few months have I relented to stack the units on top of each other. I had to do this because i was experiencing problems with RFI from the nearby KOIN radio Transmitter which looms in my backyard. (See top right corner) The preamp cables are only one foot long.
The button came off about 4 years ago (around 2002). I actually kept the button attached with duct tape for about a year(I lost it) to turn the unit on and off. Since then, i had to resort to poking the black lever with a small shim or dime or screwdriver get the idea...
The top right front of the unit which had a loose connection that started an intermittent low buzzing sound (around 2001)...its somewhere in that circuit board.
I tried once to inspect the solder joints on the back and i noticed the little black transistor was loose from the board. My shitty soldering iron worked kinda clunky in the tight area, and i wasn't able to get solder joints to my liking. However, this cleared up the intermittent buzzing sound - for about 6 months. Then it started up again!
My finger is pointing at a socket that when wiggled, stops the intermittent buzzing sound. I was not able to establish if the problem was the socket, the wires the socket was connected to, the little black transistor to the right of it, or just the circuit board itself...needless to say, I've been wiggling this little bugger for over four years now.
The speaker terminals. There are four cables here, each having round ferrite beads (to help eliminate RFI) wrapped around them. The amps are connected to Polk Fours (8 ohms) and Tannoy NFM-8s (8 ohms) hopefully operating at 4 hard working ohms. The Tannoys are in the front room, so there cable length is approx 40 feet. The polks are nearby, and their lengths are aprox 10 feet.
Preamp connector. I used gold audio cable and shortened to 1 foot, again because of RFI. At the other end are a stack of ferrite beads.
CURRENT STATE: 1/28/06. Earlier in the week i installed a Corcom 20ampVQ1 inline EMI/RFI power line filter to the power strip that runs the amp and the computer. (Its basically a big hunk of iron 2x4 inches wrapped around copper ground...does a good job of eliminating inline RFI while the ferrite beads eliminate airborn RFI) I also installed an internal 20AVB1 EMI/RFI filter to my Denon Turntable, since that was the worst offender to the noise problem...I was hearing live broadcasts of 107.9 K-Lite classic rock station...

I was moving components around in an emphasis to shorten all non-speaker cables (interestingly, the HK mono blocks were NOT susceptible to the RFI. It was only in the computer, the Rotel, and the phono preamp)...I used a small 2ft cable to connect the turntable to the Atus mixer preamp and then to the Rotel (The Rotel phono preamp was HORRIBLE regarding RFI and unusable)

The RCA connector out of my soundblaster card of the computer was about 6 ft long. There was no way to minimize space here, and the RFI problem wasn't that bad in the computer.

The AC power : The turntable plugged into the top HK, the Atus mixer amp into the bottom HK, and each HK into the Rotel. Only the Rotel plug was connected to the power strip. This is the maximum shortness of all power cables and RCA cables to reduce airborn RFI.

This application was my
most successful way of defeating RFI. It was virtually gone. I was very what happened next.

The speaker cable from the top (injured) HK mono block to a Polk Four was too short, so i lengthened the cable, not compromising for the other mono block. I added about 8 feet of low gauge wire (All the speaker cable is
12 gauge) but just to this amp, meaning that its length was about 18feet, while the other speaker was about 10 feet. I know they are supposed to be similar length, so this probably started the problem. (As if having a 40ft load and a 10ft load wasn't problem enough).Having the units stacked on each other was obviously creating more heat. Ther were in a similar configuration like this for two months without a problem. Prior to that, for years i kept them on a seperate shelf side by side so heat couldn't be a problem then.

Lastly, I ran them hard. I started a recording session at about 3 in the afternoon til 6PM with them pretty cranked. No problems during the session, and i turned everything off and went to dinner. Later that night, i returned and put on a DVD and turned on my projector which is plugged into the same power strip. Then i started a fire in the wood burning stove so my cottage was getting warmer. Nothing out of the ordinary here, i have been doing these same applications for over a year. After a while, the warm cottage caused the projector to shut off. In its location, the projector fan likes to suck in the warm air of the fireplace and occasionally will shut off. (It did that on hot humid days in Baltimore as well; but thats a projector problem, not an HK problem) I had to turn the power switch on/off several times and at some point, i noticed that one speaker was out.

NOW here's the weird part: The speakers to this amp made a "pop" sound, and did so in a steady 5 second cycle..."pop" (5 seconds silence) "pop" (5 seconds silence) "pop" (5 second silence) "pop" ...and would do this repeatedly...I disconnected the preamp RCA cable and it still did it, which means the problem was internal to the top amp, the bottom amp continued as if nothing happened. I haven't tried to see if one of the speakers is shorted, my reasoning being that the same "pop" is heard in each polk and each tannoy connected to that amp.

I touched each amp. The one on the bottom (the healthy one) was only a little warm, even with having less room to breathe than the one on top. But the top unit was
too hot to touch...the fins on the side...the power supply...there was no smoke, smell, or anything else to indicate that something was wrong; just the overall temperature of the unit.

It is hard to say what happened, but i can only conclude some of the following:

1) general wear and tear at an extreme load
2) complications from that solder joint
3) that added length of speaker wire
4) the reconfiguration of RCA cables and minimal AC cord length (but why not the other amp?)
5) a surge from the projector being shut off  (but why not the computer, or other amp?)

CLOSING: I believe i first used these amps in 1989 for DJing gigs. Since then, their setup has been pretty much the same, side by side in my shelf unit from 1990 - 2005. No problems whatsoever. I've used the twin speaker loads at several locations for at least 4 years. In 1994, the filter capacitors were replaced, and some "Mugami" (sic) wiring was installed internally by my tech friend Tim Hatfield in Baltimore.

...and now the saga continues...