Technics SA-EH760 overheating?

[Benscott]

Hi everyone, newbie here I picked up a Technics sa-eh760 so not really vintage, sorry, but it was a great price. I wired it all up with Miles of speaker cable lol never had these tri wiring speakers before. It all works great and the sound seems decent to me. My concern or lack of understanding is with the amp, it seems warm which I think is fine but when I picked it up to move it back a bit I noticed that one area underneath is very very hot. I took the cover off to investigate the hot spot, and the area that is hot is a little circuit board with 3 transistors and a heatsink clamping it directly to the bottom chassis/cover. So because of the heatsink it's obviously supposed to get hot, but could someone tell me how hot is to hot? And what it does, it doesn't seem to matter if the machine is actually playing or just on it gets hot and cools very fast when turned off.
Is some kind of regulator that just does get hot or is something wrong somewhere and it's getting to hot.
I can hold my finger on the under side of the amp for a while but it really is hot

[Chris55000]

Hi!

Service Manual:-

https://www.manualslib.com/manual/81...-Sa-Eh760.html

Do you have a good digital voltmeter or DMM?

Open Page 74 of the 80 page PDF I've provided a link for - this will show you the circuit diagram of this small board, Power Supply Board C, that provides switched +10V on W701B pin 1, switched -7.5V on W701B pin 5 and an "always on" +15V supply from W701B pin 9.

The three series regulator transistors have plastic (insulated) TO-220 cases and these will get very warm/hot in no time if you try to run the panel in your hand as your picture shows, because plastic transistors have a much higher "thermal resistance" than the normal TO-220 metal-tab devices, so yes, it is normal to get a "hot-spot" underneath the metalwork where this PCB is fixed!

If you're doubtful about the temperature of these three transistors, a little heat-sink compound or thermal-grease applied to the back surfaces of the devices before screwing the small PCB back to the bottom chassis will help them run a little bit cooler!

The +15V supply should always be present when the unit is plugged into the mains power supply, whilst the switched +10V & -7.5V supplies are only enabled when you use the Customer On/Standby button!

If any of these supplies are missing your unit will be stuck in standby or the display out, depending upon which is missing!

Check the three supplies at W701B pins 2, 5 and 9, and if they're within +/-0.5V of the marked value, the fault is elsewhere in your unit!

If you have any fault symptoms, please report back and myself or another Member'll gladly assist!

Chris Williams

[Benscott]

Thank you, the stereo works great, I was just concerned about the amount of heat generated when it was in its clamp (didn't run it with the board out) I reckon I can hold my finger directly underneath the location of the transistors for about 5 seconds before it's to hot to handle. Thank you so much for your help, time and explanation

[Radio Wrangler]

Don't believe everything you read in hifi magazines/websites.

Some decades ago some people started claiming they could hear differences between different types of speaker cables. The sorts of people with serious qualifications in electronics, and decent test equipment pooh-pooed it saying you had to do something terribly wrong to make a cable bad enough to create an audible effect. But the people who claimed to hear it would have none of this. They clam it makes a great difference and that it revolutionises your hifi system. Then someone had the bright idea that in the speaker there is a crossover network separating high, low and sometimes middle frequencies to send them to appropriate drivers in the speaker cabinet, so why not have separate cables right back to the amplifier. Then you can have specially optimised cables for each of the bands! Bi-wiring and trui-wiring was born!

The scientists still say it's a load of cobblers

The true believers say it makes even bigger differences.

The speaker manufacturers took to placing extra terminals on their speakers to cash in on the trend but supplied shorting links so single-cableists would buy them too and not have to do something they thought stupid.

Special speaker cable seems to be very expensive and tri-wiring is a great way to triple (or more) your expenditure in this area. And there' I think is the underlyin reason... good old profit.

So your quantity of cables is a nuisance, one pair of conductors per speaker is enough, feel free to strap the binding posts together on the speaker.

That little regulator board is used to drop the voltage of one power supply to make other, lower voltage ones. By its very nature it makes heat. The reduced power it puts out has the amount of reduction turned to heat. It relies on good thermal conductivity to the metal case to cool its transistors. Regulators are designed to survive higher temperatures than most other devices and good ones have internal temperature sensors to shut them off if they get too hot. But, the cooler they are run, the more reliable they are. It's best not to run it with the board not thermally fastened to the metalwork.

David

[Benscott]

Thanks for the reply, I found the amount of speaker wire ridiculous, its a crazy idea to me, I'm just happy to know that the heat is "normal" I'll probably use some thermal paste, maybe even improve the heatsink.
Just out of interest, if I did bridge the terminals to make them standard 2 posts, what terminals on the stereo would I use to place my now just 2 wires? Wouldn't I have an impedance issue? But good to know I could bridge the speakers if I wanted to use them on a standard stereo. Many thanks for your help