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Help understanding this component in a hifi amplifier

Bensco

New Member
Hi, I picked up an old technics sa-eh760 hifi system, it works great. I have a concern with a component getting very hot, it's a pcb with 3 transistors d2374 and 2 b1548's the service manual calls them regulator transistors. The 3 transistors are clamped down to the body of the amp, I'm guessing for heat dispersal. If it is a heat sink it's not very substantial and no sign of any thermal paste ever being used. So I would like to know what it is? A voltage regulator?
And should it get hot? How hot is go hot.
Many thanks Ben
 

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audioguru

Well-Known Member
Most Helpful Member
The resistor in front of the D2374 transistor is broken which might cause transistors to get too hot.
The heatsink is Mickey Mouse simple so the transistors should not get too hot.

A detailed schematic shows us what things do, not a photo.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
They are the power transistors used as part of the voltage regulator circuits.
Two of them are dropping a fair voltage so will get quite warm even at low currents.

I'd try adding heatsink compound and then keep an eye on the temperatures in use..
After it's been on for a while, isolate power then damp your finger then tap the heatsink to leave a single drop, without it running in to the electronics.
If the water boils it too hot!

It it evaporates in 30 second or more, it is fine. Inbetween those is debatable, some things used to be designed to run at 80'C or more...

AG - the circuit / service manual is easy to find from the given model number. eg. https://elektrotanya.com/technics_sa-eh760_sm.pdf/download.html

From that, the short yellow items are 47nF caps, not resistors.

Edit - ps. That voltage regulator board is on page 55 of the service manual.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I agree with rjenkinsgb, no sign of a 'broken resistor', however I wouldn't recommend drops of water, far too much to go wrong.

The simple method, used throughout the service trade, is to lick your finger (to protect it) and briefly touch the transistor body (or the heatsink itself) - if it's too hot then you will hear it 'hiss' as the thin layer of saliva boils away. If you don't lick your finger, the thermal delay of your nervous system means your finger gets burnt before you feel it's hot - I'm not talking major damage or anything, but it will hurt enough for the rest of the day to remind you to lick first in future :D
 

JimB

Super Moderator
Most Helpful Member

Bensco

New Member
When I open it back up to put the paste on I'll give it the lick'n hiss test lol, I was thinking the heat sink is a bit lame but thinking about it the body of the machine is probably doing a good job of drawing some heat away, where the 3 resisters are clamped to the base of the stereo, it's pressed in/raised up, I presume to keep more distance from the table or surface the stereo is on so it doesn't cause any damage. That works fine I can't feel any heat transferred to the table, if I put my finger under the stereo directly where the transistors are, it's hot, very hot can probably leave my finger there for 5 seconds before it gets to hot to handle
 

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rjenkinsgb

Well-Known Member
Most Helpful Member
however I wouldn't recommend drops of water, far too much to go wrong.
That's why I said "not enough to wet the electronics" :rolleyes:..

I just lick my finger, but I believe that's bad manners in some cultures so I was trying to work around it, to the same effect.
 

tvtech

Well-Known Member
Most Helpful Member
A wet hissing finger equals about 80 degrees Celsius on a hot heatsink. Wayy to hot lol
 

dr pepper

Well-Known Member
Most Helpful Member
I've done the hiss test 100's of times.
T'other day I went it cant be this device thats hot and pressed it hard, i now have an 8 pin dip square mark on my finger.
And i type this with one finger in the air.
Occupational hazard.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I've done the hiss test 100's of times.
T'other day I went it cant be this device thats hot and pressed it hard, i now have an 8 pin dip square mark on my finger.
And i type this with one finger in the air.
Occupational hazard.
I refer you to post #9 :D

As I've mentioned previously on these forums, I still have a perfectly working PIC with my fingerprint melted in the top of it!.
 

be80be

Well-Known Member
I can't see now without classes I was working with a pic16fl 3.3 volt chip and I had mixed voltage 3.3 for the pic and 12 volt for a relay
The red 12 volt wire came lose I hook it back up smoke rolled off a wire melted the molx off the end
I turned it off fast looked I had had hooked the 12 volt on the 3.3 side the pic still works.
 

dr pepper

Well-Known Member
Most Helpful Member
Yes I had #9 in mind at the time.
This chip was smoking hot, and unbelievably it too still appears to work.
Probably some kind of zener or something, or maybe the o/p transistors all turn on at too high a supply.
 

tvtech

Well-Known Member
Most Helpful Member
Well, the LM317T is pretty amazing. I've seen them in self destruct mode. But they don't. Well designed and still around after all these years. Noone has managed to make a better chip yet.

Even RS Components still sells them. Never going obsolete. Nothing can replace it.
 

be80be

Well-Known Member
Lm317t kind of hard to hurt it seeing it has all this

Features
  • Output Current In Excess of 1.5 A
  • Output Adjustable Between 1.2 V and 37 V
  • Internal Thermal Overload Protection
  • Internal Short Circuit Current Limiting
  • Output Transistor Safe Operating Area Compensation
  • TO-220 Package
 

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