Philips AH682 safety resistor keeps burning

Thread Starter

daeiron

Joined Feb 28, 2022
11
Hello,

Im kind of new to repairing electronics and can't seem to figure out this one.
Its about a Philips AH682 record player/tuner. I have the partial schematic of the area of interest:
ah682.png

So its about R623. I already replaced the BD135 transistor and checked voltages between the transformer and burned out resistor, which are all correct. Also tried replacing the resistor with a new 1W 22 ohm resistor, but that one got burned right away too.
Can anybody point me in the right direction?
 

LesJones

Joined Jan 8, 2017
3,706
I agree with Albert. TS507 is probably mounted on a heat sink (As it will be dissipating about 3 watts.) and the heat sink could be connected to ground. If this is the case the insulating washer may be damaged shorting the collector to ground. Measure the resistance between TS507 collector and ground.

Les.
 

Thread Starter

daeiron

Joined Feb 28, 2022
11
Welcome to AAC, @daeiron
Something is drawing too much current from TS507.
It could be C559 shorted or from one of the +15V rails.
Thanks for the help. I'll check that capacitor. It might be worth mentioning that fuse F791 also broke when powering on together with the burning resistor.
 

Thread Starter

daeiron

Joined Feb 28, 2022
11
I agree with Albert. TS507 is probably mounted on a heat sink (As it will be dissipating about 3 watts.) and the heat sink could be connected to ground. If this is the case the insulating washer may be damaged shorting the collector to ground. Measure the resistance between TS507 collector and ground.

Les.
Yeah TS507 is mounted on a pretty big heat sink. Didn't check it for ground connection, but thats a good one. The back of the transistor (metal part) contacts the heat sink directly, but that should be the case I guess.
 

AlbertHall

Joined Jun 4, 2014
12,020
The back of the transistor (metal part) contacts the heat sink directly, but that should be the case I guess.
The collector of the transistor should not be connected to ground. Eithe rthe heatsink should be insulated from ground (unlikely), or the transustor should be insulated from the heatsink with a mica or rubber washer.
 

Thread Starter

daeiron

Joined Feb 28, 2022
11
Thanks for all the help, the collector was indeed shorted to ground via the heatsink. So i put a thermalpad behind it and now it works. Thanks!
 
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