STV3H 60B?

that is it .. no idea about markings .. just dont bend the wires or they will break next to it and cause you headaches .
 
that is it .. no idea about markings .. just dont bend the wires or they will break next to it and cause you headaches .

Ha yeah - I've already learnt that in the past!

I've already got a headache trying to figure out what is wrong with this thing.

I'm just getting a loud "beating" noise out of both speaker terminals.

I've replaced every transistor and every electrolytic cap and am having trouble working out what it could be
 
needs logical testing starting with all power supply voltages .. if something is wrong there it can be supply itself or a good clue to what is wrong .
 
needs logical testing starting with all power supply voltages .. if something is wrong there it can be supply itself or a good clue to what is wrong .


So I've just been checking the voltages along the power supply path - starting at the transformer and working my way back down the PCB. I have the schematic and it's laid out in a pretty clear way to see the power supply path running down the centre of the schematic. Certain points have voltages marked as well for me to reference.

I'm fairly sure the transformer is ok as the first voltage check point was correct (close to correct anyway - "less than 1v off).

However - it didn't take long for the voltages to start reading way off. A reading of 39v instead of 32v at the second voltage spot I checked.

I was even reading about 38v at a spot marked as 22.6v on the schematic - which was then running through a capacitor that's rated for 25v!!!!

The power supply path is made up of diodes, caps and resistors.

So - if the voltage isn't dropping along the path like it should - I am I to think that it's most likely resistors causing the problem, - and should I start at the point where the voltage reads fine and work my way back testing the resistors to find the faulty ones?
 
yeah, that is a real strange power supply arrangement. A test for transistor circuit analysis.
To make it easy for me and others to follow when you state your measurements, you should indicate where you are taking them, like saying, reading taken at Q16,base(D4 cathode) instead of "A reading of 39v instead of 32v at the second voltage spot"
If that is this place, then the 32V zener(d4) seems to be acting up, replace it with 771-BZX79-C30133
Little hard to read my schematic, so you changed C67(220uF/50V)?
Also check those 1.2K ohm resistors,R100,R?
Q17 acts a shunt regulator with it bias set by the negative voltage doubler made up with C64,D3,C63. It certainly would help if they printed more voltages on the schematic, what would it have taken them to do this, nothing, bunch of fools. :)

Good luck
Rick
 
Since there is no reference voltage in the Q17 circuit is it a shunt regulator OR just a mute circuit for the input stage of the power amplifier? Q17 is turned on at power up which shunts to ground the B+ for the input stage and as the negative supply comes up Q17 turns off and the shunt goes away.

Craig
 
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