SC480 Troubles....

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#41
Sc480 test voltage problems

Hi all, first time poster long time electronics nut!

I have recently purchased an SC480 kit as my old ETI480's are dying off slowly.
I have the unit at test stage to set the voltages & the test resistors are in place.
There is a good supply voltage of 40V to the board but I have 0V across the resistors.

I have double checked the power transistors to make sure they are all insualted & also checked other components but still no go.

The adjustable varistor seems to be ok when I mesure it in circuit & try to adjust the screw up & down (the readings vary as I turn the screw).

Can anyone help to advise why I would not be getting any voltage across the test resistors?

Thanks
Darren.
 
#42
Darren, you have presumably removed the fuses! If not the test resistors will be being shorted out by them resulting in no voltage drop across the resistors. If this is in order maybe you have transposed the output devices, PNP where NPN should be and vice versa.

Keith
 
#43
Getting more info but still no go!

Hi Keith, thanks so much for the reply.
Fuses are not installed yet & I have double checked the output trannys (plastic type) for their correct position.

I have since checked the voltages across the board & compared them to the diagram.
A few are correct but most are zero volts where I should have various readings.

2 in particularar are confusing...
Where I should have +38V in one place I have +18V that drops slowly to +8V
Where I should have -38V in one place I have -64V.

I have gone over the board for dry/shorts etc but still no go.

Cheers

Darren.
 
#46
Darren, there is something very strange about those voltage readings. It is almost as if the whole power supply is floating (the transformer centre tap is not joined to earth). Make sure you have stripped off enough insulation to get contact with the screw terminals and check the continuity from the CT to 0V on the amplifier boards. Also check the 10 ohm resistor between the cold side of the input and 0V.

Keith
 
#47
No better!

Hi Keith, thanks again for your suggestions.
I have a good earth to the 0V line & the 10ohm resistor also checks out ok.
I have removed Q4&5 trannys for a quick check (I had different readings in circuit) both ok out of circuit.

The battle continues!

Darren.
 
#48
Nearly there! A bit more help needed.

Hi all, I have still been trying to find the fault with my recently built amp.
I have just found a broken track....YAY! 😎

All the suggested voltages on the circuit diagram are now correct...

However, when I do the final test of checking the voltages across the fuse holders (with test resistors in) I have around 39V (just under the power supplys output), I can't adjust this back to the suggested 28V, regardless of the position of the VR1 trimpot the voltage remains the same.

Could the final bit be a faulty trimpot or would it be not that simple???

Any help appreciated.

Darren.
 
#49
Hi Darren, If there are no faulty devices from Q8 thru Q13 it seems you have too much bias voltage, which means Q7 may not be turning on enough. VR1 would not be a likely cause, as if it were open circuit it would assist turning on Q7. It could be short circuit, but this seems unlikely. Try measuring it with an ohmmeter. If you were to short out the collector and emitter of Q7 you should be able to turn off the output devices and get rid of your high voltage drop accross the fuse test resistors. This may need "four hands" and could be done by shorting C to E with the tip of a ball point pen when powered up.

It is not as brutal as it may seem as the current is defined by Q6 etc. If this biases off the output stages and the voltages across Q6 emitter resistor and the "tail" resistor of Q4/Q5 are around 0.9V and 1.6V respectively you must have a problem around Q7. You definitely have a BD139 in Q7? Good luck.

Keith

PS, have you checked that Q7 is insulated from the heatsink. Sometimes whiskers of metal from hole drilling can pierce the insulating washers. Check this first before shorting C to E of Q7 and make sure you don't short the transistor leads to the heatsink behind.
 
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#51
Got it !!!

Hi Keith & Greg (& others interested).
I have found another broken joint which was under the emitter of the Q7.
Once it was repaired I was able to adjust the VR1 & get 28V across the test resistors. 😀

Thanks for pointing me in the right direction Keith.

I am now in the process of putting it all back together & tidying up, hopefully I will make some noise later this week!

Thanks again.

Darren.
 
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