SC480 Amplifier Kit Problem

[Stefan1]

Hi everyone,

If someone that has built this amplifier could they help me out please.

The problem is, when the manual asks me to set the amplifiers quiescent current level.
I put the 560 Ohm 5 watt resistors accross the fuse holders then I put power to it and, the resistors have a voltage of 40.05 volts and they get quite HOT ( after about 30 seconds). Then when I try and I vary the variable resistor (VR1) it has no affect whatso ever and it stays as 40.05 volts.


Any Ideas??

THANK YOU very much for any help.

THANK YOU
Stefan

[jmateus]

Hi
There is definitively a problem with the amplifier that is sucking
the major part of the supply voltage to be dropped across the
resistor...
I built two different versions of that amplifier but I haven't used
so high value of resistor at the initial power up, I used only 47
ohms/5W. You have to try to troubleshoot the whole amplifier
for shorts on the PC board or wrong transistors on/or wrong wired.

This amplifier like any of AB class is not forgiven to any of the discrepancies I mentioned, in order for the amplifier to function
first of all you have to make sure the bias adjusting pot has to
be initially to its high resistance position and then - after the
voltages are within reason - to be adjusted to the right bias
voltage over one of the emitter resistors.
There is a chance your amplifier is wired correctly and only the
bias pot is all the way to its MINIMUM resistor value, and in this
case the amp is drawing to much current from the supply and
consequently making it to be dropped across the resistor.
If everything is correct, the drop across the resistor is minimal
like less than a couple of volts, depending on the the resistor.
I use a smaller value, as I said 47 ohms, because is much easy
to see the drop across it...
Hope this helps, good luck.

[Stefan1]

Thanks very much for your reply jmateus.

I will tell you my whole story with what happened.

The variable resistor (VR1) that came with the kit, was a faulty part to begin with, It was a 200 Ohm variable resistor.

So Then I put my own Variable resistor in it ( which was 500 Ohm used in the ETI480)).
When I put that in and put power to it, it would start to very the voltage ( accross the 560 Ohm 5 watt Resistors) and I put it to 28 Volts and everything was running perfectly. Then I put my fuses in and everything powered up fine and I hooked up speakers and the input and everything sounded great. BY THE WAY this whole time I didnt have ( Q9, Q7, and Q8 on the heat sink ), the reason for that is because I beleive they were shorting onto the heat sink.

They didnt get hot what so ever with them off the heat sink by the way. Then after my test ( which worked perfectly) I decided to put the transistors ( Q9, Q7, and Q8) onto the heat sink and screw them on, but for some reason this time I had music VERY dull and distored coming thorugh the speaker.

I quickly unplugged it and then took the Transistors back off the heat sink to try again( to make sure I didnt do any harm) but this time, fuses burned straight away and so I put my 560 ohm 5 watt resistors accross and they get hot and stay at 40.05 Volts and the Variable resistor ( 500 OHM) doesnt vary at all ( refer to post 1 )

Sorry for not explaining the whole story before....

DO you think I should replace the transistors ???

Any ideas??

Thank you Very much

Thanks
Stefan

[Greg Erskine]

Originally posted by Stefan1
... BY THE WAY this whole time I didnt have ( Q9, Q7, and Q8 on the heat sink ), the reason for that is because I beleive they were shorting onto the heat sink.

They didnt get hot what so ever with them off the heat sink by the way. Then after my test ( which worked perfectly) I decided to put the transistors ( Q9, Q7, and Q8) onto the heat sink and screw them on, but for some reason this time I had music VERY dull and distored coming thorugh the speaker....

Hi Stefan,

I can't imagine there is anyway you could test one of these amps without a heat sink. As well as taking heat from the output transistors, it is vital to the thermal stabilisation of the amp via the Vbias transistor. The fact that the output transistor didn't get hot without a heatsink points to a problem IMHO.

I have one of these kit amp, the TO3 version from Dick Smith, and I just followed the detailed instructions and had no problems.

Check and double check everything. I have a feeling you have made an error but you may have been lucky and not destroyed anything.

Are you having the problem with both channels?

regards

[Stefan1]

Hi everyone,

I just replaced Transistors Q9, Q7, and Q8, But it has made no diffrence to my problem, I think I will replace the other BIG transistors aswell. Would Q13, Q11, Q10, and Q12, have ANY affect on power at all ?

I remember feeling one of the big transistors get quite hot when they got shorted, So I will just replace them all.

What do you think??

THANK YOU very much

Thanks
Stefan

[Stefan1]

Hi Greg..

The kit is just a single channel kit. I done everything right the first time, just when I put the transistors onto the heat sink I shorted them, sooo I have a feeling I have to replace all the transistors which I will do shortly..


Thanks
stefan

[rabbitz]

Just a thought.... did you use mica washers with thermal grease or silicon washers to isolate the transistors from the heat sink or did you install them without anything. Trying to figure out how you shorted them.

If in doubt, throw them out and for the few extra dollars, not worth the hassle.

[jmateus]

Hi Stefan1

It is imperative that when assembling an amplifier (any) care must
be taken in installing the bias and driver transistors on a proper
heatsink with washers and thermal compound.
If you don't do that chances are you are going to get what is
called thermal runway and eventually the destruction of those
transistors and possibly the outputs. This procedure is a must.

Now you have to check everything including the outputs and
the blowing of those fuses might be an indication the outputs
are shorted.

What I would do is to start fresh, replacing all the xistors
from Q6 on just in case you don't have a way to check them all.
Sorry for the mishap, this unfortunately is a way to learn and
believe me I've learnt a lot with problems like this in the past.

[Stefan1]

Hi everyone,

Thanks for all the replies.

Well I have definatly learnt from this mistake, and I will replace some further transistors in the next couple of days.

I will tell you how I went.

THANK YOU for all your help everyone.

Thanks
Stefan

[Stefan1]

Hello, Everyone..

IT WORKS, I replaced the 4 big transistors and everything works perfectly.

Thanks for all your help everyone.

Thanks HEAPS
Stefan