Audio Amplifier Repair

[n45048]

This should be an easy one for those of you into your audio gear.

A friend of mine handed me a basic audio amp for me to look at. All I've done is crack the cover and have a look inside in case there was anything obvious like a fuse or something charcoal. I'm yet to actually power it up and check voltages.

He said as he was listening to music, the audio became crappy, then just cut out completely. I've attached a photo of the inside. It doesn't look all that complicated (I think there are some components on the other side of the board.

Any ideas where I should be looking first?

[netdudeuk]

Find a schematic ?  If this affects both channels then surely it's a common thing - power supply ?

[tautech]

Find a schematic ?  If this affects both channels then surely it's a common thing - power supply ?

+1
Same as any powered device, always check the PSU voltages and ripple first.

[Shock]

When a diodes goes that can really mess things up, then check the transistors/fets 3rd one down has a small bubble in the image you supplied, then check the omps which is when you start breaking out the signal gen and the scope.

[Tim F]

Diode check on all the power transistors would be a good start - the four TO264 packages on the main heatsink, then the four transistors with clip on heatsinks. If one of the output transistors has failed short circuit it usually blows everything all the way back to the input stage, or in your case probably those DIP8 packages (opamps?).

If those seem ok then power it up and check supply rails. There will be a + and - rail for each audio channel and maybe some zener regulated supplies for the opamps. There are some devices mounted on the other side of the board just left of the transistors with the clip on heatsinks and the board looks suspiciously toasty around the solder pads, if they are resistors check they haven't burnt out and gone open circuit.

That relay probably kills the whole show if there is excessive current draw and/or DC present on the output which is probably the reason for it ceasing to work completely but not having any blown fuses.

[oldway]

Powers the amp at reduced voltage with a variac and checks if nothing overheats.

What brand and type of this amplifier?

[n45048]

It's a 'BK HIFI' CR100. Appears to be made by these guys: http://www.bkelec.com/
This looks very close to the model: http://www.bkelec.com/HiFi/Amps/marcato_A.htm

I thought it might have been a cheap Chinese knock-off at first as the casing is rather plain. Seems like the build quality isn't too bad actually, but looks hand-made.

[oldway]

Why not send them a mail explaining your problem and asking for diagrams ?

http://www.bkelec.com/Contact/Contact.htm

[Seekonk]

Since the sixties I have fixed audio amplifiers.  I always place a 100W lamp in series with the power to limit current from destroying the output transistors.   Often you will find a bad output, replace it and then destroy the new one because you haven't corrected a problem in the bias circuit.  A lamp will prevent that and allow you to diagnose problems safely with the power on.

Given that one channel probably works, you have a reference.

[voltz]

+1 for testing at reduced mains voltage either using a variac or a lamp. You get more control with a variac of course starting from 0 slowly bringing it up. And obviously check all fuses.
Measure the HT +and- rails as you go, check that both rise together. If one is clamped or much lower than the other, its a fault. Disconnect the power amplifier section and test the PSU by itself. So locating which section is shorting or faulty. If both left and right are down, then yes as already said, good chance its PSU related. Although a shorted power amplifier section could bring the whole supply down (blowing fuses normally).
After that, its the usual fault finding path to the actual component.