Cambridge A500 problems. Please help?

[jamiet92]

Hi,

I'm going to start by saying I'm pretty new at this so this could be frustrating for some of you but I would really appreciate any help you can give me.

I recently bought a Cambridge A500 on ebay. It arrived at the end of July but I didn't have any speakers at the time besides one blown one. I plugged this into the right output just to test if the amp was working fine, it was.

My gale speakers arrived yesterday but upon hooking them up I found the left output wasn't working properly. It would only work when applying pressure above the output on top of the amp.

My Dad and I have opened it up and frustratingly the output works when applying a tiny amount of pressure on the circuit board.

By judging where the movement is happening we believe it could be one of the transistors but I just don't know.

It could have taken a knock in the post but as I, stupidly, didn't try both outputs it will be too late to send it back now.

Any help you could offer me to diagnose the problem would be greatly appreciated.

Jamie

[underwurlde]

Sounds like a dry joint to me.

Can be very difficult to spot where though. Applying pressure or movement to individual components and looking at their corresponding solder joints for any movement on the reverse of the PCB can spot these problems. Then just touch up the solder joint with solder & soldering iron.

Could also be a crack in the PCB itself - have seen this before usually around mounting points or heavy components.

Andy

[Jan Dupont]

Look for a bad solder joint in the area where you need to press to get sound from the bad channel

[jamiet92]

Thanks guys but it seems I can't get any sound of it now. I'm not sure how hot the circuit is supposed to get but I'm not sure if it's getting too hot. Are there any resistances I should check first?

[underwurlde]

Essentially the amplifier works, in so much that no components are 'broken' - however, as you've discovered by applying mechanical pressure somewhere, something is not making a connection. All things considered then, this *should* be a relatively simple fix (he says).

I suppose a blasé approach to the problem you have at hand may work: Touch up ALL solder joints!

This MAY of course not even be the problem, and if so at least this is something that could be crossed off the list of potential faults and you can concentrate on an off-PCB connection fault in the wiring, switches, potentiometers etc.

Andy

[Mooly]

I'll second underwurldes suggestion of going over the whole PCB with the iron. Although its nice to identify and fix the fault sometimes you just have to adopt other tactics. And cracked print is high up the list of suspects too.

It's very very (very very very ) unlikely to be an output transistor that responds to physical movement.

I would say it shouldn't get very hot at all when not driving the speakers with a signal.
If you can come up with a circuit diagram and have a multimeter and know how to use it then all things are possible...

[dry joint]

If the output binding posts is connected to the main PCB, I'd check there first, simply connecting and unconnecting can over time wiggle a joint into being dodgy, had this with pcb mounted headphone sockets and phono inputs.

Removing the pcb will require removing the output tranistors from the heatsinks, before returning everything back in its place the thermal pad/paste on the output transistors will have to be replaced.

[jamiet92]

Hi there,
Thanks for all your help guys. Have finally got it working. There were some dry joints around the capacitors. There was also a small crack in the PCB which was fixed by linking it out with some wire.
Cheers,
Jamie