Auditec Power Amplifier Module

[Raff]

Hi everyone.

I'm trying to repair a 1970s home made guitar amplifier.

Inside it has 2 Auditec 001 MK2 modules.

They have 2 2N3055 as output devices, and the diode rectifier bridge is on board, but the filter caps are off board.

I've found a few faults such as a shorted 2N3005 and its driver a BD140, but when I power it up carefully using a variac and dim bulb tester, I get over 5 volts DC across my 8 ohm dummy load on the speaker terminals.

I suspect something else is faulty although all the semis check ok on my meter.

Anyone have a circuit diagram for these?

From memory I think Delsound in Brisbane at least used to sell these in the 1970s.

Thanks,
Raff

[SeanB]

Check the low value resistors on the power transistors, if the one dies as a short then the resistor would have either changed value or gone open circuit. The other transistors could test as fine, but could still be leaky at full operating voltage.

Pics of course will help.

[Raff]

Thanks for the info.

I'll grab some pictures tomorrow and post them up as well as checking the resistors.

Thanks,
Raff

[DrGeoff]

Auditec, that was Cyril Murray's company. I remember them from back in the 1970's.
If you have replaced the 2N3055 transistors, did you check the bias circuit?
There is probably a transistor in there (eg BC549) use to tempco the driver bias.

[Raff]

Here are a couple of pictures of the modules.

One has had the driver transistors changed to BD139 and BD140 but is otherwise identical to the other.

I've had more of a muck about and it seems that maybe the power amp modules are actually working fine, as when I move the volume control, that's when I get a DC output on the speaker terminals.

Its not just when I move it but the DC stays there after as well.

I've put it on the back burner for now as the pre amp module is pretty rough looking and Id hazard a guess that the electros I see on it are probably causing the DC out problem.

Raff

[picandmix]

If you are looking to keep using those amps then I would spend an hour or so drawing out the circiut diagram for future use.

When repairing such old boards, it often cheaper and simpler to replace all the transistors / diodes and electros, as replacing just one part often leads to a daisy chain effect of failures.
On some old boards I find it easier just to strip everything off and use all new parts, most are just cheap resistors and caps.

If the pre amp is rough, you can probably find a suitable circuit here, some designs also have a ready made pcb; have bought them before and they are good quality.
http://sound.westhost.com/projects.htm