ADR Compex Problem

[thebunker]

Hi Folks,

I have an ADR Compex which has developed a fault on channel 1. The gain reduction meter needle is deflecting when the unit is idle.

I've cleaned the input/output pots & all the card edges for that channel with deoxit. No improvement.

The fault follows card A of the 3 cards on the channel. I did a swap between channel 1 and 2 and the fault went with it. The deflection also increases with the ratio control being turned up.

(pdf of schematic and flow chart is attached)

There is +24v between the -ve & +ve rails to the card at A4 & A8.

Can anyone please help me get any further with this? I've checked for volts all over the circuit, but am not really sure how to narrow things down.

Thanks for looking

[nickb]

If the meter is moving quite slowly you should be able the track down the problem using a DVM. If faster, you might need a 'scope.

I would check for variation in voltage wrt 0v at the -ve meter terminal first. If that is changing, then look around the BC212B on pin A10 - check the wiper of the zero pot for stability.

If -ve meter is steady then it must be the +ve end that is unsteady and you should be able to track it from the top end of the Meter Range pot, to the test point on the wiper of the control volts pot, and then to the control voltage on A15.

[thebunker]

Thanks Nick,

The voltage on the top end of the meter range pot is fluctuating around between 1 - 3.5v. ( on the good channel that is 0.7v )

Also at the wiper for the control volts it swing between 1.5v - 9v. ( on the good channel it is steady at 1.7v )

I can't see the voltage fluctuate on the +ve or -ve terminals of the meter at A9/A10. It may be too fast for my DVM 9 it reads a steady 0.9v )

[nickb]

Thanks Nick,

The voltage on the top end of the meter range pot is fluctuating around between 1 - 3.5v. ( on the good channel that is 0.7v )

Also at the wiper for the control volts it swing between 1.5v - 9v. ( on the good channel it is steady at 1.7v )

I can't see the voltage fluctuate on the +ve or -ve terminals of the meter at A9/A10. It may be too fast for my DVM 9 it reads a steady 0.9v )

OK. Good so we know it's on the +ve side - now, from above:

"track it from the top end of the Meter Range pot, to the test point on the wiper of the control volts pot, and then to the control voltage on A15."

Put your meter in each of those places to narrow it down further.

[thebunker]

There is the same sort of deviation all the way out that path.

I noted that A15 on the good board reads 0v. On the faulty one I can see it move between 0-5v on the DVM.

[nickb]

There is the same sort of deviation all the way out that path.

I noted that A15 on the good board reads 0v. On the faulty one I can see it move between 0-5v on the DVM.

OK. Look at the schematic. After the input A6 is an amplifier BC182B. It feeds two buffers each of two BC182B's and forms a full wave rectifier with the pair of back to back BC182B's. This rectifier is buffered by a pair of BC212B's to drive the attack/decay signal on A16. Hope you're following this! Why oh why don't that have any references on the schematic?


Look at the base of the BC212B with the 1k and the 33k resistors it should be steady. If not lift one end of the two 8uf capacitors that feed the BC182B's and see if the problem goes away. I'm guessing now that the 8uf caps will turn out to be leaky.

[Jazz P Bass]

As I am having trouble following the logic, I had to ask.
How does this affect A15, which is varying 0-5 volts (nominal voltage is zero)?

[nickb]

As I am having trouble following the logic, I had to ask.
How does this affect A15, which is varying 0-5 volts (nominal voltage is zero)?

Jazz, look at sheet two. The attack/decay timing resistors hang off A15 and are driven from A16. A16 is the output of the rectifier buffer.

[Jazz P Bass]

Thanks for that info.

[thebunker]

Ok, finally managed to get back to this.

Yeah Nick, it is a bit of a nightmare not having any component numbers on the schematic. It does not make finding components on the board too easy either!

The 8uF caps are actually 4.7uF caps (looks like a revised design as the board has that value written on it). I've unhooked a leg on each & it looks like you're absolutely correct!

Channel 1 meter is now steady as a rock.

I'll get some of these caps on order straight away.

Thanks for the help & explanations Nick, yer a gentleman!

[nickb]

Ok, finally managed to get back to this.

Yeah Nick, it is a bit of a nightmare not having any component numbers on the schematic. It does not make finding components on the board too easy either!

The 8uF caps are actually 4.7uF caps (looks like a revised design as the board has that value written on it). I've unhooked a leg on each & it looks like you're absolutely correct!

Channel 1 meter is now steady as a rock.

I'll get some of these caps on order straight away.

Thanks for the help & explanations Nick, yer a gentleman!

Well done!