A Faulty Neve 33609 / N

[smilan]

Hi, I have here a faulty Neve 33609 / N.
The unit turns on, both meters jumps all the way to the right and there is no signal at both outputs.
I measured +24V at the point where the PSU connecting to the main board.
When turning the unit on / off I can hear the relays switching.
Since I don't have the service manual of the N version and I don't know what voltages I should expect to measure in the various test points, there's anything I can do to troubleshoot this issue?
Attach a photo of this unit

 

[RuudNL]

Does this also happen with the compress and limit switches 'off'?
I get the impression that there is a permanent control voltage, causing a high amount of gain reduction.
You could measure if there is a permanent voltage over the diode (attenuator) bridge, without signal.

Does the 'link' switch make any difference? (It is rare that the problem is the same in both channels.)

 

[JohnRoberts]

are the PS rails OK and getting where they need to go.

 

[smilan]

Does this also happen with the compress and limit switches 'off'?

Yes, the meters are stuck to the right in all of the switches configurations.

I get the impression that there is a permanent control voltage, causing a high amount of gain reduction.
You could measure if there is a permanent voltage over the diode (attenuator) bridge, without signal.

On both channels there's a 1.34 DCV at the junction where the tow anodes are meeting. On the PCB the diodes with the 1.34V are D68 and D69 (marked with a yellow stripe), I'm not sure what are the corresponding diodes on the schematic I have (D18 or D19)?

Does the 'link' switch make any difference? (It is rare that the problem is the same in both channels.)

No, it acts the same with both channels linked / unlinked.

are the PS rails OK and getting where they need to go.

At the point where the +24V enter the main PCB I've measured +24V, so the PS rails seems to be OK.

 

[RuudNL]

1.34 V over the diode bridge is enough to cause a fair amount of attenuation, I suppose.
As far as I know, there is no 'bias' or 'pre attenuation' used for the diode bridge.
This means that without any signal, the voltage over the diode bridge should be low, near zero V.
Something else to look at is the release circuit. The release control doen't go to ground, but to a diode. (In some versions two diodes in series.)
The diode gets a low current through a resistor connected to +24 V. If the diode has failed (='open') there would be ~24V there, causing permanent gain reduction. Is is possible to unsolder one wire of the release control(s) and measure the voltage over the diode bridge again?

 

[smilan]

Something else to look at is the release circuit. The release control doen't go to ground, but to a diode. (In some versions two diodes in series.)
The diode gets a low current through a resistor connected to +24 V. If the diode has failed (='open') there would be ~24V there, causing permanent gain reduction. Is is possible to unsolder one wire of the release control(s) and measure the voltage over the diode bridge again?

It's not a simple task to find this diode without a proper service manual.
I'm quite sure I've found the components marked in red, but so far I couldn't locate the blue ones, (those are the resistor and diode you've mentioned?)

 

[RuudNL]

You can simply measure the voltage on both sides of the release controls.
If there is a higher voltage than 1.34 V, this is suspicious!

 

[RuudNL]

The 'proof of the pudding' would be to short one of the wires connected to the release control to ground.
I don't see that this could do any harm. (Switch in 'dual mono' position, so channels not linked.)
If the signal comes back and the meter falls back to 0 gain reduction, you are on the right track!