Joe Meek VC6 Opto Comp not working

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Hi everyone!

New member here. I have acquired a somewhat functional Joe Meek VC6 preamp/comp/enhancer for free off a friend as it isn't operating entirely as it should. Mainly the optical compressor section doesn't seem to be working. Signal passes through but no noticeable compression no matter what the controls are set to. The compression (threshold) control does knock down the signal as it is turned up but not in a noticeable compression type effect.

Opening up the enclosure I noticed that when the compression section is switched on and as soon as the compression control is turned off zero that the LED's that are part of the optical vactrol section stay permanently lit up regardless of any signal present. From what I understand, these should only light up when signal is present in the circuit, not stay lit up constantly with no signal present which I'm guessing i part of the reason I'm not hearing any compression effect?

Anyone here have any idea where I might troubleshoot to find the cause of this issue? Is the compression control part of the problem?

Cheers for reading everyone!
 
Might help.



I have replied privately, but for the interest of others, these optical compressors very occasionally fail because the transistor driver circuits can be quite stressed under very heavy continuous compression. The LEDs that make the compressor work are driven by transistors that control current from the power rails. It’s usually the PNP transistor (often a 2N3906) that fails. I hasten to add that this is by no means a common fault, but I have seen it. But most of these compressors are getting on for 20 years old now and there have been very few failures!
Original Joe Meek TwinQCS compressor not working - tfpro


[FONT=tahoma,arial,sans-serif]By now many audio DIYers would be familiar with "What Compressor?", a project that apparently had its origins in one man's dissatisfaction with Alesis' 3630 compressor and his fascination with a friend's Joe Meek device. [/FONT]


Weak Joe Optical Compressor



joecheep_schem.gif
 
Thanks for the reply Printer2. There's a 2N3906 and two 2N3904's (one 2N3904 is connected to one of the vactrol LED's). Might have to replace all three and see if it gets it working again. At least a cheap way to find out, my local electronics supply has all these in plentiful supply so at least worth a try. I'm confident with normal size components but all the SMC's on the underside will not be worth the hassle if it goes beyond replacing the transistors.

At worst I'll gut the chassis, use the power supply and build a Weak Joe in there and maybe a input transformer led preamp in the front end with a meequaliser on the end instead of the enhancer circuit.
 
Just to add: probing around with my DMM on the circuit board, as soon as the compression control is turned up from full anti clockwise, the wiper has direct continuity with the clockwise leg of the potentiometer as if it's just an on and off switch instead of the gradual normal control like a potentiometer should be. What this means at this point I'm not sure but thought this information might be of use to someone reading.
 
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