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Help! Conrad Johnson PV2 relay not working

Posted by pastito (A) on November 22, 2015 at 22:33:15

Hi folks!

A friend of mine brought me a Conrad Johnson PV2 preamp that is having HV relay blues.

I checked and the timer circuit seems to be pretty simple. There is a 220 uF cap that gets filled slowly and then there are two NPN transistors in darlington configuration. The first one is 2N3904 and the second one is a MJE182. The relay is a double inversor - two pole with a 12VDC coil. I removed the relay and tested it with a 12VDC power supply and it works perfectly.

I measured voltaje across the relay's diode and it nicely ramps up until it hits 6.82 volts and then it stays there... and that is not enough juice to fire the relay.

Where do you think the problem lies? Should I replace the cap only?
The 2 switching transistors? All of the above?

This pre amp is built like a tank, but PCB access is impossible from above to measure voltages... and I find it way too dangerous to turn it on dissassembled... I am afraid I cannot measure the voltage feeding this very simple but seemingly effective timer.

What you you think I should do?

Regards
PS. Sometimes it works... and the preamp sounds very nice indeed.

Edits: 11/23/15

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