Diamond Compressor

[RickL]

I'm trying to repair a Diamond Compressor (schematic here: http://revolutiondeux.blogspot.ca/2012/06/diamond-compressor.html), no sound when the tone control is engaged, and I think I've tracked it down to Q2, a 2N5088.

If I short the base and collector of this transistor, either with a wire or a diode, anode to collector, cathode to base, it works. Anode to base, cathode to collector doesn't work. The simplest thing to do would probably be to just replace the transistor, but I'm wondering why this is working.

Any thoughts?

[PRR]

Q1 alone can't pass signal. It needs DC current and something to work against.

It appears that your C-B short on Q2 makes D3 D4 work as a "load", but a very crude load with limited level and high distortion. At a glance, it would seem that a Q2 C-E short would bring R16 into play as a "load", but without parts values (I am not going to go back-n-forth to suss it out), R16 may be too small to work.

A real question is: how could Q2 fail?? It is not subject to any abuse in normal life. Maybe it is just a bad solder joint??

Transistor DO fail when users apply shorts across their legs. Not always, and with full understanding of the circuit a short can yield clues. Not sure where you are finding anode and cathode on a transistor.

A deeper question: why is the Q1 Q2 stage there at all?? It comes from the strong output of IC2B and drives a plain EQ stage, other end driven by the other half of IC2. You should be able to jumper SW1B to R10R12C5 and work dandy. Maybe this plan just-grew from a simpler plan.

[RickL]

I'm not sure why they chose to do it that way. I was probing the circuit trying to find where the signal stopped and I think I accidently shorted the base and collector of Q2. When they're shorted I get signal there, but not at either pin when they're not. Because it's a pain to get the circuit board out of the enclosure to get at the trace side I've been working just from the component side.

When shorting the base and collector I wondered if shorting them with a diode would work. It did, but with the diode the opposite way that I thought would work. Paralleling the base and collector pins of another 5088 with the one in the circuit also works, but only base to collector and base to collector, not collector to collector, base to base.

In case it wasn't obvious, this is a commercial unit, not one I built.