Dunlop Phase 90 Repair

[RickL]

I am trying to repair a Dunlop Phase 90. The original symptoms where no sound or LED lighting in effect mode, passed sound in bypass mode. I am using the schematic at Tonepad as a reference. http://www.tonepad.com/getFile.asp?id=42

I found a shorted protection diode (D1 at Tonepad) and thought I had solved the problem but I am getting a weak, distorted output. Using an audio probe I get modulated signal at the outputs of all the phasing op amps (quad on the board), clean signal from the output of IC1a, half Vr at the inputs and outputs of the phasing op amps and IC1a and close to 4.7 volts at all the points attached to the zener (sources of FETs and emitter of Q5).

I am guessing either reverse voltage or over voltage was applied to the adaptor jack and damaged some component before the protection diode was shorted. Does anyone have suggestions on likely candidates? I'm hoping it's not the FETs (I'll have to match another set) or the quad op amp (Dunlop layout means I'll have to remove the speed pot before I can even begin to unsolder the (unsocketed) op amp). I did try replacing Q5 with no improvement.

I tried tweeking the trim pot with no luck (original position gives the strongest effect) and I did notice a couple of capacitors and a resistor that appeared to be clipped at one end and lifted (at the factory?). I'm not sure what they correspond to on the schematic since the board is double sided and awkward to trace. I can try to determine where they are in the circuit if someone thinks they are important.

[George Giblet]

A couple of things:

Is the signal at the output of last opamp in the phaser chain clean or distorted? (ie. the output of the last opamp which connects to the 150k resistor to the transistor.)  If it's clean then the whole phase-shifting section would get the thumbs up.  At which point I'd be looking at the transistor.  The collector is obviously at 4.7V, but what about the B and C.  The base should be at about  4.7-0.6=4.1V.  The collector should be at about 2.7V.

Try shorting the two switched ground wires on the input socket with a solid piece of wire, just in case there's some weird contact burn out caused by the reverse voltage problem.

Is the bypassed signal OK?

[thedefog]

Hi there,

I had a similar issue with a Phase 90 repair and might have an answer for you.

You mentioned you replaced the Diode that was fried. What type of diode did you use when you replaced it? One diode on the block logo is a  1n914 (the one going directly to the I think 10k resistor), but the other is a 5.1v Zener. If you didn't use a Zener, then there is your problem. I know because I did this, and when I caught it and replaced it with a zener, the problem went away.