Phase 90 clone problem

thermionic

I've finished wiring up my Fuzzdog Phase 90 clone and it doesn't work. Starting at the most basic point, I looked at the power suppply first. The bias supply didn't change with the trimpot, so going one step back I find Vref reads 0.63V, not 5.1V:

mxr-phase-90-power-supply.png

Bit of a stab in the dark, but any ideas what could be wrong here?

I think when I first measured Vbias it was around 4V, but I didn't check Vref at that point, I was just trying to adjust the trimpot to get Vbias down to about 3V.

springhead

0.63V sounds like silicon diode forward voltage drop, unless it's just coincidence!  Check D1 and D2 are both soldered correctly, no dry joints and check the polarity of both.  Bad connection on D2 and D1 the oterh way round would give you 0.6V, but then you said you had higher voltage initially.  That being the case it's likely a bad joint that needs reflowing.

thermionic

Thanks, I thought that voltage sounded familiar! The pcb is very compact, with small pads, so it wouldn’t surprise me that one of the joints is suspect.

thermionic

OK, I reflowed some of the connections which looked poor, but the result is the same. Here is the actual circuit:

https://i.imgur.com/YDUgQG1.jpg

D2 is a reverse polarity protection diode, D1 is a 5.1V Zener that should set VB at 5.1V, with VA then being the bias voltage fed to the FET gates. The node common to R26 & R27 reads around 8.6V, so something is going wrong after R27. I'm going to try and replace D1 because it's either this or C12 that looks to be the culprit, and I'm more likely to have damaged a semiconductor device than a capacitor. This is where I miss Maplins, I'd just pop round and get a couple of diodes, now I have to buy them online and wait a couple of days!

springhead

Is D1 definitely a zener?  And definitely reverse biased? Bottom leg of the Bias pot making good connection to Gnd?

Headphones

This circuit is rather different from the one posted earlier.

From your decription of the symptoms I'd suggest that D1 is in backwards, rather than a short or open circuit.  It miight even be a standard diode, as the forward volt drop of zeners is often not the typical 0.6V.  The symbol, as it appears on my screen at least, is not a zener in any case, but a normal diode.

On a separate note, I've never worked out how D2 is really supposed to protect, rather than merely warm itself and the power supply...

thermionic

You’re right, the symbol is an ordinary diode and not a zener. I also agree that it sounds like D1 is in the wrong way from what I’ve been measuring. I checked the orientation of both though, and probed at either side of R27, ground etc. and can’t find anything wrong. I’m going to try a new D1 and try it from there.

Headphones

If you lift D1 the voltage will change to that based on the pot value, R26 and 27, which will be a clear diagnostic (no values there so I can't offer a voltage).  If that doesn't go up it'll be the cap, pot or something very dodgy.

thermionic

Looks like it was the zener diode, I’ve replaced it and now it’s producing sounds - not the chewy phasing I was hoping for, but it’s a start. However there’s an even more fundamental problem - when I switch it off it isn’t passing any signal. I can hear some popping that increases in frequency as the speed is turned up, but no guitar sound.

Could this be a problem with the switch? I’ve destroyed one in the past with two much heat but I was quick and careful when soldering this one in.