Arbiter Trem Face Fixup (advice wanted)

[GammaFlex]

So a good friend of mine asked me to fix his Trem Face. these are really cool little pedals. What this one is doing is that when it's plugged in it makes noise, the caps have some effect but it's more like a filter. There is a crackle on one of the pots (needs cleaning) but the effect itself sounds like it's just trying REALLY hard to do it's thing but it's not, just sort of crackles but with no tremolo effect. Battery was fresh and everything. Any thoughts?



I took it apart and here's a few pictures. I was hoping to run this by some folks who know a little more than I do about this rather than me just trying things until it works.

One thing I noticed right off is that several components are not period correct. I have yet to check the values against a decent schematic but I will soon.



I took a multimeter to all the parts, the only one that didn't read quite right was the second cap. It's a 320uf that's rated at 2.5v. My gut tells me that that might be a problem considering that it's a 9v circuit and with the FET it might be regulated to a lower voltage. So wouldn't the cap need to be rated for like 6-7v? Plus it just looks..... odd. This is the negative end of the cap, typically on these I've seen this end is flat, leaky?



And here's a picture of the solder-side.

[midwayfair]

Don't just assume that a cap's voltage rating is too low: Measure the voltage in the circuit on both sides of the cap. It could, for instance, be an emitter bypass cap, in which case it probably isn'y seeing more than a volt.

RG Keen traced this effect, I'm pretty sure.

[GammaFlex]

Looking at the schematic and the circuit it apparently goes to ground from the emitter of the BC148. And according to Keen this cap should be a 6.3v but if it doesn't matter then I guess it doesn't.

looking at the soldering it looks like the FET on this example might be a replacement, perhaps that's part of the problem?

[midwayfair]

Did you take voltages of the whole effect? Did you check continuity against the schematic? Use all the standard troubleshooting procedures. Guessing that a part might be bad is not the way to go about it. If you suspect a transistor might be bad or oriented wrong, look up its datasheet, pull it and measure, etc. Reflow solder first thing.

[GammaFlex]

So finally got back to this. Voltage readings showed that the low voltage cap apparently is getting a full 9.38v from a fresh battery. The FET is reading the same, 8.28v on the G and 7.2v on the D, S is reading at 9.2v

[midwayfair]

So finally got back to this. Voltage readings showed that the low voltage cap apparently is getting a full 9.38v from a fresh battery. The FET is reading the same, 8.28v on the G and 7.2v on the D, S is reading at 9.2v

Come on, post a schematic so we're all looking at the same thing.

http://www.geofex.com/article_folders/trem_face/tremfc2.gif

http://www.geofex.com/article_folders/trem_face/tremface.htm

For your FET: Source is supply. How can you be reading .3V lower on the source than the supply voltage from the battery? You need to check that connection. Gate is connected to a 1M trimpot that balances between supply voltage and the oscillating voltage out of the other transistor, which you didn't give any voltages for, so I can't tell you if there's a problem there. The drain shouldn't be above 5V plus whatever the diode drop of the zener diode is. (D1 is the zener. Check that it's not broken.)

The "dry" path is passive, so with no effect happening, you should just hear the guitar, slightly quieter. If you're hearing crackling, you should also be looking for DC in the dry path.

[GammaFlex]

I've been looking at the schematic on the geofx page too, I didn't change anything literally I just took the measurements as they were

The diode according to my meter reads at .704 and no flow in the opposite so apparently it's not broken, it's a 5.7v zener according to the schematic.

The BC184 apparently is reading 9.15 at the base, I'm going to remove the board again and some more readings in a bit. I'm beginning to think this thing is screwed beyond belief.

I'm going to look for dc in the signal path since it's just crackling, there is a slight loss of sound when it's on but not severe at all.

[midwayfair]

There's just not enough in the pedal to be screwed beyond belief; even a problem in the circuit board itself would be fixable. Keep at it.

You still haven't given the voltages on the BC184. The base is only one pin.

[GammaFlex]

woops you are correct.

Emitter is reading at 9.28v, collector is reading at 8.58

[wgc]

Without even touching a meter, visually there are some bungled traces under that tranny, like gamma highlighted. Granted that may not be the problem, and the previous guy may have swapped it without getting to the original problem. But I'd still verify orientation and the soldering. There also looks like a broken trace nearby and various cold joints throughout.

There are many questionable solder joints on some wires too. Verifying that they are all going to right places would be a good idea, as well as touching up those joints.

Good luck!

[midwayfair]

woops you are correct.

Emitter is reading at 9.28v

How can your emitter be HIGHER than your collector? It's also a driode drop higher than your base, which is ... not how transistors work. The emitter should be a couple volts at most. It's not at supply voltage. It's .1V lower. Which if your battery has drained a little bit from the original battery reading COULD be about ... 1 diode drop below supply. That means there might be a short between the emitter of the BC184 and the anode of the zener.

I agree with WGC. You need to get out the flux and desoldering braid and clean up the soldering.

And then you need to go through with your multimeter on continuity and check every single connection against the schematic.

[GammaFlex]

It's more than possible that it was replaced and they it flipped the thing, but it's odd because according to the pinout and the schematic it's oriented correctly

I am planning on desoldering those janky points on the fet, I'll also check for a short Jon, there's a few points I'm going to go over and cut as they look to be crossed with each other. I'll check back later today after I'm done.

[GammaFlex]

And for posterity's sake I took the voltages on the 1M trimpot, pin 1 is coming direct from the power supply according to the schematic.

Pin 1: 9.2v
Pin 2: 7.9v
Pin 3: 8.6v

[GammaFlex]

So with some fiddling I managed to get rid of the crackling, now it just doesn't work at all. Still working on it in my spare time.

reflowing the solder seemed to do the trick, apparently there was a bridge or something somewhere that was crossing from the positive side and making some noise. still more work to do