Aion FX GAIA Mk. I (Catalinbread CB30 clone) problem

[danmc_27]

Hey All,

I put together an Aion FX Gaia MK. I awhile back. Pedal worked fine at first. I used sockets for all the JFETs and did a bit of swapping in-and-out of them. Pedal suddenly stopped working.

After a bit more swapping FETs around, pulled out my component tester and mutimeter and found that one of the JFETs had an internal short and showed continuity between the outside (drain & gate) legs

I replaced the faulty FET and now the pedal works but it is now lacking in gain. I can turn the gain knob all the way to max and while it does get a bit louder, the signal is still nearly completely clean.

So, my question is, is there any component that could be damaged by having an internal short in one of the FETs that could cause this loss of distortion/gain?

The damaged FET was likely swapped across several different positions, likely Q1, Q2, Q7 and Q8 but I may have put it in other positions so if there is ANY position a shorted FET might have been in that would potentially cause such a problem then that'd be good to know

Ny help in narrowing the problem down before I start pulling components would be greatly appreciated

Have included the schematic for reference

Cheers,
Dan

[PRR]

A JFET is normally-ON and will show a low resistance D-S on an ohmmeter.

In-circuit voltage readings might tell us so much more.

Even before that: take it in the sun with a magnifier and look for dubious connections. It happens.

[danmc_27]

The continuity reading/short is between drain and gate, not drain and source. All the other FETs I tested read fine and are reasonably closely matched aside from that single one which shoed continuity between drain and gate.

voltages are as follows:

Q1: D: 4.9  S: 1.0 G: 0.0
Q2: D: 8.0  S: 4.9 G: 3.85
Q3: D: 4.9  S: 1.3 G: 0.25
Q4: D: 8.0  S: 4.9 G: 3.85
Q5: D: 4.9  S: 1.3 G: 0.5
Q6: D: 8.0  S: 4.9 G: 3.85
Q7: D: 5.0  S: 1.6 G: 0.37
Q8: D: 8.0  S: 5.0 G: 3.84

As I said in my OP, pedal worked fine up until I swapped some different FETs in and out which included the one which shows continuity between drain and gate. The pedal SEEMS to be working fine now as it is except for it now having no real distortion present, even when the gain knob is full. Would work great as a clean boost haha. Pedal has been looked over and sprayed and given a good cleaning already

Cheers,
Dan

[PRR]

I'm curious why *three* JFETs show a positive gate voltage (nominally all should be zero; maybe RF pickup?).

I'll leave this here, it may help someone smarter.

[antonis]

Leaky 2.2μF caps, perhaps..??

[duck_arse]

Leaky 2.2μF caps, perhaps..??

all those caps on Paul's circuit are backwards. well, with his voltages on, they are, but if the gates are supposed to be at 0V each, then the lower of the pairs will be backwards with (+) to the gates. C9, 18, 21.

[amz-fx]

Yes, all the electrolytic pairs in the C9/C11 position on each booster are backwards.

C26 does not show polarity but the negative side goes to the Volume pot.

regards, Jack

[aion]

Yes, all the electrolytic pairs in the C9/C11 position on each booster are backwards.

C26 does not show polarity but the negative side goes to the Volume pot.

regards, Jack

I did the trace on this one and can at least confirm that the electrolytics are oriented this way in the original CB30 pedal. Whether that's a design mistake is another question, but I would at least affirm that cap orientation is not causing the issue in this particular build.

[amz-fx]

The caps are still backwards even if your trace is accurate, which is a little odd. I looked at a bunch of other Catalinbread pedal schematics using similar circuits and they all seem to have the caps oriented properly. It might be that the assembler of the particular pedal that you traced was having a bad day, or the board was designed wrong, etc.

It is unlikely that the backwards caps are causing the problem but it should not be ruled out. The jfets would be the first components to look at.

The gates of Q5 and Q7 in particular, have too high voltage. The gates of all the lower jfets should be very close to 0v. I looked at the voltages that I measured on a Booster 2.5 (similar circuit) and all of the voltages that danmc_27 measured should be similar but are not. The source pins of the lower jfets seem to be too high. The bias voltages on the gates of the upper jfets should be around 4.3v (when Va is 9v) and not 3.8v.

The Booster 2.5 that I built was made with genuine jfets purchased from Mouser.

The first thing to do is to remelt some of the solder joints, especially on the transistor sockets.

Best regards, Jack