GCI Apostle - Troubleshooting

[pangeadestructor]

I can't make sense of this one, I built it a while back and it didn't work, gave up after a while. The other night I came across it and wanted to give it another go, but still can't figure this out. There's no output, audio probe gives me signal at r1/r2, nothing further into the circuit.

Voltage at the jfet test points wouldn't get up to the 16-18v with the trimmers maxed. Thought my charge pump IC might have been bad, so I pulled it out and replaced with a socket, dropped in a new LT1054. It seemed to fix the problem and I had range on the jfets from 0 up to ~35v, but by the time I finished biasing the last one, q1 had dropped way down. Q1 btw currently has the same voltage at all 3 legs same as the test point, 15.667. This is the voltage that appears at all 4 transistor test points now.

If I unplug the power supply, give it some time and plug it back in, it appears to fix the issue and the jfet voltages are back up to the correct range, but if I keep my DMM probe at the test point for q1, I can see it slowly dropping by about 0.001 per second. Pics attached, including the IC voltages.

Any and all help appreciated, I'm determined to get this thing up and running.

[duck_arse]

what does the circuit diagram look like? any sign of build docs?

[idy]

[pangeadestructor]

My bad, I thought I attached the schematic page, added it to the attachments!

[idy]

R3 really is 10M? No short to the gate of Q1? Because the signal dies before it gets to the gate...

The JFETs are reputable source? Tested? Correct pinouts? 5457?

If you have more of those Qs might be worthwhile breadboarding that first stage with one....see if it amplifies...

[antonis]

Off topic, but..

OK with R2 68k "grid" stopper resistor but what's the purpose of R1..??
(or R3 in case of R1 is a Gate bias resistor..??)

[pangeadestructor]

R3 really is 10M? No short to the gate of Q1? Because the signal dies before it gets to the gate...

I'm not getting a short at q1 gate, r3 is indeed 10M - i have a ground at one end of r3, and continuity with q1 gate at the other.

The JFETs are reputable source? Tested? Correct pinouts? 5457?

yes, they're either from Mouser or guitarpcb, forget which but definitely reputable source. q1 is a Fairchild, just looked at it under magnification.

If you have more of those Qs might be worthwhile breadboarding that first stage with one....see if it amplifies...
[/quote] I can prolly try later tonight

[PRR]

> i have a ground at one end of r3, and continuity with q1 gate at the other.

But is it 63k from input jack to Q1 gate? (Don't get lost down side-paths.)

[pangeadestructor]

> i have a ground at one end of r3, and continuity with q1 gate at the other.

But is it 63k from input jack to Q1 gate? (Don't get lost down side-paths.)

63.5k from input to q1 gate.

[PRR]

....audio probe gives me signal at r1/r2, nothing further into the circuit. .....

> 63.5k from input to q1 gate.

Hard to reconcile "signal at r1/r2" with an OK 68K and no short at Q1 gate yet "nothing further into the circuit. ....."

[pangeadestructor]

Agree, it's got me scratching my head. Just checked again and same thing, no audio at r3 or q1. I feel like it's a power supply issue due to the strange dropping jfet voltages.

[idy]

We know those feelings... but no signal at the gate is definitely not a power supply problem. Even unplugged you should find a signal at the gate of Q1.

[pangeadestructor]

I just noticed that c19 appears to be backwards, will be fixing that when i get home from work though I can't see how that would have any impact at q1.

[pangeadestructor]

Turned c19 around and obviously no change, audio signal goes into r2 and doesn't come out. BTW, I've also replaced r2 a while back and it didn't help anything.

Wondering if there's a bad trace or something going on, if I should pull r2 out at one end and connect it directly to r3 with some solder, though I doubt that would change anything now that I'm thinking about it. I have a connection between that other end of r2 and r3, just no signal passing through r2 at all after entering it.

[idy]

No, trust me, pulling things out on vague suspicions is hard on the PCB. Maybe I've spared you my little "That guy on your shoulder that tells you to pull random components out is not the one with the halo and wings."

Better to test for continuity, and try to figure out what is wrong. The signal does not even seem to reach the gate of Q1.

You could try to use one of those transistors, with their pinouts, to breadboard a stage like the one in this circuit. This is guaranteed to be educational and may help trouble shoot the board. Just an idea.

Q1 btw currently has the same voltage at all 3 legs same as the test point, 15.667. This is the voltage that appears at all 4 transistor test points now.

So that sounds like all three legs of the transistor are shorted out. What says your meter? Power off, measure R between different legs of Q1. There will be six possible measurements. I don't know which ones are important....

[pangeadestructor]

Q1 btw currently has the same voltage at all 3 legs same as the test point, 15.667. This is the voltage that appears at all 4 transistor test points now.

So that sounds like all three legs of the transistor are shorted out. What says your meter? Power off, measure R between different legs of Q1. There will be six possible measurements. I don't know which ones are important....

I'm getting the following:

D-G 5.6R
D-S 244.7
G-D 5.4R
G-S 244.9
S-G 251.7
S-D 251.3

[idy]

All those numbers are ohms, not Kohms?

I am no FET wizard, but I'm pretty sure in some directions you should see no conductance. At my house it works that way.

What about your extra transistors? How does one of them test?
D and S should be low R both directions.
You should see like a diode between G and the other legs. Red on Gate, about 1v vf. Black on gate nada.

[pangeadestructor]

Correct, those are just regular ohms.

I checked another 5457 that I have (this one from central semi) that has different measurements, not sure how much of that is due to being in vs out of the circuit:

D-G 343.9
D-S 0
G-D ~360 but moving (hard to get a reading here for some reason)
G-S 0
S-G 8M
S-D 8M

[idy]

So.
What do you assume the pinout of your 5457s?

Mine are gate to the right, facing the writing.

You seem to be seeing the same R from G to D in...in both directions.

Your numbers would make sense, I think, if you re-arranged the naming of the legs.
Let's see: instead of D G S, let's call them "D", "G" and "S."
"G"  and  "D" are the same low R in both directions, Oui?
Just like a DS channel...
"S" shows conductance like a diode in two directions, towards "G" and "D".
Just like a gate to the other two, non?

[pangeadestructor]

 Crap, I labeled them wrong.


With the flat side facing me, it's D-S-G. So based on my notes, the readings should be:

jFet in circuit:

D-S 5.6R
D-G 244.7
S-D 5.4R
S-G 244.9
G-S 251.7
G-D 251.3

jFet Tester:
D-S 343.9
D-G 0
S-D ~360 but moving (hard to get a reading here for some reason)
S-S 0
G-S 8M
G-D 8M