Chop Shop Troubleshooting

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jrhevron

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HI all... I build a pedal or two a year, so I'm by no means an expert, but not a total noob.

I just put together a Chop Shop/Barbershop drive. I populated the board with components from smallbear and reused a power jack and in/out jacks from an old build. Got it all set... and no signal... signal on passthrough/when footswitch off. Pots do nothing and trimmers show no voltage and don't change anything.

I went through and probed and audio signal seems to be going all the way through, just no effect. I am having a voltage issue. I get 9V at R100 but no voltage at the trimmers/pads or at R11 or Q3.

I've reflowed and took the switch out just to make sure that that isn't an issue.

Any ideas on what to do? Clearly seems like a voltage issue... something is killing it after R100, I guess?
 
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jrhevron said:
I have reflowed a few times... also had a pack of 5 smd jfts from pedalpcb. I went in and replaced the first one. Is it possible to have bad SMD JFTs? I mean, I know that anything is possible.
Click to expand...
It's very possible to torch SMD jfets, especially if reworking.
Do you have any on adapter boards? Or through hole ones?
Might be worth a try. Though, Identical V could also be a solder bridge/short on the jfet or elsewhere
 
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jwin615 said:
It's very possible to torch SMD jfets, especially if reworking.
Do you have any on adapter boards? Or through hole ones?
Might be worth a try. Though, Identical V could also be a solder bridge/short on the jfet or elsewhere
Click to expand...
I don’t have adapter boards, but I do have 2 jfts left. Can I use cut off component ends to make them through hole?
 
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jrhevron said:
Yes.
Click to expand...
Thank you.

Your FET voltages indicate shorted or near shorted FETs, perhaps from overheating when soldering or you received a bad batch. They should be something around:
D = 8 VDC
G= 4 VDC (mid-way between D & S, for symmetrical clipping)
S = 0,6 VDC (all voltages depend on the bias trim setting).

When the gate is close in value to the drain or he source, the device is acting like a switch instead of like an amplifier. This sometimes occurs when the gate's reference to ground (R3 & 'Drive' = R6) are the wrong values, or for designs with a fixed 'Vref' - a problem with the voltage divider (not in this design), or the FET has some type of short going on (which I think may be the case here).

In the future, I suggest buying PedalPCB's adapters and mounting them to the PCB using the T092 mounts. It is a safer, more controlled way of applying surface mount parts (SMD) with a soldering iron. SMD parts were intended to be loaded on a tape with a dab of adhesive, loaded into a robot machine that picks the part and installs it to the location, and when all the parts are loaded, the whole PCB gets a quick microwave soldering. They were not made for direct heat. I've personally burned a few.

Also remember that FETS and MOSFETs are very sensitive to static and again, were not designed to be handled by human hands. Grounding your body with a wrist strap to ground, or touching something which is grounded to earth while handling them is great insurance.
 
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andrewsrea said:
Thank you.

Your FET voltages indicate shorted or near shorted FETs, perhaps from overheating when soldering or you received a bad batch. They should be something around:
D = 8 VDC
G= 4 VDC (mid-way between D & S, for symmetrical clipping)
S = 0,6 VDC (all voltages depend on the bias trim setting).

When the gate is close in value to the drain or he source, the device is acting like a switch instead of like an amplifier. This sometimes occurs when the gate's reference to ground (R3 & 'Drive' = R6) are the wrong values, or for designs with a fixed 'Vref' - a problem with the voltage divider (not in this design), or the FET has some type of short going on (which I think may be the case here).

In the future, I suggest buying PedalPCB's adapters and mounting them to the PCB using the T092 mounts. It is a safer, more controlled way of applying surface mount parts (SMD) with a soldering iron. SMD parts were intended to be loaded on a tape with a dab of adhesive, loaded into a robot machine that picks the part and installs it to the location, and when all the parts are loaded, the whole PCB gets a quick microwave soldering. They were not made for direct heat. I've personally burned a few.

Also remember that FETS and MOSFETs are very sensitive to static and again, were not designed to be handled by human hands. Grounding your body with a wrist strap to ground, or touching something which is grounded to earth while handling them is great insurance.
Click to expand...
Thank you for this description. You make good points and it's likely my error. I'm going to just harvest whatever parts I can but start with a new pcb and jfts from pedalpcb on breakout boards. Every now and then I've had one of these kinds of builds. As frustrating as it is, it's very instructive and I've actually learned a lot from it.
 
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Ended up getting a new board and new board mounted jfts... it fired up first time. Must have been a jft issue the first time-- I must have overheated them or something.

I just biased the JFTs to about 6V and it's sounding good.
 
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