Fixing my Sansamp Classic, help appreciated

[PenPen]

Hey there everyone. It's been years since I've posted around here, I won't bore everyone with the long story but I took an extended break from building circuits and focused on being in a band. Unfortunately, I'm in a position now where I have to come back and revive my circuit skills again to fix my Sansamp Classic, which just suddenly decided to stop working just before a show.

I own a old-issue SA Classic and it's a wonderful dirt box, however now it's suddenly acting up with little to no bypass signal (intermittent) and when engaged spews out a high noise somewhat clean signal with some gating. To me it sounds like it's misbiased, to give you an idea. I'm almost sure that the buffer FET is bad, or something in the biasing of it has gone awry.

1. Does anyone have an actual true schematic of this thing? The one floating around by "unknown japanese guy" isn't really correct, and I did find a cleaned up version from Gila Crisis (is he still around?) but I noted he doesn't list where VCC came from on his version.

2. Can anyone give me voltage readings on the pins of the FET? I'm getting 8.53, 8.57, 8.63 on the FET which really does not seem right to me. I haven't looked up the pinout for this K117 GR 2G FET yet to be certain of the pin out, but those readings do not seem right.

If anyone has any kind of info that might help that would be great.

[Earthscum]

2SK117GR is D-G-S pinout. Ironically, Tayda has them in stock. I would guess that, being a buffer, the Source should be somewhere around 5V if it's half biased.

Is there a protection doide from Gate? if so, you may pull a leg of it first to see if it re-biases correctly. If so, the diode is bad.

Does the Gate pulldown resistor connect to half V+? If so, check the voltage of the half V+ to make sure it is where it should be. If it's not, a smoothing cap may be bad.

If neither of those, then I would guess the Fet is bad.

Hope that helps!

[PenPen]

Thank you! There are protection diodes on the Gate, two back to back, and I think they are okay as it appears the voltage in the middle of them is 0v which indicates that they are doing their jobs, though I will triple verify that. The 1m pulldown to Vb will need to be verified but I recall in my checking last night that it was constant with the voltage at the Gate. I haven't been able to actually locate where Vb is on the board yet, this thing is has a very overly complicated layout and I'm still trying to sort out what is what, for instance I simply cannot find an input cap at all when tracing from the jack to the Gate, which makes no sense because there is no DC leakage at the jack. Then again, trying to take this thing apart and getting an accurate look at the path is difficult. I might just resort to taking photos of top and bottom then lining it up in image software.

Thanks for the tip on Tayda, it's been so long since I've ordered parts that I don't even know if the old suppliers are even around anymore. Though I do still have some JFETs in my parts bin from the old days to see if I already have a replacement, though I think I might only have J201's left. I haven't built a pedal in about 5 years now.


EDIT: Ok, I took the pedal completely apart to verify my measurements and indeed, there's 1.24v in the middle of the two protection diodes, so one of them must be bad. Also, Vb is indeed right at 5v. Looks like I need to figure out what kind of diode these are and replace them. I'll report if that fixes it, but that might be the problem! I'm still baffled at the lack of input cap, though. I took some pics of the front/back of the input/buffer board to figure out later. I was thinking of doing a full photo shoot of the entire thing but the main effect board is completely covered in opaque plastic on both sides and it's not possible to remove it without desoldering parts. I need this thing back together by Thursday's show so no complete tear down unfortunately.

[Earthscum]

You may try lifting the diodes out and testing the fet's voltages first. If everything settles back to around "normal", then you should be able to replace both diodes with 1N914/1N4148's. Just remember they are reversed (or should be) in that the negative end (with the stripe) should be pointing towards the positive voltage.

Good luck!

[PenPen]

Well, I got around to working on this tonight. Did as you said, just lifted the leg of one of the diodes, confirmed no voltage in the middle as before, but the biasing didn't correct.

I figured I'd go ahead and get my replacement diodes ready to just swap in some new ones, and rechecked the schem. These are 1N755 zener diodes, which explains why there was voltage in the middle of them. It's supposed have voltage there I suppose.

Best I can tell the FET needs to be replacing, after going through my parts I don't have the 2SK117 FET, but I do have a ton of J201. I'm sure it would be fine but just to confirm, you could technically swap the J201 in there, right? Only thing else I have are a bunch of MOSFETs and regular transistors.

EDIT: Nevermind, as per http://www.diystompboxes.com/smfforum/index.php?topic=67651.0 I should be able to swap that in there. Trying it out.

EDIT2: Okay, I'm close to just pulling my hair out. I noted the different pinout on the J201, put it in, and now I'm getting 2.5v at gate, 8.5v at drain, and 0v at source. Now getting 0v at the middle of the diodes so they aren't even turning on to reverse the flow as before. Is it possible these weird diodes have some part in the biasing? I looked at ordering replacement 1n755 zeners to go ahead and swap those out also, but only Futurlec has them and if I recall they have minimum order requirements, or they take forever to ship, can't remember now.

Thanks for the help, it's very much appreciated. At least I do have it narrowed down to the FET/diodes. I'm looking at ordering some 2SK117s to swap in as well, if I can find a place other than Futurlec that has those diodes.