Troubleshooting Rullywow Fo-Sho - PCB from OSHpark

[Toy Sun]

Hi all,
I've had some email exchanges with Ben from Rullywow, but unable to get this to work. I've now built 3x of them on the OSH park PCB and breadboarded one, too. All have the same issues...


Very low output
pot doesn't "crackle" when turned, more of a swooshy/wooshy sound.
No output unless I reverse the BS170 in the socket (from what is indicated on the PCB)

Breadboarded one - same issues.

Really checked everything, measured all the resistors, checked capacitor and diode polarity. I tried to check the transistors (not a technique I know well) and they seemed to check out ok, but not confident about that.

I have a bag of BS170s I got from Barry at GuitarPCB.com I've tried a bunch, no luck. Is there a chance they are ALL bad?



https://rullywow.com/fo-sho-simple-diy-booster/

Thanks
John

[antonis]

Very low output.. No output unless I reverse the BS170 in the socket (from what is indicated on the PCB)

BS170 pin-out is D-G-S so you can check on PCB pins connections to respective items..
(e.g. Drain to C2, R3 & R4, etc..)

As for low output, does BOOST pot setting alter output amplitude..??
(if not, middle lug (wiper) might not be shorted to lug1/GND..)

P.S.
If all the above are OK, lift one leg of R4 and check for volume increase..

[Toy Sun]

Thanks for the quick reply...

The boost pot does "alter" the output amplitude, but not by much.
I had already checked the wiper to lug 1 continuity, it's ok.
I'll try the R4 lift and see what happens.

John

[antonis]

On a second glance (first was without enough coffee..) you better insert a 22μF or so cap between BOOST pot wiper and GND..
(as it is, it alters gain together with bias..)

[duck_arse]

can you please post a photo of the pcb, preferrably without components? there is something screwy about the etching artwork shown in their build doc.

have you checked the datasheet for BS170 re. pinouts, and compared to component placements on the board?

[Toy Sun]

Hi,
I think that you (duck_arse - wow, I feel bad calling you that) are on to something. This PCB may not be verified, though I believe that others on this board have used it successfully

Attached are images of both sides of the PCB and a datasheet for a Fairchild BS170 https://dtsheet.com/doc/280924/fairchild-bs170 - I'm not sure who made the ones I have.


I'll take to the blank PCB with a VOM and ensure that it's all correct. Just have to wait for work to be done.

Thanks
John

[rockola]

This PCB may not be verified, though I believe that others on this board have used it successfully

I've built one, works just fine.

[davent]

I've built sho's, my own pcb using 2N7000 or BS170, the pinouts are reversed, when you say you get audio with the bs170 reversed does that means it's working or sound but still screwed up.

Like mentioned, good clear pictures of both side of the board for all to peruse should sort it in no time.  Is it boxed or still out of the enclosure, tested out before boxing?
dave

[Toy Sun]

Great to have all this engagement - Rockola, I knew someone had built one, that's good to see.
I posted a ton of pics, hope they help. I think that we can establish that the PCB is OK, based on Rockola's work.
I suspect the BS 170s or maybe the diode? I included the bag from Tayda. The diodes I have are larger than the pitch on the PCB, so I had to float it above.

2 versions shown. One is with the pot soldered in place, the other with leads tacked on so I could try different pots. (tried a few different values, though I didn't have them in reverse taper) - no luck.

Thanks all, this is an awesome community.

[rockola]

1N4728 is a 3V3 zener. You need a 9V1 zener (1N4739).

[Toy Sun]

Ok, gotta talk to Ben about fixing his BOM.
But hey, now I know more about diodes!

Going down to check my inventory and maybe fire up the iron - I'll report back.

Thanks so much,

John

[Marcos - Munky]

The build doc doesn't say 1N4728-1N4758 as per your image, but just 9.1V zener. Anyway.

This zener is to protect the mosfet. You can use a 9.1V zener, a 12V zener or even a red led, as used on ROG Peppermill.

[duck_arse]

Hi,
I think that you (duck_arse - wow, I feel bad calling you that) are on to something. This PCB may not be verified, though I believe that others on this board have used it successfully

Attached are images of both sides of the PCB and a datasheet for a Fairchild BS170 https://dtsheet.com/doc/280924/fairchild-bs170 - I'm not sure who made the ones I have.


I'll take to the blank PCB with a VOM and ensure that it's all correct. Just have to wait for work to be done.

Thanks
John

it's odd the reaction some people have to the name I chose. anyway, sorry, looking at the build docs, the pcb pattern is shown as viewed x-ray from the top, without any mention of the need to reverse it to etch your own, so I thought that might have been the problem. which would have explained the mosfet reversing business.

always check your datasheet first - you get no points for trying it both ways. you can just snip that 3V3 zener to test for working goodness, as the circuit does not rely on it to function. it is only there to protect the mosfet from, well, things. if the board works without it, you can replace w/ the correct at your leisure.

[davent]

The build doc doesn't say 1N4728-1N4758 as per your image, but just 9.1V zener. Anyway.

This zener is to protect the mosfet. You can use a 9.1V zener, a 12V zener or even a red led, as used on ROG Peppermill.

Three of the 3v3's in series, you have lots to use up.

[Toy Sun]

Ok, totally embarrassed here....

Reading the threads, I'm saying to myself, who could have added wrong info to the build doc? Oh no, are there two versions out there?
Well, thought about it for a minute and realized that I was the one who added the wrong info...

I researched a 9.1 zener and the source I checked might have been bad, or maybe I transposed a line or something....

Anyway, snipped it and now it works. That also solved the transistor pin orientation "issue", of course. I'll try the 3x 9.1v in series, too.
I also made a list of all of my diodes and will study some data sheets as penance.

Thanks to all for the help, really appreciate it.

[Marcos - Munky]

Nothing to be embarraseed. If you solved the issue and learned something along the way, it's a win-win.

Just put in a 3mm red led and call it a day. And don't need to study datasheets as penance, you just need to read some datasheets when you're replacing parts for others where you're not really 100% sure about it.

[iainpunk]

I also made a list of all of my diodes and will study some data sheets as penance.

read some application notes instead, they are more usefull for learning.

cheers, Iain