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Help! Synthrotek 308 kit trouble
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Help! Synthrotek 308 kit trouble
So I have been doing diy synth modules and guitar fx pedals for quite some time now and haven't had any issues. So I built the 308, which is probably one of the easiest modules i have done, but for the life of me I can't get it to work. I plug into it and get nothing. I have double and triple checked the polarity in my components and everything is right. Don't know if I have some blown ICs or what. If anyone has any suggestions, I would be grateful for any help I can get.
maybe post pictures of the front and back so we can spot any errors and/or compare with working builds
ok. sorry. here are the pictures

everything except the 1% resistors are from the synthrotek kit.
hopefully someone can help
The CA3080E IC is supposed to be an LM308N
Either op amp would work fine.

Thought the LM is what an actual Rat pedal would use, so for that sound you would want that chip. Did Synthrotek send you a CA? That seems odd, since this is a rat clone?

The input trimmer should be fully CW for modular level input, maybe that?

I can't tell how you have the diode switch set, but you should have only one set selected, ie switch down, or none at all, ie all up.
TheDegenerateElite wrote:
Either op amp would work fine.

No. A 3080 is not the same as a 308.
Yes, the CA came from synthrotek. Everything but the resistors are from the kit. I will try to get ahold of an LM chip. And right now none of the diode switches are down. Hopefully I can get it to work. Thanks for the help and advice. Please keep it coming. thumbs up
OK, lm308n has been ordered. Should be here in a few days. I will keep you posted.
Have you emailed Synthrotek to ask about the chip?

If you bought a full kit from them it should come with the right chip or they will send a replacement.
Yeah. Haven't heard back yet, but I just emailed them this afternoon.
Most faults with their DLY kits are traced back to the Vactrol being out of spec, so it might be worth asking about what the Vactrol is doing in this design.
I thought a 3080 was more $ than a 308? obviously the 3080 won't work in this module it needs a 308 but I think you scored a free 3080 (!) hoping its not fried by now.
It might be ok the the v+ v- pins are the same. There are a lot of cool things you can use the 3080 for. nanners
Yeah it's a couple of bucks more than a 308 I found. It's peanut butter jelly time! nanners Rockin' Banana!
Hmm. Still no response from synthrotek...... sad banana
got the 308 chip in the mail today. going to drop it in after work and hope for the best. thanks to everyone for all your help! thumbs up
Score!!! Module works!!! Thanks again guys!!! nanners Rockin' Banana! Guinness ftw!
Hi all,
Adding to this thread to say that posters solder work on that transistor is amazing.

Mostly the 2N3904 transistor on this (right near the printed word “synth” on the back side of the pcb) kit, with the three super close pads w/ almost no space between was so hard to solder. It was hard to touch one pin without remelting the pin next door. I did my sloppy best with pointy tip iron and wiping off flux between each pin, but the solder itself is about the width of the pad so it was miserable. However - I do not get a continuity beep between the solder lumps on each pad, so i am hoping that means it is ok. Youd never know it from looking at a pic or even magnified in person, though.

My finished 308 kit made sound for 20 mins or so distorted copy of the input, and the knobs had an effect, nice, but on the next day before doing calibration, the sound cut out after a minute (coincidentally after i dropped a heavy flashlight on the floor next to the test rack hmm), but I was thinking about those transistor solderings, and how to test if that transistor was really working. Like with power on, should i read a voltage between emitter and base, etc. I read -0.8v between emitter to base, positive probe on emitter.

I am trying to understand NPN transistors better, in between staring at this project. Thanks for reading!

Maybe the trick is to cut the transistor pins so short they barely poke through?

EDIT: Magic happened after i posted and there is sound now. I think despite trying everything and flipping amongst channels all the time, i have an unhappy mixer channel i never suspected. Really sorry very frustrating hate it when this happens.
Guys I revive this threads because I have finis assembling a 308, I can't get any sound out of it. Not even a buzz. I have double checked all polarities confirm the OP Amps are powered correctly. I don't see any issue with solder points...

Since Woodster said that the Vactrol might be an issue, I removed it and tested the anode and cathode with my meter. I got open circuit on both direction. I guess the vactrol is dead. I'm not sure if a dead Vactrol will make the module sound silent. Because the Vactrol is in the neg feedback loop of the LM308 in // with R and C at the end of the CV in chain. So a dead Vactrol should not really affect the audio ? (I have attached the drawing for reference)

Any thought ?
Seeking some help here.
I have done some troubleshooting tonight.

I have traced the Audio signal up to the pin 3 of the OP amp using an oscilloscope probe and an RCA adapter plugged into my mixer.

I could ear the sine wave up to the pin 3 of the 308, then at the output I noticed a very low buzzing sound.

So I decided to plug back the output of the module to my mixer, and I have turned the gain of the track until I could ear something.

With the gain @ 60 (FULL !!!) I can ear something but still very low !

I can ear that the 3 controls seams to act as they should, but again, that's a guess because I can barely ear the sound with the gain full.

Any thoughts ?
John64 wrote:
I could ear the sine wave up to the pin 3 of the 308, then at the output I noticed a very low buzzing sound.

Might have a blown LM308... try replacing that. Note- you can also use an OP07 as a replacement as well if you have one on hand.
I think I found the issue. The deep switch was at fault. I removed it and re soldered it, now it seems to work all fine.
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