# Help with CEM3340 **URL:** \ **Category:** KOSMO MODULES\ **Created:** [February 12, 2022, 4:21pm UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644 "2022-02-12T16:21:53Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![Pat.bayle](https://avatars.discourse-cdn.com/v4/letter/p/9dc877/32.png) [@Pat.bayle](https://lookmumnocomputer.discourse.group/u/Pat.bayle)\ **Post date:** [February 12, 2022, 4:21pm UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/1 "2022-02-12T16:21:53Z")
Hi not sure if this is the correct place to post about this topic Recently put together the stripboard layout for the cem3340 VCO and not having any luck. Not getting any sound from any of the outputs nor any wave forms when using my scope. And ive been following the troubleshoot guide ![3925C122-17C6-4B6B-869F-3BF48F6A4771](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/3X/d/6/d62fcfc80b37627ddf8c40bdd24373a57ee28f23.jpeg) Using the scribled version of the stripboard layout as reference all voltages are checking out other than pin 3 of cem3340 it says there should be -7.2v but im getting -1.5v after the 620ohm resistor and at pin 3. There is -12v before the 620r from the power suplly When scoping the saw,square and triangle at cem3340 im not getting any wave form nor at the out puts of the opamps If anyone could shed any light it would be much appreciated. My soldering isnt the greatest as The solder i was using at the start was pretty poor as i ran out of good stuff . I bought some better quality solder which definitely helped but at that point it was nearly built. Ive been staring at the solders for hours and i can see any bridging or anything that could cause issues ![20220212_143855](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/3X/b/0/b007b54ac3bada36c69ebe50e07949a9dcf4e97b.jpeg) ![20220212_142758](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/3X/4/0/40b0b35beb204194d6aae5243d5e575f714cfac8.jpeg) Thanks ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [February 12, 2022, 4:51pm UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/2 "2022-02-12T16:51:28Z")
It’s really hard to see on the solder points if there is something, and sometimes it’s not necessarily visible to the eye. Personally I check with a magnifying glass after each weld, and after I always pass a cutter blade or knife between all the lines of the stripboard (and after a little brush stroke). But I don’t know if the problem comes from there. ---
**Author:** ![analogoutput](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/analogoutput/32/29663_2.png) [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)\ **Post date:** [February 12, 2022, 5:03pm UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/4 "2022-02-12T17:03:14Z")
> [@Pat.bayle](#): > > Using the scribled version of the stripboard layout as reference all voltages are checking out other than pin 3 of cem3340 it says there should be -7.2v but im getting -1.5v after the 620ohm resistor and at pin 3. There is -12v before the 620r from the power suplly That’s definitely not good, and there’s very few possibilities. First, are you sure of those voltage measurements? They all should be measured between the point in question and ground. If so… Make sure the 620R resistor really is 620R and it’s really connected to pin 3. There’s also a 5.6k resistor and a jumper to the 10k trimmer at that pin and you should check those values and connections. (You may not be able to make a reliable resistance measurement in circuit but you can at least examine the color bands and see if they’re right.) See if the voltage on pin 3 changes when you change the trimmer setting. It seems to me either one of those components much be completely wrong, or there’s a bad connection on one of them, or a solder bridge but I see no sign of one (but definitely reflow the solder joints on the pin 3 strip), or the 3340 chip is defective or damaged. I mean, there’s so little that could be wrong. Inside the chip at pin 3 there is a Zener diode. Put -12 V on a resistor connected to it and you have to get about -7.2 V, unless there’s a short or low resistance path to ground that shouldn’t be there. ---
**Author:** ![SCGator](https://avatars.discourse-cdn.com/v4/letter/s/48db29/32.png) [@SCGator](https://lookmumnocomputer.discourse.group/u/SCGator)\ **Post date:** [February 13, 2022, 6:42am UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/5 "2022-02-13T06:42:36Z")
It looks like the resistor from pin 2 to 3 is 5.5/5.6 ohm. ---
**Author:** ![Pat.bayle](https://avatars.discourse-cdn.com/v4/letter/p/9dc877/32.png) [@Pat.bayle](https://lookmumnocomputer.discourse.group/u/Pat.bayle)\ **Post date:** [February 13, 2022, 11:01am UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/6 "2022-02-13T11:01:52Z")
Okay i just checked and it definitely is a 5.6ohm resistor. oops seems i accidentally bought the wrong values. i’ve checked the other resistors and they are correct. thanks for spotting that! ---
**Author:** ![Pat.bayle](https://avatars.discourse-cdn.com/v4/letter/p/9dc877/32.png) [@Pat.bayle](https://lookmumnocomputer.discourse.group/u/Pat.bayle)\ **Post date:** [February 13, 2022, 11:08am UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/7 "2022-02-13T11:08:59Z")
HI thank you for you in depth response. it was brought to my attention that i have used a 5.6ohm instead of a 5.6k between pin 2 and 3. i purchased the wrong values. my question is i don’t have any 5.6k resistors at hand are there any other values i could use in place? closest i got is 4.7k and could that resistor be the root cause for it not to work at all? Thanks ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [February 13, 2022, 11:30am UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/8 "2022-02-13T11:30:44Z")
> [@Pat.bayle](#): > > my question is i don’t have any 5.6k resistors maybe you can put several resistors in series to have it ---
**Author:** ![analogoutput](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/analogoutput/32/29663_2.png) [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)\ **Post date:** [February 13, 2022, 1:07pm UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/9 "2022-02-13T13:07:09Z")
> [@analogoutput](#): > > unless there’s a short or low resistance path to ground that shouldn’t be there. 5.6Ω is definitely low resistance! It doesn’t go straight to ground but still, I think it’s likely to be the cause of the wrong voltage. > [@Pat.bayle](#): > > my question is i don’t have any 5.6k resistors at hand are there any other values i could use in place? > > closest i got is 4.7k I don’t think the value’s critical, but if you have a 1k you could put in series with a 4.7k that’d probably work better. ---
**Author:** ![Pat.bayle](https://avatars.discourse-cdn.com/v4/letter/p/9dc877/32.png) [@Pat.bayle](https://lookmumnocomputer.discourse.group/u/Pat.bayle)\ **Post date:** [February 13, 2022, 8:51pm UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/10 "2022-02-13T20:51:40Z")
success! i put a 4k7 and a 1k in series measured the combined resistance and had 5.66kohms measured the voltage at pin 3 and found -7.2v. hooked up the jacks to the speakers a finally got some noise from all outputs! initially had some background noise which later found i wired the ground of the jacks incorrectly but ive now corrected that and is now working perfectly! so gratefull for everyone who went out there way to help especially SCGator for being keen eyed for spotting the 5.6ohm resistor. hats off! i was so close to giving up but that wasn’t an option considering how much i’ve all ready spent on this project so far haha lesson now learned always check for the correct values! ---
**Author:** ![handsomegenius](https://avatars.discourse-cdn.com/v4/letter/h/898d66/32.png) [@handsomegenius](https://lookmumnocomputer.discourse.group/u/handsomegenius)\ **Post date:** [February 14, 2022, 11:30pm UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/11 "2022-02-14T23:30:54Z")
> [@Pat.bayle](#): > > my question is i don’t have any 5.6k resistors at hand are there any other values i could use in place? Are there no shops nearby that have them? I mean, offline retail tends to be quite limited in what they offer, but ordinary resistors are just about the most generic component ---
**Author:** ![Pat.bayle](https://avatars.discourse-cdn.com/v4/letter/p/9dc877/32.png) [@Pat.bayle](https://lookmumnocomputer.discourse.group/u/Pat.bayle)\ **Post date:** [February 15, 2022, 10:54am UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/12 "2022-02-15T10:54:00Z")
Not any more. There used to be a local shop called maplins but they shut down. And online vendors want atleast £5 for postage so its not worth it even buying a pack of 100 unless you spend the required amount then its free p&p Ans I have good selection of common values already. I put a 4.7k and 1k in series and its working like a charm now. ---
**Author:** ![handsomegenius](https://avatars.discourse-cdn.com/v4/letter/h/898d66/32.png) [@handsomegenius](https://lookmumnocomputer.discourse.group/u/handsomegenius)\ **Post date:** [February 15, 2022, 11:13pm UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/13 "2022-02-15T23:13:27Z")
Wow… what a state of affairs. Brick and mortar shops around here all switched to consumer electronics ages ago, but there’s still a couple of shelves with resistors and caps. You’d think it would be economic for some business to carry them… like you say, there are times when the wait and the postage don’t make sense ---
**Author:** ![analogoutput](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/analogoutput/32/29663_2.png) [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)\ **Post date:** [February 15, 2022, 11:33pm UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/14 "2022-02-15T23:33:33Z")
I can remember when there were not only Radio Shack but Lafayette Electronics stores all over the place, both carrying large selections of parts, but those days are gone. There used to be a local electronics parts store in Syracuse, NY fairly near the University, but that closed about a decade ago. There is another electronics store outside the city I’ve been meaning to visit sometime, but currently I generally don’t go into places I don’t need to go into, so it’ll have to wait. Looks like either a disaster area or a wonderland based on the [pictures](https://goo.gl/maps/L2r4RZxvAUrwVBq3A). But that’s the only place in the area I’d be reasonably confident of having any basic electronics parts. ---
**Author:** ![eric](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@eric](https://lookmumnocomputer.discourse.group/u/eric)\ **Post date:** [February 16, 2022, 6:42am UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/15 "2022-02-16T06:42:13Z")
That reminds me of a (former) store called “L’electron Bayard” in Grenoble… I bet the tenants are asian and know exactly in which box to find that strange part you’re looking for ? ---
**Author:** ![jaradical](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/jaradical/32/12069_2.png) [@jaradical](https://lookmumnocomputer.discourse.group/u/jaradical)\ **Post date:** [February 16, 2022, 8:11am UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/16 "2022-02-16T08:11:20Z")
This place had most of the bits I needed to build a switching power supply! > **[OM Electronique](https://www.omelectronique.fr/)** > > Boutique propulsée par PrestaShop Cheers ---
**Author:** ![irregularshed](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/irregularshed/32/30374_2.png) [@irregularshed](https://lookmumnocomputer.discourse.group/u/irregularshed)\ **Post date:** [February 16, 2022, 10:48am UTC](https://lookmumnocomputer.discourse.group/t/help-with-cem3340/4644/17 "2022-02-16T10:48:14Z")
I’m lucky enough to have an RS trade counter a couple of miles away, so I can get things there. Until very recently their website had free next day shipping on everything - I used that a LOT, getting things like a pack of absurdly specific capacitors for under two quid, delivered - but there’s a minimum spend of £30 (+ VAT) now on certain lines (including packs of 10 5.6k resistors). _Edited because I discovered I could still get free shipping on some things 🙂_