# MS-20 filter stripboard troubleshooting **URL:** \ **Category:** DIY STUFF\ **Created:** [May 15, 2021, 2:59pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509 "2021-05-15T14:59:44Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Sonosus](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sonosus/32/13679_2.png) [@Sonosus](https://lookmumnocomputer.discourse.group/u/Sonosus)\ **Post date:** [May 15, 2021, 2:59pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/1 "2021-05-15T14:59:44Z")
Hi all, I’ve built Sam’s MS-20 filter (layout 1.0, without the lo/hi pass switch) on stripboard, and I get nothing on the output - I get some very very quiet unfiltered wave if I turn up the gain on my mixer to maximum, but the cutoff pot does nothing. I have tried: - Checking power on all 3 rails - ok - Checking for shorts between power rails - ok - Swapping out the TL074 and LM13700 - Checking the cutoff pot - swings between ± 12v at the 100k resistor. - Measuring the current between pins 1 and 16 of the LM13700 - Problem! As @fredrik says here: > [@LMNC MS20 Filter Low Output](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/32): > > So after [a local oops](https://lookmumnocomputer.discourse.group/t/my-build-this-weekend/345/163) smiley that required a bit of debugging, it struck me that a quick way to check the CV converter is to measure the bias current (aka “Iabc”): set your multimeter to microamperes (µA) if it supports that (otherwise mA might work, if it can measure with at least 2 decimals). power off the board, unplug CV in, and set the cut-off potentiometer to somewhere in the middle carefully remove the 13700 from its socket power up the board, and measure the current between pin 1 (an… There should be some current flowing, but I have none I think there must be an issue with the CV converter. I tried reflowing all the joints around the transistors but no luck. When I move the cutoff pot, the voltage on the other side of the 100k resistors swings between +0.2 and -0.2v. Is this an issue? Should this voltage be larger/smaller? Are there any other points that I should check? Bear in mind I have next to no knowledge of how an OTA works(they convert voltage to current?) or how the CV converter works, and only a vague understanding of filters themselves(thank Moritz Klein!). Power is coming from 2 (admittedly noisy) DC wallwarts but I doubt they are the problem as all my other modules work fine. I unplugged everything apart from the VCO and the filter to test it. Here are some pics. ![IMG_20210515_145330](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/7/7d403e900aec4ae4f2c7a72bfb0e5420e667639a.jpeg) ![IMG_20210515_145129](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/6/6113d0a2d5a800a83e5f3f44c0a822c41dd712cd.jpeg) Would scope readings be helpful? I have a cheap dso138 that can be forced into service. Any help is much appreciated. Cheers! ---
**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:** [May 15, 2021, 3:08pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/2 "2021-05-15T15:08:19Z")
> [@Sonosus](#): > > - Measuring the current between pins 1 and 16 of the LM13700 - Problem! I don’t think there should be current between pins 1 and 16. @fredrik says > [@LMNC MS20 Filter Low Output](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/32): > > - power up the board, and measure the current between pin 1 (and/or 16) and ground, while sweeping the cut-off pot between the end points ---
**Author:** ![Sonosus](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sonosus/32/13679_2.png) [@Sonosus](https://lookmumnocomputer.discourse.group/u/Sonosus)\ **Post date:** [May 15, 2021, 3:14pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/3 "2021-05-15T15:14:35Z")
D’oh! I’ll check again ---
**Author:** ![Sonosus](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sonosus/32/13679_2.png) [@Sonosus](https://lookmumnocomputer.discourse.group/u/Sonosus)\ **Post date:** [May 15, 2021, 3:16pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/4 "2021-05-15T15:16:18Z")
Ok, still no current between pins 1/16 and ground. ---
**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:** [May 15, 2021, 3:21pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/5 "2021-05-15T15:21:37Z")
The bias input current pins (1, 16) are usually around two diode drops above your negative rail. So something like -10V - is that the case with our stripboard? Max current into those pins should be something like 1mA - so you should be able to measure at most 10V across the 10k resistors that feed into them. Do you see any voltages across those? In my experience it’s frustratingly easy to make the LM13700 go pop by feeding too much into the Iabc bias current pins. ---
**Author:** ![Sonosus](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sonosus/32/13679_2.png) [@Sonosus](https://lookmumnocomputer.discourse.group/u/Sonosus)\ **Post date:** [May 15, 2021, 3:31pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/6 "2021-05-15T15:31:16Z")
Thanks for replying 🙂 There isn’t any voltage across the 10k resistors. Is there a simple test circuit I can use to make sure the 13700 isn’t fried? My instinct is ‘no go build a VCA’ but I could be wrong. Edit: There is about 0.02v across the resistor when I squeeze the probe cables with my hand but I think that’s just noise. ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [May 15, 2021, 3:44pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/7 "2021-05-15T15:44:22Z")
If you’re not getting any Iabc current, the problem is the CV circuit, not the LM13700. It won’t let any signal through if there’s no control current. I’d start by checking that the transistors are mounted the right way. (that, or the current is too low for your multimeter.) ---
**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:** [May 15, 2021, 3:45pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/8 "2021-05-15T15:45:15Z")
Per @fredrik, “You should see currents between ~0 and 60-80 µA”. So that’s a maximum drop of 0.8 V. > [@Sonosus](#): > > Is there a simple test circuit I can use to make sure the 13700 isn’t fried? My instinct is ‘no go build a VCA’ but I could be wrong. Well, you could go build a VCA as a simple test circuit: ![image](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/7/7826925e53acbf167170573c9de4ecc04aa7d047.png) (from the [datasheet](https://www.ti.com/lit/gpn/lm13700)). Or rather, build two of those, one for each half of the LM13700. ---
**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:** [May 15, 2021, 3:51pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/9 "2021-05-15T15:51:29Z")
If those are BC558s they’re mounted the right way. ![image](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/d/d05e7529a0240d807bbae6dcbf00b2fd5dd6cef3.png) ![image](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/e/e2bae7cb47d87da43ece4e232a874f14ad92c81f.jpeg) ---
**Author:** ![Sonosus](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sonosus/32/13679_2.png) [@Sonosus](https://lookmumnocomputer.discourse.group/u/Sonosus)\ **Post date:** [May 15, 2021, 3:53pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/10 "2021-05-15T15:53:19Z")
Yep, the transistors are definitely the right way round Edit: oops, just saw @analogoutput 's post ---
**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:** [May 15, 2021, 3:54pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/11 "2021-05-15T15:54:30Z")
~~You could try building the simple VCA from the lm13700 data sheet:~~ @analogoutput beat me to it But… if there’s nothing across the resistors, the next thing to check is the transistors that do the CV to current conversion. The more I look at the [stripboard](https://lookmumnocomputer.discourse.group/t/lmnc-ms20-filter-low-output/660/2) in the original post the more confused I get about that part of the circuit though so I’m afraid I haven’t much to suggest. Looks like a kind of current mirror but it’s confusing. ---
**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:** [May 15, 2021, 4:02pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/12 "2021-05-15T16:02:09Z")
Here’s the schematic if it helps ![image](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/4/44a9f873d1e530f93aa70a398f5fcb164caaa4f9.png) ---
**Author:** ![Sonosus](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sonosus/32/13679_2.png) [@Sonosus](https://lookmumnocomputer.discourse.group/u/Sonosus)\ **Post date:** [May 15, 2021, 4:20pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/13 "2021-05-15T16:20:50Z")
I just tried replacing the transistors - no luck ☹ Next i’ll replace some of the resistors - i have a pile from the 90s which have been reliable so far but you never know… ---
**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:** [May 15, 2021, 4:35pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/14 "2021-05-15T16:35:37Z")
Ha - cool, thanks. Makes a lot more sense now, I was missing the opamp driving the common emitters… So, it’s an exponential converter, right? The negative feedback from the op-amp should be forcing the collector of the transistor connected to 470k to zero. So, with a -12V rail there should be something like 25µA through that 470k when all the CV inputs are 0. Then, if I’m thinking about it correctly, we should get 25µA out of the other transistor into the two 10k resistors - so each one should get something like 12.5µA - and hence 125mV of drop across the 10k resistors. Stripboard layouts always make me go cross-eyed. ---
**Author:** ![Sonosus](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sonosus/32/13679_2.png) [@Sonosus](https://lookmumnocomputer.discourse.group/u/Sonosus)\ **Post date:** [May 15, 2021, 4:38pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/15 "2021-05-15T16:38:57Z")
What 470k resistor? Do you mean the 4.7k? edit: slaps face ![Untitled](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/a/a0c1dde302ebe4354b70adc1a287899736b14ee7.png) ---
**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:** [May 15, 2021, 4:40pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/16 "2021-05-15T16:40:11Z")
![image](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/c/c31a3474e60006dc77466be2187d72d675a0e117.png) That one. You’ve got that one, right? ---
**Author:** ![Sonosus](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sonosus/32/13679_2.png) [@Sonosus](https://lookmumnocomputer.discourse.group/u/Sonosus)\ **Post date:** [May 15, 2021, 4:40pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/17 "2021-05-15T16:40:55Z")
Warming up the iron now… ---
**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:** [May 15, 2021, 4:40pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/18 "2021-05-15T16:40:59Z")
Heh - it’s kind of important that one 😃 ---
**Author:** ![Sonosus](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sonosus/32/13679_2.png) [@Sonosus](https://lookmumnocomputer.discourse.group/u/Sonosus)\ **Post date:** [May 15, 2021, 4:42pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/19 "2021-05-15T16:42:27Z")
I even crossed it off the stripboard layout for some reason 🙃 ---
**Author:** ![Sonosus](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sonosus/32/13679_2.png) [@Sonosus](https://lookmumnocomputer.discourse.group/u/Sonosus)\ **Post date:** [May 15, 2021, 5:05pm UTC](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509/20 "2021-05-15T17:05:34Z")
I replaced the resistor and now on the output all I get is switching PSU noise, and no sign of the original output. Time to build a decent power supply I guess. [Next page](https://lookmumnocomputer.discourse.group/t/ms-20-filter-stripboard-troubleshooting/3509.md?page=2)