VCF-1 Troubleshooting Help

chumfree · Joined: Apr 28, 2015 Posts: 1 Location: Southampton

Hello everyone, I'm really hoping you will be able to help me out here. I've been lurking on these forums for a little while and I am always amazed at the support that you guys give. I'd be very grateful if you could lend me a hand too!

I always wanted to build a synth since I started playing them and I saw Thomas Henry's designs for the VCO-1, VCA-1 and VCF-1 on birthofasynth.com and thought they seemed like perfect places to begin into the world of DIY. I began with the VCO-1, breadboarded it with no problems and soldered it to veroboard with no hiccups at all. With my confidence at a high I then started on the VCF-1.

I put it on breadboard and it seemed to work fine with the tests that I did, only a small issue with the resonance pot not doing much but I decided that soldering would sort out any blips, which I assumed were caused by the quick'n'dirty layout I did. I made a plan for component placement using DIYLC, which I have attached, and began putting it all together.

I've now constructed the VCF and it is not working nearly as well as it was before. The pots for Coarse and Fine control do nothing at all (or at least nothing perceptible) and the Resonance control has no effect until it is rotated fully to the left which then causes the audio to cut out. Plugging my speaker into the low-pass and band-pass sockets result in no audio whatsoever and the high-pass allows signal through but no filtering seems to be happening. At least the Attenuation pot is working...

I'm really at a loss here as I've checked over the component placements, checked their values, made sure track cuts are all present where they need to be and used an IC checker to verify that the chips are all working too. I now don't know where to look. I replaced the 2SA798s with two 2N3906s, would that be an issue?

Find attached my veroboard layout plan - I've built exactly according to this so what you see is what I have. The red squares with circles in them represent track cuts. The grounds, positive and negative connections for the pots and jacks were left out to keep looking clean - but I have them connected.

This is where I got the schematics from http://www.birthofasynth.com/Thomas_Henry/Pages/VCF-1.html

I'm sorry if this is a long post but I'm really stuck!

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blue hell · Posted: Wed Apr 29, 2015 4:58 pm Post subject:

Hmm .. no replies eh Shocked

chumfree

I can only offer some moral support I'm afraid .. there must be an error in your stripboard layout .. likely at least ...

It is not fun but the only way to find it is check and recheck Confused

Some folks here are really good at spotting errors in layouts .. but they seem to be busy beeping Laughing

Anyway. hope you will find it.

elmegil · Posted: Wed Apr 29, 2015 6:08 pm Post subject:

So I don't know what might be wrong with your layout (I do intend to look further) but first and foremost: all those bypass caps, while they are at the upper left of the schematic, are actually intended to be as close as reasonably possible to the power pins of the various chips. They are segregated in the schematic mostly as a matter of clarity. Some folks draw them near the power pins of the various components, others don't. Thomas is one who doesn't, generally.

elmegil · Posted: Wed Apr 29, 2015 6:49 pm Post subject:

So I've gone over the layout vs the schematic, and it looks right to me, electrically.

Do note that Q1 & Q2 should be matched, in thermal contact, and in thermal contaact with R11, for the temperature compensation in the exponential converter to be effective. It does not look like you did that -- however this should not cause any functional problems with your build aside from a tendency to drift in tuning.

Next steps would be to hook it up, measure power and ground pins to make sure they're actually at the right values (measure on the pin, not at the PCB), and then just look carefully at everything, make sure you have the right values, etc.

I'm *assuming* that all your pots have the correct ground or +/-15V on them, and that all your jacks are similarly grounded. There are no connections from your PCB that are called out as "panel power" but that could be coming from the same place as the three wires for power into the layout.

prgdeltablues · Posted: Thu Apr 30, 2015 5:27 am Post subject:

Another tip is to calculate the approximate resistance you'd expect to see between V+ and Ground, V- and Ground, and V+ and V-. (good practice in resistor network thinking!). Then, with the power off of course, measure the actual resistances between the power pins. If the actual isn't close to your estimate, you have a problem. For example, on sheet 2, there are two 100k pots wired between V+ and V-, so (if there is nothing else - I haven't checked the whole schematic) you'd expect 50k between V+ and V-.

Peter

Thomas_Henry · Posted: Thu Apr 30, 2015 11:20 pm Post subject:

Hi gang,

Yes, indeed, the decoupling caps should be sprinkled about as Elmegil reminds us. If you look at the schematic again, you'll note I state that the capacitors should be located as close as possible to the chips.

But, with regard to the problem: consider temporarily disconnecting the exponential stuff and replacing it with 27k resistors to the Ic control inputs of the OTAs, both driven by a single 100k pot strung between -15V and +15V. The manual response will be clunky, but you should be able to hear the sweep over the entire range. If so, then you know the problem lies in the expo stuff and have narrowed things down.

Hope this helps,

Thomas Henry