Thomas Henry LM VCO problem

MrZu · Joined: Mar 23, 2017 Posts: 10 Location: Taiga

Hi everyone!

Has anyone any experience with the Thomas Henry LM VCO?

I breadboarded one after purchasing a copy of An Analogue Synthesizer for the 21st Century.

I double checked all soldering connections, ground and power as well. Everything seems proper. But it won't work, just silence. Confused

I couldn’t find LF444, so I replaced it with TL074, I also used 2N5457 instead of the MPF102. Maybe the LF444 and MPF102 is critical for the layout?

I also noticed that if I touch pins 5 and 7 of the LM13700 I can hear a signal out - for sine out is a sine, for ramp out is a ramp … etc… Fine and Coarse control pots do nothing.

Is anyone familiar with this, or has any idea why I'm getting these results?

Thanks in advance!

elmegil · Posted: Thu Mar 23, 2017 10:13 pm Post subject:

What's your C3?

Polystyrenes are VERY easy to melt. Sounds vaguely like you have no (or insufficient) capacitance until you add your own with your fingers.

Edit:

As for the pots, see whether you get any voltage change at IC2a pin1 when you adjust those pots. If not, you may have a hook-up problem. If you do, perhaps the problem is the 2SA798. Those are fairly rare, you may have gotten a bogus one?

MrZu · Joined: Mar 23, 2017 Posts: 10 Location: Taiga

Hi!

elmegil · Posted: Fri Mar 24, 2017 5:08 am Post subject:

Ceramic caps can be ok, but you need good quality and stability there, so a C0G type. Classic "disc" type ceramic caps wouldn't be a good choice.

You should measure from ground to pin 1.

A matched pair of PNP transistors would work in place of the 2SA798. Hand matched or something like the SSM2220.

MrZu · Joined: Mar 23, 2017 Posts: 10 Location: Taiga

elmegil · Posted: Fri Mar 24, 2017 8:13 am Post subject:

What's your supply voltage?

If I assume 12V, then that sounds like the op amp is stuck at the + rail.

It also sounds like the wiper voltage is changing? (measure before R34/R52, not after, the other side is supposed to be a virtual ground courtesy of the op amp, so you'll see no voltage change there).

That really sounds like something wrong with the op amp itself if your wiring/soldering is ok. In this particular case I would not consider the LF444 to be critical, a TL074 should work just fine.

Are you sure the tempco is in correctly? I could see a problem with your feedback there leading to this kind of thing possibly.

Edit: and don't worry if you don't have a tempco, or don't have another to try. A regular resistor there should work too, it's just that you won't have temp stability.

MrZu · Joined: Mar 23, 2017 Posts: 10 Location: Taiga

elmegil · Posted: Fri Mar 24, 2017 9:37 am Post subject:

MrZu · Joined: Mar 23, 2017 Posts: 10 Location: Taiga

elmegil · Posted: Sat Mar 25, 2017 5:49 am Post subject:

MrZu · Joined: Mar 23, 2017 Posts: 10 Location: Taiga

Here is the layout Rolling Eyes

THLMVCO.pdf

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MrZu · Joined: Mar 23, 2017 Posts: 10 Location: Taiga

And the PCB... it is without ICs, Q1/Q2 and Q5 Confused

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elmegil · Posted: Sat Mar 25, 2017 8:27 am Post subject:

I found 2 problems with your layout, neither of which I'd expect to have anything to do with the issue you've reported Smile

1) you reversed the designators for R7 & R45. I think you have the right values rather than reversing them in your build, but there's not enough resolution on the top-side photo to be sure.

2) The sine round trimmer, R37, is supposed to have the wiper connected to one of the two sides of the trimmer. The symbol with the cross arrow indicates a trimmer that is wired as a rheostat, and it is common to tie the side and the wiper together on one side or the other. I don't think it matters a whole lot which side, it just determines which direction the adjust goes CW vs CCW.

Even if you swapped R7 & R45, neither of these should prevent all outputs.

I think the next thing I'd do is lift one side of R1 and see if adjusting the pots *then* will make that voltage go up and down. That completely disconnects the input section from the expo converter. If the problem is with the input then you still won't get any change, if the problem is with the expo converter or downstream it will start working at pin 1.

elmegil · Posted: Sat Mar 25, 2017 8:30 am Post subject:

Also, you should note that the voltage on pin 1 is not expected to go up and down VERY MUCH. If you have a low resolution meter, it could be that it's been going up and down, but you didn't notice.

The gain on IC2a is 2K / 100K for the coarse adjustment and 2K / 3.3M for the fine adjustment, so a swing of 30V on the coarse is only going to swing about half a volt on pin 1 from all the way up to all the way down.