1550 LFO Problems

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valis
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1550 LFO Problems

Post by valis »

Greetings and thanks as always for taking questions about 30 year old products.

I bought a Stringz and Thingz that wasn't working quite right and I've run into a little trouble trying to get it to function as it should. Everything works okay now except for the LFO does not seem to have an effect on the audio (i.e. no vibrato effect). When I first got the 1550 the Rate LED just stayed on and there was no vibrato effect. Upon inspection I determined that the builder has installed a 566 (IC15) backwards. I installed a new chip which now makes the LED blink with regards to the rate knob position, but I'm still not getting any vibrato effect.

So in my reasoning, I figure the problem has to lay in the IC15 and IC16 portion of the schematic. Pin 8 of IC15 and IC16 each gives the correct voltage (about 11v) and pin 1 is properly attached to ground on both chips. The problem is that I'm not seeing any wave shape out of pin 3 or 4 of either IC15 or IC16. The 3rd pin of IC15 is making the LED blink but I'm not seeing the square wave on my 'scope. In contrast, I get a nice square and triangle out of IC13 pins 3 and 4.

Furthermore, I'm getting what I think is the correct variable voltage measurements out of the Rate knob (R58) and test point "S."

What really confuses me though is that Pin 5 (the Modulator Input) of IC15 and IC16 gives me about 11v. Pin 5 of IC13 also gives me about 11v. In looking at the 566 datasheet it appears that the Modulation Input responds to only 3 - 5.5v. So that leaves me to think that maybe it's getting hit with too much CV. But looking at the schematic, R127/R126 and R122/R121 (the timing resistors) are connected to the +V so that voltage seems intended.

I can measure that the side of R128 and R123 that connects to the Rate knob gives me a variable reading (0v-10.8v), but when I measure the other side of the resistor, it sits very close to +11v and varies by only a fraction of a volt when the Rate is changed.

So I've been looking for a short, or maybe faulty wiring but I haven't found anything. Maybe I'm barking up the wrong tree. Another brain to process this would be helpful.

Thanks as always!
David
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PAiA-Scott
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Re: 1550 LFO Problems

Post by PAiA-Scott »

Maybe "seeing" the waves on the scope is a matter of the probe coupling being set for DC and them being off the screen for the selected v/div. They come out of the part on a dc offset and not centered about ground.

You mention seeing them on IC13, but since it has a negative supply on pin 1, the offset output would be closer to 0 V DC.

In any case, since waves are occurring at IC13, and likely IC14, the no-chorusing trouble is probably after the sections with the 556 VCO ICs.

The pulsing from ICs 13 and 14 goes to IC12 which serves to provide the complementary clocking signals to the delay ICs. If the 0-12V pulsing is on pins 1/2 and 12/13 of IC12, then maybe it's just a matter of tweaking the Bias trim R66 so the audio passes through the delay ICs. There is only a small area in the adjustment range of this trim that the dc bias of the audio sent to the delay ICs is in the appx 2/3 V+ range for them to pass/delay the audio for the chorusing effect.

The dc on the middle, wiper terminal of the trim should vary with adjustment of the trimmer disk. Same at the output of IC10, pin4.
Scott Lee
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valis
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Re: 1550 LFO Problems

Post by valis »

oh....my...goodness

Thank you so much Scott. You were right. It turned out that the bias controlled by R66 wasn't properly calibrated. With a little tweaking I got it going in no time. It all seems so simple now!

I can't even begin to thank you. This project had been on my bench for ever. I really used some brain power on this one. As always, you learn more in troubleshooting than building.

Now I can finish up the PSU on my 4700 and FINALLY get everything playing together. I'll post a pic when I get it all up.

Whew. I feel like a weight has been lifted with this one!
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Re: 1550 LFO Problems

Post by PAiA-Scott »

Way to Go! You are a weight lifter, hence your profile image???
Scott Lee
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valis
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Re: 1550 LFO Problems

Post by valis »

Haha

Thanks Scott! :D
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