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Help debugging RYO 3x VCA
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Help debugging RYO 3x VCA
tioJim
Afternoon (well it is here anyway)

Stumped on a failed build of an RYO 3x VCA. It won't calibrate and one channel behaves bizarrely and the other two can't even

Here's what I've done so far

1. Confirmed IC, diode and electrolytic cap orientations
2. Confirmed IC, diode, cap and resistor values with meter (as applicable) and visual inspection
3. Confirmed ICs are receiving expected voltages at V+, V- pins and confirmed continuity of ground pins
4. Confirmed continuity of IC sockets through to reverse of PCB
5. Replaced all ICs *
6. Checked trimmers aren't beyond rotational limits and that they have correct pin outs (this was an issue somebody encountered in another thread. The trimmers are 3296W compatible. Farnell #9353305, #9353178)

* Had only one spare LM13700 so changed one (waiting on new) but they are both showing correct voltages at supply pins and are running cool so not convinved they're bad

Things worth noting

1. On first power-up one IC became too hot to touch. I powered down and replaced that IC. I don't know if the IC was bad or if I had it reversed (unlikely). It's possible junk on my bench caused a short. This hasn't happened since. What else could this blow out have fritzed?
2. The bottom channel is spitting out negative voltages - high too, -10V
3. I can't get the first channel output above 1.5V for step 4 of calibration
4. Weirdly when in situ my 100n caps read 10u on my meter but they're definitely 100n - the numbers on them say so, those left in the bag measure 100n and Farnell's bag label says they're 100n and I don't think there's any such thing as a 10u ceramic dipped cap! I assume it's in situ/meter weirdness

Pics





Schematics here

BOM/calib procedure doc here

Build docs here

Any help appreciated!
Jim
flts
tioJim wrote:
4. Weirdly when in situ my 100n caps read 10u on my meter but they're definitely 100n - the numbers on them say so, those left in the bag measure 100n and Farnell's bag label says they're 100n and I don't think there's any such thing as a 10u ceramic dipped cap! I assume it's in situ/meter weirdness


Yeah, you can't really measure caps on PCB like that. I'm assuming the 100n caps are standard bypass ones next to IC power pins, going from power rails to ground. This means they will also have the big 10u reservoir electrolytics connected in parallel (the fact they're on the other side of the PCB physically doesn't really matter), so you are, in effect, measuring them as well.

So at least that's nothing to worry about.
u0421793
Probably not a problem, but check the solder joint on the inboard leg of R12 (looking up on the circuit diagram, pcb layout and component list you supplied, it's the 10K resistor dead in the middle, lining up alongside pin 7 of the LM324).
waveglider
I have had the exact same problem and it is caused by the trimmers not set correctly.
I emailed Andreas when I had the problem and he sent instructions on testing the circuit, which I have included below..
I had VR1 set too high and once that was trimmed properly, everything fell into place.




Quote:
Some measure points:
For VCA1 when there is 5V applied to the CV in there should be 0V at pin 8 of the LM324 and when there is 0V on the CV in there should be around -11V at pin 8.
For VCA3 it's pin 1 on the same LM324 that should behave the same way.

The 50k trimmers settings will affect this voltage. If it's possible to get the mentioned pins on LM324 to behave somewhat correctly then we will know it's not the CV circuit that's faulty.


Quote:

Is it possible to trim VR1 until it gives 0V on pin 8 of the LM324 instead of 5V when the CV in is 5V? You might need to retrim VR4 also until you get the correct voltage range on pin8 for CV 0-5V. Where 0V CV gives -11V on pin 8 and 5V CV gives 0V on pin 8.
search64
Don't forget the normalisation jumpers. They won't solve this, but they're nice smile
tioJim
waveglider wrote:
I have had the exact same problem and it is caused by the trimmers not set correctly.
I emailed Andreas when I had the problem and he sent instructions on testing the circuit, which I have included below..
I had VR1 set too high and once that was trimmed properly, everything fell into place.

Quote:
Some measure points:
For VCA1 when there is 5V applied to the CV in there should be 0V at pin 8 of the LM324 and when there is 0V on the CV in there should be around -11V at pin 8.
For VCA3 it's pin 1 on the same LM324 that should behave the same way.

The 50k trimmers settings will affect this voltage. If it's possible to get the mentioned pins on LM324 to behave somewhat correctly then we will know it's not the CV circuit that's faulty.


Quote:

Is it possible to trim VR1 until it gives 0V on pin 8 of the LM324 instead of 5V when the CV in is 5V? You might need to retrim VR4 also until you get the correct voltage range on pin8 for CV 0-5V. Where 0V CV gives -11V on pin 8 and 5V CV gives 0V on pin 8.


Thanks all

Waveglider, I can confirm the measure points in the first quote but when I calibrate I can't get beyond 1.5v for step 4 (of VCA1)

Tedious and a waste of time. Shouldn't be this difficult to calibrate a VCA :(
tioJim
Sigh, one for the bin then

No response from Thonk or RYO

Well one of the purposes of a forum is a searchable record. Whilst a nicely designed board that was easy to stuff and an effective budget front panel, in my experience the RYO 3x VCA was impossible to calibrate and there doesn't seem to be many resources available if things go wrong

Many thanks to those who have responded above
AlanP
For me, the first and second calibrated no problem.

The third needed a good deal of messing around, including sending the trimmers in random directions, until it all started to gel.
babblefrog
It took me a while to get mine right. It is possible to turn those trimmers way past the end of useful values. I ended up starting over by turning them a dozen full turns counter clockwise and then trying the whole calibration again.
mike.synthesizer
I'd start troubleshooting it by leaving out the OTA's (LM13700's.) With those out, the Op amps will be easy to characterize. Simply run an aligator clip from ground to the input of each op amp and the output should be zero for every op amp. Do you understand how input / feedback resistors work to change gain characteristics? Applying a small voltage to the inputs should help verify that your circuit is working properly.

Also, 25 turn cermet trimmers keep spinning even if you go all the way to the end of the track.
mike.synthesizer
Another thing, measuring 100n caps in circuit is almost impossible. Hand held multimeters are not very good for measuring caps. Most use a pulse discharge time constant method that doesn't scale very well. Good cap testers use a sine wave and compute phase / amplitude deviations.
Monobass
tioJim wrote:
No response from Thonk or RYO


Hi tioJim, maybe you can help me identify you? We are careful about not letting people go unanswered.

Steve
Thonk
tioJim
Ach, thanks all. Sorry to vent frustration. It's a fine balance isn't it with DIY? Obviously it shouldn't be as easy as just buying a module as what would be the fun in that? But the other end of the spectrum isn't ideal either smile

Steve at Thonk. Thanks, yes you've always been prompt/responsive in the past. I sent the email at 1AM 15th Feb. I'll resend it, though everything's here really.

mike.synthesizer, thanks, I'll pop out the LM13700s and get the meter on the op-amps.
Monobass
Ah yeah you were still on my to do list, sorry I haven't had a chance to reply sooner, i'm going through all my mail today.

Will keep an eye on this thread and poke me again if you get to a dead end!
charonme
was it resolved?
mxzr
Hi there I'm struggling with the 3x VCA as well. The bottom VCA is working fine, I was able to callibrate properly. However, VCA top and middle are giving me - 9.23V on the output, regardless which trimmer I turn. Any ideas? Thanks in advance!
mawoka
Hi,

i also had some trouble with the RYO 3xVCA module. Maybe my solution might help someone, if not, feel free to laugh about my stupidity :-)

So putting the module together was fine, i checked the voltages on each Ic (was fine) , put 5V on CV and IN , oscilloscope to out, but didnt get any voltage near to what it should be. I realized, that the opamps did get warm.

it took me a while until I realized that I put the frontpanel the wrong way (upside down) on the module ...... so what i REALLY did, was putting 5V on OUT and IN , and tried to measure the voltage on CV ... very frustrating very frustrating

Even with the Panel the wrong way mounted you can mount the module in the rack, so i didnt realized my error.

The good thing is, the module survived this procedure without harm , and i was able to do the calibration fine, as soon as i only put voltages on inputs :-)

Nice Module !
tds
Hi mawoka

Quote:
i also had some trouble with the RYO 3xVCA module. Maybe my solution might help someone, if not, feel free to laugh about my stupidity :-)


I want to thank you for this insight - you just saved me hours of troubleshooting...
mawoka
tds wrote:
Hi mawoka

Quote:
i also had some trouble with the RYO 3xVCA module. Maybe my solution might help someone, if not, feel free to laugh about my stupidity :-)


I want to thank you for this insight - you just saved me hours of troubleshooting...


happy to help :-)
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