Multiplex Jr Modulation not working

[neil463]

Have the 1776 Multiplex Jr board.  Delay is working fine, and I hear a slight modulation when it's engaged.   I made one only one modification - I added a DPDT footswitch along w/ the SPDT so that you can turn the modulation on or off with the regular switch or foot switch.  I removed the modification and still experienced the issue.

This is the documentation from 1776: http://1776effects.com/wp-content/uploads/2015/03/Multiplex-Jr-BOM.pdf

Here are the voltage readings I've taken (IC# taken from the above doc)

IC1
1 4.24
2. 4.24
3. 4.25
4. 30-40 mV (fluctuating)
5. 4.27
6. 4.27
7. 4.25
8. 8.48

IC2 - This IC had fluctuating values.  The notation I'm using below indicates fluctuating between value although the numbers aren't exact. The fluctuation generally isn't slow, it's like it's bumping around ~1 or 2 v at a time
1. 5.24/3.5 V
2. 5 / 4
3. 4 / 3
4. 48 mV
5. 5.3 /3.3
6. 5 / 4
7. 8.5 / 0
8. 8.5

IC3
1. 2.55
2. 2.55
3. 2.55
4. 2.55
5. 2.55
6. 2.55
7. 2.55
8. 2.55
9. 5.06
10. 2.55
11. 22.2 mV
12. 22.2 mV
13. 2.93
14. 2.54
15. .675 V
16. .711 V

Reg
5.06 V
22.2 mV
8.5 V

[gtr2]

If you bridge the SW1 pads do you have the same effect?
Does the LED light?
If not double check the LED polarity.
if not that check R20 etc and the depth pot for any weak solder joints.

Your voltages look good assuming that the PT2399 was just read as pin 9 being pin 1...
IC2 is the oscillator for the modulation, hence the voltage swing.

Josh

[neil463]

If you bridge the SW1 pads do you have the same effect?
Does the LED light?
If not double check the LED polarity.
if not that check R20 etc and the depth pot for any weak solder joints.

Your voltages look good assuming that the PT2399 was just read as pin 9 being pin 1...
IC2 is the oscillator for the modulation, hence the voltage swing.

Josh

Thanks for the reply.  I will bridge the pads tonight and report back.

The LED does light up at a rate that changes based on the speed pot.  If I turn the mod off (with either switch) the LED also turns off.

Sent from my Pixel 2 using Tapatalk

[gtr2]

Oh, well that is clearer then and good news.

Something is amiss from the vactrol onward.  Check every spot from the vactrol to the delay time pot by following the schematic.  Check your value of R20 too, as if you have an incorrect higher value there it will reduce the modulation.  Use your continuity setting on your DMM to make sure you have continuity where you should.  It's likely something simple as there really is not much between the indicator LED and the delay pot.

You don't need to worry about jumpering the switch pins.  That was before you mentioned it was blinking in time with the speed pot.

Josh

[neil463]

Oh, well that is clearer then and good news.

Something is amiss from the vactrol onward.  Check every spot from the vactrol to the delay time pot by following the schematic.  Check your value of R20 too, as if you have an incorrect higher value there it will reduce the modulation.  Use your continuity setting on your DMM to make sure you have continuity where you should.  It's likely something simple as there really is not much between the indicator LED and the delay pot.

You don't need to worry about jumpering the switch pins.  That was before you mentioned it was blinking in time with the speed pot.

Josh

Last night I replaced the Vactrol and R20 - I wasn't happy with the Vactrol soldering and the leg broke off when I tried to re attach.  When I removed R20 and tested w my DMM it showed no resistance so I replaced with a new 180k.  The modulation is still a bit weak but it has def improved.  I may further replace R20 w a lower value-added haven't decided.

Thanks for your help.

Sent from my Pixel 2 using Tapatalk

[gtr2]

The modulation shouldn't be weak at all so there is definitely a problem somewhere.  Make sure that the vactrol polarity is correct and/or that the LDR an LED side of the vactrol is correct.

If you were measuring the resistor in the circuit it won't give you a proper reading.  The chance of an open circuit resistor is slim to none unless it gave up smoke.  Reflow the applicable solder joints.  If you work methodically using the schematic you should find where you are losing signal.  One check you can do is with R20 out you can measure the Ldr side legs of the vactrol and you should have a varying resistance when the modulation is on.  Once you confirm that move onward.

Reflow the depth pot as well.  Maybe a bad joint is causing the weak signal.

[neil463]

Thanks for taking the time to reply.

I'll have to validate the vactrol again - maybe I have it wrong.  I'm traveling so it will be a couple days.

I measured the previous resistor outside of the circuit that measured as 0.  At this point I've reflowed most of the solder joints as well as replaced off board wiring to the pots (I didn't board mount them).  When I previously troubleshot this I had a bit of a heavy hand w/ the soldering iron so perhaps I've damaged the board.

Once I get back from my travel I'll try to follow the schematic (not particularly experienced but now's a good time to start).   Thanks again for your replies.