Help me troubleshoot DJ Thomas White Lopass Gate

From circuitbending to homebrew stompboxes & synths, keep the DIY spirit alive!

Moderators: Joe., lisa, luketeaford, Kent

User avatar
wendallsan
Wiggling with Experience
Posts: 342
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Tue Mar 17, 2020 9:48 pm

Hi all,

I have the 1st of 4 LPG boards installed in my DJ Thomas White Quad LPG module and have so far managed to blow up the TL084 that was installed backwards on first plug-in. I've now got a replacement in the socket and it hasn't exploded, so progress has already been made! I am not getting any audio out of the module, though, so there is still some troubleshooting to do yet.

I have reflowed all the solder points on the board, and per my last debugging session, I was told I should always do a pass at the opamps on a problematic board and make sure they have power going to them. In this case, things look good on that front, in that there is 11.83V going to pin 4 of both IC's on the board and -11.75V going to pin 11 of each IC, which is what the data sheets are telling me they want to see.

I THINK have the wiring and jumpers on the board set up properly, but would not be too surprised to learn I have something wrong with one or the other of those-- this offboard wiring is tough stuff!

Here are the voltage readings on the IC's:
TL084:

1 0
2 0
3 0
4 11.83
5 0
6 0
7 -7.58
8 0
9 0
10 0
11 -11.76
12 0
13 0
14 0

TL074:
1 0
2 0
3 0
4 11.83
5 0
6 0
7 0.01
8 0
9 0
10 0
11 -11.76
12 0
13 0
14 0

Here are pics of the board:
Front
Back
Wiring
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

Synthiq
Veteran Wiggler
Posts: 514
Joined: Mon Feb 06, 2017 3:10 pm
Location: California

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Wed Mar 18, 2020 3:44 pm

In the comment below I will assume the schematic for your version 3.3 of the pcb is similar to version 3.2 of the schematic found in the zip file here:
https://naturalrhythmmusic.com/lopass
Without any input signals, the voltages on the two ICs looks reasonable. The output on pin 7 of the TL084 will depend on the offset potentiometer and should always be negative.

If you see no audio output signal, it can depend on either on a failure in the audio path or the control of the current to the vactrols. To check the current to the vactrols, set the offset potentiometer to the maximum value which should result in the most negative voltage on pin 7 of the TL084. If the vactrols are turned on, you should see a voltage drop across the 470 ohm resistor R6 due to the current in the vactrol diodes. I think this resistor is the one closest to pin 7 but you have to check.

If you apply an audio signal and measure the ac voltage with your dmm, you should see the same amplitude on pin 2 & 3 on the TL084 and the same or twice the amplitude on pin 1 depending on the mode switch. If it works well so far, you should then see roughly the same ac voltage on pin 9 and 10 of the same IC as on pin 1. There should also be an signal on pin 8 but the amplitude will most likely depend on the setting of the resonance potentiometer.

User avatar
emmaker
Veteran Wiggler
Posts: 511
Joined: Sat Mar 10, 2012 5:07 pm
Location: PDX

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by emmaker » Wed Mar 18, 2020 4:14 pm

Don't know what board rev you have but you might check this out (top post with the PCB bottom). This is a good example of why you shouldn't do plane fill unless you know what you are doing.

LPG PCB Issue.

Good luck.
Jay S.

Synthiq
Veteran Wiggler
Posts: 514
Joined: Mon Feb 06, 2017 3:10 pm
Location: California

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Wed Mar 18, 2020 4:58 pm

emmaker wrote:
Wed Mar 18, 2020 4:14 pm
Don't know what board rev you have but you might check this out (top post with the PCB bottom). This is a good example of why you shouldn't do plane fill unless you know what you are doing.

LPG PCB Issue.

Good luck.
Jay S.
This issue was with version 3.2 of the pcb but was supposed to be fixed in version 3.3 used here. I first connect all grounds manually with traces and check they are all connected before adding ground planes. Maybe that's why I have not had this problem.

User avatar
wendallsan
Wiggling with Experience
Posts: 342
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Wed Mar 18, 2020 8:11 pm

Synthiq wrote:
Wed Mar 18, 2020 3:44 pm
If you see no audio output signal, it can depend on either on a failure in the audio path or the control of the current to the vactrols. To check the current to the vactrols, set the offset potentiometer to the maximum value which should result in the most negative voltage on pin 7 of the TL084. If the vactrols are turned on, you should see a voltage drop across the 470 ohm resistor R6 due to the current in the vactrol diodes. I think this resistor is the one closest to pin 7 but you have to check.

If you apply an audio signal and measure the ac voltage with your dmm, you should see the same amplitude on pin 2 & 3 on the TL084 and the same or twice the amplitude on pin 1 depending on the mode switch. If it works well so far, you should then see roughly the same ac voltage on pin 9 and 10 of the same IC as on pin 1. There should also be an signal on pin 8 but the amplitude will most likely depend on the setting of the resonance potentiometer.
Hi and thanks very much for the assistance!

If I measure the voltage on the 470 ohm resistor and adjust the OFFSET pot, the voltage does go up and down at that point, between 0 and about -7V. It sounds like if I'm seeing values change there that vactrols might be working.

I do indeed see the input audio signal at pins 1, 2, 3 9, and 10 on the TL084. The signal is also on pin 8, but adjusting the RESONANCE pot doesn't seem to adjust its amplitude as it sounds like you expect.

Hopefully this narrows things down a bit, please let me know if you have other questions about what the circuit is doing and I'll try my best to get those questions answered!
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

Synthiq
Veteran Wiggler
Posts: 514
Joined: Mon Feb 06, 2017 3:10 pm
Location: California

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Wed Mar 18, 2020 9:43 pm

Pin 8 on the TL084 drives the output from the module, so if you see a signal at the IC but not the output jack, the problem seems to be in the wiring between them. Plug a patch cord into the output jack and measure the resistance between the cable tip and pin 8 of the TL084. You should see 1kohm.

User avatar
wendallsan
Wiggling with Experience
Posts: 342
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Wed Mar 18, 2020 10:43 pm

Hi and thanks again, I checked the resistance between those point and was able to trace back a loose wiring joint going to the output jack. I am now getting audio through the module, and am able to adjust the amplitude of it via the OFFSET knob! :sb:

The amp setting definately sounds like a clean VCA effect while the filter setting has a nice lowpass filter effect as I would expect from such a circuit. Very nice sounding!

I am not getting any response when feeding a signal into the CV input with the CV attenuator cranked to full left (or right for that matter, in case I got my pot wires crossed or something), and the resonance knob doesn't seem to adjust the resonant peak of the filter any.

Still, its good to hear audio coming out! :party:

I'll attempt to troubleshoot the CV control portion of the module tomorrow and see what I can track down, otherwise feel free to spitball ideas.

Many thanks again for your help, it is good to hear those vactrols in action!
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

User avatar
wendallsan
Wiggling with Experience
Posts: 342
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Thu Mar 19, 2020 10:19 am

Alright, I took a look at the CV input bits this morning and here are my findings:

I had the wrong lug soldered to ground on the CV input jack, according to pic I found in another thread (at the bottom of this page: viewtopic.php?f=17&t=49842&start=175).

I desoldered it and connected it to the other lug, but am still not getting any different results on the module. When I plug a signal into the CV input and then connect my scope up to cable with one lead grounded, I can't press the other lead to any of the 3 lugs on the CV jack and see the input signal. Pretty sure I have a wiring problem still. :hmm:

Here is what is hooked up with the offboard wiring at the moment around the CV input and pot, maybe someone can identify what is wrong here:

CV input jack has the 'ground lug' (the one not sticking off the bottom of the jack, but is instead stick out off the side of the jack near the top by the panel) connected to the same lug on the audio input jack next it it, which is then connected to the bottom-most lug on the CV level pot, which then goes to ground.

The lug furthest from the ground lug on the CV input jack is connected to the topmost solder lug on on the CV level pot. The lug nearest to the ground lug on the CV input jack has nothing connected, and previously had the CV input pot connected to it rather than the other lug, and that wasn't doing anything.

The middle lug of the CV input pot is connected to the 1V/Oct direct input on the board.

Does this sound right? I THINK that this is what I'm seeing on the photo in the post I linked to above, but I've already been wrong once ... :doh:
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

Synthiq
Veteran Wiggler
Posts: 514
Joined: Mon Feb 06, 2017 3:10 pm
Location: California

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Thu Mar 19, 2020 1:44 pm

wendallsan wrote: When I plug a signal into the CV input and then connect my scope up to cable with one lead grounded, I can't press the other lead to any of the 3 lugs on the CV jack and see the input signal. Pretty sure I have a wiring problem still.
Unless you have a battery operated scope that is not grounded you have to make sure it is the negative oscilloscope lead you connect to ground, not just _a_ lead, or you will not see the correct result. Again, if you have a wiring problem, you should plug in a patch chord into the CV jack and measure the resistance from tip to the top of the potentiometer and make sure you see less than 1ohm.

It sounds right to connect the CV input to the top of the potentiometer and and then the wiper to the 1V/OCT direct input on the pcb. I assume the audio jack goes directly to the AUDIO direct input. I did notice something that looks wrong in your pictures: the red wire from the output jack correctly goes the the output on the pcb, but there is also a red wire going to one of the 3 audio mixer inputs which doesn't make sense to me so please check that again. And I can't stop wondering why the components for the audio and CV mixer are even there on the pcb since the outputs don't seem to be used anywhere. If you have some free time, you can remove the TL074 and see if you see a difference.

User avatar
wendallsan
Wiggling with Experience
Posts: 342
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Thu Mar 19, 2020 3:35 pm

Hi and thanks again for the assistance. I tested the resistance between the input jack, through the CV pot and to the 1V/oct pad on the board and there is no resistance when I have the pot cranked fully clockwise, and then resistance builds to 100k as I adjust it to the left. As it seemed like something was indeed getting through to the 1V/OCT input on the board, I decided to test things out again, and found that the CV input and attenuator are both working, but the pot is 'backwards', so no CV comes through when it is turned fully Cwise rather than when it is fully CCwise. I'll look up which wires I've crossed and take care of that.

The last mystery to unravel on this board is that while the filter modes are definitely filtering, there is no change in the resonance when adjusting the RESONANCE pot. :hmm:
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

Synthiq
Veteran Wiggler
Posts: 514
Joined: Mon Feb 06, 2017 3:10 pm
Location: California

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Thu Mar 19, 2020 5:27 pm

wendallsan wrote:
Thu Mar 19, 2020 3:35 pm
The last mystery to unravel on this board is that while the filter modes are definitely filtering, there is no change in the resonance when adjusting the RESONANCE pot. :hmm:
I think the resonance circuit is only active in only one of the 3 modes so make sure to set the switch to frequency. I simulated the circuit with 3 resistances (1M, 100k & 10k) in the vactrols and 5 gains between 1 and 2 in the resonance circuit and see quite a bit of resonance at higher gain settings. You need to click on the figure to see the details.
LPG AC-sim.gif
You do not have the required permissions to view the files attached to this post.

User avatar
wendallsan
Wiggling with Experience
Posts: 342
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Thu Mar 19, 2020 11:59 pm

You 'simulated' it?

You, sir/madam, humble me. :hail:

I reread the build notes and looked over the boards again to find that R18 wasn't installed on the board, which is part of the resonance mod. I also double-checked the jumpers on the board, as they are involved in enabling the various mods.

I'm betting as soon as I can sit down with a solder iron and a 50k resistor, that will get that resonance knob doing something! :sb:

A couple of posts ago, you had asked about the mixer and the connection on the output jack to the mixer input. That is supposedly for the "Mix Out" on the panel. There is a 3 channel(?) mixer circuit on each board that is not connected at all to the LPG circuit, but there are some ways to use it to get the 'mix out' for multiple LPG's, and of course other purposes if you want to use the mixer just a mixer, ect. It's a weird design, imo. I guess its meant to be a really flexible board, but its probably wasted on me. :omg:

I'm really curious what this resonance mod will sound like! I'll let you all know what comes of my adding that last resistor. Once again, thanks for getting me to this point where I can whittle away at the last odds and ends, and hopefully re-use all these tricks as I get the other 3 boards built and working in this QUAD MODULE. :love:
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

Synthiq
Veteran Wiggler
Posts: 514
Joined: Mon Feb 06, 2017 3:10 pm
Location: California

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Fri Mar 20, 2020 11:04 am

wendallsan wrote:You 'simulated' it?
I think quite a few people on this forum uses a circuit simulator to test new circuits without actually building a real circuit on a breadboard. I use a free program called LTSpice where I create a schematic of the circuit I want to test and it calculates the response to an input signal(, or just the circuit itself in the case of an oscillator.)
wendallsan wrote:I'm betting as soon as I can sit down with a solder iron and a 50k resistor, that will get that resonance knob doing something!
Me, too. Without the fixed 50kohm resistor the gain will be 1 no matter what the pot setting is.

The reason I mentioned the audio and CV mixers was because they don't seem to be used in your case so you could save some time and components when you build the other 3 boards if you are not going to use them. And you would save some power as well, if you care about that.

User avatar
wendallsan
Wiggling with Experience
Posts: 342
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Fri Mar 20, 2020 8:07 pm

Still no resonance! :deadbanana:

Hooking up my scope, I can SEE the resonance knob affecting the output of pin 14 of TL084, and can trace that to one of the RESONANCE pot connections. The other 2 pins (including the wiper/middle pin) is showing what I'm guessing is the 'non-resonant' signal, or maybe its the VCA output. It looks like the same signal but stays consistent as the RESONANCE pot is adjusted. The wiper on the pot seems to always output the the signal that is on the pin that doesn't change when the RESONANCE pot is adjusted. I tried swapping the wiring on pins 1 and 3 of the pot to see if I had them backwards or something, but that didn't seem to change the behavior.

Do I have a wiring problem going on, or maybe a bad pot? I'm guessing I should see the signal at the wiper crossfade between the two signals on the other 2 pins or something like that? :hmm:
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

Synthiq
Veteran Wiggler
Posts: 514
Joined: Mon Feb 06, 2017 3:10 pm
Location: California

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Sat Mar 21, 2020 12:28 am

If you see the amplitude on pin 14 double when you change the resistance in the resonance potentiometer from 0 to 50kohm, then I think that part of the circuit is correct. To get a resonance, the feedback via capacitor C8 must also be there, so check that you have a low resistance between pin 14 and the capacitor. The latter is connected to S5 in the mode switch which may be easier to access.

User avatar
wendallsan
Wiggling with Experience
Posts: 342
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Sat Mar 21, 2020 11:24 am

Thanks for the continued assistance!

I can trace a connection between the TL082 Pin 14 the C8 capacitor through S5 and S6 when the switch is set to "FILTER.Testing the resistance between these two points shows no resistance, so that much seems good. Here is the result of the trace-- the green line is what I was able to trace out with no resistance between any point along the line.

Image

So far so good, I think, what else can I check here?
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

Synthiq
Veteran Wiggler
Posts: 514
Joined: Mon Feb 06, 2017 3:10 pm
Location: California

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Sat Mar 21, 2020 12:45 pm

The trace you did looks correct so that's good. Next I would measure the C8 capacitance just to make sure it's 4.7nF and make sure there is a good contact between the other side of C8 and the 220pF capacitor next to it. Another way to test it would be to select a different mode where S5 is disconnected from S6 and then connect an external signal to S5 and see if it comes out at the out jack.

You haven't mentioned what type of input signal you are using, but to see the effect of the resonance pot it's also important to have an input signal that has frequency components at the resonance frequency so a sawtooth is better than a sinewave.

User avatar
wackelpeter
Wiggling with Experience
Posts: 457
Joined: Fri Apr 17, 2015 5:56 pm
Location: Germany, MD

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wackelpeter » Sat Mar 21, 2020 2:26 pm

without looking too much into the posts, the resonance works only in the VCF mode, in VCA no resonance and in combined (VCA/VCF) setting, assuming this version has that feature, also no resonance.

User avatar
wendallsan
Wiggling with Experience
Posts: 342
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Sat Mar 21, 2020 2:27 pm

Synthiq wrote:
Sat Mar 21, 2020 12:45 pm
The trace you did looks correct so that's good. Next I would measure the C8 capacitance just to make sure it's 4.7nF and make sure there is a good contact between the other side of C8 and the 220pF capacitor next to it. Another way to test it would be to select a different mode where S5 is disconnected from S6 and then connect an external signal to S5 and see if it comes out at the out jack.

You haven't mentioned what type of input signal you are using, but to see the effect of the resonance pot it's also important to have an input signal that has frequency components at the resonance frequency so a sawtooth is better than a sinewave.
I don't have a meter that measures capacitance on hand, but I visually inspected the cap in question and see that it says '4700' on it, which seems correct.

I was able to confirm a good contact between the other side of C8 and the 220pF cap, so that connection seems good.

I tried to do your other suggested test, but didn't get any signal out the jack, but am wondering if I did the test properly. :hmm:

I connected an audio signal to S5 with the switch in "VCA" mode, which I think disconnects S5 from S6, then connected the output of an oscillator to S5 by clipping the tip of a patch cable to the switch terminal, then checked the output jack and didn't see any oscillator signal.

I tried this test again using a CV rather than an audio signal and again didn't see the CV output at the output jack.

I'm not sure if there is a grounding problem in my test, as the oscillator was in another case with another power supply and maybe things aren't properly grounded or something. Let me know if I need to perform this test again with everything on one power supply to see the proper results or if something else needs to be adjusted to perform this test correctly. Otherwise, if this sounds like a valid test of what you had described, it sounds like this test is failing, which hopefully further narrows down the problem.

I'm using my Sid Guts as the sound source for my tests, so am able to easily test with SAW, TRI, and PULSE signals, and haven't been using sine waves in my tests, so we should have some nice harmonic content to work with.
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

User avatar
wendallsan
Wiggling with Experience
Posts: 342
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Sat Mar 21, 2020 2:34 pm

wackelpeter wrote:
Sat Mar 21, 2020 2:26 pm
without looking too much into the posts, the resonance works only in the VCF mode, in VCA no resonance and in combined (VCA/VCF) setting, assuming this version has that feature, also no resonance.
That's interesting that the Resonance circuit wouldn't be in the combo mode, but I'll take your word on it! I've been testing in both combo mode and VCF mode, and even in the 'pure' VCF mode, there is no effect from adjusting the Resonance pot. I'll make sure I do my testing in the "Filter" mode as I continue to troubleshoot this.

Many thanks!
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

User avatar
wackelpeter
Wiggling with Experience
Posts: 457
Joined: Fri Apr 17, 2015 5:56 pm
Location: Germany, MD

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wackelpeter » Sat Mar 21, 2020 2:44 pm

forget it, i mixed something up... the jumpers seem to be installed correctly and my initial response was wrong...

Synthiq
Veteran Wiggler
Posts: 514
Joined: Mon Feb 06, 2017 3:10 pm
Location: California

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Sat Mar 21, 2020 4:45 pm

wendallsan wrote: I tried to do your other suggested test, but didn't get any signal out the jack, but am wondering if I did the test properly.
Thinking more about it, I realized that it is also possible to use S5 as an output instead. So set the mode switch to the mid position and input a signal to the normal audio input jack (which would also connects the grounds of the signal source and LPG) and then use your scope and look at the signal on both sides of the 4.7nF capacitor. On the side connected to the vactrols you should see a signal with the same amplitude as the input signal if you have set the offset pot to max to turn on the vactrols. With the mode switch in the middle position, the node connected to S5 should only be loaded by the oscilloscope probe so any signal seen one side of the C8 capacitor should also be seen on the other side. A 4.7nF capacitor and a 10Mohm probe resistance will act as a highpass filter with a 3.4Hz corner frequency so any reasonable audio signal should pass through the capacitor un-attenuated. If this is not the case, the capacitor is probably faulty and should be replaced or at least you can do a test and solder another one in parallel on the back side of the pcb.

User avatar
wendallsan
Wiggling with Experience
Posts: 342
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Sun Mar 22, 2020 10:27 am

Hi and thanks for your continued assistance!

I was able to hook an audio source to the audio input, turn up the OFFSET pot, set the mode switch to its middle position (VCA+VCF), and was able to probe both sides of that 4.7nF cap. Here is what I see:

Here is the side that does not connect to the vactrols:

Image

And here is the side that is connected to the vactrols:

Image

Sounds like you're not expecting any attenuation here, so can I assume this is what a bad cap looks like?
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

Synthiq
Veteran Wiggler
Posts: 514
Joined: Mon Feb 06, 2017 3:10 pm
Location: California

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by Synthiq » Sun Mar 22, 2020 12:52 pm

These pictures would make sense if the clipped triangle wave is the signal at the vactrols and the squarewave is after the 4.7nF capacitor and the oscilloscope input resistance is about 50kohm. I couldn't find a spec for the scope input resistance, but you can measure it if you plug a patch cord into one of the inputs and use the DMM to measure the resistance between ground and tip.

From the picture it looks like the triangle wave would have a peak amplitude of +/-8V even if the scope clips it at 6V. My concern is that the resonance circuit has a gain of 2 at max resonance setting which would require an output amplitude of 16V, well above the 12V supply and the 10V practical limit for a TL084. Try to reduce the input amplitude to less than 5V peak to see if that improves the performance.
C8_test.gif
You do not have the required permissions to view the files attached to this post.

User avatar
wendallsan
Wiggling with Experience
Posts: 342
Joined: Thu Jul 04, 2019 1:06 pm
Location: Olympia, WA, USA

Re: Help me troubleshoot DJ Thomas White Lopass Gate

Post by wendallsan » Sun Mar 22, 2020 6:13 pm

Yes, the triangle wave was over 6V so was appearing clipped on the scope, sorry for not adjusting it to show the entire waveform.

The resistance of the scope appears to be 100k based on the test that you described.

I repeated the last test using a waveform that was less than 5V, here are the results:

Here is the 'non-vactrol' side:

Image

And here is the vactrol side:

Image

Thanks again!
Student at Evergreen State College's "Advanced Electronics In Music" for the 2019-2020 school year. Built a Roland System 700 clone out of Pharmasonic PCB's. Currently building or troubleshooting a DJ Thomas White Quad Lowpass Gate, a Barton Bytewise Operator + Expander, Timo Rozendal Steiner Parker VCF, Timo Rozendal Joystick, NLC 1070 Resonate, NLC 1058 Triple Sloths v2, Fonitronic PS3100 Triple Vactrol Resonator, Befaco BF-22 Sallen Key Filter, and Befaco A*B+C.

Post Reply

Return to “Music Tech DIY”