Dead End Fx Turbotrem issue with triangle wave (more diagnostics provided)

[spacekid71]

The short answer is NO

As long as you wired the TRS jacks in accordance with the build doc (Sleeve connections to Sleeve/Tip connections to Tip) then you should be OK. I have built many of these and always used TRS jacks in every position with no issues. The big thing to note is that the LFO Pedal Sleeve and Sleeve Switch and LFO Trigger Sleeve and Sleeve Switch do NOT make contact directly with the enclosure (as well as the Sleeve connection of the Right Stereo Out)
As an aside, you may want to verify your soldering of the LFO Pedal and Trigger wiring at the PCB. Just make sure nothing is shorted where it ISN'T supposed to be.

Just wanted to clarify the grounding that is going on before verifying that the sockets aren't shorting out.

I have a wire running from the enclosure (attached with a screw and nut) to the enclosure GND pad (LFO pedal socket - middle pad), and all sockets are plastic so they don't come in direct contact with the enclosure. But the enclosure GND is connected to the power jack GND (I checked with continuity) and through that to the GND of the input and outputs. Is that correct?

When I check continuity on the LFO pedal socket I get the following:

Without a cable plugged in:

GND to SCN: yes
GND to TIP: yes
TIP to SCN: intermittent

With cable plugged in:
GND to SCN: yes
GND to TIP: no
TIP to SCN: intermittent

I guess this makes sense as without the cable, the tip is being shorted to ground so it has no effect on the LFO?

[Govmnt_Lacky]

What is “SCN”?

[spacekid71]

What is “SCN”?

I just checked using continuity and it gives me the following silkscreen to schematic mapping:

TIP -> PAD 12
GND -> PAD 11
SCN -> PAD 10

Not sure what SCN would stand for but maybe SCAN?

[spacekid71]

Well, the circuit is proven so we should start by checking things:

- Check proper values of R29 through R39, C18 through C21.
- Check to make sure that Q4-Q6 are correct and in their proper locations
- Verify the Mode and Shape rotary switches are making good and proper contact
     (When the A-pole is contacting Pin 1, then B-pole should be on Pin 5... C-pole should be on Pin 9. As you rotate the numbers should move up 1 with each rotation)
- Verify that you have a Linear taper pot (B-taper) for the LFO Rate pots
- Verify you have Vref voltage on IC4 Pins 2, 5, 9, and 12.
- Verify you have Vref voltage on Mode rotary Pins 1-3
- Swap out IC4 and retest
- Swap out IC3 and retest

- I have gone through the resistors and capacitors by visually confirming their values and they check out.
- I have verified that Q4 is the J113 and that Q5 and Q6 are BC549C transistors. They are also the right way around based on the silkscreen.
- I have used continuity to check both the Mode and Shape switches to make sure that pins correspond to correct poles.
- I have checked that both LFO rate pots are B100K.
- I have checked on the specified pins of IC4 and the Mode switch that they have the VREF voltage of 4.30V from pin 1 of IC1.
- I have swapped out IC3 and IC4 multiple times and the triangle wave just won't behave.

I have made a new video to illustrate the triangle mode problem: https://drive.google.com/file/d/1f8QkTaSL3imUIGuzdfav6-vwStCFjt_C/view?usp=share_link.

[Govmnt_Lacky]

When I check continuity on the LFO pedal socket I get the following:

Without a cable plugged in:

GND to SCN: yes
GND to TIP: yes
TIP to SCN: intermittent

With cable plugged in:
GND to SCN: yes
GND to TIP: no
TIP to SCN: intermittent

I guess this makes sense as without the cable, the tip is being shorted to ground so it has no effect on the LFO?

Are you talking about the LFO PEDAL jack -OR- the LFO TRIGGER jack? YOu should not see GND on the Tip of the LFO Pedal jack at all.

[ElectricDruid]

What is “SCN”?

"Screen", I would guess - e.g. the shield on a shielded cable.

Maybe. Don't know whether that makes sense with the connections given.

[anotherjim]

Well, sure Scrn=Screen=Sleeve is the "S" in TRS, but for control/switching it can be anything that's safe. Doesn't have to be an audio cable shield/ground/0v.

I don't like the PCB having wire pads at a minimum distance apart. Stranded wire can hide broken strands that pop out and short to a neighbour. Unless you're going to use crimped connectors, solid wire is a better choice. Anyway, the only thing I'd look for there is shorts between the wires to the control pedal jack. The LFO trig needs to ground normally so that shouldn't be an issue.

[spacekid71]

Are you talking about the LFO PEDAL jack -OR- the LFO TRIGGER jack? YOu should not see GND on the Tip of the LFO Pedal jack at all.

The continuity info that I provided was for the LFO Pedal jack. I think I should redo the offboard wiring, starting with the LFO Pedal and LFO trigger jacks, it looks like that could be killing the oscillation.

Is there a reason that it would only kill the oscillation in triangle mode when the rate is below 2 o'clock?

[Govmnt_Lacky]

The continuity info that I provided was for the LFO Pedal jack. I think I should redo the offboard wiring, starting with the LFO Pedal and LFO trigger jacks, it looks like that could be killing the oscillation.

Is there a reason that it would only kill the oscillation in triangle mode when the rate is below 2 o'clock?

Perform this quick test:
- Remove all plug from any jacks on the pedal
- Set Left Rate pot to center
- Set Right Rate pot to center

With your Multimeter on Continuity or Resistance check, place Black probe to Ground lug. With the Red probe, check ALL 3 of the solder points on the Left and Right Rate pots.

You SHOULD NOT get continuity or a direct short to Ground on any of them.

EDIT: Just to make sure we are on the same page, based on your build you sent pics of.... your LFO Trigger jack is on top of your pedal above the LFO Rate pots.. The LFO Pedal jack is above the Depth pots.

[spacekid71]

Perform this quick test:
- Remove all plug from any jacks on the pedal
- Set Left Rate pot to center
- Set Right Rate pot to center

With your Multimeter on Continuity or Resistance check, place Black probe to Ground lug. With the Red probe, check ALL 3 of the solder points on the Left and Right Rate pots.

You SHOULD NOT get continuity or a direct short to Ground on any of them.

Thanks Govmnt_Lacky! I am in Canada and I am currently working but I will have a look at this when I have some dedicated time.

[spacekid71]

Perform this quick test:
- Remove all plug from any jacks on the pedal
- Set Left Rate pot to center
- Set Right Rate pot to center

With your Multimeter on Continuity or Resistance check, place Black probe to Ground lug. With the Red probe, check ALL 3 of the solder points on the Left and Right Rate pots.

You SHOULD NOT get continuity or a direct short to Ground on any of them.

EDIT: Just to make sure we are on the same page, based on your build you sent pics of.... your LFO Trigger jack is on top of your pedal above the LFO Rate pots.. The LFO Pedal jack is above the Depth pots.

I have tested continuity between the ground lug and the solder points on both RATE pots, and there is no continuity for any of the solder points to ground.

There is continuity between both pads of the LFO trigger socket. When retesting the LFO pedal socket, I am unsure what is happening.... With the pedal powered on, I get continuity between GND and TIP (not good), NO continuity between GND and SCN, and intermittent continuity between TIP and SCN. When the pedal is turned off, there is no continuity between any of the three pads. I am probably doing something wrong...

I have made an image that shows the PCB pads on top of the photo of my build and I have indicated which socket is which, I hope this helps.

[anotherjim]

Do not test continuity in a circuit with its power on. It will clash with the DMMs own probe voltage if the circuit is powered.
To properly test sockets, check continuity with a plug fitted and test to the plug contacts or the other end of a suitable cable fitted with that plug.
The LFO trigger socket has a switch contact used opposite the tip contact. This contact breaks when a jack plug is inserted and makes when it isn't. Have a look at what happens to the contacts when a plug is fitted in and out -  you should see the movement that works the switching.
So, the tip is grounded with no plug fitted and that will allow the LFO to work. When a footswitch pedal is attached, the socket ground is removed from the tip and it's then up to the external footswitch to ground the tip or not.

The LFO speed pedal socket connects an external potentiometer in an expression/volume pedal to the LFO rate pot, so it will depend on the setting of the rate pot how much a fault on the pedal socket will affect it.

[Govmnt_Lacky]

When the pedal is turned off, there is no continuity between any of the three pads. I am probably doing something wrong...

This is correct with NO power applies (as Jim said) and with the LFO PEDAL jack.

Lets try to call in reinforcements....

[spacekid71]

The issue with the triangle wave is still not fixed..

[spacekid71]

I did some more investigation on IC4 to see what voltages are like when the triangle is working and isn't working. Here are the values:

rate below 2 o'clock (triangle wave not working)

IC4
1   7.81V
2   4.31V
3   4.37V
4   8.59V
5   4.30V
6   4.30V
7   2.06V
8   1.44V -> 6.04V *
9   4.31V
10   2.05V -> 8.73V *
11   0V
12   4.31V
13   4.31V
14   1.72V -> 5.23V *

rate above 2 o'clock (triangle wave working)
1   4.17V -> 4.41V *
2   4.29V
3   4.13 -> 4.17V *
4   8.56V
5   4.29V
6   4.29V
7   4.04V -> 4.06V *
8   1.2V -> 7.56V *
9   4.29V
10   1.99V -> 8.99V *
11   0V
12   4.29V
13   4.29V
14   3.37V -> 8.56 *

* voltage constantly changing, values to give an approximation

[spacekid71]

Hi everyone,

I have been continuing to investigate the triangle wave problem. So as far as I understand, the Turbotrem circuit has two waveform generators, LEFT and RIGHT. It uses one TL074 to generate both LEFT and RIGHT waveforms and for each it generates square and triangle waves. Now when I am in Tremulant mode using the triangle shape, there are oscillations when the LEFT rate knob is passed 2 o'clock, otherwise it is a static voltage.

So this morning, in the static voltage scenario, I thought I would see if the RIGHT waveform generator is having any effect on the LEFT one freezing up, so I shorted the RIGHT triangle (pin 14 on IC4) to ground. The result was that the LEFT triangle waveform started to work as intended.

Looking at the schematic, it seems that the only way that the RIGHT waveform generator is being used and connected in the whole circuit, is in the Dual Tremulant mode, in which it connects through pin A of the Shape switch to pin 5 of the Mode switch. It seems odd then that it would have any effect in Tremulant mode.

I am using a IC socket for the TL074 and thought maybe there was a short there. I checked but the only connections are the pins 2, 5, 9, 12 which are connected to VREF. Finally, I did some measurements directly on the LEFT rate pot in various positions to illustrate the oscillator lock up (which I attached as an image).

I hope someone will be able to point me into the right direction. Any help would be appreciated.

[Govmnt_Lacky]

Everything about this just screams that there is either an incorrect resistor value somewhere in the circuit -OR- the rate pot(s) are the wrong value or taper 

[anotherjim]

Could the rate pot just be faulty?

[spacekid71]

Thanks Lacky. I will do a sweep of the resistor values in the whole circuit, and perhaps take out the pots and check their values as well.

I was thinking that the two vactrols have LDRs in them so if they are faulty, that could also mess the circuit up resistance-wise. Do you know what function the vactrols have the circuit? I have attached images of both places where the two vactrols are used. I also spoke to Matt Harrison (GiveUsYourBones) by email yesterday and he had built the Turbotrem and experienced some issues because of the vactrols.

[Govmnt_Lacky]

I was thinking that the two vactrols have LDRs in them so if they are faulty, that could also mess the circuit up resistance-wise. Do you know what function the vactrols have the circuit? I have attached images of both places where the two vactrols are used. I also spoke to Matt Harrison (GiveUsYourBones) by email yesterday and he had built the Turbotrem and experienced some issues because of the vactrols.

Any time I have built a Lovetone-like project, I have always used LDRs and LEDs. Not vactrols.

In this case, with the TurboTrem (Lovetone Wobulator) you need to use either Diffused Green or High-Brightness Clear Green (NOT Superbright) LEDs and 10-30K On/20M Off LDRs.