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

[spacekid71]

Hi all,

I have built the DeadEndFx Turbotrem and most of it seems to be fine except for the triangle shape. I have indicated the path that doesn't seem to be working on the schematic. I have also made a video to illustrate the issue. Any help would be great.



Schematic:

Video: https://drive.google.com/file/d/1ZEjflX3k6r_vhyk5GshDNh353fP5xwUf/view?usp=sharing

DEFX build document: https://drive.google.com/file/d/1zfFb6p8sl1gLMdpkSlTrNNS-i0e7dZRp/view

[ElectricDruid]

At a guess, I'd say there's something wrong in the switch wiring that's causing the triangle output to be tied to some static voltage and is killing the LFO.

Certainly I'd start by making sure that all the soldering around that rotary switch is really clean and there's nothing that's shorting to the ground plane or anything else. A tiny little blob of "lost solder" would be enough to do it. With the power off, scrape the gaps between the tracks clear with a small screwdriver bit to make sure that you've got gaps where you should have gaps.

HTH,
Tom

[spacekid71]

I have cleaned around the rotary switch area including IC4, but the triangle wave still doesn't function. I have now also taken out the rotary switch and I am manually connecting pin C to pin 10, but it still doesn't change anything.

When pin 12 is connected to pin C (square wave selection), pin 7 of IC4 is showing the voltage going up and down in sync with the left LED. When pin 10 (triangle wave selection) is connected to pin C, pin 7 of IC4 reads 2.4V constantly. In both scenarios IC4 is getting 9V power.

Anyone have any more ideas on what I can look at? I can provide more readings if it helps.

[anotherjim]

Does the square wave work? Really, the square depends on the triangle which is enabled by a JFET. Bad JFETs happen all too often but a good one is normally conducting and stopping the cap C19 from charging and discharging. To get the triangle running the JFET needs to be switched off, so what voltage do you have on Q5 collector? It will be 0v to switch it off and run the triangle. What is connected to R39 off to the right of the snippet?

[spacekid71]

Yes, the square wave works fine. The voltage on the Q5 collector is 7.1 mV independent of the setting being selected (square or triangle). This doesn't seem right based on what you said as it should be 0V for triangle to work.

The pins indicated on the schematic snippet are related to two sockets, the LFO pedal socket, and the LFO trigger socket. I have attached another image that shows the offboard wiring. Pad 13 is connected to the LFO Trigger socket and the silkscreen indicates TRG on it. Pad 10 and 12 are connected to the LFO Pedal socket and pad 11 grounds to the enclosure.

I can provide additional info if needed.

Your help is much appreciated!

[anotherjim]

I think the triangle must be working. It and the squarewave are two outputs of the same oscillator. IC4 pin7 is Triangle and IC4 pin1 is Square. This is a common oscillator circuit using two opamps known as a "Function Generator" if you want to read up on it.

Check there is continuity between IC4 pin7 and the two contacts it feeds of the Shape switch.

[spacekid71]

Thank you for identifying that the LFO is a function generator, I will have a look at it tonight.

There is continuity between pin 7 of IC4, and pin 9 and 10 of the rotary switch.

When I measure voltage on pin 1 and pin 7 of the IC4, I see the following:

1. Square wave mode

Both pins are showing fluctuating voltages between 3V and 6V that seem in sync with the LEDs going on and off.

2. Triangle wave mode

Both pins show a constant voltage and the light from the LED also remains the same.

Pin 1: 7.7V
Pin 7: 2.4V

[ElectricDruid]

There is continuity between pin 7 of IC4, and pin 9 and 10 of the rotary switch.

This is good!

When I measure voltage on pin 1 and pin 7 of the IC4, I see the following:

1. Square wave mode

Both pins are showing fluctuating voltages between 3V and 6V that seem in sync with the LEDs going on and off.

This means the LFO is working, at least in this mode. That means there's nothing fundamentally wrong with it.

2. Triangle wave mode

Both pins show a constant voltage and the light from the LED also remains the same.

Pin 1: 7.7V
Pin 7: 2.4V

In this mode, the LFO stops working for some reason. The Shape switch seems to be ok, or at least you said you'd checked it out and it seemed to be all soldered cleanly. Could the problem be the mode switch? In the triangle mode, the triangle is supposed to only be connected to IC3 via R25/100R. Is it possible there's some other connection being made there? Could the switch have been fitted the wrong way around? (is that possible?) Are the Mode and Shape switches identical? Could they have been reversed if not?

It still seems to me most likely there's something shorting the LFO out in the triangle mode. The question is finding it!

[spacekid71]

In this mode, the LFO stops working for some reason. The Shape switch seems to be ok, or at least you said you'd checked it out and it seemed to be all soldered cleanly. Could the problem be the mode switch? In the triangle mode, the triangle is supposed to only be connected to IC3 via R25/100R. Is it possible there's some other connection being made there? Could the switch have been fitted the wrong way around? (is that possible?) Are the Mode and Shape switches identical? Could they have been reversed if not?

It still seems to me most likely there's something shorting the LFO out in the triangle mode. The question is finding it!

Both the Shape and Mode switches are 3P4T rotary switches and they are installed the correct way around, I double-checked them. As far as I understand, in triangle mode, the triangle connects through R25 and R24 to pin 2 of IC3, and also through pin 4 and pin A of the Mode switch to pin 3 of IC3. Pin 2 and 3 of IC3 being inputs and pin 1 being the output.

I measured pin 1 of IC3 and in Square wave mode, the voltage is fluctuating. In Triangle mode the voltage stays at 2.5V continuously...

[Govmnt_Lacky]

Are you sure that R34 is a 10K resistor and not a 1K?

[anotherjim]

^ That's a very good one to check. The triangle amplitude will be very low if R34 is too small but the squarewave could still appear. Conversely, R29 could be too large and have the same impact. It's the balance between these two resistor values that sets the amplitude of the triangle wave but the squarewave is always going to have full amplitude.

[spacekid71]

Thanks anotherjim and Govmnt_Lacky! I will check these resistors and report back. Hopefully they are wrong and it will fix the issue.

[Govmnt_Lacky]

Thanks anotherjim and Govmnt_Lacky! I will check these resistors and report back. Hopefully they are wrong and it will fix the issue.

Looking at the pic you uploaded, I am not so much concerned about R29 as it looks to be the correct value of 15K (Brown, Green, Black, Red) however, the R34 resistor "could be a 1K (Brown, Black, Black, Brown). It should be Brown, Black, Black, Red.

[spacekid71]

Looking at the pic you uploaded, I am not so much concerned about R29 as it looks to be the correct value of 15K (Brown, Green, Black, Red) however, the R34 resistor "could be a 1K (Brown, Black, Black, Brown). It should be Brown, Black, Black, Red.

I just unsoldered R34 and checked its value, and unfortunately it measures in at about 10K. I must admit that the colors are very close between the Brown and the Red, I couldn't tell the difference by looking at it. Any other parts that I should check?

[ElectricDruid]

When I measure voltage on pin 1 and pin 7 of the IC4, I see the following:

1. Square wave mode

Both pins are showing fluctuating voltages between 3V and 6V that seem in sync with the LEDs going on and off.

2. Triangle wave mode

Both pins show a constant voltage and the light from the LED also remains the same.

Pin 1: 7.7V
Pin 7: 2.4V

Jim, Lacky,

It's not that the triangle wave is low in volume, it's actually *stopped*, as evidenced by the quote above. In triangle mode, even the square signal goes dead - the LFO stops oscillating.

What could that switch be doing that's killing the oscillation?

[spacekid71]

I diagnosed a bit more tonight and the triangle wave is not completely dead it seems. Hooray!

When I turn the LFO-L-RATE knob (B100K) to 2 o'clock or further, the triangle comes alive! If I turn the knob back down below 2 o'clock, the IC4 pins show a constant voltage again (Pin 1: 7.7V,  Pin 7: 2.4V) and the LEFT LED goes dim.

Another thing I noticed is that 2 o'clock and further both LEFT and RIGHT LEDs are on, and when below 2 o'clock, only the LEFT LED is on dimly.

It does seem that in triangle mode, the oscillations are being killed...

[Govmnt_Lacky]

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

[anotherjim]

Just a thought. Are the rotary switches the Alpha type 3P4T? If the turn limit tab washer was in the wrong hole giving 4 positions at the wrong end of the 12 possible positions these have, what do the contacts make to, if at all?

[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

Thank you for the list of things to check, Govmnt_Lacky, I will have a look at this and report back.

Just a thought. Are the rotary switches the Alpha type 3P4T? If the turn limit tab washer was in the wrong hole giving 4 positions at the wrong end of the 12 possible positions these have, what do the contacts make to, if at all?

These are the Alpha 3P4T switches but I have checked that the switches are the right way in and that the turn limit tab washer is properly in place (both in position 4 of 12).

I did have a problem earlier with the Mode switch where I had overturned the nut too much and it broke the rotary switch. For this reason I have replaced both rotary switches to make sure that they function properly.

One thing I did wonder though. I have all isolated TRS sockets for this build and in the build document it uses TS sockets for all sockets except the input socket. Would this cause any potential issues? I am wanting to check this with you because both the LFO pedal and the LFO trigger sockets are hooked into the LFO. Just a thought I had in the back of my mind for a number of days so I thought it would be better to check.

[Govmnt_Lacky]

One thing I did wonder though. I have all isolated TRS sockets for this build and in the build document it uses TS sockets for all sockets except the input socket. Would this cause any potential issues? I am wanting to check this with you because both the LFO pedal and the LFO trigger sockets are hooked into the LFO. Just a thought I had in the back of my mind for a number of days so I thought it would be better to check.

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.