Guyatone TD-X Tube Echo

[GuitarFreak302]

I have a Guyatone TD-X Tube Echo that does not have delay signal only dry. I have gone through with my scope and I have signal everywhere I am supposed to. I also checked for a clock signal which I have. I know how the circuit works up until right after the NE570 but once I get to the LM311 I am not 100% sure what the circuit is doing which makes it hard to troubleshoot. If anyone is willing to help that would great.

[Digital Larry]

The 311 should just be buffering the SA571's output to drive the input of the delay chip. 

OK I take that back.  The 311 is a comparator.  Not sure then, although in context it seems like it should just be buffering the compander's signal to go into the delay chip (which is a PLD so there's no telling what it has been programmed to do).

https://www.alldatasheet.com/datasheet-pdf/view/7413/NJRC/NJM311.html

This article suggests it might be used as a 1-bit A/D.

https://components101.com/article/difference-between-op-amp-and-comparator

https://www.ti.com/lit/an/slyt423a/slyt423a.pdf

Sorry about all the faffing about, I'm pretty rusty on this stuff any more.  The PLD is a digital device so I'm guessing all in all this is what is going on (1-bit A/D).  What is the clock frequency range of the cascaded invertors?

What are the voltages/signals on the 311?

[Rob Strand]

This article suggests it might be used as a 1-bit A/D.

https://components101.com/article/difference-between-op-amp-and-comparator

https://www.ti.com/lit/an/slyt423a/slyt423a.pdf

Sorry about all the faffing about, I'm pretty rusty on this stuff any more.  The PLD is a digital device so I'm guessing all in all this is what is going on (1-bit A/D).  What is the clock frequency range of the cascaded invertors?

Sounds right to me.

One comparator input is Vref.   The other is the sum of filtered FOUT from the PLD (R32, C37, R33) and the audio (via R20).    The sum goes to the + input of the comparator so the 311 can only be working as comparator, so a 1 bit A/D.

The PLD output FOUT is likely to be PWM.    I suppose alternatively it could drive and up/drive down a blip to the filter (R32, C37, R33).   I'd have to read-up on the datasheet to see if FOUT could be analog.

[GuitarFreak302]

Pin 3 of the LM311 I am getting around 8 volts and on pin 2 I have around 4.5 volts.

[Rob Strand]

Pin 3 of the LM311 I am getting around 8 volts and on pin 2 I have around 4.5 volts.

Maybe check pin 7 of the LM311.   If pin 3 (in+) is 8V and pin 2 (in-) is 4.5V then the output pin 7 should be high.

You might need to look at the signals with the oscilloscope.  Especially around R31, R32, R33, C37.   You might also need to input an audio signal.   The FOUT output, which goes to R31, is likely to have a complex pulse nature - higher frequencies than audio.

The signals around R35, R36, R37, C38 come from DOUT.   If you are lucky there might be similar looking PLD signals on DOUT and FOUT.
[EDIT:  What I mean here is: DOUT is a D/A output and FOUT is a D/A output but the connection of FOUT and the LM311 turns the FOUT D/A into a 1 bit A/D
See p149 (pdf p13)
https://www.analog.com/media/en/training-seminars/design-handbooks/microprocessor-systems-handbook/Chapter7.pdf
]

[GuitarFreak302]

I checked FOUT at the IC and also at the other side of R31. I checked DOUT at the IC and at the other side of R36. Both FOUT and DOUT looked similar. The signals however looked like very noisy sine waves but were in the Mhz range.

[Rob Strand]

I checked FOUT at the IC and also at the other side of R31. I checked DOUT at the IC and at the other side of R36. Both FOUT and DOUT looked similar. The signals however looked like very noisy sine waves but were in the Mhz range.

I guess you would have to assume DOUT, FOUT and the comparator are working.

So getting back to your original problem.    You stated earlier all the expected points show audio but if no effect is getting through there must be somewhere where there is no audio?  I probably should go over the schematic again.  It's bit messy so it doesn't stick in my head long.

[GuitarFreak302]

The comparator always has a high output so DIN on the PLD is always pulled low.

[Digital Larry]

The comparator always has a high output so DIN on the PLD is always pulled low.

Pin 2 of the 311 looks like it should be a DC voltage "about" halfway.  Pin 3 should be switching over/under this level to get any action at the comparator output.

So what's going on at pins 2 and 3 of the 311?

DL

[GuitarFreak302]

Pin 2 of the 311 is around 4.5 volts like it should be but pin 3 always stays at around 8 volts so the output at pin 7 is always staying the same. The voltage on pin 3 is fed from the compander IC and FBOUT. I installed a new compander IC and checked the voltages of the compander and they seem correct based off of the components around it and the voltages its connected to. This leads me to believe that FBOUT may be the issue. I'm not great with digital circuits which is why I came here for help.

[Digital Larry]

FOUT is wiggling but FBOUT is stuck?  I'd trace that circuit then through the little filter network.  R31, R32, R33, C37.