A twisted tale about a logarithmic relay attenuator

[BrianDonegan]

Roy, can you check if there is any conductivity betyween the chassis and any of the pot pins?

[rpafenberg]

Sorry to be so confusing Russ. Previously I had not attempted to "play" the preamp through speakers, I had only passed signal through the preamp and attempted measurement.

In this case what I was referring to was moving the Pot to activate the relays. When I moved the pot sufficiently to hear music, I had constant relay run on. It sounded something like very low frequency osculation.

I am in the process of building a ribbon cable adaptor that will allow me to move the controller around while leaving the relay boards in place to see if this makes any difference in performance.

Since the stacked JT structure works without problems outside the preamp this might isolate the problem to interference with the controller.

Roy

[cjd]

If you have a constant relay switching (I have that on occasion) try tapping the end of the pot, or adjusting it another notch up or down. I've not yet tried any other pots in mine yet, since I have bigger issues. But I think that the supplied pot and setup can sometimes hit a spot where a relay will constantly be in an open/close cycle.

C

[Russ White]

C,

If I read Roy's posts correctly Roy's issue does not occur when the JT is outside his case, So I think the pot istelf is fine, but testing with another POT would be a good test in any case.

If you get to the end of your rope you can send your JT to me and I will do my best to debug it.

Cheers!
Russ

[rpafenberg]

I have done additional testing and found the moving the entire stack outside the case but maintaining all power connections reduces the instability but doesn't completely remove it. If I disconnect the Balanced Darwin Stack power it becomes a little more stable but their are still places where one relay (on both relay boards) will continue slowly clicking every second or two. I am looking in my scrap box for a replacement pot to test and will reply with results.

Roy

[rpafenberg]

installed a replacement pot. It didn't change the results.

I do have new information that changes things somewhat.

I can no longer duplicate relay stability outside the case. I still get some relay chatter even with the TJ stack disconnected from the preamp, the boards separated from each other and the stack running on a different AC power source (my varistat)

Brian, the pot pins were not grounded to the chassis.

Could this be a problem with the PIC?

[Russ White]

Hi Roy,

At this point I would no rule out anything, the PIC included.

To me it sounds like the PIC is reacting to external influences. A couple of things to check.

1) The diodes on the relay PCB. Make sure they are only contacting their pads. It is ok if they contact the power pins of the realay (pin 1 and pin 8).

2) Make sure the diodes are oriented correctly.

3) Check continuity between each pin of the IDC header and signal out and signal ground. There should be no continuity from any IDC pin to either signal GND or signal hot.

4) Check the supply voltage to the PIC with the pot at various positions.

5) Check the voltage from V+ to GND on the pot at various positions.

6) Check for shorts to any of the relay at the controller end by checking for continuity at the controller end betwen D_GND on the controller and each of the relay output pins on the IDC connector (the pins with traces which run to transistor collectors).

Cheers!
Russ

[Russ White]

One other thing to try, though it should not be necessary, would be to plus another 47uf or so cap on the PCB in the place I have marked with a yelow box.

This should further smooth out any voltage dips when relays turn on/off.

Anyway its worth a shot.

[rpafenberg]

Russ,

I have gone through your checklist and have the following results:

"1) The diodes on the relay PCB. Make sure they are only contacting their pads. It is ok if they contact the power pins of the relay (pin 1 and pin 8)."

All diodes check out ok with no shorts to the signal relay pins.

"2) Make sure the diodes are oriented correctly."

The diodes follow the orientation on the PCB and the picture on your web site.

"3) Check continuity between each pin of the IDC header and signal out and signal ground. There should be no continuity from any IDC pin to either signal GND or signal hot."

Checked continuity and confirmed that there were no shorts.

"4) Check the supply voltage to the PIC with the pot at various positions.

5) Check the voltage from V+ to GND on the pot at various positions."

The voltages for tests 4 and 5 are the same which should be expected since they are connected to the same pins on the voltage regulator. Therefore, I am combining the results for both.

Testing was done with the JT in its most stable position. It should be emphasized, that the chatter/run-on is much worse with the JT stack fully installed in the preamp.
The Balanced JT Stack was removed from the preamp and powered through an external varistat. The stack was not connected to the Darwin through either signal or power. I was using a replacement pot which was also 5K; however, it was log rather than linear. The pot change did not impact the stability of the JT; however, it did push the chattering/run-on further to the clockwise side of the pot.

In this configuration, when the pot was moved slowly clock wise chatter/run-on was found at a spot just before the full clock wise position. Interestingly enough the run-on stopped in the full clockwise position. Also, the results were the same with either or both relay board in the configuration.

The voltage with the pot "off", i.e., in the far counter clockwise position was 5.003V. This voltage remained constant until just before the end of the clockwise movement when it dropped to about 4.996V during relay run-on. The voltage dropped to 4.939V at the full clockwise position of the pot.

“6) Check for shorts to any of the relay at the controller end by checking for continuity at the controller end between D_GND on the controller and each of the relay output pins on the IDC connector (the pins with traces which run to transistor collectors).”

Assuming that by D_GND you mean pin 20 of the PIC, no shorts were found during testing.

[Russ White]

Originally posted by rpafenberg
Testing was done with the JT in its most stable position. It should be emphasized, that the chatter/run-on is much worse with the JT stack fully installed in the preamp.
The Balanced JT Stack was removed from the preamp and powered through an external varistat.

The voltage with the pot "off", i.e., in the far counter clockwise position was 5.003V. This voltage remained constant until just before the end of the clockwise movement when it dropped to about 4.996V during relay run-on. The voltage dropped to 4.939V at the full clockwise position of the pot.

Hmmm... Intersting. The Regluation seems a bit poor. Tell me more about the Varistat input. What is the AC voltage in. And how much current can the unit supply.

I can send you a new PIC with some new firmware to try, but I am still confused as to why this is happening. I cannot replicate it here.
So I think you might have a deeper problem than the PIC.

I just measured mine and my voltage stays right at 5.087 Volts +/- .01 volt the entire range.

I am starting suspect a bad relay or a bad transistor.

Try this... measure the total current the JT consumes from volume 0 to full volume. If there is one bad transistor or relay it will spike the current draw when it is activated. If you turn the pot slowly and measure one click at a time you can tell which trasistors are active by measuring the voltage at the base.

I hope we can get this thing squared away.

:EDIT: Max current draw for the JT alone will be about 150ma for each relay PCB. So 300-310ma or so for two relay PCBs. Add 20ma or so for the controller itself.

Cheers!
Russ