SOLVED Tommy III - hums and squeals

[Better Button]

Morning All.

Any assistance gratefully appreciated.

The build works in by-pass mode, and the LED lights, but all I get is lots of hum and squeal, increasing with gain.

My soldering skills are quite good and I have a general knowledge of parts etc, but can't read the schematic to any degree.

I've confirmed the voltage at the obvious points, and also at Pins 4 and 8 of the TL072.

I've checked the 3PDT, I / O, and all ground points for continuity.

I've reflowed the whole PCB, including underneath the bottom 2 pots.

Apart from the hums and squeals, touching the enclosure, or other metal parts has an effect on the hum, like there's a grounding problem, but I can't find it.

Again, thanks for any help.

Glen

 

[Locrian99]

Morning All.

Any assistance gratefully appreciated.

The build works in by-pass mode, and the LED lights, but all I get is lots of hum and squeal, increasing with gain.

My soldering skills are quite good and I have a general knowledge of parts etc, but can't read the schematic to any degree.

I've confirmed the voltage at the obvious points, and also at Pins 4 and 8 of the TL072.

I've checked the 3PDT, I / O, and all ground points for continuity.

I've reflowed the whole PCB, including underneath the bottom 2 pots.

Apart from the hums and squeals, touching the enclosure, or other metal parts has an effect on the hum, like there's a grounding problem, but I can't find it.

Again, thanks for any help.

Glen

Have you tried putting r5 in. I dont really think it would cause the issue but

 

[Better Button]

Yes, I wondered about R5. I read here that in lieu of an a25K pot for VOL, a 10K log pot could be used, omitting R5, which is what I did.

Glen

 

[spi]

Did you reflow the middle pin on the A500K jack. That one looks suspect, but could just be the picture.

What's up with the clipping switch. Mounted on the opposite side of the board?

 

[Better Button]

Did you reflow the middle pin on the A500K jack. That one looks suspect, but could just be the picture.

What's up with the clipping switch. Mounted on the opposite side of the board?

I actually de soldered all the pots and re did them; getting good with the solder sucker.

re the clipping selector, I couldn't source a slide switch locally so am dicking around with the placement of an SPDT; might just solder it directly to the PCB.

Thanks for your reply.

Glen

 

[Lytt Effects]

You may still need to jumper R5?

 

[Better Button]

You may still need to jumper R5?

That one is on the 'check later' list. The hums / squeals are intermittent; the pedal some times works and I can't track down why!

I've pretty much replaced all the wiring, checked all the PCB conponenant values, replaced a few of them, but the problem persists.

It's been a loooong few days; hope I'm actually learning something.

Glen

 

[Lytt Effects]

Hmm. Yea looking at the schematic I’m not sure R5 would have that effect.

Hum sounds like a ground issue to me. Squealing seems like a feedback issue to me.

Have you tested it IN the enclosure and it’s not working? Have you tested it OUT of the enclosure and it’s not working? Both?

 

[Better Button]

Yes, tested in and out of enclosure. Intermittent in both!

I thought there might be something shorting when it was in the enclosure, but I can't find anything conclusive using a multimeter, checing for continuity and voltage at all the places I know about.

I'm currently totally rewiring the 3PDT breakout board to the PCB. If the problem persists after that, I'm out of ideas.

Thanks for your ideas.

Glen

 

[Better Button]

Sorted!

The intermittent nature of the hums always suggested a grounding problem, but everything tested OK with the multi meter.

Anyway, after rewiring all the off-board connections with 22 AWG solid core wire, and replacing the OpAmp socket and one 2k2 resistor which was measuring albout 900 Ohms, it works and passes the gentle 'technical thump' test.

It would have been better if I'd actually identified the problem, but as usual, there were probably multiple things going on.

Great sounding pedal. I normall go for transparent silicon diode based overdrives, but this one Rocks! I whish I had it back in the pub rock days.

Thanks for all replies.

glen