SOLVED Frost Drive - Funky behaviour

[EGRENIER]

Ok, I was hoping never to have to do this, but here we are… my first troubleshooting thread… I think

I built a combo; Frost Drive in a Raincoat.

RainCoat seems OK. But the frost drive for a TS, doesn’t do what I would expect. And there‘s a few this that are just odd.

Here’s the list of odd behaviour:

- Tone knob goes from muddy to super bright in the last 1/20th clockwise.
- Level and Drive maxed out does induce much clipping, it more like just a boost
- Freq and Impedance trimmer doesn’t affect the sound at all
- Mid doesn’t seems to change the tone at all, I used a C100K Dual and applied the recommended changes to R4-5 and C3-4.
- Hi-Cut and Lo-Cut seems to work in different direction. Ex: Lo-Cut gets bright when going CW and Hi-Cut get dark going CCW. I used C10K from the same batch, so it’s not like one is a reverse pot.
- Dip switch seems to have no impact.

Troubleshooting performed so far:

Check the voltage at the MPSA18 +9V is coming in, and check the voltage at R14, -9V is there, so I concluded the charge pump is working fine.
Swap the MPSA18, no impact.
Swap both 4558 and the 1044, no impact.

So at this point’ I’m looking for pointers… In my list of options:

- Reflow the C100K Dual as the mid seems not to work. But inspection let me think it’s fine.
- Replace the C10K of the Tone, maybe it’s defective hence all the bright just at the end (I should measure it resistance to see if it’s just jumping at the end.

I’m attaching picture of the board before assembly, maybe you could spot a mistake.


Thanks in advance….

 

[TravisM]

When you subbed the tantalum caps in, did you validate polarity, I'm guessing you did since you didn't smoke 'em but nothing else visual jumps out at me at the moment.

 

[EGRENIER]

When you subbed the tantalum caps in, did you validate polarity, I'm guessing you did since you didn't smoke 'em but nothing else visual jumps out at me at the moment.

Hmmm, in all honesty I did not. So that one thing I can look at. Would they smoke automatically if the polarity is reversed ? Maybe I just got lucky…

 

[EGRENIER]

After checking I believe these are monolithic ceramic. I probably have some film box, I could replace them.

 

[Feral Feline]

Try not to introduce new variables — ie No need to replace the monolithics if they’re working; you can change to film caps if you like once the build is sorted and working well.

Have you got some tape or other barrier between the dual-gang and the board?

Sometimes it’s the simple things:
- recheck solder-joints hidden under the pots;
- look for fine solder bridges with a magnifier (to do this you’ll have to be extra careful cleaning the flux off, make sure the pots don’t get contaminated by cleaning agents);
- reseat the ICs & transistor (don’t just press on them, pop them out and reinsert);
- reflow any suspect solder-joints…


That’s all I’ve got.

 

[EGRENIER]

Try not to introduce new variables — ie No need to replace the monolithics if they’re working; you can change to film caps if you like once the build is sorted and working well.

Have you got some tape or other barrier between the dual-gang and the board?

Sometimes it’s the simple things:
- recheck solder-joints hidden under the pots;
- look for fine solder bridges with a magnifier (to do this you’ll have to be extra careful cleaning the flux off, make sure the pots don’t get contaminated by cleaning agents);
- reseat the ICs & transistor (don’t just press on them, pop them out and reinsert);
- reflow any suspect solder-joints…


That’s all I’ve got.

I modify a standard Tayda dust cover to fit so yes the dual is covered.

Cleaning it with the trim and Dual pots put will be difficult... But yes, I will for sure do a close inspection for solder bridge.. however, it is working just not as expected. So solder bridge seems unlikely... Based on the information I got so far, I will have to deep dive the schematic and see if I can come up for a reason for certain of those to happen as a first lead.

What I'm thinking is trim pot with no effects, I have to check if I could figure out if they actually have an impact on current to see this can lead me somewhere.

 

[music6000]

I modify a standard Tayda dust cover to fit so yes the dual is covered.

Cleaning it with the trim and Dual pots put will be difficult... But yes, I will for sure do a close inspection for solder bridge.. however, it is working just not as expected. So solder bridge seems unlikely... Based on the information I got so far, I will have to deep dive the schematic and see if I can come up for a reason for certain of those to happen as a first lead.

What I'm thinking is trim pot with no effects, I have to check if I could figure out if they actually have an impact on current to see this can lead me somewhere.

I think C4 - 120nf is 12nF on your PCB???

 

[TravisM]

Can you post pics with it boxed? Just to double check the offboard wiring. Really, I'm grasping at straws here though. Also, you can validate grounds and VCC per the schematic as an extra layer of validation.

 

[EGRENIER]

I think C4 - 120nf is 12nF on your PCB???

It is as directed by the replacement of the Dual pot to C100K instead of C10K



For more info, I built the Mint version that also include substitution:

Can you post pics with it boxed? Just to double check the offboard wiring.

 

[music6000]

It is as directed by the replacement of the Dual pot to C100K instead of C10K

View attachment 23846

For more info, I built the Mint version that also include substitution:

View attachment 23847

View attachment 23848

See you confused me But C3 - 4n7 looks like 47nf on your PCB!
I haven't checked your Resistors yet!

 

[EGRENIER]

See you confused me But C3 - 4n7 looks like 47nf on your PCB!
I haven't checked your Resistors yet!

Ahhh... well then there's an unforgivable mistake... I'll address that for sure. And it is sitting in the MID circuit section of the schematic. This will surely fix my Mid no impact issue... Now I'm usually pretty accurate in my build, what I mean by this is, because I prepare all my parts in a box, there are now a very high odd that I may have used a 4.7n for a 47n somewhere else on the board...

So Now I need to look into this as well....

 

[EGRENIER]

OK, so my mistake could have been to have swap C3 and C19, but C19 is correct. So I guess I just had a brain fart when pulling my parts out. And a second brain fart when installing the C3....

Grrr...

I'll subs when time permit and provide an update...

Thanks @music6000

 

[EGRENIER]

So I found some time to work Ont the Frost Drive…. Replaced the cap for a 4.7K thanks @music6000, that solved my mid issue.

I also tested my W20K tone pot, it was a dod, one lug was not just working. So I re-laced that with a new one and got normal tone behaviour.

Back to my list of issues:

- Tone knob goes from muddy to super bright in the last 1/20th clockwise. SOLVED
- Mid doesn’t seems to change the tone at all, I used a C100K Dual and applied the recommended changes to R4-5 and C3-4. SOLVED

REMAINING WIERD;
- Level and Drive maxed out does induce much clipping, it more like just a boost, a not a big one, barely a few db.
- Freq and Impedance trimmer doesn’t affect the sound at all
- Hi-Cut and Lo-Cut seems to work in different direction. Ex: Lo-Cut gets bright when going CW and Hi-Cut get dark going CCW. I used C10K from the same batch, so it’s not like one is a reverse pot.
- Dip switch seems to have no impact.

Any advice welcome, or measurement that could help you point me in the right directions ?

 

[music6000]

So I found some time to work Ont the Frost Drive…. Replaced the cap for a 4.7K thanks @music6000, that solved my mid issue.

I also tested my W20K tone pot, it was a dod, one lug was not just working. So I re-laced that with a new one and got normal tone behaviour.

Back to my list of issues:

- Tone knob goes from muddy to super bright in the last 1/20th clockwise. SOLVED
- Mid doesn’t seems to change the tone at all, I used a C100K Dual and applied the recommended changes to R4-5 and C3-4. SOLVED

REMAINING WIERD;
- Level and Drive maxed out does induce much clipping, it more like just a boost, a not a big one, barely a few db.
- Freq and Impedance trimmer doesn’t affect the sound at all
- Hi-Cut and Lo-Cut seems to work in different direction. Ex: Lo-Cut gets bright when going CW and Hi-Cut get dark going CCW. I used C10K from the same batch, so it’s not like one is a reverse pot.
- Dip switch seems to have no impact.

Any advice welcome, or measurement that could help you point me in the right directions ?

Check for Continuity on these matching Coloured Circles
This is the ''MINT'' version , Correct? :

 

[EGRENIER]

Check for Continuity on these matching Coloured Circles
This is the ''MINT'' version , Correct? :

View attachment 24186

All points check out… continuity on all colours.
And yes, this is the mint version.

 

[music6000]

All points check out… continuity on all colours.
And yes, this is the mint version.

Check for Continuity & confirm which other Clipping Diode connects to Green circle with ? in it :

 

[phi1]

Your description of lo-cut and hi-cut matches the schematic, I get what you’re saying though it’s counter-intuitive that they don’t both ‘cut’ hi or lo in the same direction.

Weird that your aren’t getting the gain. Dip sw probably aren’t having an effect because you’re not getting a big enough signal for clipping, regardless of setting. Either the gain stage isn’t boosting enough (maybe check the Ohm reading with a meter on the 1M drive pot and the 4k7 max pot. Or, you’re somehow loosing a bunch of signal over the first two op amp stages. This could maybe be checked if you’re loosing signal over the first two stages with an audio probe.

 

[EGRENIER]

Check for Continuity & confirm which other Clipping Diode connects to Green circle with ? in it :

View attachment 24199

For the reds. All continuity confirmed.

For diodes green circles confirmed.
Sw1/sw2 On
connectivtity from D2 (?) right side to:
D4: both points
D3: left point
D1: Nothing

Sw1: off Sw2: on (based on the sw number as installed, see board pic)
D4: right side only.
All others nothing

sw1/ sw2 off:
D4: Right side only
All others nothing

sw1 on sw2 off:
D4: right side only
All others nothing

 

[music6000]

For the reds. All continuity confirmed.

For diodes green circles confirmed.
Sw1/sw2 On
connectivtity from D2 (?) right side to:
D4: both points
D3: left point
D1: Nothing

Sw1: off Sw2: on (based on the sw number as installed, see board pic)
D4: right side only.
All others nothing

sw1/ sw2 off:
D4: Right side only
All others nothing

sw1 on sw2 off:
D4: right side only
All others nothing

This one has me scratching my Head???
Sw1/sw2 On
connectivtity from D2 (?) right side to:
D4: both points
I think it's time to pull that 1M Drive pot & put a temporary 470k resistor across legs 1 & 2 & see if you get a better result!

 

[EGRENIER]

This one has me scratching my Head???
Sw1/sw2 On
connectivtity from D2 (?) right side to:
D4: both points
I think it's time to pull that 1M Drive pot & put a temporary 470k resistor across legs 1 & 2 & see if you get a better result!

Well on schematic, we can’t tell what diodes is what. So if this is as below, that would explain why D4 get continuity on both legs.



As for replacing the 1M pot, which one are you thinking of, the C1M (Drive) or the Trim pot ?

And what would that do in terms of connectivity ?