Weird pot behaviour?

andy_k

Little bit of background....
Last night I was playing around with one of my finished Frankenstein builds.
This one has a single push-push pot to select between the neck stacked Seymour Duncan and the bridge EVH Frankenstein.
I think it sounds fine, the pot seems to be working ok, down is neck, up is bridge - which gives me a bit more of the knob to do volume swells.
Anyhoo, I was playing it fairly quietly through a Katana head, with a high gain brown sound and I noticed that the volume does not go to zero, never noticed it before, and I am using it to clean up by turning down-range feels pretty smooth.
I don't want to open it up yet, as the sound is okay-but was wondering what I could have got wrong, it is probably level 2 on the complexity level, being just a switch and a volume.
A quick search led me to some suggestions of it either being a bad ground solder joint, or a defective pot.
I tried taking some readings with a multimeter set to 20k, and don't really see the effect I am hearing.
Bridge measures 13.14 on 10, but between 10 and 9 resistance rises and then reads 1 until i get to 1 on the knob where it reads 19.9 and up until it settles at 10.16 with the knob fully off. Very strange?
I am not sure of the spec of the pot, it is the first time I have used a push-push, more familiar with a push-pull, and I am pretty sure I am using the meter correctly, but that is another thing I am a bit hazy about.
With it set to 20k, an open circuit reads 1 and closed reads zero, anything else should be resistance, right? ie-normally half way down on a pot should read half the full volume.
I actually like the effect I am getting, I think I may have discovered the secret of the break in Hot for teacher, and I definitely can still use the pot to get vol swells, so it just seem a bit strange with the readings, setting the meter to 200k and 2000k gave me even stranger numbers, and the behaviour is the same with the other pickup selected, resistance at zero is only slightly lower than full output but sonically it is almost silent, if I was using a loud clean sound I probably wouldn't notice it.
Strange???

ICBM

That sounds like you've got a bad connection between the pot track and ground, with an effective resistance of 10.16K ohm. 

This can happen when the pot terminal makes a poor contact with the trackboard - this especially happens if you bend the terminal back to solder it to the casing, which strains the rivetted connection. Try squeezing the rivet hard with a pair of narrow-nose pliers to clamp it tighter to the trackboard.

The other pot behaviour is normal because the pickup is in parallel with it when it's turned up full. You can find the pot value by measuring the peak resistance in the middle of the turn - you'll need the 200K scale - multiply by four, and subtract the pickup resistance (13.14K if the bridge is selected).

Dave_Mc

(Hopefully someone who knows more than me will be along soon to contradict... er, I mean confirm what i've written here )

Push-pushes (any I've used) work the same electrically as push-pulls- it's just the switching mechanism which is different. That shouldn't be the problem.

It'll only read half-resistance at 5 on the volume if it's a linear pot. If it's logarithmic, it won't, it will be lower than half.

Do you have the volume pot wired backwards? I had a couple of guitars wired backwards, and IIRC they didn't go to 0 when the volume was at 0. When I changed them to the correct way round, the problem disappeared. 

Actually to tell the truth I can't remember if one of those guitars did that with the volume at 0, but I remember for sure that one of them did and rewiring fixed it. Also, backwards wiring is horrible, as you lose a ton of treble when you turn down- especially since you don't even have a setting with both pickups on, which is the only real (slight) bonus of backwards wiring- the volume doesn't cut out when both pickups are on and you turn one volume to 0, as happens with regularly-wired volumes, you get the sound of one pickup alone.

Just to be clear, backwards wiring has the hot wire from the switch going to the middle lug of the volume pot, and the output (to the jack) wired to the first lug (not the lug which is grounded). To fix this, wire the hot wire to the volume pot's input (the outer lug which isn't grounded) and wire the wire which goes to the jack socket to the middle lug of the volume pot.

You're also right, incorrect grounding could well cause the problem, too (and it's a bit more awkward to ground to push-pulls or push-pushes, I find, there's not as much room to do it). It should be pretty easy to check, just by looking with such a simple circuit, whether your volume is wired backwards or not.

andy_k

Thanks for the pointers, I'll have a look inside when I have the time, as I said, otherwise it seems to be working fine, sounds great in fact.
The neck position is something I have never really liked, but this particular mix of pickups is good.
Neck is powerful enough to be a balance with the Frankenstein,  but doesn't sound too muddy-which is usually the case.
I find the knack is getting the distance from the strings right, and weaker pickups get too quiet to balance with a hotter bridge.
I usually use push / pulls, and I think the quality is not as good as plain pots, so this was my first go with a push / push, and I do like the way it is working out.
I don't need the vol to drop to zero-and I do use the pot to control output, but I haven't been able to try anything at full volume for a long time-hopefully soon.
cheers
andy k

Dave_Mc

Ah I got ninjaed by @ICBM I didn't even notice earlier. Listen to him, basically. 

ICBM said:

That sounds like you've got a bad connection between the pot track and ground, with an effective resistance of 10.16K ohm. 

This can happen when the pot terminal makes a poor contact with the trackboard - this especially happens if you bend the terminal back to solder it to the casing, which strains the rivetted connection. 

That's very interesting. I've always just soldered a separate wire to ground the third lug on a volume, because I was scared of breaking it by bending it back- I thought I was being hyper-cautious, but apparently not! (Also I figured, maybe wrongly, that it was easier to solder a wire to the back of the pot than the lug!)

ICBM

A wire is better, yes. I do usually bend the terminal back just because it’s easier and ‘vintage correct’ if I’m rewiring an old guitar, but I do it by actually bending it over with the pliers rather than just pushing it with my fingers, and I always give the rivet a nip afterwards just to make sure.

It’s actually a very common problem, I’ve fixed hundreds of them probably.

Dave_Mc

^ Wow! That's nice to know that, by complete accident, I hit upon the better way to do it, lol.

andy_k

I had the guitar at a rehearsal last night, quite like the effect I was getting, but decided to have a look today- thinking I have probably got it backwards.
It actually looked ok, except that this was a re-used pot from something old, quite messy.
I have braided wire going from output jack, braid is soldered to case of pot, with bridge ground, and a single wire(bridge ground ) going into lug 1, this looks like a healthy solder joint and the lug is not bent. I had core from output lead to middle lug, and hot output from switch section to lug 3, this all looked ok but I was thinking it could be backwards, so I swapped middle and lug 3, all other grounds, ie, braid from bridge humbucker and unused leads from neck humbucker ( not inc coil splits) are going to another healthy looking ground point on the end of the switch section.
Hot leads from pickups are going to top and bottom lugs of switch, with centre lug being hot (same side of switch)
The effect now seems less obvious, something is obviously wrong as I now just seem to lose a bit of treble when the vol is rolled off.
I honestly CBA right now to undo everything, so I think I will get another push / push switch and re do it fresh.
Strange how some things should just be left alone, and I will first re do my now backwards wiring when I have a clear head.
I can't see how I got something so simple wrong in the first place, maybe it was just a funky pot to begin with?

ICBM

andy_k said:

I have braided wire going from output jack, braid is soldered to case of pot, with bridge ground, and a single wire(bridge ground ) going into lug 1, this looks like a healthy solder joint and the lug is not bent. I had core from output lead to middle lug, and hot output from switch section to lug 3, this all looked ok but I was thinking it could be backwards, so I swapped middle and lug 3

It was correct before - output jack to middle terminal.

But even if the ground terminal isn't bent back, try squeezing it hard onto the trackboard with pliers, since there is still a resistance there.

andy_k

Nailed it @ICBM, took a breath and went back in there, swapped the 2 wires back round, cleaned up a bit of mess, and squeezed hard on the lug while I had it plugged in - volume pot restored.
I think push / pull and push / push pots are not really built for regular use, and in my case here, the pot is being used for volume and pickup selection so I think I will try and get a few in. Push / push works so much better for this application as the strat knob is not designed to be easily grippable so I will have to source something fairly well made, not a lot of choice out there really. Do Bournes make one I wonder?
andy k

ICBM

CTS make a really nice quality push-pull now, using a full-size pot and with the switch connections on a PCB, which actually makes it easier to wire.

https://www.allparts.uk.com/products/push-pull-audio-pot-cts-500k

As far as I know it's available in 250 and 500K, split and plain shaft, long and short bushing - although you may have to search various vendors to get the combination you need.

Not sure if Bourns do a push-push, but Alpha do.

andy_k

Thanks, plenty of choice for push / pull, not so for push / push, I'll do a bit of digging sometime.
cheers.

Dave_Mc

ICBM said:

CTS make a really nice quality push-pull now, using a full-size pot and with the switch connections on a PCB, which actually makes it easier to wire.

https://www.allparts.uk.com/products/push-pull-audio-pot-cts-500k

As far as I know it's available in 250 and 500K, split and plain shaft, long and short bushing - although you may have to search various vendors to get the combination you need.

Not sure if Bourns do a push-push, but Alpha do.

Bourns does a push-push. Seem pretty nice, however...

I ordered a couple of log ones, but I think I got sent linear ones by mistake I need to open the guitar to check (i usually check before i fit it, of course this one time I forgot to ). but they feel very much like linear, unless I duffed up the wiring somehow...  

Does CTS do push-pushes? I haven't managed to find any...