Panel Meter Ref. Pots

[engrguy42]

I've got a cheapo panel meter (V/I) I'm going to use with my power supply remodel, and I noticed it's quite a bit off compared to my multimeter. Amps show 0.0 when it's actually 0.1, and voltage is low by maybe 0.5 volts or so sometimes. Maybe that's within specs, not sure I care.

Anyway, I noticed some super tiny reference pots with metal casings, one for V and one for I. So I figured what the heck lemme see if I can tweak them.

Turns out when I touch the pot with my metal screwdriver the amps reading goes thru the roof. Like up near max rating of 30A. Apparently they want a plastic screwdriver or something? Good luck finding a tiny plastic screwdriver....

Is this normal and I'm just out of the loop?

Thanks.

[ColdKeyboard]

Can you maybe share some pictures?

[schmitt trigger]

Back when radios and TVs had tuneable RF and IF stages, assembly lines employed plastic or ceramic screwdrivers to tune the variable inductors, capacitors and trim pots.

Repair shops would also use them.

I haven’t seen them since the late 80s.

[WattsThat]

It’s picking up mains noise conducted through you.

As mentioned, you need a non-conductive adjustment tool. Search eBay for “ceramic screwdriver”, tons from China for a buck apiece or thereabouts.

[ITman496]

We have a million different kinds of those at my office (land mobile radio business) so I kind of forgot how rare they are.  I forget where we get them, most of them are older then I am and say motorola on them!

[wizard69]

Back when radios and TVs had tuneable RF and IF stages, assembly lines employed plastic or ceramic screwdrivers to tune the variable inductors, capacitors and trim pots.

Repair shops would also use them.

I haven’t seen them since the late 80s.

These are still available from suppliers in a variety of forms.   I make use of them for tweaking motor drives and stuff many or which are still tuned via pots.   The plastic shafts add a bit of safety.   If there is a metal blade (that is a possibility) you have only a tiny bit exposed.

[engrguy42]

Here's a photo of the culprit...

[ITman496]

Last week at work we wished we had one, we were tuning the oscillator on a Master III receiver shelf and we at first were using just a regular screwdriver and, well..  Wow, that changed it a lot just bringing it near it!  Eventually it turned into look where it was, tweak a bit, pull it back out and look again, rinse, repeat, until tuned.  Regret leaving the special plastic/ceramic drivers at the other building.   

[engrguy42]

It’s picking up mains noise conducted through you.

As mentioned, you need a non-conductive adjustment tool. Search eBay for “ceramic screwdriver”, tons from China for a buck apiece or thereabouts.

Thanks. Yeah, I think my tolerance for monkeying with this is pretty low since it's such a cheapo meter. I might just cut out a piece of plastic and dremel it to a fine point or something.

BTW, here's an image of the meter itself. It's one of those very common Chinese devices on Amazon, listed under 27 different rebranded names.

I would think this is a fairly common issue here since these are so common, and assuming these unlocked/unglued pots are probably off when received from the mfr?

Or maybe the folks here can't be associated with a cheap $10 panel meter? 

[digsys]

Just bought a heap of very similar DPMs, but I didn't pay that much for them ! You been ripped off :-) .. just finished making up a dremel router table to fit them into a plastic case ... but back to your issue ... in years gone past - we would cut up a strip from acetate / plastic / etc sheets and wedge it into a strip of wood etc

[rdl]

Ceramic alignment tool/screwdriver.

First Amazon hit when I searched as an example, there are others:

https://www.amazon.com/Hanperal-Slotted-Ceramic-Alignment-Screwdriver/dp/B01AJC29OI

"ceramic screwdriver" on ebay.

https://www.ebay.com/sch/i.html?_nkw=ceramic%20screwdriver

I may order a set just to have them around.

[engrguy42]

Okay, problem solved... 

I only had to do a few repeats of the "turn everything off, tweak the pot, turn it on and see what you get" cycle, and now my V and I pots are dead nuts on.

Sometimes the simplest is the best. 

[schmitt trigger]

Or maybe the folks here can't be associated with a cheap $10 panel meter? 

Of course not. Once that one becomes aware of its limitations, they are perfectly good monitor meters.

My experience when these drift, is because the open frame nature of its trimpots. The resistive element might also be cheap carbon film instead of the better tempco of cerment.
A simple upgrade is to remove these cheap trimpots and replace it with a good sealed cermet. A multiturn type is even better.

Of course, these can cost almost as much as the meter in the first place! But it is a simple and very significant improvement.