VU meter repair

[smorphet]

Hello,

Can anyone give me any tips on repairing a VU meter?  This is a proper Ernest Turner moving coil meter, with its own rectifier.  I've pulled the meter out, so buffer amp problems, etc., can be ruled out.  I don't recall doing anything that might have caused damage.  It used to work fine, then the unit sat on a shelf for a while, and when I plugged it in this week, one meter wasn't moving.

There's absolutely no needle response to a signal.  From the point of view of the driving circuit, the meter seems normal, with a reasonable looking drop across the external 3K6, so it's not completely open circuit.

The needle moves freely, but bounces around more than expected, appearing under-damped, for example if I pick up and move the meter.  My working meters don't bounce like this.  Is the damping likely to be mechanical or electrical?

I don't really know what components are inside a meter like this, and they're mostly hidden behind the movement.  Incoming signal first encounters a resistor on each side.  The rectifier must be hidden.  Is there anything else in there?

I suspect I'll have to have it apart, but while I'm not a complete sausage-fingers, this is smaller and more delicate than anything I'm used to working on.  I'd appreciate any advice from anyone who has done it before.  What am I likely to find, and what are the chances that I'll be able to repair it?

Steve.

 

[MikeClev]

There are very fine wires in there. I broke one about 6 months ago leaving the soldering iron on the (external, input) terminal too long, and the wire (inside) came off, that wasn't much fun to fix!

 

[s2udio]

This may help

http://www.tpub.com/neets/book3/7b.htm

The diode bridge is more than likely on the inside of the meter...
mounted on the terminals...

Take it easy ...

 

[sodderboy]

Meter does not work, passives are OK
needle "bounces around too much"

Sounds like one of the two tension coils is broken.  They are on the front and back of the assembly and serve two purposes- they act as a ballistics control and spring return, and they pass the + and - wires from the stationary magnet side to the moving coil side.

Get a jewelers loupe and take a look in there.  Quite a marvel
Mike

 

[smorphet]

Sounds like one of the two tension coils is broken.

Yes, I think you're right.  I've just been having a look at it through a lens, and I *think* I can see that the top spring is broken at the centre.  It's definitely a miniature mechanical marvel.

Is it possible or sensible to try to repair something like this?  The break looks to be at or near the soldered connection, so perhaps it could just be re-soldered.  I don't fancy my chances at doing it myself.  Does anybody repair meters (in the UK), or is it the sort of thing a watchmaker would have a look at?

Steve.

 

[sodderboy]

You could try VU repair specifically, maybe Brian Gibson at TG Electronics knows of one and yes,a real horologist would have a small welder or brazier.

It really is a matter of having the correct tools for the job.  Some good magnifying glasses and a tiny tipped iron would do the trick.

Mike

 

[smorphet]

Thanks guys for your suggestions.  I will report back if I have any success.

Steve.

 

[lassoharp]

I've got a VU meter that I recently took apart to remove the rectifier and upon reassembly found that the needle will not return completely to the far left rest position (stops at -10).  It seems to move normally for VU use but is sluggish on GR and not very crisp for small movements on lower input levels.

I'm not sure if I damaged anything during rectifier removal and unsure of what if any mechanical adjustments I can make to restore the full left resting position.  The needle does not bounce freely and still has what looks to be otherwise normal tension beyond the elevated resting point.  Any suggestions for what to do/look for?  


EDIT:  Question amended.  Looks like it's and issue of having the turn screw properly refitting into the adjustment fork.  Kind of tricky because you can't see exactly how it was positioned originally.  The fork being out of position was apparently the cause of the needle resting too high.  Off for some more tinkering . . .