Analog DC Volt Meter Repair?

[garytoosweet]

Hi,

I have a power supply from the mid 80's with an analog Volt meter that isn't measuring correctly. This is my first time encountering an analog volt meter.

The meter is a Beede MCB FS300VDC. Display read out is 0-3000V.

Originally, there was a resistor array between positive power supply output and positive meter input, 3 carbon comp. resistors spec'd at 2.55M, now reading 2.9M.

Internal resistance is of the meter is 301k. With the 'FS300VDC' reading I deduced that there needs to be 1k of resistance per volt on the input, so I changed the resistor array to fresh resistors measuring 2.7M.

The meter is still off by around ~25%. 400V on output is reading just under 300V. Originally reading lower, it came up to 300 with a few hard taps on the front and is now staying there.

Is the metre faulty, or is there something I've missed? If faulty can it be repaired, or am I looking to replace?

Some Photos: https://imgur.com/a/CJdeOTK

Thanks for any help,


 

[George Edmonds]

I will not look at photographs that need you to accept the hosting sites advertising cookies first.  From what is said the meter is a 1K per volt one or 1ma full scale.

George G6HIG

[TheMG]

Is the meter sticking at all as you increase/decrease the voltage or does it come up and down smoothly and consistently?

If it appears to be binding, or gives non-repeatable readings, best to replace it.

Otherwise if it moves smoothly, consistently, and gives repeatable results, I would just adjust the resistor values until the reading is correct, you could even throw a potentiometer in there to make that task easier.

It does sometimes happen that the magnet in the meter movement can get weaker after many years, which would cause the meter to always read too low.

[edpalmer42]

As TheMG says, if the meter is mechanically sound, just adjust resistors as necessary to get the proper voltage reading.  I'd be cautious about using a potentiometer.  I don't think I'd like to stick a screwdriver into a pot that was sitting hundreds of volts or more above ground!

Either my calculator is broken or there's something odd here.  The meter is 300 Vdc.  It's reading a voltage of 3 KVdc.  That means the external resistors have to drop 2700 Vdc.  The resistors are supposed to be 3 X 2M55 = 7M65.  That works out to 2K833 ohms/volt, not 1K ohms/volt.  However, 2700 V and 7M65 gives a current of 352 ua full scale.  I'm used to nice, even numbers like 100 ua, 500 ua, 1 ma full scale for an analog meter.

Normally, a meter just has a resistor in series to set the maximum voltage reading.  Maybe in this case, there's a voltage divider to ensure that the meter movement doesn't see a high voltage under any circumstances.  But whatever is inside the meter, those internal resistor(s) could have drifted over the years. 

If the meter is sticking, then that's a completely different project!  You could consider replacing the meter with another analog meter or a digital meter.  If the sticking is minor, the meter might just need cleaning.  Move very carefully!  An analog meter is a very, very delicate device!  Before touching it, see if you can find a web page or forum post on analog meter cleaning / repair.

[Jay_Diddy_B]

edpalmer42,

My understanding is that the meter is a 300V FS analog meter.
The OP measured the internal resistance as 301K

So the meter is a 1mA movement with 300k series resistance.

An additional 2.7M is required to change the meter to 3kV FS.

He says there were three carbon comp resistor specified as 2.55M This must be the total for the three resistors in series.
It should in theory be 2.7M

Carbon comp resistors are not a good choice for this application and have drifted to 2.9M .

This all make sense.

There may be the additional problem of the meter sticking.

May be the OP can share more information on the power supply?

Regards,
Jay_Diddy_B


Regards

[garytoosweet]

@Jay_Diddy_B you have it all correct.

@edplamer42 The value I gave  for the original resistors was the total of the 3 resistors in series, specified as 2.55M and have drifted up to 2.9M.

Tomorrow I will double check that the meter is not sticking, and see if I can determine a resistance value that will read accurate voltage.

It is an old electrophoresis power supply from the mid 80's. Very simple, just a big transformer attached to a 10A variac with minimal filtering (6 40uf@500V caps in series), shoved in a box with a volt and current meter.

[Jay_Diddy_B]

Hi,

The good news is that those meters are fairly common I think they are modutec.

You might be able to find a 1mA movement and swap the scale.

Regards,
Jay_Diddy_B

[bob91343]

If you can open the meter it may be feasible to repair it.  I have repaired many meters (and ruined many others) so it's possible.

A sticking pointer may be due to rubbing on the scale or other mechanical interference.  The bearing pivot may be damaged or maladjusted.

Best to test the meter alone, without any multiplier resistors to see if it is truly 1 mA F.S.

[edpalmer42]

I knew I was missing something!     Thanks for the explanation.  It's amazing how easy it is to misinterpret even simple things.

Another trap that analog meters can throw your way is that the clear cover can, particularly if it's plastic, pick up a static charge that can affect the movement of the pointer.  Touching the cover can help dissipate the charge.  So can wiping the surface with a damp cloth.

[garytoosweet]

I have verified that the main issue is a sticky meter.

Opened it but could not see any reason for it to be sticking. Tried to blow some air through it carefully but the problem remains.

I found a Modutec that's physically the same size but for AC Volts. If I pick this up, it should be possible to swap the scale right? Would the only thing required to modify to read DCV be adjusting resistor values?

https://www.ebay.ca/itm/Modutec-Jewell-2S-AVC-250/222249082814?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2763.l2649

[edpalmer42]

No, don't buy that meter!

Don't buy an AC meter.  There's too much 'stuff' inside that you have to remove.  Buy a DC voltmeter.  In a perfect world, buy a 300 Vdc meter.  I saw this one:  https://www.ebay.ca/itm/142997478434 , but it looks like it's too big.  There's another one but it costs about $50 and still might be too big.

It may or may not be possible to move the scale.  If the new meter is a different brand, it gets much more complicated.  Your meter looks like it's Modutec, but it says Beede on the back.  I can see screws holding the scale in place, but similar-looking Modutec meters don't have visible screws.  Some people change the numbers on a meter and change the internal resistor(s) as necessary.  Some people make custom scales.

Swapping out a non-standard analog meter like this one can be a nuisance.  In a case like this where you've got two that are supposed to match,  it can get downright ugly.  It might be easier to swap both meters so they match.

I'd take another look at repairing the meter.  After all, you've got nothing to lose.  Remove it from the panel and then remove the movement from the plastic case by removing all the nuts and bolts on the back.  Leave the scale in place.  The entire unit should lift out of the case.  Now do a good visual inspection under strong light.  Is the pointer bent?  Is the scale bent?  Does the pointer look like it might be touching the scale?  Is the pointer bent away from the scale so it might be touching the inside of the cover?

If your inspection doesn't show anything obvious, confirm that it still sticks when it's outside the case.  You can either move the pointer with a toothpick or something similar (so you don't scratch the scale) or ideally, by using power.  But don't use 300V!  You measured 301K resistance which means that the meter is 1 ma full scale.  So, if you connect your cliplead to the point between the internal resistor and the meter movement, it will take 1 ma of current to get a full scale reading.  Use any convenient resistor and power supply to accomplish this.

If nothing has worked, try putting one drop of 99% isopropyl alcohol on each pivot.  Exercise the needle a few times with power or the toothpick.  If it still sticks, the problem is more serious and replacement starts to look a lot more attractive!

[bob91343]

If there is no debris or other obvious interference with motion, it could be an issue with the pivots.  At the worst, a cracked jewel or bent pivot, and at the best a loose or overly tight pivot adjustment screw.

An ac meter might have the fundamental movement you need, and converting it shouldn't be a big deal.  If it's cheap enough, take a gamble and see.  But the two meters must be from the same manufacturer; otherwise it won't fit.

[garytoosweet]

Another repair attempt today was unsuccessful and I think I may have damaged the meter. Now I understand why people say they are delicate.

I will look to replace. Either replace both meters, or perhaps could just put a smaller digital rectangular meter over the logo on the top left and keep the analogues for looks.

 This may make the most sense in terms of budget and ease of replacement. Since it has no collectible value and was meant to be a budget HV supply that will rarely be used.