Replacement meter for Omnipressor?

[hodad]

Anybody have any experience with repurposing a meter for the Omnipressor? It's an oddball size (2 5/8" high by 3" wide), made by Honeywell, with a three-stud mounting configuration. I've read from one source that it's a 50 ua meter, elsewhere that it's a 1 ma meter. I do have the meter face. Also, this is the later blackface model Omnipressor.

 

[Walrus]

Not had to replace a meter on our Omni, but the schematic I have says 50uA on the meter.....

 

[hodad]

Thanks for that.  I'm also wondering if, when the omnipressor is turned off, the needle sits at the center of the scale, or all the way to the left.  I was looking at a meter that might work sizewize, but the needle rests at center scale, which I'm guessing the Omni doesn't (even though that's where 0 is on the dial.)

 

[Walrus]

Hold on, let me check...

[Runs upstairs to Studio 2]

[Runs back down again]

When off, the needle rests at the -30 position.

 

[hodad]

Thank you so much, Kevin.  I think I'm on the right track now.

 

[mjrippe]

So did you have any luck with finding a replacement?  I've got an Omnipressor with a broken meter in my shop 

 

[joaquins]

50µA is the standard amp metter, most bigger ones use a 50µA one and a shunt... Shouldn't be difficult to find one 50µA but if you don't you are a diy person, open a bigger one (the cheaper you find in the size you need) and take out it's awfull resistance, in this time you can change the back paper so the metter has it's original back plane and no 5A linear full scale.

JS

 

[mjrippe]

I have some Honeywell VU meters that are 100uA fs, but they do not have resistors.  Perhaps the circuit could be modified to drive these?

 

[joaquins]

They should work, but I don't have the schems... I'm in a measuring class and this week we are with measuring techniques... so may be a good real world excercice and have no truble to see if I can help... link me the schems and I see them.

JS

 

[PRR]

> 50µA is the standard amp metter, most bigger ones use a 50µA one and a shunt...

1mA is usually a dollar cheaper than 50µA, so that's what most cheap meters are.

At least how it used to be. ALL analog meters are drying-up at the usual sources.

> a meter that might work sizewize, but the needle rests at center scale

When down to your last alternative, a center-Zero meter *might* be convertable to an end-Zero meter. Or a small paper-weight. Open it up. See where the face zero-screw hits the guts. That slotted tab rotates. Turn it over. There is usually another one on the back. Rotate it a bunch in the desired direction. if there's no back-spring and adjuster, bugger the front spring adjuster; but you will probably lose the face-zero function so get it as close as you can before you close it up.

 

[mjrippe]

So I dropped in the movement from the 100 uA meter (exact match physically).  Unfortunately even with the trim pots maxed out there was not enough current to drive the meter properly.  I ordered another 50 uA meter from a different manufacturer so hopefully I can fit it into the original housing.

 

[joaquins]

Don't you have the schem for this part of the circuit at least?? maybe changing one or 2 resistors it will work with the 100µA one...

JS

 

[mjrippe]

Have the schem, but with trims turned all the way, resistors don't matter.

 

[joaquins]

How it would not matter... R47 is limiting the current in the ameter, Put a trimmer there, maybe a 100K trim and a 50K resistor in series, will keep the max position if you change it for a 50µA again sometime and give a usable range for your 100µA you have now... put all the other timers half way so you can do a fine trim for each metering mode after you have the metter working.

The other option is what I show here. 100K should be fine. With this scheme you should be able to pot almost any ameter (not 5A one), just short R47 and put this trimer across the meter.

JS

 

[PRR]

> R47 is limiting the current in the ameter

No; ALL that mess top-left limits the current. The virtual-earth opamp just reflects it into the meter. The 150K *limits* the current to 15V/150K = 100uA, the maximum safe over-load for a 50uA meter.

We need to change ALL that mess of resistors top-left.

OR: we need a Current Multiplier.

Which in this case is simple. Take the meter out of the 150K loop and close that loop. The opamp output voltage to ground will slew to I(in)*150K. Add a new resistor in the meter loop to ground. Value is lower in proportion to meter FS current is higher.

To be fussy: the meter internal resistance should be accounted. The error will be very small, a couple percent. I am not an advocate of "trims turned all the way", but that's already happened. In re-trimming the error will get trimmed-out. (If there's detectable residual error, try 73K or 77K. Or 69K plus 10K trim-pot.)

 

[mjrippe]

Sorry, what I meant was that I put resistors in parallel with some of the existing ones and it did not change the way the meter responded.  Probably because I would have had to do that to ALL of them as PRR pointed out!  If I get a chance today I'll try the current multiplier method.  Much obliged!

 

[joaquins]

I don't know what I was thinking, you are right PRR.

mjrippe, you can still use my schem but connected from opamp output to ground as PRR said. To have a bigger trimable range, but not safe to your 100µA in which case a 56K resistor in series with the meter will take it to a safe limit.

JS

 

[mjrippe]

So I tried PRR's method - although R47 in this unit is actually 100K, not 150k.  Used a 68k plus 10k trim to ground.  No luck.  Can't get the meter to Zero in GR mode.  Swapped R47 for a 150k, still not happening.

Is this drawing what you are describing joaquins?

 

[joaquins]

Let's go again... To be clearer... I don't know exactly why is the switch there, it makes the meter measure one or the other half wave and loosing the other. always you will be loosing half way, the only important thing is when you are measuring something that's already DC (like most compressor CV) so in one way you measure somthing and in the other just nothing. I guess they put it there for something so I let it there.
Note I use an inductor symbol for the meter because I don't found any analog meter in my library.

Well, in the metering range, at the output of the OA you'll have a voltage output, and with a resistor in series it make a current flows, "protect" will (as it name sais) protect the meter in any range you put the trim. So you can have something more than your metter full scale but not tooooo much more or the meter will slam and get damaged. You can use it or not, but if not used be nice when turning up level till calibrated and be carefull to start calibration with trim all the way down (so metter is shorted by the trim) and then go up slowly. This will give a range that's limited by the current limit of the opamp and the voltage range it's excited because the input. If protection resistor is too big you won't have as much range as you will want, so if you want to be sure you can put something around 56K and go lower if range is not enoght. Big steps, don't loose to much time, if 56K donesn't work and meters goes 1/4 way put 10K or lower and see what happens is just to not put 100mA to a 100µA meter. trim it down to be safe before testing and then up slowly again. This trim will be usefull to change to 1mA meter or return to 50µA meter when you want. 10K to 100K trim should work, 10K probably gives you a more usable range if you have 1mA to loose here.

Let R47 where it is so the op amp get stabilized and controled gain, how much will depend on the input connected and the trim aplied to each metering section.

Just ask if I miss something or you want to know something more i can tell you or we can learn together.

JS

 

[mjrippe]

So I went back to this unit today and put everything back to stock.  The meter was responding differently - I could get to zero on the GR setting, but still could not cal everything.  Something had changed!  Suspecting solder joints, I re-attempted PRR's method and made sure to make solid joints everywhere as opposed to tacking things in place.  IT WORKS!  Not that I should be surprised with PRR's reputation, but I am really *REALLY* happy to have this problem solved!  Thank you all so much, the next rounds are on me  ;D