Author Topic: Fluke 415b (2.5KV) Power supply - a techician is often his or her worst enemy  (Read 2938 times)

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Offline Excavatoree

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I was working on a Fluke 415b I bought in a lot of two  parts units.  (415B and 412B)  Turns out the 412 was OK - it just needed a meter.   The 415 had minor problems, so I fixed the two "parts units" with parts from the original unit I was going to fix, which had physical damage, bodged resistors soldered in, etc.

The first problem with the 415B was some of the voltage selections for the X100 volts were wrong.   These work by switching in resistors of a voltage divider, and it as obvious one was burned open.  I robbed my other unit and fixed that.  (mechanical disassembly to get to it was the worst part.)  The second problem was that the calibration pot didn't QUITE get the unit in spec.  Turned all the way, it was a few percent low.  (198 volts in 200 v setting,  396 in the 400 v setting, etc.)

The manual mentions trimming jumpers that short out resistors in the divider.  I saw that one was open, so I re-applied it.  I then had the opposite problem.  The voltage was too high even with the trimmer turned in the other direction. Ah - ha, I thought, all I need to do is cut a jumper for a smaller resistor, and I'll have fixed it.

Not so fast, I thought.  I better go through the manual procedure and do it right.   I measured the reference voltage (the trimmers are to compensate for the varying vacuum tube voltage reference) and it was off quite a bit.  I removed the tube and checked it, but it was in spec.  "Something's wrong with the circuit." I thought.  I fiddled around checking transistors and diodes, but I couldn't find the problem. 

Worse, I created a solder bridge on the unmasked PC board.  I found that when the voltage suddenly changed by quite a bit.    At least that got me to inspect the board, and i found a diode I installed backwards.  I took a break, and another look at the service manual and saw I had the incorrect test point.  Great.   It was a couple of volts off.  If it has been something quite different, I'd have caught it, but I thought the reference or the surrounding circuit was bad.

After one more check to be sure I hadn't messed anything else up, I measured the reference voltage, and noticed that for the voltage I have, I should cut the jumper for one of the smaller resistors.    I was correct all along.  I did that, and it was fine.  I adjusted it with my best meter, and the pot was near the center of the adjustment range. 

A couple of the resistors for the 2000 and 2500 volt ranges are a bit off, but they are close enough for what I'll do with the thing.    Otherwise, it seems to work very well.

I'll mention the odd meter problem I have with the three supplies here instead of starting a new thread:

I have a 412b with a bad meter. (cover gone, needle bent to heck.)  It's of type "A"
I have the now working 415b with a good meter, also type "A."
I have the "parts" 415b with a good meter, but it's type"B."

I'd like to put the meter from the parts 415b into the 412b, but they are differing types, so I can't exchange the scales.  (412 is a lower voltage supply)
I could put the parts 415B meter in the working 415b, then use the meter from the working 415b in the 412 after changing the scales.  (same meter type, no problem)

However, I don't want have to play "musical meters" so I found a "type b" meter on E-bay for too much money and bought it. 
 

Offline Smith

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I have a Fluke 412B at work. These things are very stable (something you don't expect when you see its antique guts). Due to the easy adjustments and stability I use it to check multimeter's a lot.

Some time ago it lost 50% of its output value, the caps of the reference where shot. That was an easy fix.
Trying is the first step towards failure
 

Offline Kryten 2X4B

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Hi Excavatoree,

I'm looking for a power transformer for my Fluke 415b, are you interested selling just a transformer?

Regards,
Peter
 

Offline Excavatoree

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Hi Excavatoree,

I'm looking for a power transformer for my Fluke 415b, are you interested selling just a transformer?

Regards,
Peter

I'm not ready to give up on that last unit - I'm hoping to find a unit with a defective transformer or damage other than the few parts I need.   Shipping from the US may be prohibitive.    If I buy a parts unit that has a good transformer, or give up on this one, I'll let you know.
 


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