Author Topic: HP 6920B Meter Calibrator  (Read 15479 times)

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Offline gilbenl

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HP 6920B Meter Calibrator
« on: January 30, 2015, 02:46:40 am »
I recently bought a HP 6920B Meter Calibrator as a tinker/restoration project. Being a stone cold genius (:palm:), I knew that the manual was available from agilent and all these old HP manuals had schematics. So, I bought without double checking. And, I was right...unfortunately the manual scan is piss poor awful and thus the schematic is piss poor awful and illegible.

Anyone have or know where I might be able to find one?
« Last Edit: January 30, 2015, 10:17:10 pm by gilbenl »
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Online tautech

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Re: HP 6920B Meter Calibrator Schematic
« Reply #1 on: January 30, 2015, 04:02:09 am »
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Offline gilbenl

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Re: HP 6920B Meter Calibrator Schematic
« Reply #2 on: January 30, 2015, 06:30:30 am »
Thanks. Hate that it was that simple. Must have used a different search than I did. This is infinitely better.
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Offline gilbenl

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Re: HP 6920B Meter Calibrator
« Reply #3 on: January 30, 2015, 10:45:13 pm »
Thought I'd throw up some pictures of this unit. After running through the service section, its pretty clear that the output is extremely unstable, although it is consistent through the ranges. I plan to clean it up and at least to get it back to original spec (0.1%). I would then like to upgrade selected components to see if I can increase that spec, at least on the DC range. Seems like a good learning experience.

Prior to powering up the unit, I took a quick run through things. After a few days of power, it seems that most of the sprague caps on the top board have cracked. The smaller sprague's are 100uF/50v and larger are 1200uF/20v. The silver are HLAB 2250uF/10v and 1400uF/30v.
« Last Edit: January 30, 2015, 10:52:23 pm by gilbenl »
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Offline wn1fju

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Re: HP 6920B Meter Calibrator
« Reply #4 on: January 30, 2015, 11:06:20 pm »
Sprague capacitor problems?  Gee, what a surprise.  If you start repairing test equipment from the 60's, 70's and even into the 80's, you will quickly learn just how bad Sprague electrolytics can be (or should I thankfully say, were).
 

Offline gilbenl

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Re: HP 6920B Meter Calibrator
« Reply #5 on: January 30, 2015, 11:38:21 pm »
I ran across some sprague's in my 6113A, but they came back to life nicely (except for one). Needless to say I'm familiar with their reputation. Unless anyone has a reason not to, I was planning to replace all the caps en masse. Many of the transistors also look rather rough and some modern equivalents should perform far better.
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Online tautech

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Re: HP 6920B Meter Calibrator
« Reply #6 on: January 31, 2015, 12:15:25 am »
I ran across some sprague's in my 6113A, but they came back to life nicely (except for one). Needless to say I'm familiar with their reputation. Unless anyone has a reason not to, I was planning to replace all the caps en masse. Many of the transistors also look rather rough and some modern equivalents should perform far better.
I'd be more concerned if they where leaking.
For the PSU ones, I'd first be checking the ripple, then maybe sub with another to prove replacement WAS necessary.
Replace working transistors in calibrated test equipment?  :o WHY
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Offline SeanB

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Re: HP 6920B Meter Calibrator
« Reply #7 on: January 31, 2015, 09:35:00 am »
The Spragues were good for 2 decades after manufacture, but now they are in need of replacement. As to changing old hermetic transistors because they look bad it is not a good thing, as you can get an "improved" transistor that is not going to work or will oscillate while the old one never would.
 

Offline gilbenl

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Re: HP 6920B Meter Calibrator
« Reply #8 on: January 31, 2015, 07:43:12 pm »
The Spragues were good for 2 decades after manufacture, but now they are in need of replacement. As to changing old hermetic transistors because they look bad it is not a good thing, as you can get an "improved" transistor that is not going to work or will oscillate while the old one never would.

I removed many of caps in the initial teardown. I'm not a fan of replacing things for the sake of it, but they're 50+ year old caps all with cracks through the endplates(?), many bulging. Worst case scenario I've spared myself the headache of replacing the caps down the road. I'm not sure if I'll go with NOS or modern replacements. I will have to do some research to better understand precisely what each one is doing and if there would be any benefit to a modern replacement other than possibly being easier to find/less expensive.

As far as the transistors go, I may have misled-There's a particular set I think might need replacement that are associated with Q1A and Q1B in the DC reference circuit. The ripple on Q1B shouldn't exceed 1mv and the 1V reference should read b/w 0.7V and 1.0V. When I tested it, the ripple at E4 on switch S1 was irregular and greater than 1mv and the 1V reference read 0.26. Per the excerpt below, my understanding is that there are some open transistors in the neighborhood. The zeners and resistors tested fine. Please correct me if I've diddled something up.
« Last Edit: January 31, 2015, 07:44:59 pm by gilbenl »
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Online tautech

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Re: HP 6920B Meter Calibrator
« Reply #9 on: January 31, 2015, 07:50:01 pm »
Quote
Please correct me if I've diddled something up.
Your detailed reply put my mind at ease.  ;)

Please continue sharing your findings.  :-+
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Offline SeanB

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Re: HP 6920B Meter Calibrator
« Reply #10 on: January 31, 2015, 08:00:49 pm »
Change the bad caps first then try again. Ripple on the supply rails will cause issues. Only change out those transistors if they are actually bad, either short, open or leaky. Hermetic packages also had issues with loose particles shorting them out, just tap and see if any show this, caused by internal metal particles floating loose. Most transistors however have a silicone grease blob inside to protect the active device or have a passivation coat of glass doing the same.
 

Offline gilbenl

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Re: HP 6920B Meter Calibrator
« Reply #11 on: February 01, 2015, 10:41:24 pm »
Spent some time researching the caps and they're rare birds indeed-Used, NOS or modern equiv. Will be following up soliciting some advice once I've identified some potential replacements. There is one cap, however, that I hoped wouldn't need replacing: Sprague D33267 4400uF/ 2VDC. Murphy has won again.

Here's where my ignorance shows-This section of the output is a DC-DC converter, specifically a push-pull configuration? After looking at it again, this is the current output, correct? The closest equivalent I've come across is 4400uF/63VDC. What do I need to consider in selecting a replacement for this?
« Last Edit: February 01, 2015, 10:45:48 pm by gilbenl »
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Online tautech

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Re: HP 6920B Meter Calibrator
« Reply #12 on: February 01, 2015, 11:23:33 pm »
2 questions:
Is the gang switch S1 a mains voltage selector?
What is the output votage at L!?
This will determine the voltage rating the cap must need.
I'd select a cap V rating of 150% min of the voltage at L1

Edit
The voltage rating of Q 34 might offer a clue.
« Last Edit: February 01, 2015, 11:46:06 pm by tautech »
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Offline gilbenl

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Re: HP 6920B Meter Calibrator
« Reply #13 on: February 02, 2015, 12:46:26 am »
2 questions:
Is the gang switch S1 a mains voltage selector?
What is the output votage at L!?
This will determine the voltage rating the cap must need.
I'd select a cap V rating of 150% min of the voltage at L1

Edit
The voltage rating of Q 34 might offer a clue.

I'm unable to measure the output at L1 given the current state of the unit. Correction, there's a math opportunity here...
Max current output is 10A
Inductor is 100uH
Ton=1ms (no clue, parts list doesn't specify the inductor)
Ipk=10A ("maximum" output of unit)

Ipk=V*Ton/L
V=1v


S1 is the Function Selector (Off-AC-DC), S2 (shown at top of attached) is range. R4 over in the error amp is the output pot. Q34 is a 2N1445 power transistor, Vcb=60V, Vce=30V. So, now I'm rather intrigued. I double checked and the cap I removed (4400uF/2V) is what is spec'd in the manual. Is this area in fact the output for the current ranges?
« Last Edit: February 02, 2015, 01:48:32 am by gilbenl »
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Re: HP 6920B Meter Calibrator
« Reply #14 on: February 02, 2015, 01:18:30 am »
OK, I thought 2V was an error/misprint.
Then it only need to have a V rating....say 100% more than the calibrators max V out so a 5 V rating should be plenty.
Just check my thinking against the rated output specs.  ;)
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Offline gilbenl

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Re: HP 6920B Meter Calibrator
« Reply #15 on: February 04, 2015, 10:43:17 pm »
Absolutely no chance of getting NOS replacements after scouring ebay and vintage radio/TV sites. I really didn't want to just solder in some modern caps-the orange heat shrink of the spragues really looked cool with the lime green PCB :P Came across the technique of "capacitor restuffing" and gave it a shot. The rest should be a bit neater, lessons learned. Polarity was maintained with the label despite whats shown in the first image, in case anyone is wondering.
« Last Edit: February 05, 2015, 03:06:24 am by gilbenl »
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Offline wn1fju

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Re: HP 6920B Meter Calibrator
« Reply #16 on: February 05, 2015, 12:55:51 am »
Great.  Until you tire of it and put it on eBay in a few years.  Then someone like me will buy it, take the cover off and, as a matter of principle, immediately replace all the electrolytics because they say "Sprague" on them!!!
 

Offline gilbenl

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Re: HP 6920B Meter Calibrator
« Reply #17 on: February 17, 2015, 03:45:03 am »
I recognize this is the "repair" section and this seems to have become more appropriate for "projects." Anyway, I've finished the cosmetic and functional restoration on the unit. Certainly not what some members on this board have achieved, but it's a start. It now also achieves its designed 0.1% accuracy. I plan to selectively replace some resistors and the reference zeners in order to improve stability and increase accuracy. If anyone is interested, here are some before and after shots. I welcome any comments or critiques and I appreciate the help I've received thus far.

http://s76.photobucket.com/user/nick_gilbert2/slideshow/HP%206920B%20Restoration
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Offline lowimpedance

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Re: HP 6920B Meter Calibrator
« Reply #18 on: February 17, 2015, 04:20:46 am »
Actually its quite a good job of restoration, down to the cap re stuff!  :-+.
(which I would not have had the patience for!!).
 A useful bit of fairly rare kit back in use saved from landfill, good job.
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 

Offline Arkham

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Re: HP 6920B Meter Calibrator
« Reply #19 on: December 09, 2015, 08:23:37 pm »
Wanted to "piggyback" on this thread - I just received one of these Calibrators, which will be a useful tool bringing my pile of old VOM's up to snuff  ;D
This one should clean up ok, but I am more concerned with getting it back into service.

Looks not too bad under the hood, at least nothing scorched

 

Offline Arkham

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Re: HP 6920B Meter Calibrator
« Reply #20 on: December 09, 2015, 08:27:22 pm »
The old Spragues are cracked, bulging, oozing, etc. and will be replaced immediately. I have a full set of long hour 105 degree caps en route (won't be restuffing the old ones, sorry!)

 

Offline Arkham

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Re: HP 6920B Meter Calibrator
« Reply #21 on: December 09, 2015, 08:33:28 pm »
I had PM'd gilbenl asking for any tips on the unit. He wrote back an entire novel of incredibly useful advice! Thanks gilbenl!
I'll be replacing the power indicator, the chewed up banana jacks, and cracked fuse holder.
Once the caps are in, any other cursory checks to make before first power up?
« Last Edit: December 09, 2015, 08:35:23 pm by Arkham »
 

Offline gilbenl

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Re: HP 6920B Meter Calibrator
« Reply #22 on: December 10, 2015, 09:53:21 pm »
oof, she's been around the block, hasn't she? Good bones though. There should be some date codes around, but can't make any out. Do you have a date?

I noticed that HV neon lamp is already missing. Any sign of what happened to it (remaining bits/leads)?

Would love to see the bottom board. There were a number of hidden "dammits" down there.
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Offline KJDS

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Re: HP 6920B Meter Calibrator
« Reply #23 on: December 10, 2015, 10:33:00 pm »
I've had one of these for a few years and I keep going back to it as my preferred calibrator, but it's starting to misbehave, so I guess I too will be replacing caps soon.

Online Cubdriver

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Re: HP 6920B Meter Calibrator
« Reply #24 on: September 03, 2016, 04:17:09 am »
Holy thread resurrection, Batman!!  I dug deep for this one.   :D

I just bought one of these on the 'bay today, so I'll be joining the club too.  It should be here in a week or so, what with the holiday on Monday. I just looked at the slide show of your restoration, and it appears to me that you did a fine job of it.  I don't think I'll go quite so far as to reshuffle caps on this, but we'll see when it arrives.

Did you ever get around to tweaking and increasing the accuracy?  I will likely be picking your brain for info once I get into working on it.

- Pat
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