WTB: HP419A MCM lens

[guenthert]

I just recently acquired a HP419A in reasonable good nick (seems like a low mileage unit: neons work perfectly, even the NiCd batteries still hold charge).  Unfortunately it arrived with the lens fractured -- it was poorly packaged  .  The MCM still works fine, I just need the glass.  Does someone here have a broken HP419A (with good lens) sitting for too long on the bench waiting for a repair?  I happily take the whole unit or just the glass MCM, either way.  Would be willing to spend a couple of bucks too.

I could pick up in the S.F. Bay Area.

[beanflying]

I would look at cutting a polycarbonate one to suit unless you know a good glass or clockmaker/jewelery shop. Be careful of the Mercury cell for leakage in case you haven't seen it yet. Just working on my fairly recently arrived 419A on the bench at present.

[pelule]

I have two 419A - an old one (still waiting for repair) and a newer one, completely refurbished.
The newer one doesn't have the mercury cell anymore. There is a lithium battery instead.
There is a difference in the MCM between the older and newer also.
So it would be good to know a bit more about the age of your's - best would be the serial number to judge the age...
/PeLuLe

[guenthert]

Hmmh, I might have a Frankenmeter: the top case has a calibration (!) sticker, expired in 1974, but the rear wall has a stamp that reads 9/78.  The serial number I found is: 948-03349.  Oops, no that's the patent number, where it also reads "patent pending", so I guess, one of the earlier models.  Can't find a serial number on the outside, but the cover of T2 (whatever that might be) on the A2 board lists 9100-1391 and 9/69.

[beanflying]

The QA sticker on mine is 9-87 and it's last Calibration date was 28/1/85 so I suspect the 9-87 is not a date code but an inspector code.

Serial number on the rear of mine (riveted aluminium panel) is 707-01649 it just has the line above it claiming patent pending. That tag is consistent with other similar aged bits of HP gear I own and their serial numbers. So your serial would be 948-03349.

The number on my meter if it helps is 1120-0312 if that helps.

EDIT The Manual for your 948 prefix unit is here https://www.keysight.com/en/pd-419A%3Aepsg%3Apro-pn-419A/dc-null-voltmete?pm=PL&nid=-536900193.536897533&cc=AU&lc=eng

I have a PDF of the 707 Prefix one but have forgotten where I got it from (old age) 

[pelule]

707-01649

That's the serial number and shows the revision (707).
707- is a quite old revision, HP build it up to revision "948-".
The link is the by Hesight/HP published scan of their lastest manual including "change/upgrade" information from older revisions.
It has unfortunatel a very bad quality.
An version for the 707- revision in a much better quality you get from "The Boat Anchor Manual Archive"
bama.edebris.com/download/hp/419a/HP%20419A%20DC%20Null%20Voltmeter.pdf
/PeLuLe

[guenthert]

Removing the glass from the MCM has been allegedly easy for the fellow over at https://www.amplifier.cd/Test_Equipment/Hewlett_Packard/HP_meter/HP419A.htm .   I had a look and to me the MCM seems sealed (I found four brass screws, but after removing those, nothing comes apart).  I even have difficulties removing the MCM, as for that I'd need to remove the front plate, for which I would have to remove the knobs.  Unfortunately where the holes for those little screws holding the knobs should be, is some hard (much like bakelite) material, sealing those.  Not sure, if that's done by HP (why would they do that, the meter is otherwise quite service-friendly) or by the University to which the meter did belong in the past (to deter halfwits from stealing knobs?).  Did anyone see such before and know a way how to get past that hurdle?

[beanflying]

Random item on evilbay today while looking for multimeters. eBay auction: #223064574713 The 427A meter this appears to be is the same form factor as the movement for my 419A

[guenthert]

That's too kind of you to look out for me there, thanks!

  I do however need to figure out how to take the meter out of the case first, not to speak of the lens out of the meter.  I'm in no particular rush though and I'm sure there are quite a few non-working 419a on the shelves collecting dust.  Someday someone will do a spring cleaning.

  I also got distracted by an almost working (meanwhile working -- I think, not entirely trivial to test whether their performance is in spec.) lock-in amplifier.  Those things are even more interesting than null voltmeters 

[beanflying]

There is a reason I had to rotate the picture of my two HP's they are upside down sitting on a shelf without their covers while I am doing 'other jobs'

Also realised I screwed up the item link  Now fixed and here again eBay auction: #223064574713

[guenthert]

Meanwhile I found another unit at a local flea market.  That one is apparently considerably older (colour scheme is more beige/brown than grey/blue), the batteries dead and the neons flicker a-rhythmically, so that would be a bit more work.

Unfortunately where the holes for those little screws holding the knobs should be, is some hard (much like bakelite) material, sealing those. 

Meanwhile I noticed that the black hard stuff isn't actually all that hard, but rather rubbery and seems to have been put in place for appearance alone.  So then I was able to take the MCM out and swap it with the other unit.
Now, I need to address the other issues:  too high input bias current and excessive noise on the 3xxx ranges only -- the 1xxx ranges seems fine 

[guenthert]

  Just in case someone else runs into similar issue (broken lens) -- perhaps the whole thread should now rather be in the repair section:

> The 427A meter this appears to be is the same form factor as the movement for my 419A
  On a flea market I acquired a MCM from what might have been a HP400EL or 400FL.  Same form factor, but those AC meters don't have a mechanical null adjustment.  The front face seems to be prepared to take such, the hole is just covered by some black rubbery stuff.  I was able to transfer the disc with the pin grabbing the null point adjustment, but I can't get the plastic button (head of screw?) in the front out of the broken meter.  Oh well, works kinda, sorta without.

>  I had a look and to me the MCM seems sealed (I found four brass screws, but after removing those, nothing comes apart). 
  Not sure what happened there, but now after removing those four brass screws in the back of the MCM, the front face with lens comes off fairly easily (held in place only by the QA sticker/seal on the bottom).

  Just replacing the glass window itself would be a bit of a challenge, as it seems to be glued in place.  Don't know how to soften or remove that glue.  As it is, the glass would rather shatter than separating from the plastic frame.