HP 410C Voltmeter repair and clean advice

[RichardM]

Hi All

I gave in yo my urge to purchase an old HP voltmeter, an HP410c.

It is complete with all probes including the AC probe but it is by far the filthiest bit of test gear I have purchased.
 I did some general checks for shorts and use a dim bulb and it does power up without blowing a fuse and most voltages look ok. More work to be done there though as it is not measuring anything at the moment.

The first bit if advice I was seeking was how to best clean the device. It had all sorts of fluff, dust and a few leaves inside that I have vacuumed out. It appears it may have been used in an auto or machine workshop as everything is covered in a thin sticky layer of oil or grease. It actually looks to have protected it from corrosion. What technique would be best suited to cleaning the insides which are a tight birdsnest of wires, gang switched and resistors etc ? A simple wipe is not going to be sufficient. I was thinking of using a small paintbrush and lots of soapy water ? Would this be safe ? Maybe a citrus cleaner to remove grease and a final clean water wash ?

Cheers

Richard

[Kleinstein]

A thin layer of clean oil is not that bad and there is no real need to remove it all. The oil is already doing it's job in reducing corrosion and the parts seem to be OK with it.
To remove mineral oil it needs quite some cleaning effort and the detergent could harm some parts.  So I would leave most of the oil and only use a dry clean with maybe compressed air (outside) and papertowls to remove excessive oil and dirt.

A point to check may be electrolytics for leakage and if there is excessive ripple as a sign degeneration. Another part to check are EMI caps at the supplies, if they show a sign of damage (cracks or swelling).

[coppercone2]

while the above works,

1) check the wire insulaton for stiffness and cracking. If its bad... well you need to rewire that sucker. Takes a while. Recommend you get alot of different color teflon wire and those tiny zip ties if you do this.
2) ultrasonic cleaner dip followed by bake with a controlled oven (I damaged equipment before when the oven got too hot)
2b) if you do this, you need to remove the dial gauge
2c) after the clean and rinse, you need to take apart every electromechanical thing with a compartment (switch, potentiometer, etc) and clean the insides and relubricate.
3) tend to all the screws, I use gunblue + lubricant after cleaning. Check all the grounds for corrosion, every screw that does electrical things should be cleaned, the thread should be cleaned with a brush, and it should be lubricated. Star washers and such are best replaced, like for aluminum grounds. Replacing all the screws with new hardware is best but its good enough to restore them for me.

And if you do a dip like... remove coils and stuff that can get dirty water sucked into it (transformer, inductors, so forth), and clean those carefully.


I find it unacceptable to have greasy crap. Don't know if its capacitor goo, PCB bearing oil, or otherwise bad stuff. And it frequently stinks when its turned on. It might be tobbaco grease too, because people smoked pipe near those things.



And prepare for a heart attack dealing with the wafer switches, even if you ultrasonic clean them, unless you physically take all of them apart... the best you can do is squeeze like deoxit on them with a long needle while their in that wire bundle. But some people take them apart and clean with salt/vinegar/baking soda, then relubricate. You need long applicators to get grease/oil where it needs to go deep inside. Very frustrating unless you take it apart.


Anyway a long process. At a reasonable pace, months of work.

As for potential bad parts:
capacitors
carbon composition resistors
electromechnical dirty things (potentiometer, switch, tube socket).


And I looked at a tear down, its not that much work to remove all that wire and rewire it. Just need careful notes. But uh.. whatever side effects and risk there is, I had good luck just dropping units in the ultrasonic with the sensitive parts removed to clean up that kind of wiring. And for random things that you think might rust after cleaning off the protective tobbaco layer, you can put car wax paste on them. If you wanna restore it nicely, I recommend you clean up a shelf and leave the unit for occasional work, its a bad idea to just schedule the restoration as the only thing you will be doing in the lab... its checklist work.

My favorite part is when you clean all the adjustment and cal pots, and then when you do the manual procedure, it works smoothly. And I also recommend getting rid of mods to keep garbage tubes in service (if you find a swapped resistor that is 'hacked' to make a out of spec tube work, just replace that unless you are an expert on the rammifications). That stuff is actually usable when its all cleaned, adjusted and nice... I find that when I had destitute gear, it would sit on the bench and the battery DMM would come out rather then the better equipment because it had too make irks from lack of service).

[Dr. Frank]

I fully confirm what coppercone2 already stated.
40 years ago, I have repaired & calibrated many of these or similar VT and solid state VM from hp and all these are very susceptible to contamination, due to their very high impedance (100MOhm). Such gunk might give rise to big reading errors, so check all ranges first.

Usual procedure was to disassemble the meter movement and the PCB with the semiconductors and put the whole switch and case in a big ultrasonic cleaning bath (Freon or similar at that time, I think).

The meter and the PCB should be cleaned differently.

 I remember one difficult case, where only a small area on the ceramic wafer was contaminated with conducting stuff, and I could not remove that. So I had to replace the whole switch assembly, which was a total loss, obviously. My Staff Sergeant did not  allow scrapping the instrument, so I had to repair it that way. That  was my masterpiece, especially even finding this deeply hidden failure.

So good luck with the repair, these instruments are really fine.
Frank

[RichardM]

Some good ideas thanks.

I may have undersold the quantity of greese/oil/tobacco. On the underside of the wafer switches it almost looks wet. I suspect a short somewhere so I will do my best to clean it without dissassembly first. The thought of re-wiring this scares me to death!!

I am finding the meter pegs high on -DCV and -DCA at present and does not seem to respond to any DCV input on the meter.

Richard

[RichardM]

I am measuring a dead short to ground on test point 96 on figure 5-8 which should be 560k.? Would love to know what other owners of the 410c measure in terms of resistance on voltage on their figure.

Richard

[coppercone2]

Well I don't have one but if you wanted to get rid of the grease the best results will be from submersion and ultrasonic or submersion and scrubbing with a correct size bottle brush that you are able to get in there. Again, with the complexity of those wafer switches, you are liable to damage something with anything but the ultrasonic. I can assure you that a damaged wafer switch will piss you off more then doing ten rewiring jobs.

You could also try spraying solvent in there for a while, but its basically a kind of plastic, unless their ceramic wafer switches.. its actually possible to damage something. For instance, I had BNC to Banana adapters get damaged from excessive 50% isopropyl solution (they got warped!). What solvent to use in excess on a 60 year old piece of equipment... I don't know if there is a good answer. If you get that freon stuff mentioned in the  thread, warm it up, and spray the wafer with it, maybe that would work instead of a dip. Like forceful spraying with a power air source.


Only suggestion I Have for shorted stuff in that kind of equipment to eliminate variables is to wire the meter up with clips and then adjust the dials, pots and manually actuate any relays to see if any mechanical state change results in a impedance change, and to also reverse polarity on the measurement incase there is a diode you missed or something. And give it a prod to make sure there is not a broken wire that is restring on a place that it should not be causing a short. But usually unless there is a temporary shunt across it, a 6 order of magnitude change in impedance is very bad.


And FYI usually the wires going to those switches are on the exterior, so its not that hard. You might only need to desolder 10% of the wiring on a wafer stack to remove it from the chassis and dip clean it. How do you not mess this up?

1) take pictures
2) open pictures in editor or print them, and put labels on the wires, every wire you de solder should be color labeled on your document. They like to use those spiral candy cane wires in HP, so its easy to get confused from camera picture since its not solid color. And the wafer stack could be represented as a circuit element, its a bunch of radial pieces that stack up, with a pin out. Just draw the wafer slices next to each other on a piece of paper in the correct order in the correct orientation with the wires labeled.
3) if there are same color wires (i.e. someones dodgy repair), put heat shrink bands on them to give them unique ID's, and label it in your document.
4) if you make a nice document, shrink and laminate it, then place it on the bottom of the chassis with tape for next time.

If you remove it, you will find it a hell of a lot easier to lubricate/deoxit/whatever the contacts, not as nice as taking it apart fully, but you still get ALOT of access for only 10% of the work. And it will fit in a small ultrasonic machine.

For undoing the wires, you need  to get some picks. And a portion of the wires might be cut so close that if you fail at removing them nicely, you will either need to resolder crap or replace the wire. With my luck I only need to replace.. maybe 1% of the wires I remove, because they were cut too short and it looks mangled and too much solder goes under insulation, for HP gear. If you damage a wire, well cut it off, measure the strand with a micrometer or caliper, count the strands, get the same AWG wire and replace it. You will only have a problem if you refuse to get the correct wire and try to put something too heavy in there or put something too thin that fuses/overheats.

Also if you see a wire that is clearly long enough, sometimes cutting it with flush cutters on the contacts very precisely and carefully, then removing the crap and restripping the wires is better then trying to unhook it, if you feel like you can break the contact of the switch because its being annoying, replace the wire!

[coppercone2]

this is how you need to treat those wafer switches

[RichardM]

Thanks for the advice. Looks like I have a lot of work to do.

If anyone has one of these meters I would love to know. I will get organised to strip it down for a full clean .

In the meantime this is the current the current behaviour of the device:

- doesn’t respond to any input DCV or ACV
- when turned on the DCV is zero and -DCV pegs high
- after about 3 minutes or so the -DVC begins to drop until it reaches zero and the +DVC climbs until it pegs high and -DCV pegs low.
- I have checked all the capacitors on C3 and the other components also appear fine.

I hope it’s not that strange chopper assembly with the four glowing lights but I have little idea how to test that.

Regards

Richard

[coppercone2]

is this 3 min after you let the tubes heat up? Any measurements you take before the tubes heat up are not going to be useful unless you are diagnosing the heater circuit.

First thing is to test the rail voltages. Are all the voltage rails in spec? If not, then zener/cold gas tubes, or regulation transistors. Always check those first. It should tell you which pin of what tube to test to check power rails.

Once you verify that the voltage rails are correct, then proceed further.

[RichardM]

OK, So I replaced some elctrolytic capacitors on the chopper board A3. I also changed the 1200uf capacitors on the power supply board.

The behaviour is unchanged and there is no response to inputs.

The rail voltages are shown attached. There is no volatge on wire 96 at the moment. There is also a small 0.7VAC 50hz voltage on wire/point 0 also.

Any suggestions what to look at next would be much appreciated.

Richard

[factory]

Something seems odd here, wire 96 goes to pin 1 on the amplifier board, on the simplified DC voltage measurement circuit it goes straight through the range switch (set to 15mV range, as per fig 5-8 in your picture) from the DCV probe (has 1M resistor inside). I can't understand how you are supposed to get +1.5V at wire 96 with +15mV input, maybe they meant wire 96 should measure +1.5mV. 

David

[coppercone2]

Damn IDK I am not familiar with the meter,  I caught typos before, usually in the parts list though. Kinda getting too specific for me to help, and I just don't have room to add this meter to my lab. At least you know there is no major problem like a blown rail though

BUt uh, back to the original point.. you could try to find the 560k resistor and see if its damaged. Because if it says 1.5V (not sure about the above, post, did not go through it), and the manual says you should be able to measure 560k, and you measure a short, and it reads 0V, instead of 1.5V (or possibly 15mV), that might be a shorted resistor

[coppercone2]

and if you are worried about what I think was the chopper block,

https://test-equipment-for-sale-wanted-or-exchange.groups.io/g/main/topic/wanted_assembly_chopper_for/80045482?p=,,,20,0,0,0::recentpostdate/sticky,,,20,2,0,80045482,previd%3D1611500578393446042,nextid%3D1608562010351487005&previd=1611500578393446042&nextid=1608562010351487005

here is mr carlson working on another meter that has a chopper block, and replacing it with LED instead of neon. I know that its infuriating to have a non replaceable circuit element.





BTW if you go into neon land, or LED land, I suggest making a block of ESD foam by buying a large sheet and stacking it in a deep tray so you can seat the bulbs upright like you would with transistors, it made me feel so much better to have like a sorted upright array of crap rather then a loose pile of long lead parts (makes the whole endeavor feel... really dodgy), something about a pile of sky scrapers (for me, photo resistors and diodes) was distasteful. Like the 'tray of glass' maybe triggers my 'cut sense". Like, call me crazy, but its nice to inspect a glass object before touching it, rather then dipping your hand into a bowl of glass bulbs.

[RichardM]

Wire 96 does have some voltage, a few mv from memory. I guess when I was looking for 1.5V I thought a few mv was effectively zero.

I haven't been able find a 560k resistor anywhere yet!! I measure 408k here.

I have measured some of the resistances and most are ok now except I measure an open on wire 0 and  zero ohms on wire 90 (which it should be right?) I think there are multiple typos in this version of the manual.... sigh.

I would like to be able to test the photo chopper but I am unsure what/where to place the 1Meg resistor it suggests in the manual.

[coppercone2]

a3 a1 v1 is valve one on assembly 3 circuit board A1 I think. I assume V1 is a two terminal neon, and that A3 is the optical assembly, and A1 is the board inside of the optical assembly

Those resistors are usually 10%. They do drift though, I found drifted carbon comp on multiple old equipment, more then 20-30% out.

[Tomorokoshi]

...
Maybe a citrus cleaner to remove grease and a final clean water wash ?
...

Watch out for compatibility with plastics and elastomers when using any of those citrus / orange cleaners.

For most electronics cleaning I use a thin solution of Formula 409, followed by a rinse, then I dry it either in front of a heater or a dehumidifier. If I'm trying to be careful I spray it down with DI (de-ionized water). Switches and potentiometers get an application of whatever contact cleaner seems appropriate.

[coppercone2]

For instance I noticed even dip of banana connectors in  50% IPA with a short period in the ultrasonic resulted in later warping. I tried to one into something and it did not fit. Ends up looking like heat damage. Found the problem like.. 3 months later. I try to just stick to simple green (heavily diluted).. seems to clean alright with little problems.

[RichardM]

Success!!!!!

Capacitor A3C1 was shorted. I just replaced it with a tiny 100nf ploycap and the meter is working for DCV, Ohms and ACV. Both A3C1 and A3C2 are radial poly caps with a central terminal and aluminium can as the other terminal in a strange arrangement with a 1.8M resistor. I am not sure if they are anything special ?

Thanks for the responses everyone. Plenty of cleaning to do.

Richard

[factory]

That is a polystyrene cap, they are usually quite reliable, but are easy to damage with too much heat when soldering, some cleaning chemicals will also destroy them. They are usually chosen for their temperature stability, i.e. they have very little change in value with temperature.

David

[coppercone2]

Did the resistance/voltage change on that wire? I am curious because it sounds like a shorted cap could make 1.5V into 0V, and did that resistance correct itself when measured?


Would be nice to know if measuring that resistance has any use in trouble shooting, and if the manual has a incorrect voltage.

[coppercone2]

So that cap is shielded, its designed to screw in like a coaxial connection. Its a high Z node most likely.

If you want to replace that, maybe cut apart the original cap to get the base of the thread, glue a new axial cap in there, then bend the top lead and bond it to the side of the threaded bit, and cover with a bit of putty, put copper foil on it, or maybe brass tubing in liue of putty, and solder it all together.

This would be to maintain the high shielding level offered by that cap.Its like a coaxial component.


Always wondered if they made caps like that, very interesting. You would just need to use aluminum soldering techniques to bond to that base, meaning cleaning and special flux.

and it might be possible to replace with a ready made feed throguh capacitor that has a end cap put on one end

FYI did a little experiment, I compared kester aluminum flux to mineral oil on aluminum foil that was scratched up with a fiber scratch brush and cleaned with alcohol. The kester flux worked great (beautiful high flow tin), mineral oil very slightly worked (blob that did adhere but very little flow or cleaning action). I wonder if that mineral oil soldering thing was a scam or if I did something wrong.

[RichardM]

I am pretty happy with the accuracy of my meter now.  Final measured voltages are as shown below for anyone else looking repair a similar model.

Main difference is with wire (5) ?

Regards

Richard

[w9gb]

@CuriousMarc (Northern California) has two (2) YouTube videos on his recent HP 410C restoration.

Rescuing two (2) HP 410C VTVM from the disposal bin -Part 1
Marc shows how he restored the Chopper Block (replacing Neon lamps)
https://youtu.be/OOqh2Wqdi4s

AC Probe restoration - Part 2
https://youtu.be/wud7ky3Fb4s
==
@atkelar (Central Europe) acquired the HP 410C from @CuriosMarc that

He rewinds the Dead “shorted” AC transformer (twice) - Part 1
https://youtu.be/VlpNK7vdQKA

Looks at the Chopper block for any issues (Part 2)
He received the matching AC Probe from viewer.
https://youtu.be/0XuVXvejFic
==
Ashley Hall at KISS Electronics has HP 410C Parts
and Replacement PC boards (you can upgrade from Tube/chopper to solid-stare)
http://www.kiss-electronics.com/hp410c.htm

Good Luck !
Restored mine in 2017/2018 from a closed Test facility, in rough shape.

greg, w9gb