Maker Pro
Maker Pro

My next project? What do you think?

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
I picked up a very sorry HP410C.

I'm not sure if it's been wet, or in a corrosive atmosphere, or what.

1547807020206159.jpg

The handle seems to have quite a bit of surface rust.

1547807164917992.jpg

I wonder what it looks like inside?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
Here's a view in the top.

1547807749942913.jpg

The ceramic cap has a lovely pattern on it, probably from stuff falling in through the ventilation holes.

I can only see a single tube, and the shield on it looks pretty rusted. Oddly enough, that's all that appears significantly rusted. I think the chassis is aluminium, so while it looks a bit crusty, it seems mostly just in need of a clean.

One board there looks to be in remarkably good condition. I expect the gold playing of the PCB tracks has something to do with that.

1547807750080916.jpg

And looking from the other direction, the various adjustment trimpots look nice and clean!

1547808393294276.jpg

At first I was concerned about the almost unreadable meter, but that seems to be just an accumulation of dust. I rubbed some of it away with my finger.

154780861640035.jpg

I think there may even have been an AC probe for this. I don't have it at the moment, but looking at where it plus in, and remembering something that was also where this was found...

Anyway, I'm sure this will require serious work to get it in a condition where I would even think about applying power. Should I take it on?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
Wow, there's some very interesting design aspects of this meter.

It uses neon bulbs and light sensitive resistors to control modulation and synchronous demodulation to (I think) allow the use of an AC coupled amplifier to amplify the input signal.

Here is a manual.
 

Tha fios agaibh

Aug 11, 2014
2,252
Joined
Aug 11, 2014
Messages
2,252
I suspect your like me and enjoy fixing things just for the heck of it.
I say it'd be worth it just for the fun of it provided you have the free time to invest.

As far as functionality, I suspect it wouldn't be the most reliable piece of equipment and certainly not the prettiest.

If you don't really need an extra meter I might pass on it because it probably doesn't have much resale value.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
Fun may be the best reason. Any reward is going to be something other than financial.

I've been skimming through the manual (on my phone -- urgh!) and it seems all the transistors are germanium. There's also lots of "high tolerance" resistors (1% carbon composition) with unusual values like 70k, 6M, etc.

Some other parts are pure unobtainium, but I've not seen any physical damage yet. I'm hoping it was put away because it was obsolete or because it had some easily fixed fault.
 

Tha fios agaibh

Aug 11, 2014
2,252
Joined
Aug 11, 2014
Messages
2,252
I would do some tests to make sure the meter itself is working properly. I imagine even a high quality jeweled movement can be compromised by corrosion even when sealed.

It would suck to rebuild that thing only to find out that the needle movement sticks.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
Very good point. I'll also do some basic checks on the transformer.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
Well... The meter movement works. Up to a point :-(

At about 80% of the deflection, something starts rubbing. If forced a little further, the movement sticks, although a tap on the front of the meter frees it up again.

I used my meter in the ohms and diode check ranges as a quick and dirty current source. The resistance if the meter is 188.5Ω, by the way.

This may be the first thing to look at.
 

Tha fios agaibh

Aug 11, 2014
2,252
Joined
Aug 11, 2014
Messages
2,252
That's a horrible sign. I'd guess there's dirt or corrosion on the bearings, magnet or spring. I'd be hesitant to cleaning it because it may throw off calibration.

You could try feeding a extremely low frequency signal (a few hz at obviously limited current) to see if a back and forth needle sweep may free it up.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
Well, it only takes the removal of 14 screws and desoldering 2 joints, but the meter is out!

It's complete with a QA sticker and an "individually calibrated" sticker.

1547970290826473.jpg

1547970290706258.jpg

I wonder what I will find inside?

And I guess this is going to void the warranty, right?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
Warranty voided!

The problem looks simple enough. There is some corrosion that the moving coil brushes against as it gets near full scale deflection.

1547972053671758.jpg

Do you think it's possible to scrape away at that rust without either damaging the movement or getting the rust stuck against the magnet?

I can't use a steel tool, and that corrosion is pretty hard.

On the other hand, it is just a 1mA meter with a custom scale.
 
  • Like
Reactions: Ian

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
Chemical rust remover?

Will that remove rust without damaging the very fine parts in the vicinity? That entire image shows a section about 1cm across.

Do you have any suggestions of what rust remover I might use? I guess application with a very small paintbrush would allow me to get it just where it's wanted.
 

bushtech

Sep 13, 2016
1,025
Joined
Sep 13, 2016
Messages
1,025
I would start with something like CorrosionX

IMG_20190120_125102[1].jpg
Just a suggestion. I'm no expert, so use at your own risk
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
That seems to be fairly difficult to get here. I do like that it's compatible with electronics though.

There seems to be both a corrosion removal (not safe for electronics) and a protection version. Does the protective version also act to remove rust?

I see there are various options containing phosphoric or hydrochloric acid. For fairly obvious reasons I'd prefer to steer clear of those. Alternatives described as "chelating" seem safer, but possibly a little messy.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
The only place within 400km or so of me no longer stocks it :-(

I may have to shop around :)
 

bushtech

Sep 13, 2016
1,025
Joined
Sep 13, 2016
Messages
1,025
Feel for you, also in the boondocks, nothing is just around the corner
 
Top