Need help to remove clear spots on HP 6177C front panel [SOLVED] plus other prob

[enut11]

I have just taken delivery of an HP6177C Current Source that has multiple blemishes on the front panel that were not obvious in the eBay listing pix.

The spots are clear, not unlike water splash, but are extremely hard to shift. Initially I tried to remove one on the polished aluminium side-rail using vigorous rubbing with isopropyl alcohol on a cotton bud. Although the spot finally came off, I am reluctant to do this on the painted and meter faces in case I damage them.

Looking for ideas on how to proceed.
enut11

[tautech]

Did you check in this Repair board sticky thread:
https://www.eevblog.com/forum/repair/vintageclassic-renovation-techniques/

[abdulbadii]

might be Acetone ~ 70-100% or just cooking oil mixed with mild/strong acid and a bit water

[Gyro]

I would avoid Acetone, too much danger of taking off the paint and silkscreen - and almost certain to dissolve and craze the plastic meter window.

The spots look a bit like flux splashes, but could be any organic material including somebody talking while eating an apple!

Start off with the mildest cleaner - a tissue lightly moistened with water and dish soap. If that makes no impact, then meths or IPA - test it on the paint and silkscreen on the rear panel first!

If you polish the meter front, you will probably build up a static charge that will deflect the pointer. A tissue moistened with a little water and fabric conditioner will discharge this.


P.S. Download and read the HP manual - they always include cleaning instructions.

[jpanhalt]

There are lots of solvents one could try.  I agree about avoiding acetone or any ketone.  The plastic on the meter would almost certainly be ruined.

Household acrylic floor stripper (not just daily cleaning stuff) might work.  The one I use is made by ZEP.  It contains butyl cellosolve. 

I would leave the spots, and if you remove them from the panel, leave the meter alone.  There is no way to predict the long term effect on that unknown plastic.  You may get immediate clouding or long term crazing.  If you insist on attacking the meter, I would try a little IPA (which you said didn't work) or VM&P naphtha, which is a low boiling version of mineral spirits with slightly better solvent properties.  Automotive paint shops may sell something called "final prep" that is similar but more expensive.  If you try the hydrocarbon solvent, be ready to wipe it off with a soft rag, if it doesn't work.  It can cause crazing later.  On CD "jewel" cases, those solvents cause slight clouding almost immediately.  Bottom line, leave the meter alone or be ready to buy a replacement from HP.   

EDIT: Buying from HP/Agilent/Keysight may be difficult.  See current thread about Keysight only selling to commercial users.

[Wallace Gasiewicz]

How about automobile headlight cleaner?

[fordem]

How about automobile headlight cleaner?

I would not recommend that - generally speaking, most headlight cleaners are a mild abrasive and are used to remove the oxidized outer surface of the polycarbonate lens, so if used, you'll be removing the silk screen, markings, etc.. - it could probably be used on the meter, but, honestly, I would just ignore the spots.

[enut11]

Hi all. Thanks for all the suggestions. And yes, I did look up the EEVblog vintage repair notes but nothing stood out for my particular problem. I will start with the least harmful method suggested and work from there.
enut11

[enut11]

SOLVED
Hot water with a little detergent assisted by some gentle finger nail persuasion got rid of most of it!
Happy

[Gyro]

Good news. It was the enthusiastic apple eater then! 

[enut11]

Not sure. The spots were not sticky - more glass-like. They were certainly unmovable without first pre-soaking with warm water and a little detergent. Much happier now

Now I have to fix the frozen 10-turn current set pot. I am guessing the shaft lube has dried out but I don't think these pots can be disassembled.

[tautech]

Now I have to fix the frozen 10-turn current set pot. I am guessing the shaft lube has dried out but I don't think these pots can be disassembled.

Use a proper penetrant and let it soak up the shaft. Be frugal with how much you use and do multiple applications.
Don't expect to free up miraculously but look for tiny signs of movement and gently encourage more.
Patience is the key here unless you bite the bullet and replace it.

[SilverSolder]

[...]
Patience is the key here unless you bite the bullet and replace it.

The pots in these units are extremely high performance (temperature stability in particular), I haven't been able to find a replacement that works as well overall...   Lost art, it seems.

[Gyro]

The HP6177C appears to use a Bournes pot, linearity 0.25%. Another case of a sticky one...  https://www.eevblog.com/forum/repair/hp-6177c-dc-current-source-repair-and-restoration-(complete)/

[enut11]

[...]
Patience is the key here unless you bite the bullet and replace it.

The pots in these units are extremely high performance (temperature stability in particular), I haven't been able to find a replacement that works as well overall...   Lost art, it seems.

Thanks @SilverSolder, I was about to replace the pot. In fact, the replacement (from eBay) does not have a tempco so it could be anything. Again, a case of easy-easy approach is best for this old gear.

[enut11]

This thread is turning out to be more than just a 'spot removal' exercise.
I just had a look at one of the blue rotary switches and the metal tracks were all matte black! I really expected a metallic look.
The photo below was difficult to take but it might give you an idea.
Question: is this normal for these switches or have the tracks turned black with age? I am thinking silver can blacken under certain conditions?
enut11

[tautech]

This thread is turning out to be more than just a 'spot removal' exercise.
I just had a look at one of the blue rotary switches and the metal tracks were all matte black! I really expected a metallic look.
The photo below was difficult to take but it might give you an idea.
Question: is this normal for these switches or have the tracks turned black with age? I am thinking silver can blacken under certain conditions?
enut11

Are you near the sea as Sulphur from sea air can certainly do this.
Places like geothermically active Rotorua in NZ are really troublesome to keep silver clean.

[enut11]

Yes, I do live near the sea (Australia) but the instrument came from the USA and I do not know its history.

[tautech]

If it were mine I'd just try exercising the switch and check if the wiper was getting through the oxidization and producing a stable connection and if so leave it be. Otherwise some Deoxit and a little more exercise would be as far as I'd go unless the switch was still dicky.

[enut11]

Yes, when I applied deoxid red, lots of black came off those switch tracks. Finished off with deoxit gold. I will see if this now stabilises the current.

[Gyro]

[...]
Patience is the key here unless you bite the bullet and replace it.

The pots in these units are extremely high performance (temperature stability in particular), I haven't been able to find a replacement that works as well overall...   Lost art, it seems.

Thanks @SilverSolder, I was about to replace the pot. In fact, the replacement (from eBay) does not have a tempco so it could be anything. Again, a case of easy-easy approach is best for this old gear.

eBay is a bad place to buy precision pots. Bournes in particular are widely counterfeited in China. Use a reputable distributor (the cost of the pot alone will probably get you near the free postage threshold).