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Author Topic: Cleaning conductive corrosion from Triplett VOM?  (Read 10629 times)
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WB3JOK
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« on: April 04, 2010, 04:32:08 PM »

I acquired an "untested"  Roll Eyes but nice-looking Triplett 630-PLK cheap. Unfortunately when I opened it up, I found out why it was "untested" - the 30 volt battery had corroded into an incredible mess. Even the wires going to its spring-pins were rusted off.

I was able to remove the remains of the battery, clean off and reinstall/resolder the contact pins, and the few other obviously copper-oxided parts like the fuse in clips that sits directly above the battery compartment. Surprisingly, the meter movement is intact, and it even reads correctly on the DC and AC volts ranges.  Cheesy Didn't have time to check the current or ohms ranges.

However... the first odd thing I noticed was when the overload protection relay was manually (finger) tripped, with the contacts visibly apart, instead of the meter indicating zero it would come UP to about half-scale (and go to zero when I removed the 30v batt, which supplies current to the overrange trip circuitry).  Oh-oh.

Worse yet, the readings on the 5000V ranges were almost identical to the 1000V ranges (as you probably know, those ranges are separate jacks with 20 and 60 meg multiplier resistors to the V-O-A input jack). Measuring across the 20 meg resistor with my Fluke DVM showed a changing reading starting at only 0.7 meg and gradually increasing.  Shocked

My theory is that corrosive battery vapors over the years gradually deposited in scattered patches around the inside of the unit, and are causing conductive pathways on the Bakelite surface. Not everywhere, since there is no visble corrosion or oxide on the rotary switch pins or the resistors mounted on the "carousel", and the DCV and ACV ranges from 2.5 through 1000V are working properly...

Any thoughts (other than throwing it away)? Would a rinse with alcohol, or detergent followed by distilled water, do any good?
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Jim, W5JO
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« Reply #1 on: April 04, 2010, 04:40:18 PM »

When we cleaned battery corrosion from Simpson 260s we applied vinegar with a soft bristled tooth brush.  Then cleaned everything with distilled water.  Of course, dry thoroghly.
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WB3JOK
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« Reply #2 on: May 01, 2010, 03:01:01 PM »

Finally got around to trying that... cleaned with an old soft toothbrush and white vinegar (lots of greenish/brown crap came out), then rinsed with warm water, final rinse with distilled water, baked in oven until dry. Looks nice.

Now it's even worse - the volts ranges don't read right any more (way high), and even with both batteries removed (and the one electrolytic that is part of the overload circuit completely discharged) the meter shows about 5 uA on several ranges!! That can only be coming from a combination of salts and metal acting like a small battery.  Tongue

Guess it's toast. Lips sealed Let this be a reminder to all of you with good VOMs to check the batteries and remove them before they spew chemical vapor all over the delicate innards. 630's go for close to $300 new these days.

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K5DBX
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« Reply #3 on: May 01, 2010, 05:27:40 PM »

A couple things to try before you pitch it completly. Remove the board from the device and if you have a dishwasher, place the board through a complete cycle with as hot a water as possible. Do not use commercial dishwasher detergents, but do use a small amount of liquid soap. No more than an ounce or so added about half way through the wash cycle. (use too much soap and your kitchen floor may be full of sudsy fun) Once this is done allow heated dry. Remember electrolytics can be on-board, but will require drying time, which is one of the primary reasons for heated dry. Many manufacturers use this exact process after wave or reflow, however, most use a cycle of freezing and baking the board a couple of times to remove all moisture. Most homes are not equipped to freeze with liquid nitrogen and bake boards at around 300F, so if left in just air, a couple of days of "drying" time is needed.

Obviously run your dishwasher through another full cycle before placing your dishes back in.
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WB3JOK
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« Reply #4 on: May 01, 2010, 06:49:15 PM »

Thanks for the general board-washing tips. I would have to remove the meter movement in order to wash the assembly in a dishwasher, though.

Meanwhile I dried it some more in a 125F or so oven, and the volts ranges "came back" to normal. Smiley The current ranges all work and agree with my other 630. The 20 Meg multiplier resistor for the 5000 VAC range measured an irregular 1.5-2M even when removed and cleaned separately! The 60 Meg (5000 VDC) resistor was completely open. Replaced them both with some HV 2% resistors I acquired years ago at Mendelson's in Dayton. Finally I found one 6.5 ohm (the X10 ohms range) open from corrosion and replaced it.

Unfortunately the 20k ohms-adjustment pot is completely corroded away inside. It looks like a standard part (in fact I have one just like it in the junk box but it's 5 Meg). Anyone know where I can find another?
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K5DBX
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« Reply #5 on: May 01, 2010, 07:53:31 PM »

I'm glad to hear a little warming over helped. What kind of pot is it? Precision 10 turn? 20 turn? Linear? My bet would be Mouser, DigiKey or Newark..
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WB3JOK
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« Reply #6 on: May 01, 2010, 09:32:58 PM »

It's single-turn, but it has solder lugs and mounts via two 2-56 threaded studs. I checked all the "major" suppliers and several surplus outlets with no luck. $urplu$ $ale$ has a similar one with a switch (for 75 cents) that looks like I can make it work.



There is still some deflection (< 10 uA) on the X100Kohm range with the leads open, so there's at least one small leakage path remaining. I notice that if I just let it sit for a while, the current slowly drops. It's not coming from the 30 volt battery either...
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KA0HCP
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« Reply #7 on: May 02, 2010, 12:50:54 PM »

Glad to hear you are making progress.   I guess this is a great demonstration of the power that corrosion cells can generate yielding damage.

73, bill
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New callsign KA0HCP, ex-KB4QAA.  Relocated to Kansas in April 2019.
WB3JOK
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« Reply #8 on: June 01, 2010, 11:26:55 AM »

Wellll... the "close enough" trim pot isn't - even after rigging the bracket to hold the new pot, the shaft's threaded hole isn't the same size.
Even the original screw (that holds the knob to the pot) is smaller than 2-56 and the new pot is even smaller.

Rant: I called Triplett and they no longer stock replacement parts for the older models. Apparently when Jewell acquired them, they didn't buy the repair parts inventory!! So their techs cannibalize parts from other old meters and they won't sell pieces from their junk box, but offered to fix it for me. Ya, right. I didn't bother to ask the price.

This is all too typical - can't get a $1 part? Throw it away, add to the landfill, buy some new Chinese-made piece of junk that will break again.  Angry
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WB6NVH
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« Reply #9 on: June 01, 2010, 01:10:16 PM »

I believe Bigelow Electronics in Bluffton, Ohio bought a large part of the older repair parts inventory from Triplett.  They advertise in Electric Radio in the classifieds.  Possibly they might have it, they are a surplus dealer.
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Geoff Fors
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WB3JOK
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« Reply #10 on: June 01, 2010, 04:36:15 PM »

thanks... I just tried Bigelow. Got a very helpful gentleman who said he'd been getting parts from Triplett for 50 years - up until the point where Jewell acquired them!

Unfortunately he too does not have that pot in stock (Triplett p/n 16-31) so it looks like I either have to "JS" it with a shaft pot & knob, or find an unsalvageable 60 or 630 for parts.  Roll Eyes
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WB3JOK
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« Reply #11 on: June 02, 2010, 07:53:42 PM »

This is mechanically identical to the 20K trimpot I need:
(Naturally I found one in the junk box... but 5 Meg)  Roll Eyes

http://tinypic.com/r/scgzmu/6
http://tinypic.com/r/2j48zk2/6

Can anyone help?
thanks!
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fscolesSWL
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« Reply #12 on: February 25, 2012, 11:07:39 PM »

WB3JOK, I just today signed into this forum for the first time, I am a SWL, but not an Amateur Operator. If you're still looking for the TTT pn 16-31 (~20K potentiometer with the knurled knob & mounting holes), I have a good used one I could mail out to you. As far as I know, Jewell didn't take any of the remaining Triplett parts inventory from Bluffton as they've shown no intention of supporting classic TTT VOM's or panel meters. According to my TTT eng files & drawings/blueprints/marked-up manuals, etc the pn 16-31 was used from the very first model 630 (1948) thru 2007 in all of the 630 series 50ua mvt/20KOhm per Volt VOM models; but not in the 630-NS, 630-M, or 60 series or 310 series.
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KM1H
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« Reply #13 on: February 26, 2012, 11:32:27 AM »

Id be willing to save it from the landfill if it comes to that as Ive 2 of them in use and one case cant survive much more abuse.

Three 9V batteries and Velcro take care of that issue and seem to last forever.

Carl
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