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 Post subject: Solved - What happened here? Cardmatic short lamps
PostPosted: Feb Wed 13, 2019 1:28 am 
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Joined: Nov Sun 04, 2007 6:01 am
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Location: Redding, CA
While testing tubes from an inoperative AA5 this past weekend, my WE/Hickok Cardmatic tube tester indicated a screen-suppressor short on the neon indicator lamps. Ah HA! I need to replace the 12BE6. But the neon short lamps remained on after removing the tube. And after removing the card! Houston, we have a problem. The Cardmatic is diagnosing itself with a screen-suppressor short. I removed the tube socket island and the short lamps remained on. Not debris in the tube sockets. I opened and thoroughly inspected and cleaned the pin bank. The short lights remained on. Okay, I have to open the beast up and diagnose the problem. Starting at the neon lights themselves, I measured approximately 2M across all lamps except the middle lamp which measured 50k (see schematic).

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Given that the circuit resistance is high in all directions from this lamp, the problem had to be local to the neon light, .005 capacitor, or 2M resistor. After removing one end of the capacitor, the capacitor indicated infinite resistance and the resistor/lamp measured 50k. I kind of expected that. I inspected the wiring and surfaces for debris and found none. I then removed one end of the resistor and found it to measure 2M and the neon lamp to measure infinite! Whatever was shorting the lamp had vanished upon the applied heat and mechanical removal of the resistor. I replaced the resistor anyway, reinstalled the capacitor, and tested the Cardmatic to find it working.

What do you suppose caused the short? I have a hunch but was wondering if anyone else may have the same thought. The resistor of concern is under the third capacitor from the left in the photo below.

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Norman

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Last edited by Braithwaite on Feb Mon 25, 2019 1:15 am, edited 1 time in total.

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 Post subject: Re: What happened here? Cardmatic short lamps
PostPosted: Feb Wed 13, 2019 8:40 am 
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Location: 253 Blanche St. Plymouth, MI USA
OK, lessee..... I know disc caps are highly reliable.... same for resistors of that era, my guess is something odd about the neon bulb But I am more thinking the switching set up in the card reader got some crud in it?


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 Post subject: Re: What happened here? Cardmatic short lamps
PostPosted: Feb Wed 13, 2019 2:33 pm 
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dirt in the flux residue on the phenolic board


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 Post subject: Re: What happened here? Cardmatic short lamps
PostPosted: Feb Thu 14, 2019 12:32 am 
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Location: Sunbury, Ohio 43074
well ... since carbon resistors can't fail towards LOWER resistance, it kind of seems it's either the capacitor (mechanical failure?) or a conductivity of some kind on the board. Or perhaps the capacitor somehow physically pressing on the resistor and "leaking" .... yea, I made that up, but hey, it's a forum .....

If it's not any of those things, I'm guessing it must be something else.

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 Post subject: Re: What happened here? Cardmatic short lamps
PostPosted: Feb Thu 14, 2019 2:21 am 
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Joined: Nov Sun 04, 2007 6:01 am
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Location: Redding, CA
Here is my take...

It was not crud in the card reader because I had inspected and cleaned it (I called it the pin bank) or the neon light because it tested to have infinite resistance and is presently working. It was not flux residue or conductivity on the terminal board because the problem disappeared upon lifting of the resistor without disturbing the board.

Note that the terminal posts are covered with black oxidation. They are silver plated. I suspect that silver migration produced a silver hair across the resistor. It is a long path but I have no better explanation.

Norman

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 Post subject: Re: What happened here? Cardmatic short lamps
PostPosted: Feb Thu 14, 2019 3:09 am 
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Location: Montvale NJ, 07645
Barry H Bennett wrote:
well ... since carbon resistors can't fail towards LOWER resistance,

I have never personally experienced it, but I have heard others tell of carbon resistors that have gone down in resistance. It only seems to have happened to the lower value resistors, like 1000 ohms or less, so that would not be the case here anyway as this one is 2 meg.
As for the silver migration, that is an awful long path.


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 Post subject: Re: What happened here? Cardmatic short lamps
PostPosted: Feb Thu 14, 2019 6:55 pm 
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Location: Monte Vista, CO. USA
I believe it was crud in between the leads of the neon lamp. The spacing is small, the mounting is located where dirt and dead skin cells can get down to that area through the push-to-test switch holes, and any type of movement or heating would clear it.

I saw a case once where the customer was using steel wool to clean tube pins. Over time steel wool dust built up in the tester, causing all kinds of anomalies and erratic shorts indications. A good thorough blowing out with an air compressor fixed the problems.


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 Post subject: Re: What happened here? Cardmatic short lamps
PostPosted: Feb Thu 14, 2019 7:32 pm 
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Location: Redding, CA
The base of the neon bulbs are sealed in the neon bulb block and hence protected from debris. At least this is the case for my Cardmatic. Only six well spaced wires go through the phenolic board into the tightly mounted neon bulb block to operate five neon bulbs having a total of 10-leads.

I examined the suspected problem resistor with a powerful magnifying lens this morning and cannot find any evidence of a silver thread so the cause of the 50k resistance remains a mystery. What I do know is that the low resistance appeared suddenly, did not falter, and did not vary in value through the entire diagnosis process until disconnecting one end of the 2M resistor.

Norman

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 Post subject: Re: What happened here? Cardmatic short lamps
PostPosted: Feb Mon 25, 2019 1:15 am 
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Joined: Nov Sun 04, 2007 6:01 am
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Location: Redding, CA
Final diagnosis... The tube tester worked fine for three days then the problem reappeared. The problem was not the disk capacitor, not the resistor, and not any external surfaces. Therefore I decided to open the sealed neon lamp block for an inspection. As expected no foreign debris was found but the once bare copper wires from the neon lamps were fuzzy black as in copper oxide with loose black fuzz within the block as well. Removal of the copper oxide dust with a soft bristled brush solved the problem. PhilF was partially correct.

Norman

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