Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: dude on September 20, 2021, 11:42:19 am
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I have a BK 2704B multimeter that I fried by mistakenly putting high voltage on the Mv setting. The meter has only one fuse and a thermistor, PCT "Positive Temperature Coefficient" in series with the red probe and the meter. This thermistor is the switching type, it's like a small cap (not a cap but a temperature sensing resistor) as seen in the picture. This PCT thermistor acts like a fuse, it reacts to temperature changes (high heat from voltage over-load). As the voltage increases the PCT senses the higher temperature and lowers the current flowing threw it, acting like a self setting fuse to protect the meter. The fuse blew too, replacing fuse the meter still doesn't work, but the continuity does with a battery installed, the only setting that works.
I'd like to replace this PCT but can't identify the value I would need, the marking are "222 with a H under the number, back side has nothing. The BK meter has a 1000vdc reading capability, 750 vac. a 9 volt battery operates the meter. I am assuming I need a PCT that can handle 1000vdc, as far as the temperature (heat from an voltage over-load) rating, I have no idea. Hopefully, someone with experience in thermistors can help me find the correct replacement. The coating on the PCT is a polymer like coating, I believe it's the switching type and the PTC thermistor was covered in a rubber boot like a roach.
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Do you have another meter? What is the Ohms of that PTC?
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Do you have another meter? What is the Ohms of that PTC?
Good idea, but most meters today don't have a PTC, two fuses instead. I'll check.I'll keep searching for more info on PCT's, possibly call BK tech support, might get lucky.../ Thanks PRR. Maybe time for a new meter... :think1:
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A cheaper meter I have is capable of the same 1000vdc readings as the BK. The cheaper meter has a PTC, the resistance is 1k. The BK meter the PTC is 2.2k. With my lack of knowledge about PTC’s, l’m thinking maybe both meters have the same measuring capabilities, and maybe the 2.2k is too high and was damaged as none of the functions work but panel does show 0.00 when turned on for every setting, manual meter.
I’ll try the 1k PTC from the $10 cheap meter if no difference, time to toss it for parts only.
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No dice, l’m just to tenacious. I need to learn to quit trying to fix things beyond my limits. :w2: The BK meter is 15 years old, probably not calibrated correctly anyway.
Gave it a shot, on to the next project, :BangHead:
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> The BK meter the PTC is 2.2k.
THAT's what I was asking. (Not what another meter's parts might be.) Is the PTC blown-open?
At 2K (small compared to the 10Meg of the total path), I think it is fine and your problem is elsewhere.
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You’re correct, the circuit board was machine made not meant to replace parts, junk now.
New second meter time, thanks