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 Post subject: Hickok 752a Calibration issue
PostPosted: Mar Sat 03, 2018 6:07 pm 
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Joined: Mar Sat 03, 2018 5:24 pm
Posts: 3
I'm having a problem calibrating my Hickok 752A, after the Line ADJ is set to give 150 Volts plate, R5 does not have enough swing to center the needle on Line Test. Curious if anyone has seen this problem.
Sharon


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 Post subject: Re: Hickok 752a Calibration issue
PostPosted: Mar Sun 04, 2018 12:18 pm 
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Joined: Aug Wed 31, 2011 11:23 pm
Posts: 435
Location: Tucson, AZ
The line adjust is based on the transformer input voltage, not it's output voltage. Most Hickok tube testers are set so the input voltage is exactly 100V AC. The plate voltage can vary a bit, and does not need to be exactly 150V (150V +/-5V is a typical range). The other usual problem is that if you measure the plate voltage with a modern multi-meter it does not provide enough load to pull down the plate voltage. You should be using a 1000 ohm/V voltmeter or emulate one by using something like a 250K resistor across the cathode to plate and then measuring the plate voltage.

Typical testing procedure on Hickock testers, although some will give voltages for other loading or specify a VTVM for specifics tests. This usually applies to the bias and signal voltage, and is indicated in the calibration procedure for that model.
"Except as noted, all of the readings are taken with a 1000 ohms per volt meter. If an accurate 1000 ohms per volt meter is not available a modern high impedance analog or digital voltmeter can be used with appropriate shunt resistors in parallel with the input to simulate proper loading. The following resistor values should be used: 10 volt scale use 10K, 50 volt scale use 51K, 250 volt scale use 250K. All resistors are 1/2 watt 5% carbon composition."


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 Post subject: Re: Hickok 752a Calibration issue
PostPosted: Mar Sun 04, 2018 2:09 pm 
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Joined: Mar Sat 03, 2018 5:24 pm
Posts: 3
Thanks for the reply, I was using my OLD RCA VTVM for the measurements that actual still works. I discovered that the 81 fuse lamp socket has given up the ghost, every time I would change the position the unit for access to the pots the socket was loosing good connection. When I tried to re-seat the lamp the socket fell apart, should be fun to find one of them. I was able to verify the proper voltages by installing a 1A slow blow for testing.


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 Post subject: Re: Hickok 752a Calibration issue
PostPosted: Mar Sun 04, 2018 5:27 pm 
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Joined: Aug Wed 31, 2011 11:23 pm
Posts: 435
Location: Tucson, AZ
A VTVM will not provide the proper load to measure the plate voltage, and as mentioned you will want to measure the line voltage to the transformer for the meter line set. I will often use a variac set to 100V and set the rheostat to 100%, otherwise you can "carefully" measure the voltage going to the transformer.


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 Post subject: Re: Hickok 752a Calibration issue
PostPosted: Mar Sun 04, 2018 7:12 pm 
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Joined: Mar Sat 03, 2018 5:24 pm
Posts: 3
I guess I need to replace the #81 Fuse Lamp socket before I can go any further, are there any suggestions where to find one?


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