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 Post subject: Hickok 752 Calibration
PostPosted: Jan Tue 19, 2021 5:40 am 
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Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
i came across a problem with my tester and that is with calibration. All voltages check out using a 250k shunt resistor but when im checking the bias voltages im gettting voltage without depressing the s5 button. Should it have voltage at all time on pins 5 and 8 regardless if push s5 button?


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Tue 19, 2021 6:09 am 
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Joined: Aug Wed 31, 2011 11:23 pm
Posts: 541
Location: Tucson, AZ
The Grid voltage is always present, pressing S5 applies the plate/screen voltages. When calibration, for some tests the grid voltage needs to be set to 0.


Last edited by mksj on Jan Tue 19, 2021 9:29 am, edited 1 time in total.

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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Tue 19, 2021 6:34 am 
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Joined: Oct Fri 19, 2018 8:30 pm
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ok so on this tester its normal to have the dc bias voltages all the time even the 2.5 vac since uses the same 5 and 8 pins? even for the signal voltage check says press s5 on my tester i get 2.5vac without pressing s5


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Tue 19, 2021 7:15 pm 
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Joined: Dec Sat 28, 2019 4:18 pm
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Location: Corinth, TX
If it helps, here are the cal instructions for the 750 and 752A that I have:

John


Attachments:
752 & 752A Calibration.pdf [654.85 KiB]
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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Wed 20, 2021 5:00 am 
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Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
Thank you very much for those instructions that verifies that no need to press s5 to check the bias voltages :) here was my calibration instructions that where slightly wrong. looks like signal voltage check in your instructions says to press s5 even though mine has that voltage all the time. wonder if i have a short somewhere in tester cause no voltages should be present unless press a test button. although in the hickok 539c calibration it says no need to press any button to test bias or signal voltages so not sure what is correct. but at least i know that for bias no need to press a button


Attachments:
hickok-752-calibration.pdf [168.92 KiB]
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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Thu 21, 2021 7:32 am 
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What shunt resistors should i use to check voltages properly?


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Thu 21, 2021 4:40 pm 
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Joined: Dec Sat 28, 2019 4:18 pm
Posts: 1076
Location: Corinth, TX
Based on a 1000 ohm/volt meter and the standard VOM ranges, you need 50 K (50 volt range), 100 K (100 volt range), and 250 K (250 volt range) resistors - or the closest standard values you can find.

John


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Thu 21, 2021 6:09 pm 
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Joined: Oct Fri 19, 2018 8:30 pm
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I read that a 250k should be used for 50v or higher. seems for different tube tester calibration instructions they call for 120k for to check the 56vdc. I used the 250k to check the 150v, 130v, 56v, and 68vdc all where perfect but to check those lower voltages with a 120k they seemed low but still within tolerance. i did order a triplette 850 VTVM so can use it to verify the voltages at 1000ohm/volt


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Thu 21, 2021 11:26 pm 
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Joined: Aug Wed 31, 2011 11:23 pm
Posts: 541
Location: Tucson, AZ
The resistor load is typically based on the scale range used, 50V would be 50K, 100V would be 100K, 250 would be 250K. It does not need to be exact, just present some loading. No need for VTVM, just about any digital voltmeter will work. There is limited to no adjustment for many of the measured voltages, you are looking more for something that is significantly out of spec. I use a Fluke 289 with a loading resistors for older Hickok's, but also have an AN/PSM-37 military voltmeter with different loading measurement volt ranges. I also use a variac to set the line voltage as opposed to the rheostat.


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Fri 22, 2021 12:01 am 
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Joined: Oct Fri 19, 2018 8:30 pm
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so to measure 56v or 68v need 50k resistor?


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Fri 22, 2021 3:41 am 
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Joined: Nov Mon 02, 2009 7:01 am
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Location: Lincoln City, OR 97367
Greetings to tysman and the Forum:

Please reread mksj's post. The shunt resistance is based on the SCALE of the METER that you use... not the voltage you are measuring. Since the voltages were measured with analog meters, these had discrete scales. A 50K ohm resistor turns a high-Z meter into a 50 volts full scale analog meter. You would not measure 56 or 68 volts with a 50 volt full scale analog meter because the meter would be pinned and might even be damaged. Common meters in use in the 1K ohm/volt era had either 100 volts full scale ranges or 250 volts full scale ranges, although there were some meters with odd scales like 150 volts.

Without knowing what meter Hickok used and what its full scale reading was on the particular range that they used, all you can do is guess. For the voltages you mentioned, you could assume 100 volts full scale (=100K shunt), 150 volts full scale (=150K shunt) or 250 volts full scale (=250K shunt).

Since meters are most accurate in the upper 30% or so of their scale, I would guess (and this is only a guess) that Hickok would have used a 100 volt full scale meter to measure the 56 and 68 volt values that you mention. In which case, their meter would have a resistance across the probes of 100K ohms. I would suggest that you use that value for your shunt.

Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Fri 22, 2021 5:47 am 
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Joined: Dec Sat 28, 2019 4:18 pm
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Location: Corinth, TX
Jthorusen wrote:
The shunt resistance is based on the SCALE of the METER that you use... not the voltage you are measuring. Since the voltages were measured with analog meters, these had discrete scales. A 50K ohm resistor turns a high-Z meter into a 50 volts full scale analog meter. You would not measure 56 or 68 volts with a 50 volt full scale analog meter because the meter would be pinned and might even be damaged. Common meters in use in the 1K ohm/volt era had either 100 volts full scale ranges or 250 volts full scale ranges, although there were some meters with odd scales like 150 volts.

^This.

Ranges for a TS-352 or PSM-37 are 2.5, 10, 50, 250, 500, and 1000 volts at either 1 Kohm/volt or 20 Kohm/volt. That's why calibration instructions say to use a 1 Kohm/volt meter. So, in the case of a TS-352, you use the pin jacks on the right side of the case. You use a 50 Kohm shunt for your VTVM or DMM to measure the bias voltage (0 - 42 volts or so) and a 250 Kohm shunt to measure all the other DC voltages.

John


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Fri 22, 2021 5:48 pm 
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Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
Jthorusen wrote:
Greetings to tysman and the Forum:

Please reread mksj's post. The shunt resistance is based on the SCALE of the METER that you use... not the voltage you are measuring. Since the voltages were measured with analog meters, these had discrete scales. A 50K ohm resistor turns a high-Z meter into a 50 volts full scale analog meter. You would not measure 56 or 68 volts with a 50 volt full scale analog meter because the meter would be pinned and might even be damaged. Common meters in use in the 1K ohm/volt era had either 100 volts full scale ranges or 250 volts full scale ranges, although there were some meters with odd scales like 150 volts.

Without knowing what meter Hickok used and what its full scale reading was on the particular range that they used, all you can do is guess. For the voltages you mentioned, you could assume 100 volts full scale (=100K shunt), 150 volts full scale (=150K shunt) or 250 volts full scale (=250K shunt).

Since meters are most accurate in the upper 30% or so of their scale, I would guess (and this is only a guess) that Hickok would have used a 100 volt full scale meter to measure the 56 and 68 volt values that you mention. In which case, their meter would have a resistance across the probes of 100K ohms. I would suggest that you use that value for your shunt.

Regards,


Thanks that makes me understand more on how to use the shunt resistors and what values to use. basically when i looked up the hickok 6000 calibration instructions they say to use a 120k shunt to measure the 56v. Thank you i now know for 50v and 68v should not use a 50k shunt :)


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Sat 23, 2021 6:43 am 
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Joined: Dec Sat 28, 2019 4:18 pm
Posts: 1076
Location: Corinth, TX
Well, the basic 5K/250Kohm shunts for the 1Kohm/volt meter is a general rule. OTOH, if the docs say to use a 120Kohm resistor (or whatever they specify), use a 120 Kohm resistor.

John


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Sat 23, 2021 7:21 am 
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Joined: Oct Fri 19, 2018 8:30 pm
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How would i set up my Hickok 752 for a mutual conductance reading test, using a 10K 10W current limiting resistor and 50V and an isolation transformer?


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Sat 23, 2021 8:53 am 
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Greetings to tysman and the Forum:

Assuming the 752 is more or less the same as the other Hickok testers, you would set it up to test a 6L6 and then connect your test setup according to Daniel Schoo's diagram below. You will have to be careful.... I don't believe that it is safe to apply power from the test setup unless the test button for Gm is depressed... so this becomes a two-handed operation at best. In other words, dial the variac to zero, press the button (in my case it would be P4 for the 534B or P3 for the TV-7), and adjust the variac for 50 volts. Note the Gm reading on the 752, then dial the variac back to zero BEFORE letting go of the test button. The connection is between pins 3 and 8 of the octal socket (plate and cathode for a 6L6) as shown on the diagram. If the 752 meter deflects backwards, reverse the connections to pins 3 and 8.

If you have Daniel Schoo's writeup, this diagram is found therein, along with explicit instructions on how to use it. If you don't have it, best get it and read it carefully.

Attachment:
Schoo's Gm Test Setup.jpg
Schoo's Gm Test Setup.jpg [ 77.47 KiB | Viewed 977 times ]


Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Tue 26, 2021 2:59 am 
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Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
Thanks so basically set it up same way as other hickoks but instead of setting red dots on english or shunt control use the multiplier switch. so should i be looking for 2000 umhos using either of the ranges (multiplying the reading by the multiplier) at same 50v?


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Tue 26, 2021 4:49 am 
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Location: Lincoln City, OR 97367
Greetings to tysman and the Forum:

I have attached a pdf from my Hickok files which contains all the data I can find on the 752. It does not have a Gm calibration procedure. In reading between the lines, the multiplier switch controls the magnitude of the grid signal, which means that its setting is irrelevant to the Gm cal procedure. You just have to press the right test button (S5?) for a 6L5.

Attachment:
752 & 752A Calibration.pdf [658.04 KiB]
Downloaded 47 times


Good Luck,

_________________
Jim T.
KB6GM


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Tue 26, 2021 7:18 am 
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Joined: Oct Fri 19, 2018 8:30 pm
Posts: 110
in my manual for the 752 it says the multiplier is for the gm scales example x2 is for 3000 scale x4 is 6000 scale x10 is 15,000 scale an x20 is 30,000 scale. take the value you get and multiply it by the multiplier to get umhos. for 6L6 it says multiplier x10 so when i test it i get around 550 so x10 = 5500umhos. if set multiplier to x4 i get a higher reading on scale and times it by 4 i get basically same 5500umhos


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 Post subject: Re: Hickok 752 Calibration
PostPosted: Jan Tue 26, 2021 12:20 pm 
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Location: Lincoln City, OR 97367
Greetings to tysman and the Forum:

I don't have a manual or schematic for the 752. I did grab the calibration instructions that I sent you. In the calibration instructions, it states to check the grid signal amplitudes versus the multiplier setting. Gm is another name for the transfer characteristic of a vacuum tube. The magnitude of the transfer characteristic is determined by the change in plate current for a given change in grid voltage. The scale of the measurement can be changed by changing the magnitude of the grid signal "swing".

I do not know whether there are any other changes incorporated in the 752 multiplier switch. If not, then its setting is irrelevant, as I said... because the GM calibration technique does not use a tube... it provides a large scale AC change in the measured plate current when the test button is depressed and the calibration 50 volts is applied. Since this AC change in current in the Hickok plate circuit is completely independent of the grid signal which the tube tester is supplying (the grid pin isn't even connected for this test), then changing the magnitude of the grid signal with the multiplier switch can have no effect at all.

Herein, I make an assumption that the grid signal amplitude is the ONLY thing changed by the multiplier switch; if the meter bridge circuit is also changed, then all bets are off. In any event, it would appear that the 2000 number would be irrelevant also. For the 600A, the test is done with the English dial set to 73, which gives a full-scale meter reading of 3,000 uMhos. Therefore, the correct calibration point is 2/3 scale (2,000 uMhos). For the 752, the calibration test should produce a reading of 2/3 scale. Figure out a convenient number for that and that becomes your calibration number.

You can determine if the above assumption is correct by simply changing the setting of the multiplier switch during the test. If the 752 meter reading changes, then the multiplier affects the meter bridge circuit as well as the grid signal and all bets are off; a fair amount of research would have to be done to establish a correct technique. If the meter reading does NOT change, then you are good to go at 2/3 scale.

Regards,

_________________
Jim T.
KB6GM


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