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 Post subject: WTB: Hickok 539A micromhos meter
PostPosted: Oct Fri 27, 2017 8:33 pm 
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Joined: Aug Wed 26, 2009 1:45 am
Posts: 2350
Location: Sioux Falls, SD 57108
The story that I got is a fellow bought this 539A on e-bay as a working tester. When he got it, it was not working. A friend tried to get it working and could not find the problem. I got it at a ham swap meet. It is not working. The meter moved a little when a good tube was tested. I checked pretty much everything and I think the meter is bad. I took it apart and the spring for the needle is messed up. I tried to move it a little but it is bent and there is no way I can get it the way it needs to be. So I am looking for a good meter for a Hickok 539A. Do not want to spend a lot of money on this because the cabinet is not in good shape. I would like to get it at least working and not part it out. Thanks for any help.

The meter is 3 3/4" tall and 4" wide.


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 Post subject: Re: WTB: Hickok 539A micromhos meter
PostPosted: Oct Fri 27, 2017 11:52 pm 
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Joined: Dec Tue 01, 2015 5:31 am
Posts: 1371
Location: Columbus Ohio
There is a 539c meter on eBay, the Micromhos scale is the same, but has added scales you don't need.
You could swap out the faceplate, VERY carefully. There are meter specialists on here who may be able to offer more info.
Roger Kennedy MAY be a resource, though that seems hit and miss lately. http://alltubetesters.com/parts.htm
Looks like the 533a is a match for the scales, not sure if it will be a direct replacement though.

good luck

Joe

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 Post subject: Re: WTB: Hickok 539A micromhos meter
PostPosted: Oct Sat 28, 2017 12:47 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: North of Mpls, Minnesota
Meter spec's are wrong. 539C will not work. Just because the face is close doesn't mean the current and resistance are the same.

Dave


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 Post subject: Re: WTB: Hickok 539A micromhos meter
PostPosted: Oct Sat 28, 2017 12:54 am 
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Joined: Aug Wed 26, 2009 1:45 am
Posts: 2350
Location: Sioux Falls, SD 57108
I have found the following. Have not found any other Hickok that used the same meter. It will need to be the exact one.

539A----97ua---250 ohm
539C----115ua--1500 ohm
533A----500ua--233 ohm

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 Post subject: Re: WTB: Hickok 539A micromhos meter
PostPosted: Oct Sat 28, 2017 5:06 am 
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Posts: 32702
Location: Maryland 20709, USA
dsturz wrote:
I have found the following. Have not found any other Hickok that used the same meter. It will need to be the exact one.

539A----97ua---250 ohm
539C----115ua--1500 ohm
533A----500ua--233 ohm
Hi Don,

You can use a less sensitive (higher FS current) meter to replace a more sensitive meter by building an amplifier.
This is a perfect application for a regular operational amplifier.

Model both meters in voltage mode, thus:
The 539A meter FS voltage is 97uA * 250 ohms = 0.02425 V = 24.25 mV.
The voltage required for the 539C meter FS = 115 uA * 1500 ohms = 172.5 mV.

The gain of the op amp stage is therefore 172.5 / 24.25 = 7.113.
That gain is easily obtained and quite stable with precision resistors.

The input resistance of the op amp stage must be accurately 250 ohms.
This connects to the original 539A meter leads.

The op amp output can be connected directly to the meter if the input voltage is limited.
A germanium diode could be connected across the 539C meter terminals as a safety device.

As a check:
24.25 mV * 7.113 = 172.49 mV.
172.49 mV across 1500 ohms = 114.99 uA.

- Leigh

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 Post subject: Re: WTB: Hickok 539A micromhos meter
PostPosted: Jan Sun 21, 2018 11:14 pm 
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Joined: Aug Wed 26, 2009 1:45 am
Posts: 2350
Location: Sioux Falls, SD 57108
Still looking for an original meter. Thanks.

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 Post subject: Re: WTB: Hickok 539A micromhos meter
PostPosted: Apr Tue 17, 2018 8:27 pm 
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Joined: Aug Wed 26, 2009 1:45 am
Posts: 2350
Location: Sioux Falls, SD 57108
Still looking for an original meter.

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