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 Post subject: Jackson 658 meter
PostPosted: Oct Thu 22, 2020 2:52 pm 
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Location: Dayton Ohio
I picked up some Jackson 658 tube testers recently, and unfortunately a few lack their meters.

The meter used is a 200 microamp movement with a 155 ohm coil. That means its only 30 millivolts for full scale.

I discovered that most meters, even sensitive 50 microampere movements aren't that low. Usually on the order of 100 millivolts.
So, what to do? After discussing a possible transistor amplifier circuit with a friend, they suggested an OpAmp circuit.

So here is what i have. It seems to work, but requires a power supply. As many people are installing digital meters in their tube testers, they require a power source too.

So far this is working with a variety of different meters. I plan to make a small supply to power it rather than a 9 volt battery.

The IC is an LM358. Sorry that didn't get onto the schematic.

-Steve


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jackson658clean1.jpg
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jackson658x3.jpg
jackson658x3.jpg [ 274.7 KiB | Viewed 286 times ]
meter-circuit2.jpg
meter-circuit2.jpg [ 653.04 KiB | Viewed 286 times ]
meter-circuit1.jpg
meter-circuit1.jpg [ 811.27 KiB | Viewed 286 times ]
meter1.gif
meter1.gif [ 3.29 KiB | Viewed 286 times ]

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 Post subject: Re: Jackson 658 meter
PostPosted: Oct Thu 22, 2020 5:52 pm 
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Location: Sunbury, Ohio 43074
Looks like it should work fine as long as you've got the power supply bypassed to keep noise out. I'd add maybe a .1 across the output filter cap. Perhaps even put in a second stage filter ... resistor/cap

Might want to make sure the whole amp/meter combo is linear when you get them put together.

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 Post subject: Re: Jackson 658 meter
PostPosted: Oct Thu 22, 2020 6:05 pm 
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Thanks Barry, actually I did put a .1 cap on the output, but neglected to put it on the schematic.

It seems nice and linear on checking it out with the meters I tried.
Followed the Fluke well. :D
Just as long as it doesn't drift out of calibration.

-Steve

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 Post subject: Re: Jackson 658 meter
PostPosted: Oct Thu 22, 2020 6:09 pm 
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azenithnut wrote:
Just as long as it doesn't drift out of calibration.

If you know the approximate gain that you need in advance, replace the pot with a lower-valued pot and a series resistor. That'll reduce the drift.

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 Post subject: Re: Jackson 658 meter
PostPosted: Oct Thu 22, 2020 6:28 pm 
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Thanks Steve, ill do that.

-Steve

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 Post subject: Re: Jackson 658 meter
PostPosted: Oct Tue 27, 2020 5:31 pm 
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This is the circuit I'm going to use. The power transformer is a dinky one I found in a DVD player.
It outputs 12 VAC at probably 100 milliamps. The circuit draws 2.8 milliamps at most, so no worries about current draw :D

Connected to the meter I found, which is a spot on exact model Jackson used in the 658A that I had.
I moved the scale and the "Dynamic Output" logo to the good meter.
This meter is a 100 microampere with a 1900 ohm coil. Not going to work.

My circuit converts it very well and it seems to be quite linear in action :D

-Steve


Attachments:
meter-power.gif
meter-power.gif [ 4.02 KiB | Viewed 193 times ]
jackson-beede.jpg
jackson-beede.jpg [ 226.97 KiB | Viewed 193 times ]

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 Post subject: Re: Jackson 658 meter
PostPosted: Oct Tue 27, 2020 7:31 pm 
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Excellent and elegant solution to a common problem! Thanks for sharing.


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 Post subject: Re: Jackson 658 meter
PostPosted: Oct Wed 28, 2020 3:10 am 
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Location: Lawton Oklahoma
Sweet! Great work!. I have several 658s and fortunately have a meter for all of them. But its great to see an alternative for the unobtanium originals.


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 Post subject: Re: Jackson 658 meter
PostPosted: Oct Wed 28, 2020 10:31 am 
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Op Amps are such versatile little critters.... ;-). As a novice, I was fascinated by them. "The Op Amp Cookbook" by Walter J. Jung ..... the bible. :mrgreen:

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 Post subject: Re: Jackson 658 meter
PostPosted: Oct Wed 28, 2020 12:10 pm 
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I found this one on Ebay. It has the wrong meter in it and the seller makes absolutely no mention of it, only says the tester is RARE! No mention of how it was modified to even work with that meter but wants $495.00 shipped for it!
Attachment:
File comment: Jackson 658-1 With wrong meter
Jackson658WM.jpg
Jackson658WM.jpg [ 682.29 KiB | Viewed 130 times ]


Last edited by fordtech on Oct Wed 28, 2020 12:13 pm, edited 1 time in total.

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 Post subject: Re: Jackson 658 meter
PostPosted: Oct Wed 28, 2020 12:12 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 8459
Location: Sunbury, Ohio 43074
Quote:
only says the tester is RARE!

Of COURSE it's RARE.... most of them have the correct meter in them. 8)

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