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 Post subject: Jackson 658 meter
PostPosted: Oct Thu 22, 2020 2:52 pm 
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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


Attachments:
jackson658clean1.jpg
jackson658clean1.jpg [ 403.36 KiB | Viewed 1450 times ]
jackson658x3.jpg
jackson658x3.jpg [ 274.7 KiB | Viewed 1450 times ]
meter-circuit2.jpg
meter-circuit2.jpg [ 653.04 KiB | Viewed 1450 times ]
meter-circuit1.jpg
meter-circuit1.jpg [ 811.27 KiB | Viewed 1450 times ]
meter1.gif
meter1.gif [ 3.29 KiB | Viewed 1450 times ]

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 Post subject: Re: Jackson 658 meter
PostPosted: Oct Thu 22, 2020 5:52 pm 
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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 1357 times ]
jackson-beede.jpg
jackson-beede.jpg [ 226.97 KiB | Viewed 1357 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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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 1294 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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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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 Post subject: Re: Jackson 658 meter
PostPosted: Sep Sun 12, 2021 11:18 pm 
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Well, so far I can say, success! :D

I took this 658A replaced the diodes and capacitors, then built a circuit above adapting a 0-10 mA meter to 0-200 μA at 155 ohms.
The tester calibrated well using Bill Eccher's procedure and so far is working great!

The meter face, I scanned an original, then printed it out 1:1 scale on a self adhesive packing label paper. Cut it out and put it on the existing meter scale (Actually the back side if it)

I think this will be a wonderful way to save these things :D

-Steve


Attachments:
658newmeter.jpg
658newmeter.jpg [ 961.3 KiB | Viewed 469 times ]

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 Post subject: Re: Jackson 658 meter
PostPosted: Sep Sat 18, 2021 10:13 pm 
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I just bought that Jackson 658-1 with the wrong meter in it! :lol: He wanted a lot less for it on Reverb, only $375 with the shipping.

This is what it looks like on the inside. The only changes that were made was a resistor was added in parallel and capacitor C5 was replaced. Nothing else had been touched. It came with a several copies of the schematic, a bunch of updates to the tube data on the roll chart, and a copy of Erik Knutson's restoration and calibration procedure. From the handwritten notes and what's checked off, you can see where the previous owner attempted to adjust the resistors then stopped when it failed to calibrate. Didn't bother trying to follow the restoration part of it, just fiddled with the adjustments and wondered why it didn't work. :roll: He also neglected to mention it was missing the 6AU6 tube, but at least it was shipped well.

Thank you for posting the image of the meter and your circuit! :mrgreen: I like the idea of using an op-amp to drive a cheaper, more rugged meter. Op-amps excel at circuits like that.


Attachments:
Wrong Meter Closeup.JPG
Wrong Meter Closeup.JPG [ 462.19 KiB | Viewed 386 times ]
Diodes & Adjustments.JPG
Diodes & Adjustments.JPG [ 652.65 KiB | Viewed 386 times ]
Jackson 658 Chassis.JPG
Jackson 658 Chassis.JPG [ 684 KiB | Viewed 386 times ]

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 Post subject: Re: Jackson 658 meter
PostPosted: Sep Sun 19, 2021 3:08 pm 
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Interesting, that has the same perfboard deal that the 658-1 I picked up has.
I was expecting to find the actual circuit board and saw that and wondered if it was original. Seeing yours convinces me that it is.

with a 25μA movement, you would think it would be possible to add a shunt to get a 200μA full scale at close to 155 ohms, but perhaps not.
I'm surprised how meters just aren't available even close to the original specs of that thing.

btw, here is a high resolution image of the meter scale. I printed it with a color laserjet printer where I work into package label paper with adhesive backing. Works great! But you will have to play with your printing software to make sure it prints the proper size.
(I used the 4X6 setting in the default image viewer in Windows 10)

http://www.theradiofox.com/temp/jackson658meter1.png

-Steve

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 Post subject: Re: Jackson 658 meter
PostPosted: Sep Sun 19, 2021 7:24 pm 
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More information is needed on the 25μA movement, since we still have 2 unknowns. I would suggest putting 1.5 volt battery in series with a 50K pot, digital multimeter measuring current and the 25uA meter. Make sure the pot is set at Max resistance before connecting the loop. First we can verify if the meter is accurate by comparing the meter with the Digital multimeter current. Now if you have another Digital multimeter check the voltage across the meter and now with current and voltage the meter resistance can be computed. If you only have 1 digital multimeter you can remove it from the current loop and use it to measure the voltage across the meter. Make sure you adjust the pot for a full scale reading. Post the results and you should get some help.


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 Post subject: Re: Jackson 658 meter
PostPosted: Sep Sun 19, 2021 8:14 pm 
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I would use a 100K pot.

1.5V divided by 50K ohms = 30μA.

I doubt it would damage the meter, but it will be beyond full scale.

31 millivolts across the meter is the goal, there can be a slight fudge factor as the sensitivity pot R29 will compensate a little.

-Steve

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 Post subject: Re: Jackson 658 meter
PostPosted: Sep Sun 19, 2021 8:33 pm 
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Sorry about the in my head calculation. Just use a 100k pot or add a 47K resistor in series with the loop. You need to find the resistance of your meter and the only safe way is to determine that is finding the voltage across the meter at full scale.


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 Post subject: Re: Jackson 658 meter
PostPosted: Sep Sun 19, 2021 9:35 pm 
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I apologize for my criticism, it wouldn't have hurt anything.

I guess I'm on edge lately...

-Steve

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 Post subject: Re: Jackson 658 meter
PostPosted: Sep Mon 20, 2021 12:51 am 
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I didn't have a 100k potentiometer, so I used a 1M one and a 12V power supply. The meter is good. With exactly 25.00 µA passing thru it, it indicates exactly full scale. The voltage drop across it is 61.85 mV, so it has a resistance of 61.85mV/25.00µA or 2474Ω. Since I want a scale factor of 200/25=8.00, the shunt resistor should be 2474/ (8-1) = 353.4. The closest 1% value would be 357Ω. (This indicates the existing 300Ω shunt resistor is incorrect.)

How sensitive does it really need to be? 61.85mV seems pretty low.

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 Post subject: Re: Jackson 658 meter
PostPosted: Sep Mon 20, 2021 1:59 am 
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wow, I'm afraid you need to get down to at least 200 ohms and 40 millivolts full scale before there is any chance of things working. Even then I'm not sure how much play there will be.

Now I know not to even bother trying to buy a 25μA meter to see how well it works.

I wish I knew how the meter manufacturers managed to satisfy Jackson's requirements.

-Steve

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