Hickok 209A calibration procedure

[WA4WAX]

If you have a good 209A, here is what to do:

1) Check the bridge tube and associated resistors for match.  Check the 470 pF caps for leakage......or just replace them with new 500 VDC dipped micas, closely matched.

2) Make sure the 6AL5 is FB, and the DC voltage coming off the DC supply meets spec.  Upgrade the 10 uF electrolytic to a new 105 C 50 uF Nichicon.

OK.........

Remove meter from case.  Oriented it horizontally with the face plate spanning a pair of properly spaced 2 x 6 or 2 x 8 boards.  This will give you nice access to the DC and AC cal pots down below.

Looking at the top of the unit, you will see a pair of pots.  They are 500 K carbon.  The outer one does AC balance for peak to peak mode.  The inner one does DC balance for normal mode (RMS).

First step:

Put the 209A in AC, normal.  Short ground lead to probe tip.  Probe should be plugged into the 4 pin jack.  Go to the 1200 volt scale, and zero the meter.  Now, switch to 3 volt scale, and zero the meter by adjusting the INNER top pot.   Voila!  You just took care of the 6AL5 contact potential on RMS.

Repeat the above, but switch to peak-to-peak.  This time, use the top OUTER pot!

If all goes well, she will now zero on ohms, caps, DC+, and DC -.   My was pegging left until I set the AC balance stuff.

Set up DC and AC calibration sources.  If you plan to do audio and/or RF work, use an AC source in the 1000 Hz to 20 KHz range.  The 30 volt scale is a good choice in my opinion.

Calibrate both AC and DC using the two bottom pots.

Have fun!

Recommended: Lose the phone tip jacks in favor of nice banana post....silver plated best.  Use heavy, low R wire for the ground lead and ohm leads.

[WA4WAX]

A little fine tuning on the last post:

1) 50 uF may be excessive.  Use 16 to 20 uF.

2) Replace 6SN7 cathode and plate resistors with metal film, 1 to 2 watts, and 1% or less. 

3) Consider ditching the batteries.  Plenty of room in the beast for a little regulated 3 VDC supply.  If you stay with batteries, put them in a little Tupperware box or something.......that way, if they should ever "spill their guts", the damage will be contained.

If any of this helped you with your 209A, please advise.

Thanks!

[WA4WAX]

Also, bake out a matched bridge tube.......that should make it gas free, or nearly so.  Please see my post "perk up your VTVM...." for details on this procedure.