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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 18th May 2023, 9:21 pm   #1
G4_Pete
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Default Greyshaw CR Bridge Type CR 50 Calibration

Last weekends car boot find now working but not without some confusion which I will try to clarify here in case anyone else has similar problems.

Firstly thanks go to R Bryan for drawing out and posting the circuit diagram.
https://www.vintage-radio.net/forum/...ad.php?t=67860

After replacing the normal leaky capacitors I found the calibration was way off . I hope the math's is right but either way it is working correctly now.

The range resistors were all in tolerance and the wire wound pot was good. I started to calculate the theory but it did not line up with the fitted resistors for example the 1K +100 Ohm in parallel should be 103.25 Ohm. This is fixed by the wire wound 1K pot only using part of its travel ie it undershoots and overshoots the printed scale enabling a balance at the 1 and 10 scale points rather than an end stop. My Pot gave 929.25 Ohms span which divided by 9 gives 103.25 Ohm. Divided by 9 as there are 9 scale segments in 1 to 10 scale.

Replacing the 1K/100 Ohm parallel combination with 103.3 ,100+3.3 Ohm and indexing the pointer on 1 with a 100 Ohm standard resistor on the 100 Ohm range completely fixed the Resistance calibration on all ranges.

The capacitor range was still way off so I ran the calcs for an AC bridge and found that the 0.1 uF standard should be 0.09685 uF . I fitted two nominal 0.047's selected from a half a dozen or so to give approx 0.097uF simply measured on a fluke DVM on C range. This worked perfectly with adequate C calibration across all the ranges.

This never was going to be a precision meter but the results of the above were really satisfying with every component I checked reading as expected so I am really pleased with this little instrument to add to my bridge collection.

So it remains a mystery why the 1K//100 Ohm combination was used.

For completeness the resistor in series with the Pot sets the range start and end overlap so that at scale reading 1 the Balance pot and series resistor is set at exactly 103.25 Ohm giving 1:1 ratio equaling the 1:1 ratio in the range arm with 100 Ohm test on 100 ohm range. I used 100 Ohm as I have a standard for that but any decade combination 10 100,1K etc will work.

To re state previous post comments , this is a fairly crude instrument capable of warming up resistors on the lower ranges , bridge feed is 33 volts limited by a 270 Ohm 5 Watt limiter with the lowest range only putting 1 Ohm in the circuit so worst case you could dissipate 1 watt in a 270 Ohm test resistor. Note also on leakage test capacitors charge to over 200 Volts .
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Old 21st May 2023, 4:57 pm   #2
John_BS
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Default Re: Greyshaw CR Bridge Type CR 50 Calibration

It's taken me some time to get my head around this, but I can only suggest that the choice of 91 ohms (assuming the 100 ohms is 1%) was made to take out possible tolerances in the rheostat.



Leaving margins at both extremes, a nominal 1k pot with 47 ohms series kind of makes sense:

47+ 44 = 91,

47 + 863 =910
Perhaps it was at one time a 10% part?
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