Hunts CRB capacitor tester

[Yorkie]

Hi All,
Many thanks for your replies.
I've now got this working, the magic eye and mains neon are awesome on this unit. BUT having tested some caps at the same time as Kevin using his and with the same type of caps (the ones from Just Radio) my tester is reading approx 40% too high for all caps which is real pity.

Can anyone suggest the best place to start looking as I am aware that there are some higher tolerance components in this unit and I don't want to make it any worse.

Many thanks

Rob

Hi Rob,

As the name of the model conveys, it's a 'bridge' and works on the Wheatstone Bridge principle. I've attached a link below which expalins that principle. Basically, if all four sides of the bridge (which can be capacitors, inductors or resistors, depending on what component it is desired to test) are balanced, then no current will glow through the galvanometer - it flows eaqually along all four legs of the bridge. Hence the galvanometer will show the null point at which the bridge is balanced. The value of the component under test is then read off on a dial attached to the balancing pot, which calibrated to show the value of the component under test at the null point. In the CRB, this null is observed on the magic eye of course, rather than on an analogue meter.

The way the CRB works is that three legs of the bridge are made up by capacitors in the CRB - the fourth leg is the capacitor under test, the value of which you wish to know. When you turn the dial, you adjust the pot in series with a cap in the bridge, and read off the reading on the dial at the null point.

I think you can be quite confident that the new caps are very close to their stated values, and given the age of the CRB, the fault is almost certain to lie with that. I've no experience of the instrument, but 40% is a large error, and if that error applies on all ranges, are you sure that the dial hasn't slipped on the shaft of the pot? I assume that it hasn't, because I guess it will have 270 degrees of rotation, and thus, the dial has to travel from end to end through that arc of rotation, so needs to be line up with the cursor. If that is so, the fault points to a cap or caps in the bridge that are way out of tolerance. The only way of ascertaining that would be to disconnect one end of each of the CRB caps in turn, and test them on a capacitance meter that is known to be correct.

The only manual I can find is that which I posted above, which is for the CRB2 dated 1945. I don't know if that's the correct manual for your model. The manual isn't very comprehensive, and has no component values on it, but it's better than nothing.

Have you tried it on the resistance ranges, and is it accurate there? If so, that will tend to confirm suspicions about the caps in the bridge. If indeed any are faulty, the problem that arises is getting replacement caps that are cose tolerance. Fine up to say 1000pf - silver mica ones are 1%, but for higher value electrolytic ones - eg, up to 50uF, the normal tolerance is +/- 20%, so you'd have to 'select on test' using an accurrate capacitance meter to find caps which are within say 1%.

All of the above is speculation on my part, not being familar with the CRB, but I hope it's of help.

This explains the Wheatstone Bridge principle - sorry if you are already familar with it:

http://www.citycollegiate.com/wheatstone_bridge.htm

Best of luck with it Rob. It's time that the God's smiled upon you, and that your luck turned!!

David