Hunts CRB capacitor tester

[Bushbaby]

Hi All,
I'm currently refurbing my Hunts CRB tester. When I removed the chassis from its case I accidently pulled the mains transformer connections from the mains wire connector. I'm in the process of reconnecting these - does it matter which connector goes from the tranformer to -ve or +ve of the mains? Originally it did not have any earth connection but I've added a connector to the chassis - will this also effect which way round the connectors to the transformer are wired?

Thanks

Rob

[attachment=2646][attachment=2647][attachment=2648][attachment=2649]

[Pamphonica]

Rob,
It should make no difference, as mains is ac (no + or -). But check the mains switch. If it only switches one side of the mains, the switch should be in the live connection. Earth connections should make no difference to mains connections (although I am sure you have checked if there are any mains click suppressor caps to chassis and made sure they are suitably rated).
Nice piece of kit by the way!
Jeremy

[Yorkie]

On the assumption that what you have is a simple primary winding on the transformer (perhaps via a switch and fuse), with no other connections on the primary side, it doesn't matter which way around the primary is connected.

The same applies to an AC only radio.

However, it becomes an issue with 'live chassis' equipment such as DAC90As because the chassis is connected directly to one side of the mains. Taking the DAC90A as an example, although, as we know, the set will work fine with the reversible mains plug (actually being female it's a socket - the plug is the two male pins on the rear apron) either way around, one way will connect the chassis to the line, so all the metalwork will be live, but the other way around, neutral will be connected to the chassis.

There are no such issies with the Hunts CRB (Capacitance Resistance Bridge'). The circuit on page 13 of the 115-page manual of the Hunts CRB is a bit sketchy, but simple shows a straighforward single winding on the primary. Not only is the mains isolated from the case, the manual states: "This instrument has been tested at 2000 V DC from mains lead to case. It is recommended however that it is earthed at the terminal provided". I think you're doing the wise thing by earthing it via a three-core mains lead Rob.

We might wonder why it didn't have a three-core mains lead to earth he case from the outset but standards change over the years, and I think it behoves us to rank safety higher than originality when renovating equipment that is now almost 70 years old.

The manual I'm referring to is 'CRB-2' dated 1945, which I asssume is the relevant one, which - in case you or anyone else needs it, I've attached to this post

Hope that helps.

Best of luck with the restoration Rob!

David

[Bushbaby]

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

[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

[Bushbaby]

Hi David,
Thanks for reply - the resistance side is also very inaccurate. I agree this will point to either:
- the dial has slipped on the shaft
- one or more components are now wildly off there stated tolerance.

Yes, the manual does not give much help as its one of the few that I have seen that doesn't indicate any component values. Furthermore, there several versions of this model and it appears that the unit became refined over time and this isn't reflected in any schematic! I have recently purchased a 2nd Hunts tester which I can compared this one to. But as I indicated in the previous sentance, the 2nd tester has a serial number of 177 and the one that is out of spec is no 3611 so I have a feeling that the circuits/components will not be the same.

Pity really as it looks so nice when its powered up and working!

Regards

Rob

[Yorkie]

I'd certainly invesigate the dial and any associated drive mechanisms if it isn't mounted directly on the shaft of the pot. It seems inconceivable that the components that make up the bridge circuit can have drifted can be so far out of tolerance,and be out on all ranges for capacitance and resistance.

Unfortunately, the manual is an 'Operators' Manual rather than a 'Service' manual, so I guess it wasn't intended to do anything more than to give operators a basic insight into how the bridge works.

Do update us on progress Rob (oer even lack of progress!).

David

[Bushbaby]

Help Please!
I'm currently working on the older of my two CRB testers which appears to have been modified. It has a switch installed where the earth terminal should be and the neon has been removed for a small bulb. My good friend Kev has supplied me with a lamp holder from a another CRB that was proported to have been working (many thanks). On inspection of mine its got a component that looks like a ferrite rod (it certainly isn't) but I cannot identify what this or its function. Can anyone offer advice how I can return this to its original condition.

Thanks as always

Rob

Picture from a working model CRB
   

Picture from the tester I'm working on
   

[Bushbaby]

[/align]....Perhaps a better pic of the one I'm working on!

   

[AlanBeckett]

Rob,
I'm not sure which bit you mean when you say 'looks like a ferrite rod'. If you mean the long brown thing it's probably a rectifier. The paxolin tube will be filled with little Selenium discs. If it's duff it's usual to stuff it with a 1N4007 or some such, plus a resistor.
Alan