Hunts CRB capacitor tester

[Bushbaby]

Hi Alan,
Apologies as the pics are somewhat confusing - I think you are correct that its the rectifier, if you study the photo its missing on the one I am working on but I cant fathom what its been replaced with?

Rob

[Yorkie]

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

I'm not sure which component you refer to as 'looking like a ferrite rod' Rob, but in the top picture of your first post, the long paxolin tube marked red at one end, looks very much like a 'sentercel' rectifier, and in the other two pics, if you are referring to the component which is half blue and half white along its length, that's most definitely a capacitor, which will be its value embossed on it near to one end, but you have to look closely for the value as they don't stand out well. (They're good quality caps not noted for failure or leakage - I've used a lot of NOS ones of that type over the years).

Hope that helps a bit.

Unfortunately, you're rather 'sailing in uncharted waters' due to the lack of a good service manual for the CRB is something of a drawback. (I don't think they ever existed - just the 'User Manual' c 1945 with the orange cover, and the annotated circuit by a member of another forum that's done the rounds).


David

[Bushbaby]

Hi David,
Its the lack of rectifier I'm concerned with

Thanks

Rob

[AlanBeckett]

Rob,
Perhaps I've still got the wrong end of the stick (sorry, those rectifiers were often referred to as 'Sticks')
In the absense of a circuit all you can do is connect a replacement between the same points as on the 'good' one.
What's the maximum voltage you expect? You could always measure either one to see what the ac is.
Alan

[tiv48]

if i get time in the next few days i will open mine up and take some voltage readings not sure how much help this will be
rob t

[Yorkie]

Hi David,
Its the lack of rectifier I'm concerned with

Thanks

Rob

The original rectifier will - as Alan has said - have consisted of a series of small discs in a paxolin tube. I've attached some pics of a disembowelled 'Sentercel' rectifier that I recently stuffed with a silicon rectifier diode in a Heathkit C3-U as it was duff. (Most such rectifiers will be well past it after more than sixty years, even if they still work at all). Because a silicon diode will give a much higher voltage than a metal rectifier, I added a series resistor, the value of which I work out on test to drop the voltage to that required by the C3-U.

However, in your case Rob, the 'phantom dabbler' was who tampered with your CRB before it fell into your hands, may have cared nothing for originality of appearance, and rather than go to the trouble of stuffing the paxolin tube, may simply have binned it, and fitted a silicon diode such as a 1N4007, which is of course very small - just as small as a .5W resistor for example, so may be lurking where you have yet to find it.

If you've got a copy of the annotated CRB circuit with hand-written notes on it, which has been doing the rounds, you'll see that there are three secondary windings on the mains transformer. One is the AC heater winding for the Y63 magc eye filament, another is the AC to the bridge, and lastly, you'll see that one end of the the middle winding in the circuit layout goes to a rectifier, where someone has annotated the circuit 'metal rectifier'. Maybe if you identify that winding, you'll see a diode lurking somewhere? (The other end of that winding goes to a 1 uF cap, and to the wiper of the rear switch wafer).

Another option would be to check that the HT voltage at the magic eye is indeed DC - not AC, in which case, somehow, somewhere, it is being rectified.

I appreciate that comparing a circuit with a rat's nest of wiring and 'switchery' is no easy task, especially when it's been subjected to undocumented bodgery by an unknown miscreant, but that there's only one way to bottom it in my view: Namely, by checking each part of the circuit against the circuit diagram, using a highlighter pen to mark the circuit as you go, noting any ommisions or discrepancies, checking the continuity of switches, transformer windings, and the value of resistors and caps as compared to the circuit.

What we'll never know, due to the poor documentation on the CRB, is whther or not during the production period, it was subjected to any factory mods along the way.

(If you don't have manual and the circuit to which I'm referring Rob, do let me know and I can e-mail it to you, but I don't think I can add it as an attachment as it's a 615K PDF).

Hope this helps a bit, rather than adding to the confusion.

[AlanBeckett]

David,
That was an excellent description of what needs to be done. The only thing I can add is to identify the points where the rectifier needs to be connected, once the 'bodge' has been removed.
Alan

[Yorkie]

David,
That was an excellent description of what needs to be done. The only thing I can add is to identify the points where the rectifier needs to be connected, once the 'bodge' has been removed.
Alan

Thanks Alan - I can be sure that if you pay me a compliment, it's genuine! :D

I've e-mailed two files of the manuals to Rob, one of which has component values marked on it, though he may already have the manuals. On the circuit, you can plainly see the metal rectifier diode on the lower end of the centre secondary winding on the mains transformer. If Rob can't find a rectifier, it must have one hiding somewhere to rectify the HT for the Y63 valve. That's the only part of the circuit that uses DC. The bridge circuitry uses AC, and the Y63 heater is 6.3V AC too.

The pinouts and ratings for the Y63 magic eye are as follows:

2&7 = 6.3V AC filament
3 Anode - absolute maximum 200V (DC)
4 = Target
5 = Grid
8 = Cathode

To achieve maximum visual change of the display requires a swing of 22V on the grid.

If the magic eye lights up and changes its display as the dial potentiometer is rotated with a test cap in place, (even if the reading on the dial isn't accurate), it's a reasonable indication that valve is working OK, so must be happy with the voltages on the grid and anode, and that there is a voltage swing on the grid, (hence the bridge part of the circuit is working), which is easy enough to check with a multimeter on the DC voltage range between pin 5 and ground. In that case, there must be a rectifier diode tucked away somewhere and doing its job.

I've no idea what the voltage on the anode of a good working CRB should be as none are stated on the circuit, but it's a fact that even if a metal rectifier is still working at all after six decades, its voltage will be down on its original voltage. It's also a fact that a silicon diode will gave a higher voltage, and thus, it's important to check that it doesn't plonk more than 200V on the anode of the Y63 magic eye, which will also be six decades old or more, so would be eligible for a free bus pass. (It was introduced in 1937). If the anode voltage is approaching 200V or even more, I'd be inclined to select a resistor on test in series with the silicon diode rectifier that will drop the HT to perhaps 175V so that it works only as a rectifier - not a 'rectum fryer' :D

[tiv48]

ok i have taken some voltage readings

on the black wire the voltage using a fluke 25 with no cap under test and dial set to 1 (cap)are
range c 19.8 ac
c1 19.5 ac
r 19.8 ac
on the red wire
range c 19.4 ac
c1 19.2 ac
r 18.4 ac
on the pink wire
range c 18.9 ac
c1 19.2 ac
r 19.5 ac
on the purple wire
range c 18.9 ac
c1 8 ac
r 18.4


voltage at red end of rec stick is 156 dc

finaly the scale is coupled directly to the pot the is no ident on the shaft

[Bushbaby]

Thanks David, Alan, Rob T,
For your thoughts and guidance which are most gratefully received. I'll have to sit down and have a good look at this unit in light of the info you have all kindly provided to move forward (I hope so!).

Rob