Hunts CRB capacitor tester

[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