Hi, I got a Healing Capitol 23 "Wide Horizon" Model 601 home recently. After a clean up replaced a couple of 600v caps, I got sound up & for a short moment a vertical line. I soon discovered I still had HT from the transformer but then it stops at what looks like a 7mm wide x 70mm long HRC fuse. It's a DIODE! Markings are TV18-Sc + Made in Germany. I've never seen anything like this before & it's open circuit both ways, it is mounted between the HT output to the CRT. Any recommendations on replacing this with a silicon rectifier diode? Also does anyone have the schematic for this beast?

Doug

Hi Doug.

I'm wondering if that part you described is actually a thermistor. I had a look at the circuit and that is only thing I thought it could be.

Anyway, send me a email & I'll shoot you off the circuit which is from one of the JR TV manuals.

Cheers

TheTV18 diode is either a Selenium or Silicon EHT rectifier stick. Fairly common for a while in the early 1970's as a solid state equivalent for the trusty old 1S2 and 1B3 EHT diodes.

I'm not sure you can test these like an ordinary diode as the forward voltage drop is quite high I believe (10's of volts).

I do have a few spare TV18 sticks somewhere I think, otherwise the set could be converted back to the original 1B3 valve EHT diode.

I have the schematic somewhere, I'll email you a copy when I dig it out.

No it's a Diode alright! I know why it there, It's to stop EHT flashback. I've just never ever seen one like this before.

Just to make sure we are talking about the same thing, the TV18 stick has been fitted in place of the 1B3 EHT rectifier that feeds to the EHT connection on the side of the CRT?

If it's connected in some other way, it sounds like a bodge repair! I'll have a play with a couple tomorrow and see if they can be tested.

I had a play with some EHT rectifier sticks today to see if they can be tested easily.

As I suspected, you can't test them with an ordinary multimeter as the forward voltage drop is far too high.

Using a bench power supply with a current limiting resistor in series, I was able to get current flow through the diode from about 10 volts and up.
The current limiting is important, these diodes are probably rated for around 5 to 10 mA maximum and an average operating current around 1 mA so you will destroy it if you attempt to test it without current limiting.

Andrew what was the value of the current limiting resistors?

Anything over 100k ohms will be safe. Don't go over 30 volts on the diode, if it's not conducting in one direction by then it's probably faulty.