testing cathode current

[timmy]

hey guys does anyone have a simplified way of checking cathode current on a ctc15 without putting jumpers everywhere. if the current was to high well then i will have to do just that, putting jumpers or whatever the folder says to do to adjust. the hi voltage adjustment is the pot in the rear and i have an eico probe for that which should be 23kv. but the current will save the fly if it were touched, i would feel better knowing that it is at the minimum. thanks...

[jpdylon]

I install chassis mount fuses where convenient and connect the fuse holder in series with the cathode circuit. Not only will the fuse blow if there is a failure, likely saving the flyback, but i can remove the fuse the connect an ammeter across the empty fuse holder to adjust the cathode current.

I advise buying an autoformer or variac and run your set at 110v like it was designed to. 120v is a little too high for these sets and will decrease their service life. You may also not be able to bring down the cathode current to or below the recommended value without doing this in some sets.

[bob91343]

I have an HP clip-on DC milliammeter that can be used for this sort of thing. The most sensitive range is 3 mA FS and it goes to 1 A. The only down side is that the probe is a bit large and won't fit some places. I think it's model 428A.

[timmy]

i have a dvom and it has m.a that goes to 10 amp could i use this and put it in series somewhere? just to get an idea what the current is. problem is every pot i believe was turned and the fly was replaced so from what i understand if this cathode current is to high it could overheat the fly and burn up so after replacing resistors and tubes and a silicon dual diode and a focus coil all burned,i would rather not take a chance. would the horiz output tube be the one area to check the current by putting a meter in series with the single wire plug on top of the tube, would this be a reliable option? i am not sure at this point.

[Findm-Keepm]

Easiest way to measure the cathode current when you are unsure of things is to get yourself one of the adaptors that Pomona made. They plug in between the tube and the socket, and allow you to measure the cathode current directly, no disconnecting anything necessary. Ensure you have the service manual or Sams folder BEFORE attempting any adjustments to the Horizontal Output cathode current and efficiency coil.

I've seen the adaptors on eBay and Playthings of the Past's website - do a google/eBay search as well. Should turn up somewhere.

Cheers,

[bob91343]

I have some of those adapters.