Pilot TV-37 HELP!! - Videokarma.org TV - Video

ghjkl67 · #1 02-22-2015, 08:45 PM

Sorry for adding yet, another Pilot TV-37 thread, but I have an issue that may or may not be rather unusual. So far, I've gone through most of the trivial restoration stuff. This includes recapping, fixing the CRT filament voltage surge, and most recently, repairing the IF audio section. As for the picture, I'm getting horizontal and vertical deflection, but the picture is extremely dim, distorted, and is not visible in a light room. The brightness control is DEAD and has no effect on the picture. All measurable voltages at the CRT base are consistent with specs. The problem may be low emissions, but it also may have something to do with the CRT cathode circuit. My testing procedure is outlined below.

One at a time, I unsoldered the wires from CRT pins 2 and 3 (grid and cathode) and reattached them using an alligator clip so that they can be easily disconnected with the set powered on. This way, I can disconnect the wire and see if it has any effect on the picture. If it has any effect at all, it would tell me that the CRT is picking up the signals from that pin. However, if disconnecting the wire has no effect on the picture, it would tell me that there may be something wrong, possibly an open circuit in the CRT base. Here are some observations:

Pin 2 (grid) when disconnected caused the picture to flutter. This seemed to indicate that the grid is being activated and that there is continuity to the grid.

Pin 3 (cathode) when disconnected had no effect on the picture at all. To make sure it was making contact through the CRT base, I melted some new solder on the pin. Still no effect on picture, so I decided to monitor voltage when the cathode is disconnected (using a less sensitive analog meter). Any change in voltage when disconnected would indicate that there is current being drawn from the cathode and that it is making a proper connection. Upon disconnecting the wire, the voltage increased by about 1v.

So, the cathode is making a connection, but it has no effect on the picture (like the brightness control). It's drawing very little current (in the microamps), but I don't have any specs to go by for the current draw. Again, the voltage range at this point is consistent with Riders, so what's wrong with the cathode? I don't know if these are proper testing methods, so please correct me if you see a mistake in what I did. I posted a piece of the schematic below for easy reference.

WISCOJIM · #2 02-22-2015, 10:02 PM

Try subbing a different 3KP4 or a 3KP1 for your current CRT, and see what difference that makes. Might as well try the obvious first before wasting a lot of time trying to diagnose what may be a CRT issue after all.

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jr_tech · #3 02-22-2015, 11:39 PM

A few years ago, I fired up a 14" DuMont portable to find that the tube had almost zero emission... in a dark room a very faint fuzzy picture could be observed. On a tube checker the needle did not move, even when the heater was cranked up to 8 and 10 volts. At 10 volts, I could occasionally get "normal" emission for a bit by tapping on the neck... hmmmm loose cathode connection. I tried re-soldering the base pins several times to no avail. Next, I tried to re-weld the cathode connection by discharging a capacitor charged to a few hundred volts between the cathode lead and the grid and anode tied together (with the heater turned up to 8 or 10volts)... No joy there either. I replaced the tube.
Later, I autopsied the tube, a DuMont 14SP4 and found the worst looking carbonized weld that I have ever seen at the connection between the cathode tab and the base pin... Somebody must have been having a bad day at the old DuMont factory.

jr

ghjkl67 · #4 02-23-2015, 07:47 AM

So a weak CRT can actually cause the cathode/brightness to be inactive? I thought the brightness would have some sort of effect on even the weakest CRT. Turned counterclockwise, shouldn't the picture turn off? Mine doesn't. The faint glow on the CRT cannot be controlled.

dieseljeep · #5 02-24-2015, 09:23 AM

Quote:

Originally Posted by jr_tech

A few years ago, I fired up a 14" DuMont portable to find that the tube had almost zero emission... in a dark room a very faint fuzzy picture could be observed. On a tube checker the needle did not move, even when the heater was cranked up to 8 and 10 volts. At 10 volts, I could occasionally get "normal" emission for a bit by tapping on the neck... hmmmm loose cathode connection. I tried re-soldering the base pins several times to no avail. Next, I tried to re-weld the cathode connection by discharging a capacitor charged to a few hundred volts between the cathode lead and the grid and anode tied together (with the heater turned up to 8 or 10volts)... No joy there either. I replaced the tube.
Later, I autopsied the tube, a DuMont 14SP4 and found the worst looking carbonized weld that I have ever seen at the connection between the cathode tab and the base pin... Somebody must have been having a bad day at the old DuMont factory.

jr

Dumont CRT's were some of worst CRT's out there. Several different manufacturers used them and there were all junk. That is, the later production. Those old CRT's, from the late 40's or early 50's, seemed to last as long as the other makes.