I’m Back! Pilot TV37

[jr_tech]

Often, the shield of a coax cable is grounded at only one end, to prevent “ground loop” induced hum.

jr

[Electronic M]

Originally Posted by Chris K

There is something odd here. There's a coax cable coming off the on/off/volume control pot. If the cable is traced all the way to its other end, both the shield and the center wire connect to a 4 terminal terminal strip near the 6AU6 and the 6AL5. But back up at the volume control, the shield isn't connected to anything. The insulation is stripped back and the weave is exposed and twisted but it's not soldered to anything. I've checked the schematic briefly but I can't find it.

Been a while since I've had mine open, but I can tell you having a coax shield grounded on only one end is a common practice for audio lines on 40s and 50s equipment...the outer shield of the coax isn't needed to complete the ground circuit because the items the coax connects are both hooked to chassis ground...The coax shield only needs grounding at one end to block the wire from noise pickup.

If the loose end looks like it was connected then something is going on, but if it looks like it never was connected it's probably fine that way.

[Chris K]

Happy holidays to everyone! All the best to you and your families!!!

[nasadowsk]

Grounding only one side is the norm in industrial control systems. But we use current loops to send signals

[Chris K]

Got it regarding the coaxial ground. So is the filament string the only, self contained AC circuit in the TV? If yes, what would account for a circuit that's continuous all the way to ground, but only the last tube in the string before the ground lights up blazing before any of the other tubes get appreciably warm?

[jr_tech]

Do both sections of the 6AL5 lite up? If the heater on one section is open, the other section would be quite bright, in a series string set. Do you have another 6AL5 to try?

jr

[Chris K]

Originally Posted by jr_tech

Do both sections of the 6AL5 lite up? If the heater on one section is open, the other section would be quite bright, in a series string set. Do you have another 6AL5 to try?

jr

Yes I do...I'll give it a try

[Chris K]

Put another tube and yes both sides are lit up. When the TV was working, this tube was not lit this brightly. When I bring it up to around 70ac volts through a 300w bulb, it's already burning brightly and none of the other tubes are getting warm or giving a hint of filament light, even with the room darkened. I'm probably going to have to go through the wiring, schematic, tubes and everything I replaced to run this down. Tics me off! The TV was working after I changed most of the caps. Then I started replacing resistors and when I powered it up, the 3 ohm resistor going to pin 4 of the 6AL5 burned up with a flash...I saw it against the bench behind the TV despite the chassis being upright. This was no slo-blo...it went off like a firecracker. Since I replaced it, I've brought it up very slowly on the Variac only to see the tube light up like it's been doing.

[timmy]

Is it possible you put one end of the resistor in the wrong place ? It happens. Is it also possible that one of the tubes has a massive heater short.

[timmy]

Well if the 3kp4 is no good you could use a 3kp1 it’s green but takes alittle getting used to.

[Chris K]

Originally Posted by timmy

Well if the 3kp4 is no good you could use a 3kp1 it’s green but takes alittle getting used to.

Thanks Timmy! The filament in the original is fine, at least testing continuity with my meter, but I do have a NOS 3KP1. Do you think it's worth a try to put it in and see how it goes?

[timmy]

Originally Posted by Chris K

Thanks Timmy! The filament in the original is fine, at least testing continuity with my meter, but I do have a NOS 3KP1. Do you think it's worth a try to put it in and see how it goes?

The 3kp4 or 3kp1 well you need to use one for testing but if the p4 is good then use the p1 since the p4 is pretty much unobtainable

[Chris K]

No difference with the new CRT. I'll get back to everyone. Like I said, I need to go through this almost wire by wire, component by component and check against the schematic. In the meantime, If anyone has any other suggestions, please chime in.

[Chris K]

...and when I get my Seco 107 later this week, I'll test all of the tubes.

[Chris K]

The tube tester came today and it works great!...I found out how great when I went through all of the tubes in the Pilot. Valve Carnage from my 3 ohm resistor flash. Filaments all checked out earlier as I posted but so many tubes got fried. The 6AU6 sound IF, the 6BA5 video amp and all 3 12AT7 tubes in the front end...oscillator, mixer and RF amp, all bad or baked. Wow. Well, now I know why I wasn't getting much when I powered it up. Time for a tube search through my stash but most likely another eBay, tube purchase extravaganza.