My First Predicta

[Chris K]

Originally Posted by bandersen

+/-20% on the voltages is fine. They are very much dependent on your input line voltages and the operating points of all the circuits, condition of tubes etc.

Like I said, go through the resistance chart. There has to be a major smoking gun (or two) for voltages to be completely absent. It's not going to be something off be a few percent - it's going to be an open circuit or something shorted.

I've started to. First I checked all the wiring from the AC line to the line filter cap, through the diodes, through the choke and the electrolytic caps that bracket it. It all checks out so that means I didn't screw up those first 3 electrolytics.

I started with V1, first tube in the chart and the first video IF tube, a 3BZ6. Pin one should be 1.5Mohm and I have that. Pin 2 should be 47 ohm and I have 65 ohm. Pin 3 should be 14 ohm and I have zero. Pin 4 should be 15 ohm and I have zero. Pin 5 should be 330 ohms measured from the 140v source and I have 420 ohm. Pin 6 is the same measurement parameters as 5 and I have the same 420 ohm. Pin 7 to ground should be zero and I have 0.1 ohm. Looking at the schematic, I don't see pins 3 and 4 connected to anything so I don't know what these mean. I really apologize asking you to take me by the hand but is there a tube in a different section I should check that would help figure this out faster?

Thanks Bob

[bandersen]

3 & 4 are the tube filaments. I wouldn't worray about them since your tubes are lighting up. I suggest checking the 12CA5 audio output tube and the 12DQ6 horizontal output tube.

[Chris K]

Oh of course they are. Duhhhhhh! Anyway thanks for the guidance. I’ll check back in to the board once I get more data tomorrow night

[Yamamaya42]

Since it does not have the direct link as you mention for the 260V C, but through the unnamed 2.2k resistor ( current limiting? ) , doing as bandersen suggested should find the problem, be it a short or whatever, if it IS a short, the voltage drop across that 2.2k resistor would be rather large, and it's prob getting a bit warm!

[Chris K]

Originally Posted by bandersen

3 & 4 are the tube filaments. I wouldn't worray about them since your tubes are lighting up. I suggest checking the 12CA5 audio output tube and the 12DQ6 horizontal output tube.

Happy Friday all and Happy Friday Bob!

I'll keep this simple. I'll state the tube and the readings will go from Pin 1 to the last pin sequentially. I won't give the pin number for clarity sake. I'll list the resistance readings as measured/should be. All were measured from chassis ground except where indicated otherwise.

12CA5: 96/68 0/480K 0/12 0/14 530K/480K 1100/1000 295/295

12DQ6: 0/0 12meg/22 0/0 192k/9k 1meg/1meg 500k/Tie Point 20/20 3/0 15/25 (Top Cap)

To me, the 260v C voltage isn't getting to the 12CA5...but I'm kinda lost! Any thoughts? I'll remeasure to be sure.

[bandersen]

Well some of those readings are way off. I'd start by tracing out the 12CA5. Is pin 2 grounded accidentally? Pin 2 and 5 should be connected together so you should have gotten identical readings but you didn't?

Also check that the primary on the audio output transformer is good. That supplies the plate voltage to the 12CA5

[Chris K]

Originally Posted by bandersen

Well some of those readings are way off. I'd start by tracing out the 12CA5. Is pin 2 grounded accidentally? Pin 2 and 5 should be connected together so you should have gotten identical readings but you didn't?

Also check that the primary on the audio output transformer is good. That supplies the plate voltage to the 12CA5

Audio output primary is good, 285 ohms. The reading on pin 2 is correct at zero. Checked several times. I thought I might get away with not removing the board but it's looking like I might need to.

[Chris K]

So I pulled the board. At some point, the socket for the sync sep was replaced. Lousy job too. All the pins have continuity with the traces on the board except pin 2. In fact, the board is so discolored from flux and heat, I can't even see where it goes. In the schematic, pin 2 is the screen of the 9BR7 and it feeds into the K2 couplate. I do not have continuity between the pin 2 solder tab and the tab just to the right and above which would be the more likely candidate for continuity...again, the board is so fouled, I can't see if that's the case. No problem jumping the pin to another spot but I don't want to jump to the wrong spot. Anyone have any ideas?

[bandersen]

That's the 10DE7. There is no connection to pin #2.

[Chris K]

I better stop embarrassing myself and taking everyone's time. I'll dive into this over the next few days and see where I can get. Thanks everyone.

[Chris K]

Sand resistor is all messed up. I cut it out and measured it. First section measuring 280 ohm...next two sections measuring in the 25-50k range. Final section is around 60 ohms. Have the appropriate 20 and 10 ohm high wattage wirewound resistors to replace. We'll see what happens.

[Yamamaya42]

Are you referring to R77? Which seems to be the only multi power resistor in the set? ( 13/17/8 ohms?)

Or something else? If it's R77, then the heater string must be very starved of power!

[bandersen]

Yeah, I'm surprised the tubes lit up at all.

[Chris K]

Yep…R77 and those resistances

[Chris K]

Well...let me make certain I'm measuring this correctly. Is it lug one to two for the first section and lugs two and 3 for the second section and so on or is it measured some other way?