My Predicta crapped out

[old_coot88]

Originally Posted by Yamamaya42

the voltage on the plate cap CAN be measured, if IF it is unhooked from the HOT...

The HO tube beginning to red plate proves there's voltage on it. With the cap disconnected, the screen grid will get cooked in short order.

[Yamamaya42]

For the very short duration that the damper heats up and gives power through the FBT to the plate cap, long enough to be measured is what I meant, there is really no risk to the screen grid or its resistor, it's not like you are going to leave it on any longer to take the voltage measurements after it first comes up, 30-50 seconds or so, I have done it many time, to confirm that power IS getting through the damper and FBT to the HOT OK.

[old_coot88]

Originally Posted by Yamamaya42

For the very short duration that the damper heats up and gives power through the FBT to the plate cap, long enough to be measured is what I meant

Correct you are amigo.

[Tube TV]

Originally Posted by madlabs

Finally where the set it. Now when I power up I get nothing.

Swapped out V8, V10, V11, V12, V13. No change. The horizontal output tube looks like it might be red-plating a little but I am not sure. It is just a dim orange.

Voltage at the rectifiers is a little low, 247 instead of 290. I couldn't find my HV probe before I came over to where the set is. How did I misplace something 12" long with a cord on it? I dunno. Anyway, I measured the flyback resistances and all are exactly as they were when I restored the set.

What do do next? I guess start trying to track down the low B+ voltage. I think I have a test socket I can put on the horizontal output tube and do some looking at the voltages there.

The low B+ could be from the horizonal output tube red plating and loading the B+ down. I would re-measure the voltage at the rectifiers with the horizontal output tube removed. I'm assuming this is a series string set with 2 rectifiers in a voltage doubler circuit?


Any chance you can post up the schematic?

[madlabs]

Sorry for the delay in replying. It has been a crazy week and now I am at work for a 72 hour shift.

I will figure out a way to get the chassis on its side to take voltage measurements. I could set up a jumper on the output of the HOT to the flyback. Then I could use the test socket.

Here is the schematic:

http://www.earlytelevision.org/pdf/p...sams_439-1.pdf

Thanks for the help. I know that slow progress can be annoying to those helping. At best I have one day a week I can work on the set.

[Yamamaya42]

You really need to hunt up your HV probe to see if you are really getting the 14-15kv or not, and check for the 435 boost. It looks like point C can be unhooked, at L14 point 20, If so, and you DO have HV and boost, unhooking that and grounding it will turn the CRT full on, giving a full raster, IF you have full scan that is, this will rule out any video problems.

[madlabs]

I will do another look. It has to be around here somewhere! But I did do the CFL lightbulb test and it showed nothing. I forgot to mention that.

I am at work but will look at the schematic to understand the test you suggest.

[Yamamaya42]

It all really depends on if you have HV or not, if you don't, your HOT may be overloading the bias as someone mentioned, this could be from a problem of the FBT side or more likely a failure of the horizontal oscillator, the Sams shows that the voltage on the g1 of the HOT is -45v, this should start positive, then go negative as the horizontal oscillator starts up, if it does not go to -45v, it's a good sign the the oscillator has failed, suspect the couplet K6, if it's not replaced if the oscillator is not starting.
Example low or no voltage pin 6 on V10.

[Notimetolooz]

Originally Posted by madlabs

I will do another look. It has to be around here somewhere! But I did do the CFL lightbulb test and it showed nothing. I forgot to mention that.

I am at work but will look at the schematic to understand the test you suggest.

Oh, if the bulb doesn't light you won't have HV. It also indicates that the HO tube isn't switching any current to speak of.
Most probably the horizontal oscillator isn't running or is way off frequency.
It could very well be that the oscillator signal isn't make it to the HO tube.

I know this guy and he does excellent work. I think he still makes replacement couplet kits.
https://tvrestorerguy.com/

[madlabs]

Thanks notime. I will be back at the set in a couple of days. The couplates have been replaced with ones made by banderson when I did the set 4-5 years ago. There might be one original underneath the chassis, I think I was told that it never went bad.

I ordered a little sains-smart 'scope that will be easier to bring over to where the set is. I'm still hoping to avoid moving it back to where my bench is. I will get some voltage readings this week. Since the FBT tests the same as when I restored it and the HOT is red plating a little my guess is as you have suggested that the horizontal osc isn't running. I will take another dig looking for my HV probe to confirm the lack of HV.

[Tube TV]

It does sounds like the horizontal oscillator is not running. Another part to check would be the resistor 15K 1 watt (R62) on the horizontal oscillator to make sure it hasn't drifted out of spec. Kind of a long shot, but it wouldn't hurt.

[old_coot88]

If the HO tube is red plating, it's in self-destruct mode. What you could do is this: leave the plate cap disconnected, and clip the wire feeding the screen grid (pin 4). Then you can troubleshoot without cooking the screen grid. Have you checked the Horz Freq coil (L22) for continuity?

[madlabs]

OK, I am here with the set and trying to test a few things.

I made a little jumper so I could use my test socket on V11, the HOT, 12DQ6A. Pin 4 has 105V instead of 175. Pin 5 is just barely negative at -.15 instead of -45. So I think we can say the horizontal multi-vibrator isn't running. I did actually get a small Sains-Smart oscilloscope to look at the horizontal multi-vibrator wave form but without it running at all it seemed pointless to try it.

Looking at the power supply side of things, the junction of the rectifier diodes (M1 and M2) starts at 150V and drops to 126 as the set warms up. The output of M1 is 245V instead of 290. So I decided to see what the current passing into the set is. The schematic shows it as 270mA. Once the set is warmed up the output current reaches 350mA. So I guess the power supply is OK and something is dragging the voltages down.

The couplates, including K6 have been replaced. I have subbed another 6CG7. I routed the sound through the set. I can get some sound with the volume cranked and the local/distance switch set to distance. However, it is weak and distorted.

@old_coot88 I just tried to check L22. There are three tabs visible from the top. Between 2 of them I get essentially 0 resistance. Between either of those two tabs and the third, I get infinite resistance. I though I had a drawing/picture that showed the traces on the back of the PCB so I could see how it is wired into the circuit. I can't find it. I do have a parts set I could dig out and see how it measures there. You may be on to something!

Thanks for the help!

[madlabs]

@old_coot88 I think you nailed it. I dug up one of my parts chassis and the inductor reads as it should, 139 out of 140 ohms. So the one on my set is indeed bad.

So, the next question is: Are there other issues that could have caused the inductor to go bad that I should check for? Or is it just a luck of the draw thing?

[old_coot88]

Originally Posted by madlabs

Are there other issues that could have caused the inductor to go bad that I should check for? Or is it just a luck of the draw thing?

lt's generally a luck o the draw thing unless there was an overcurrent condition that made the coil go open. Check that 15K 1 watt plate resistor (R62) that's in series with L22. Does it look charred by any chance?