My First Predicta

[Chris K]

Originally Posted by bandersen

What about the rest of the set? Are you getting anything out of the speaker? Can you tune a station in?

There is some static from the speaker but no change when the channel is changed. All tubes are lit.

[Chris K]

Oh and Bob, it's been completely recapped...all electrolytics cut out and replaced with Nichicon caps and all waxies replaced. I installed new couplates from Mr. Rigotti's website. I guess I need to consider the possibility the TV suffered some sort of catastrophic failure, like a flyback short, early on in its use and was put away. That may account for what looks like, to me, minimal use.

The safety "glass" is another issue. The inside had the well described white powder residue. I've been using some fine automotive polishing compound and an electric buffing pad and it seems to be working but with considerable effort. It smells bad. I don't know if that's a common thing. It's also pretty dark. Do they yellow with age as well? I do know some plastic yellowing reversal techniques using hydrogen peroxide and UV light that I have successfully used on old computer equipment and vintage games. I would want lots of opinions and advice before I take any steps like that.

Chris

[Chris K]

I changed the 1B3 out of another set that works and replaced the one in the Predicta but no change. I've got all of the B+ voltages...all maybe 10% low due to the age of the rectifier diodes that I didn't change. I'll have to look at the schematic as I can't get to the bottom of the H Osc socket, The choke or the VOT is mounted right under the tube and I don't have a test socket so I can check the bias. The resistor between the HVR and the HV lead to the CRT is right on at 5.5K and the HV lead has continuity. I did find something interesting as one of the wires going into the yoke chassis plug had pulled out of the pin socket but that's probably something I did moving the chassis around. It's been repaired.

[bandersen]

They used tined plastic - do NOT try to chemical treat it. I've worked on about 200 vintage TVs at this point and have never found a bad flyback so that's the last thing I'd consider.

Yes, they smell - it's the Tenite plastic Cleaning them exposes fresh material and it will smell worse for a while. I've had good luck with ScratchX and Novus #2 plastic polish.

As for the CRT looking pristine, that's great, but not that unusual because the plastic shell protects them from the environment.

If you're not hearing anything when you change channels, you have at least one other fault - maybe related to the lack of HV like something is off with the power supply.

I always recommend the easiest thing first. Grab an ohmmeter and go through the resistance chart published in Sams. No need for socket extenders or an isolation transformer.

Everytime I've made a wiring mistake, I found it very quickly with this method.

[Chris K]

One thing that makes this hobby more difficult than it probably should be...I am significantly colorblind. Resistors and bumblebee capacitors are very difficult for me to interpret and I rely on Sams chassis pictures, what the installed capacitor is labeled as and the parts list. Below is the only capacitor in the main chassis that I have not replaced. I can't see it in the small Sams picture and I can't find it on the schematic. It reads a very weak .015 and leaks at 25 volts. Can I get some help?

Thanks
Chris

[Chris K]

Originally Posted by bandersen

They used tined plastic - do NOT try to chemical treat it. I've worked on about 200 vintage TVs at this point and have never found a bad flyback so that's the last thing I'd consider.

Yes, they smell - it's the Tenite plastic Cleaning them exposes fresh material and it will smell worse for a while. I've had good luck with ScratchX and Novus #2 plastic polish.

As for the CRT looking pristine, that's great, but not that unusual because the plastic shell protects them from the environment.

If you're not hearing anything when you change channels, you have at least one other fault - maybe related to the lack of HV like something is off with the power supply.

I always recommend the easiest thing first. Grab an ohmmeter and go through the resistance chart published in Sams. No need for socket extenders or an isolation transformer.

Everytime I've made a wiring mistake, I found it very quickly with this method.

Thank you very much Bob. I'll check the resistance readings

[Chris K]

OK there is an issue with the B+ voltages in 3 of the 6 DC sources. There is no 260v C source, the boost voltage is 225v (should be 435v) and the 140v source is reading 41v. Sorry about that. Looks like there is an issue!

[bandersen]

That's a 0.01uF cap rated for 600v

[Chris K]

Originally Posted by bandersen

They used tined plastic - do NOT try to chemical treat it. I've worked on about 200 vintage TVs at this point and have never found a bad flyback so that's the last thing I'd consider.

Yes, they smell - it's the Tenite plastic Cleaning them exposes fresh material and it will smell worse for a while. I've had good luck with ScratchX and Novus #2 plastic polish.

As for the CRT looking pristine, that's great, but not that unusual because the plastic shell protects them from the environment.

If you're not hearing anything when you change channels, you have at least one other fault - maybe related to the lack of HV like something is off with the power supply.

I always recommend the easiest thing first. Grab an ohmmeter and go through the resistance chart published in Sams. No need for socket extenders or an isolation transformer.

Everytime I've made a wiring mistake, I found it very quickly with this method.

Thanks for the information and the translation on the bumblebee capacitor! In my CRT evaluation, I was referring more to the heat damage I've seen on yokes after prolonged use. Just about every yoke on an almost dead CRT I've come across shows obvious signs of heat related degradation. Plastic warped and brittle...applied tape dried out and burned...wires with heat baked brittle insulation...discolored ceramic capacitors and resistors etc. Nothing of the sort on this one. Do damaged or dead 21EAP4s test as good sometimes? I really hope that's not the case with mine. It tested with good emissions and cutoff on my Sencore at 2.3V

[bandersen]

Keep in mind the 21EAP4s have a design flaw that resulted in a short life - less than two years. Long before the issues you mention show up. For example, here's a dead 21EAP4 and yoke that look brand new.


No, it would not test good if it had issues. I'm not saying all 21EAP4s are duds or doomed to early failure - just that that had a high failure rate initially. I replaced this dead EAP4 with another that tested like new. It had no label - so I don't know who made it though.

[Yamamaya42]

I'm not sure which Predicta you have, nor have I had a chance to work on one yet, but they should be mostly alike, found this, so.

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

Obviously the missing voltages are the first task.
No 140v = no IF, thus no video/ sound.
No 260v C will kill the horizontal oscillator/ HV, and thus you have no boost and it will be at low of 200v ish.

Looking at THIS schematic anyway, there has to be something really weird wrong for 260v C to be missing, without that, the cathode of the audio output will be all wrong, and thus 140v will be wrong as well.

[Chris K]

Originally Posted by Yamamaya42

I'm not sure which Predicta you have, nor have I had a chance to work on one yet, but they should be mostly alike, found this, so.

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

Obviously the missing voltages are the first task.
No 140v = no IF, thus no video/ sound.
No 260v C will kill the horizontal oscillator/ HV, and thus you have no boost and it will be at low of 200v ish.

Looking at THIS schematic anyway, there has to be something really weird wrong for 260v C to be missing, without that, the cathode of the audio output will be all wrong, and thus 140v will be wrong as well.

That's the Sams I'm working from. I see what you're saying about Source C and the 140v. I'm not all that great with schematic symbols...is there anything significant to that X on the line between 260 B and 260 C line right next to the .047 capacitor designation? Could resistor R75, the 18K 2 watt be an issue? I'm going to replace the rectifier diodes anyway but I have 250 v DC coming out of them and not the 280-290 V that should be there.

[Yamamaya42]

An x / broken line ( so on ) indicates an optional way of the circuit being wired, depending on run / make/ year / model.

[Chris K]

Thank you...so if the model I'm working on does not have that jumper, could issues with the resistors I mentioned be a possible source of the lack of voltage on C and half the voltage on the 140v source?

[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.