Predicta Holiday - First power up attempt

[madlabs]

HI All,

Well, the XYL and I tried powering up the Predicta Holiday last night. Ramped the power up slow and got the tubes to glow. No sound or raster. Everything smelled and looked fine. Could hear a slight noise from the flyback and up at the CRT, a regular beat. Looking at the troubleshooting chart it says power supply is the most likely culprit. So I pulled the HV can off and measured the flyback for resistance. With the can off I can see a drip underneath the flyback.

5-D, 11.6 Ohm out of expected 12.
4-3, 10 Ohms out of expected 13
3-B, 8.6 out of expected 11
L17, spot on, 1.7 ohms

Although it says on the schematic not measure between 5 and the HV output, it gives an expected measurement of 385 ohms. I figured that the do not measure was for older VTVM's, so I checked it anyway, 366 ohms. So the top coil seems good, lower coil a little low in resistance. Shorted turns? When we pulled the HV rectifier tube, it was completely cool to the touch. With the really low filament voltage I am not sure how warm it should be.

So, any suggestions where to start?

Jonathan

[Kevin Kuehn]

Originally Posted by madlabs

Although it says on the schematic not measure between 5 and the HV output, it gives an expected measurement of 385 ohms. I figured that the do not measure was for older VTVM's, so I checked it anyway, 366 ohms. So the top coil seems good, lower coil a little low in resistance. Shorted turns? When we pulled the HV rectifier tube, it was completely cool to the touch. With the really low filament voltage I am not sure how warm it should be.

I'm pretty sure that warning means not to measure the voltage with the set powered up, because of the presence of high voltage spikes. There should be no harm in measuring the DC resistance when the set is powered down.

[N2IXK]

So, any suggestions where to start?

Measure the B+ voltage at the output of the rectifier, and on the various power supply rails? Fusible resistor OK?

With the horizontal output tube plate cap removed, is DC supply voltage present on the cap lead?

[bandersen]

All those resistance readings are well within spec. Dripping wax is common and no cause for concern. When working normally, the flyback will be a little warm after maybe 30 minutes. Don't expect the set to work until you get to around 110 or more on your variac.

[Phil Nelson]

If your TV has all of its original capacitors, I'll put on my magical wizard hat and predict that you likely have a bad capacitor -- and more likely, a few dozen of them.

I wouldn't fret over the flyback at the moment. Some wax drip underneath it is common.

Many restorers would plunge ahead and start recapping, beginning with the (duh) electrolytic capacitors in the power supply. The schematic shows the voltages ("280V source" and so on) that the TV is supposed to produce downstream of the rectifiers and p-s filter caps. Set your variac at 117 volts and test those points.

Until your power supply is producing the basic voltages needed by the rest of the TV, trying to troubleshoot problems on a symptom-by-symptom basis can be frustrating and confusing.

What have you done to this TV up to this point? Tested tubes? Cleaned controls? It's helpful if you confine all discussion of your TV to one thread, so that people can read about it in context instead of searching around in multiple threads to get a clue about what's happening.

This article has some advice about basic first steps that may be useful:

http://antiqueradio.org/FirstStepsInRestoration.htm

If you have already done all of those good things, then recapping is next on the agenda. The only question is whether you want to do them all at once, or one-by-one (or few-by-few), powering up the TV periodically to see if you're making progress (or at least, not making things worse).

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[madlabs]

Fusible resistor ok. Will check B+ and DC on the horiz. cap. I did bring the line voltage up to 117. I do feel silly for not doing a complete voltage check before posting. I'm sorry, my enthusiasm ran away with me.

Not much has been done, other than a good visual inspection and basic cleaning. I am going to jump into the recap, I gotten the list together and will be ordering that Monday. So, soon I will pull the PCB, replace all paper and electrolytics and measure all resistors and replace as needed. I'll test all the tubes, but I don't put that much faith in my cheapie Eico tester. I just got ants in my pants and wanted to see what it did as is. Since I am waiting on caps I'll poke around and check voltages.

Thanks for all the help! I'll try to take a deep breath and be more methodical.

Jonathan

[reichsrundfu]

Originally Posted by Phil Nelson

If your TV has all of its original capacitors, I'll put on my magical wizard hat and predict that you likely have a bad capacitor -- and more likely, a few dozen of them.

I wouldn't fret over the flyback at the moment. Some wax drip underneath it is common.

Many restorers would plunge ahead and start recapping, beginning with the (duh) electrolytic capacitors in the power supply. The schematic shows the voltages ("280V source" and so on) that the TV is supposed to produce downstream of the rectifiers and p-s filter caps. Set your variac at 117 volts and test those points.

Until your power supply is producing the basic voltages needed by the rest of the TV, trying to troubleshoot problems on a symptom-by-symptom basis can be frustrating and confusing.

What have you done to this TV up to this point? Tested tubes? Cleaned controls? It's helpful if you confine all discussion of your TV to one thread, so that people can read about it in context instead of searching around in multiple threads to get a clue about what's happening.

This article has some advice about basic first steps that may be useful:

http://antiqueradio.org/FirstStepsInRestoration.htm

If you have already done all of those good things, then recapping is next on the agenda. The only question is whether you want to do them all at once, or one-by-one (or few-by-few), powering up the TV periodically to see if you're making progress (or at least, not making things worse).

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Quite agree. I simply do not believe in powering up any radio or television receiver until ALL of the filter and paper capacitors--"all of them" being the operative-- have been replaced.

Did I mention all of them?? Powering up a receiver with original caps is in my very humble opinion, silky, lazy and risky. Heavy on the risky. Recapping your set fully will reduce risk tremendously. Then you can power it up and go looking for whatever else may be Ailing it, such as mica caps, resistor issues, etc.

[Electronic M]

Originally Posted by reichsrundfu

Quite agree. I simply do not believe in powering up any radio or television receiver until ALL of the filter and paper capacitors--"all of them" being the operative-- have been replaced.

Did I mention all of them?? Powering up a receiver with original caps is in my very humble opinion, silky, lazy and risky. Heavy on the risky. Recapping your set fully will reduce risk tremendously. Then you can power it up and go looking for whatever else may be Ailing it, such as mica caps, resistor issues, etc.

It is not all that risky if you do the power up right. A slow start with a variac, the back off so any smoke or signs of trouble can be seen, and periodically feeling the cans and transformer to make sure they are not getting hot is about all that needs to be done to give a safe test power up. (and of course having the sense to pull the plug if signs of trouble manifest)

It is always good to have a baseline before you start changing caps so that if you screw it up you know to check your own work, and not simply blame the set and start chasing shadows.

[reichsrundfu]

Second part is true. However, why waste your time with components that everyone knows are kaput, or will soon be!

Just do it right the first time, be careful--don't rush--and more than likely you'll have a nice and SAFE set to tweak on the backend!

George

[Electronic M]

Anything newer than 1960 I power up replace what (if anything) is preventing it from working right and run it with as many original parts (including caps) as it will run properly with...If an original cap goes in the course of later use I find it change it, and return to using the set. Much of the fun is troubleshooting...Why waste it with a shotgun recap?

If your set is being used properly (with someone watching it at all times) if a lytic goes bad it will be noticeable and unless you let it run in that state for several minutes almost nothing ever gets damaged.

Your claim of fire, and danger rings of trumped up alarmism/over caution to me. I've seen several dozen caps that have gotten hot and puked their guts, but never any cap fire evidence....Hell some sets had problems with the power switches starting REAL house fires when the sets were new and NOBODY here changes a power switch unless it has failed. It's as goofy to me as saying we should gut all our sets and install flat panels cause someone might get hurt from a random CRT implosion. Life is all about calculated risk.

[holmesuser01]

The HV rectifier tube won't get warm with the other tubes. The horizontal circuit and flyback have to be operating for the tube to even work. Normally, you won't even see the filament glowing on the HV rectifier when the set is running.

[madlabs]

As an ex-firefighter, for me it is about the risk VS gain equation. Unfortunately I don't know enough in this situation to really asses the risks or mitigate the hazards. So I should be more cautious.

I'm putting together a parts list and I have a couple of questions. Looks like 1 watt metal oxides should fit for the 1/2 watt resistors. How often do these go bad? On a Hallicrafters SX-44 panadapter I did recently, 90% were bad. On an SX-43 they were pretty much all good. What are these Predictas like in that regard? I can order the exact values called for on the video PCB or buy a variety pack of values and see how it goes. Thoughts on that?

I'm not planning on messing with the tuner for starters, just the video board and all the paper and electrolytic caps under the chassis. I can't seem to find a PDF of the Philco manual anywhere. I saw that Phil had a PCB diagram with top and bottom views but mine is different. I'd love to have one of those.

I will keep this project in this thread to make it less confusing.

Thanks!

Jonathan

Edit: Bruce, thanks for the info. That's what I figured, with such a low filament voltage.

[DaveWM]

Video boards on predictas rarely have issues, no paper caps there, and resistor values are generally not critical. In fact most of the resistors can be off a lot and still be fine.

Generally speaking if you have a lot of exp with diagnostics you can get away with shot gun replacing. If not you are best to base line and then replace a few. Of course with a predicta pulling the PCB is a bit of a pita, but often you can just unstake the grounds and lift a few wires to get access.

I also like to base line a set to determine its overall status before touching it, and that includes cleaning it. I have gotten sets that's that were real dogs due to a single misplaced cap on a botched recap.

I have a series on a 10L chassis, there are several parts. You may find it interesting as I talk about my approach.

https://www.youtube.com/watch?v=PbKmydNJzg0

If you want to jump right to pulling the pcb ck part 5

[tom.j.fla]

You can find the Sams folder #439-1 at the early television web site. All the best,Tom

[madlabs]

Dave, maybe I am mis-speaking. I mean the main PCB, which does have paper caps. I don't have any tv diagnostics experience at all. That is why I was trying to see what it would do as is, to get a baseline. Considering the PITA to get that PCB off though, shot-gunning might be called for.

Doing some more poking around I see the multi-tap sand resistor is waaaay off. The 8 ohm section reads 30. The 13 ohm section reads 62 and the 17 ohm section is reading 93! Furthermore, I have discovered that R76, the 400 cold, 11 ohm hot, has been replaced with a 10 ohm resistor. Yikes! What is the best modern replacement for that?

Thanks!

Jonathan

EDIT: Tom, thanks but I have the Sam's. It doesn't have the same PCB diagram that Phil had for his set from the Philco manual.