Predicta Holiday restoration

[wa2ise]

Originally Posted by bandersen

There's a 47K and 4.7K in series between pins one and 2 for a total of 51.7K. I measured 70.7K.

Between pins 2 and 7 I should have 3300pF but measured around 4300pF.

Seems to me those are pretty far off and I should replace this network. I'll pop out K2 next and see how it measures up.

These things were not that accurate when they were manufactured back in the day. The resistance may have increased some over they years, but it's hard to figure out how the capacitance went up (unless leakage is fooling your test equipment, but that equipment likely also has a leakage test). Replacing them with modern parts certainly won't hurt, and may fix a problem if one of these modules did get too far out of spec.

[mpatoray]

Bob,

thought you might be interested that diode package you removed is a Germanium diode(like you suspected) the max forward voltage is 380V and current is 0.4A.

I think it looks rather cool.

Matt

[bandersen]

I've finished creating replica K2, K5 and K6 networks and should be ready to try firing it up soon

[snelson903]

that turned out nice and neat .

[Down Under]

Hi Bob,

I noticed one of the caps on your networks looks to be 600VDC. For some reason, I thought that all the caps on these things needed to be 2KV. What are the voltages on the brown caps?

Very neat job, by the way!

Cheers,

Adam

[bandersen]

That's actually 800VDC. I don't know what the voltage rating is on the original caps in the K networks. I'm going with the experience of other restorers and common sense.

For example the K4 network connects to a cap on the main board rated for 1KV so it's been recommended to use a 1KV cap in the replica network. I used 2KV caps because that's what I had on hand.

As for the other networks, they don't connect to any circuit nodes running at more than a few 100 volts so I figure 500 volt mica and 630+ VDC ceramic and film caps should be more than sufficient.

[Down Under]

Dang these old eyes! It IS an 800V cap! Thanks for the info, I can't remember where I thought I read about the caps being 2KV, but I remember thinking at the time that seemed really high compared to everything else in the circuit.

I have a 17in Princess giving me woes, and I suspect it's the couplate feeding the grid of the HOT. The voltage is too high (approaching zero instead of -45V) and I'm getting red plates. Your solutions really do look top notch, very tidy.

Cheers,

Adam

[bandersen]

Here's what I came up with for the K3 network. Two 47K plus two 1200 and a 1600 to replace the 90K + 4000pF sandwich in the original.
I would have liked to use a small ceramic 5100pF cap, but the closest I had on hand was a big 2,000V 5100pF film cap.



It's tempting to coat these networks in epoxy or silicone, but I'm afraid I may need to tinker with them to get the set running properly.

[snelson903]

that turned out nice ,i wouldnt dip them in nothing you never know when you might have to replace something, or get at it for a test.

[Kevin Kuehn]

Looking good. You could use heat shrink over those little boards. Can even do that after they're installed and tested. But who's going to see them after the hoods on.

[bandersen]

It's finally time to reinstall the main board. Hopefully, I won't need to pull it out again anytime soon.


I popped the HV cage off to get a closer look at the flyback. Some chunks of insulation have fallen off, but the resistances match the schematic.
I picked up some RTV133 silicone to patch it up.


I wonder why the 15 ohm resistor has been shorted out



It's R72 on the schematic.

[bandersen]

I've been going through the tubes and found a 12CU5 in the 12CA5 socket.
They have similar specs, but I don't see 12CU5 listed as a valid sub only the 12EH5

[AiboPet]

I'm SO waiting to see this beauty lit up!

It's an AWESOME meticulous restore you are doing here....I hope you are KEEPING it so it runs another 20-30 years or so.

[Boobtubeman]

Yeah predictas are cool

Just saw one in a garage of a friend of mine over the weekend. He had the last tv shop here in town, closed months ago. Was doing my impression of american pickers in his garage.. Told him about AK and VK here, Hope to see alot of his gear for sale on here soon... Wont get rid of the predicta as of yet, (was his dads) Hope he'll fix it one day

SR

[bandersen]

I just hit a minor roadblock. The push on/off power switch is bad. Perhaps it got fried along with the fusistor and thermistor. Probably when some knucklehead tried to turn the set on.

I'll try working it for a while and hopefully the contacts will work themselves clean. Otherwise, I'll just jumper it for now.