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Recapped My Predicta Tandem, I don't know what to do - Printable Version

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RE: Recapped My Predicta Tandem, I don't know what to do - Mondial - 01-19-2022

As Bob says, the Hitachi is the way to go if you can get it for $100. Its a lot of scope for that price, with a lot of nice features including on screen alphanumeric readout.

http://manuals.repeater-builder.com/te-files/MISCELLANEOUS/HITACHI%20V%20Series%20V-1585,%20V-1565,V-695,%20V-1085,%20V-1065A,%20V-665A%20Operation.pdf

RE: Recapped My Predicta Tandem, I don't know what to do - apERNA1985 - 01-20-2022

Ok I'm going to reach out to the seller and see if it's still available

RE: Recapped My Predicta Tandem, I don't know what to do - apERNA1985 - 01-28-2022

So I'm home on quarantine and I decided to poke around in my set. I replaced all the resistors in the video amp and 3 had drifted up significantly, but it made no real difference. Then I noticed I'm not getting 400VDC on the 3CB6 video amp tube in the CRT shell. So first I checked the wire going into the .22 cap and only had 203VDC. According to the drawing I should have 400VDC going into the cap. Just to make sure I took the lead that goes from the cap to 3CB6 off and checked but the voltage going into the .22 cap didn't change. Next I noticed the blue wire that goes through the 120ohm resistor is supposed to have 250v on it. So I disconnected the 120Ohm resistor and checked it has 223v on it. Then I checked the other wire that feeds the .22 cap and it has no voltage on it. It would be the one marked at 1 on the drawing. I checked the 3300ohm, and 1500ohm resistors on the main chasis and board they test good. It looks like everything is fed by the N1 network which is brand new. How can I test it? By the way I'm using the philco drawings.

RE: Recapped My Predicta Tandem, I don't know what to do - Mondial - 01-29-2022

Why do you think there should be 400V on the .22 uF cap at the 3CB6? I don't see any connection to the 400V supply.

From what I can see, voltage to the .22 uF cap is sourced from the 275V supply through the 120 ohm resistor. That's why there is no voltage on the circuit when you disconnected the resistor. So the voltage supplied through the 120 ohm resistor then flows from the common connection at the .22 cap back through the cable to the main chassis then through the 1500 ohm and 3300 ohm to the 6AN8 video amp and to the N1 network. So the 120 ohm resistor supplies voltage to the 6AN8. The network supplies no voltage as it has an internal cap at its input and only receives a video signal from the 6AN8.

The voltage on the .22 at the 3CB6 is slightly low, probably because the 275V supply is low, but not maybe that significant.

How much voltage do you measure at the 3CB6 plate and CRT cathode?

RE: Recapped My Predicta Tandem, I don't know what to do - BrendaAnnD - 01-29-2022

Usually safety margins are built into the engineering of a circuit, so the WVDC will always be higher than what is across it. Another reason higher WVDC ratings are used is because of spikes and transients, where the instantaneous voltages can be much higher than the quiescent DC across the cap.

RE: Recapped My Predicta Tandem, I don't know what to do - apERNA1985 - 02-12-2022

Hey Guys, I decided to throw the towel in and send it to a pro. I just wanted to thank all of you for your help. I am going to try a princess next. It seems a little easier. I'll make a post today.