Rca 721tcs

[N2IXK]

That is a type 5890 tube. A seriously obscure device that is essentially a 5TP4 electron gun firing into a 2X2A style anode. I stuck it in there to load the CRT filament winding. and give a visual indication of filaments lighting before I installed the rest of the tubes.

Datasheet here:

http://www.shinjo.info/frank/sheets/049/5/5890.pdf

The 5890 was a sort of grandparent to the 6BK4 or 6BD4 HV shunt regulator tubes. It was used as an HV regulator in RCA theater/industrial projection sets. You could connect the second anode to the cap, as well, if you wanted to load test the HV circuit without a CRT installed. I have a pair of them kicking around the shop, and they are also useful for testing a CRT tester.

[Electronic M]

Originally Posted by N2IXK

That is a type 5890 tube. A seriously obscure device that is essentially a 5TP4 electron gun firing into a 2X2A style anode. I stuck it in there to load the CRT filament winding. and give a visual indication of filaments lighting before I installed the rest of the tubes.

Datasheet here:

http://www.shinjo.info/frank/sheets/049/5/5890.pdf

The 5890 was a sort of grandparent to the 6BK4 or 6BD4 HV shunt regulator tubes. It was used as an HV regulator in RCA theater/industrial projection sets. You could connect the second anode to the cap, as well, if you wanted to load test the HV circuit without a CRT installed. I have a pair of them kicking around the shop, and they are also useful for testing a CRT tester.

Cool tube! I'll have to keep an eye out for one.

Good luck on the restoration.

[N2IXK]

Ok. Got the ratty speaker and lamp harnesses fixed. The wire was just too brittle to withstand the handling it will surely get during the restoration process, so I just replaced them with new wire, spliced to the ends from the chassis which haven't been flexed and are still sound. Have some black nylon expandable mesh sleeving that will cover the colorful new wire.

After this, I decided to bring up the B+ very slowly. I decided to leave the field coil/filter choke disconnected, so I was only energizing the 2 caps right off the 5U4 tube. Solid state 5U4 plugged into socket so it will start conducting at low voltages. Milliammeter connected at speaker connector B+ interlock pins, to monitor leakage current in the caps. Voltmeter connected across caps (betwen the +280V bus and the -85V bus). Plug set into variac and watch cap current. Bring variac up until meter shows ~5 mA leakage. Voltmeter says ~20V across cap. Leakage drops pretty quickly to <1 mA, when I bump the variac up again to show 5 mA again. Voltage now ~40 V and leakage drops off again.

Lather, rinse, repeat.

Eventually got it up to full working voltage (365V), with about 0.8 mA of leakage after 30 minutes at full voltage. Caps staying nice and cool, and ripple looks pretty low. Cut off the power, and caps hold charge like they should.

Swap in a 5U4, and do a "hard start" from wherever the variac was resting (~90V), and it comes right up, but somewhat lower voltage because of the tube rectifier. Goose up the variac to get about ~380V to simulate a bit of overvoltage or cold start , and everything looks great. Leakage < 1 mA after 30 more minutes idling.

Next up...connect the field coil and hope that the other caps come back as well as these 2 did. That's probably tomorrow evening's adventure...

[N2IXK]

Well, spent a while trying to run the set up slowly to reform the rest of the caps, but didn't have as much luck as I did with the first 2.

Pulled the 5V4 and 6BG6, reinstalled the solid state rectifier, connected the speaker, and started ramping up the applied voltage while monitoring the current being drawn, as well as checking the voltages on the +225, +150, and -85V supplies.

Managed to get the +225V supply up to ~120V before leakage current stopped dropping off and C127 started getting warm. That one refuses to reform the way the input filters did. Just placed a big order for enough caps (electrolytics and film caps) to do both chassis, so no progress expected until the caps get here sometime next week.

On the positive side, all the positive and negative rails seemed to be coming up proportionally, so the voltage divider string, focus coil, centering pots, etc. all appear to be good.

[N2IXK]

Ok, the new caps have arrived, so I started with the electrolytics. Took about 4 hours, but got all 12 twistlock sections replaced, along with the one axial lead lytic under the chassis. Fairly standard installation in the bases of the original twistlocks. Used Rubycon 105C rated caps. 2 of the 5 cardboard insulator sleeves crumbled during removal, but I have a bunch of old caps in the junkbox to salvage from.

Am going to call it a day at this point. Will try running the B+ up again tomorrow, and see if I have any signs of life before tackling all of the paper caps under the chassis.