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#1
08-10-2014, 05:37 PM
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Rca 721tcs
Just picked up a whole flock of 721 sets from another VK member. Got the console 721TCS, the tabletop 721TS, and an additional chassis (minus CRT) from a TS. Also picked up a nice fairly complete Crosley 9-408 which I put aside for a future project.
Pulled the chassis and drop the cabinets off at another shop where I will tackle the cabinet work. First up onto the electronics bench is the 721 TCS chassis. Pulled all the tubes, and ran them through tests on a TV-10 tester. Found a few weak 6SN7s and 6AG5s, a 1B3 with essentially zero emission, and a 5V4 damper tube with an open section (cathode tab broken internally). Rest of the tubes looked good, and put aside. CRT tested good for emission, but lousy cutoff according to my CR31. Have a really good spare 10BP4 here, so no problem. Seeing no oozing or other signs of damage to the electrolytics, I plan to bring it up slowly to see if there is any chance at them reforming long enough to at least evaluate the rest of the set. I knew that the voltage divider resistor string is a usual problem here, so I popped open the resistor box, and sure enough, the 1125 ohm section of the upper resistor is wide open. A quick transplant from the donor chassis fixed this problem pretty easily. The candohm resistor checks good on all sections. Resistance checks to ground show no dead shorts of any filter caps. Putting an ohmmeter across the AC input showed no continuity at first. A good shot of deoxit and working the power switch a dozen or so times broke down the insulating "patina" from the contacts, and restored proper operation. Looked like the transformer primary was good! Took the time to clip out the 2 micamold caps from the AC power inlet to ground. Have had these things go "bang" from time to time when hit with line voltage, so just wanted to avoid pyrotechnics. They will be replaced with proper XY rated ceramics. The removed caps (.01/400V) test around 10 M leakage with a multimeter, FWIW. Connected a DMM across the 5U4 socket plate pins (tube removed), and plugged the set into the variac. Brought voltage up a bit at a time to 115V, where I had ~750VAC across the rectifier plates, 6.3 on the heater lines, and no hums, smoke or other unwanted drama.  Put all the tubes back in just for the heater load, and let the set run at full line voltage for about 30 minutes with no unpleasantness. All tubes lit properly, and gave off enough heat to release some of that wonderful "hot dust, beeswax, and rosin" smell that we all love. While things were cooking, I grafted a replacement width coil from the donor chassis to replace a broken one, and put some heatshrink over the brittle and cracked ion trap magnet leads. Ion trap coils tested good on ohmmeter. Have to do similar heatshrink job on the wiring harnesses going to the speaker and the pilot lamp, as well. The speaker field coil is used as a filter choke, so those leads need to be insulated well before trying to bring the B+ up. Will get some long lengths of color coded heatshrink tomorrow. More to come... Last edited by N2IXK; 08-10-2014 at 05:58 PM. 
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#2
08-10-2014, 07:02 PM
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Uhh, as an aside, we have an EXCELLENT Furniture Refinisher here in Bugtussle... He's done a BUNCH of radio & TV cabinets for me, he & his wife are VERY meticulous, aren't cheap, but aren't an arm & 2 legs, either, & usually DON'T keep you hangin' til next St. Swithin's Day.... PM me if interested..
__________________
Benevolent Despot
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#3
08-11-2014, 05:08 PM
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What is that plugged into the CRT socket in the last two pics?...It don't look quite like any tube I've seen before.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4
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#4
08-11-2014, 09:25 PM
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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. 
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#5
08-12-2014, 01:46 PM
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Quote:
Good luck on the restoration.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4
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#6
08-12-2014, 08:13 PM
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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... Last edited by N2IXK; 08-14-2014 at 11:22 AM.
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#7
08-14-2014, 11:36 AM
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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.
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#8
08-22-2014, 06:58 PM
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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.
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#10
08-23-2014, 06:05 PM
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Back at it for a bit this afternoon. Ran The B+ up slowly again, while monitoring the +225 and -85 rails. Everything came up nicely, with no drama from the new caps. Brought a screwdriver near the focus coil, and the magnetic field seemed nice and strong.
Installed the 5V4 damper, and hung a voltmeter from the 6BG6 plate lead to ground. Fired the set up, and got about 260VDC at the plate cap. Quick check at the 6BG6 socket showed reasonable looking DC voltages on cathode, grid, and screen pins, and hooking a scope to the grid pin revealed a nice looking 30Vpp sawtooth drive signal! Installed a 6BG6, shut off the bench lights, and applied power. Heard the horizontal come up, and bringing a neon bulb near the plate cap gave a dim glow., and a much brighter glow near the 1B3 plate lead. No signs of redplating or smoke, and flyback seems to be staying cool. So far, so good... Installed a 1B3, and hooked my HV probe to the anode lead. Set is making about 4.9 kV. Low, but somewhat encouraging. No sound from the speaker other than some crackling from a dirty volume control. Rotating the tuner gives no sound at all. May try installing a CRT into the chassis tomorrow, to see if I get anything at all on the screen. Have to cut a foam block or something to hold the front of the tube up while testing. The it's on to the rest of the recapping work... Last edited by N2IXK; 08-23-2014 at 06:11 PM.
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#11
08-23-2014, 06:10 PM
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I find that Duct Tape works well as a CRT holder.
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#12
08-23-2014, 07:56 PM
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Quote:
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4
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#14
08-24-2014, 05:33 PM
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Stuck a 10BP4 in, and got a whole lot of nothing on screen at any adjustment of brightness and ion trap. On with the recapping...
Just closed a deal on a 5AXP4 which will simplify testing. Hope it gets here soon...
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#15
08-27-2014, 08:14 PM
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Well, spent a couple nights recapping the underside of the chassis. Nice easy access to most of them. 2 of them required demounting the horiz osc. transformer and shifting it aside.
Only strangeness found concerns C139 and 140. Service data showed C139 as .035 /600V. What I pulled out was .035/1000V. Replaced with a .033/2kV. Data showed C140 as 0.1/1000V. Installed cap was 0.1/600V. Replaced with a 1kV unit. Also replaced R197, a 10K 10W wirewound. Tested OK, but was pretty cracked and flaky looking. A 5AXP4 arrived on my doorstep today, just in time. Installed it and fired the set up-- first light at about 7:30 this evening!  Had a badly defocused horizontal line at first, but playing around with height centering and linearity pots got a sort of squished raster. Had some static from speaker, and flashes on the screen when changing channels. Tweaked things a bit to get a decent sized raster on the 5" tube. Hooked up my VA62 to the antenna input and fed in a crosshatch on channel 3. Couldn't get anything there, but channel 4 seemed to come in somewhat. Tuner needs a serious cleaning, but I was able to get a signal through it. Vertical and horizontal sync were both way off, but fiddling with lock and frequency trimmers got a recognizable pattern on the screen. Audio comes in OK, but slightly mistracking with video. Vert linearity pretty bad and sync very touchy, so will start checking resistor values in the vertical circuits. Will need to get into the tuner for some cleaning. Multiburst pattern only shows resolution up to 3.5 MHz, so may need to go through an IF alignment eventually. All in all, a nice bit of progress for 2 days. Time to plot strategy from here... Last edited by N2IXK; 08-27-2014 at 08:31 PM.
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