Rca 721tcs

[M3-SRT8]

Ahhh...721s. I've done two, a TS and a TCS, with very little problems.

Good luck in the home stretch.

[N2IXK]

Started looking at resistor values in the sweep and sync circuits, and found a bunch that have drifted pretty badly, some to almost double their nominal value. Have to place an order for some of the 1W units, as I don't have on hand.

HV now measures 8.5 kV with zero brightness, about 7.2 kV at full brightness. Haven't dug into the HV cage to clean and check the 1B3 socket resistors yet. Schematic says 8.5 kV is normal. Any input on the advisability of adding a B+ fuse in the horiz output circuit?

May try injecting a substitute IF signal at the tuner output. Did the EPROM mod so my VA62 should work for these sets, although this is the first actual test. I assume 21.25 MHz?

[Zenith6S321]

I used 25.75 MHz on my 721TS to feed the modulated IF signal into the tuner. I did have to add the manual IF/RF toggle switch because the VA62 function switch outputs RF (microvolt) level when in the TV RF channel positions. The EPROM defaults to 25.5MHz, due to my misunderstanding when I did the programming. Here are the steps to get a modulated 25.75 MHz IF signal to drive the IF/detector stages:
1. set the RF/IF - SIGNAL function switch to PROG CABLE
2. enter 42 on the keypad, because I only reset channel 42 to 25.5MHz
3. press the +/- to program the offset frequency
4. press 025 to tell it to add 0.25 MHz to the 25.5 to get 25.75MHz
5. Set the manual RF/IF toggle switch to the IF position to get millivolts of output
6. Use the RF-IF LEVEL and RF-IF LEVEL VERNIER to set the output level you need.
7. Third IF input needs higher level, second less, and first less due to the amplification of the IF stages.
8. Set the VIDEO PATTERN to the test pattern you want modulated on the IF signal output from the VA62.

Dave

Link to RF/IF manual switch mod: http://videokarma.org/showthread.php...ht=va62&page=3
Its really hard to see, but in the pictures you see a hand drawn switch diagram followed by two pictures of the back of the front panel. I added the manual RF/IF level toggle switch between the AUDIO and RF-IF LEVEL rotary switches.

[N2IXK]

Thanks for the info, Dave! Will give it a try when I get back to it. Already have the RF/IF switch mod installed.

[N2IXK]

After a bit of a break, got back on the set for a bit this weekend.

While waiting for the resistors to come in, decided to use the time to start the cleaning and mechanical type work now that the set is showing some positive signs of life.

Removed the HV cage and gave everything inside a good cleaning with IPA to remove the thin layer of dust and crud. The 2 resistors (3.3 ohm and 1 meg) under the 1B3 socket test spot on after all these years. The HV anode lead looked really nasty, but it was just a layer of sticky film and dirt on the insulating tubing threaded over the wire. A rubbing down with IPA cleaned it right up. Rubber anode cap still in good shape. Reassembled the HV area after cleaning the chassis inside.

Went over the entire chassis with a bit of naval jelly followed with a wipe with ammonia then clean water to neutralize the acid. Cleaned up most of the corrosion and oxidation pretty well to an even finish. Removed, cleaned, and reinstalled the yoke and focus coil brackets. Reinstalled the cardboard tubes over the restuffed capacitor cans, securing them with a bit of RTV sealant.

Opened up the tuner, and gave the switch wafers a thorough cleaning with Deoxit. Little bit of lube on the fine tuning mechanism to get it freed up, and everything seems to be working smoothly. Grafted the fishpaper cover from over the brightness/contrast control from the donor chassis to replace one that was cracked and damaged. Powered everything back up, and once things warmed up, had some immediate arcing from one of the vertical yoke leads to the chassis hole it comes through. Killed the power immediately...

RCA didn't deem a grommet necessary here, and the flexing of the leads while working on things must have chafed or cracked the insulation. Unsoldering the lead and slipping some heatshrink over it fixed the problem, and I'm back where I was with a much cleaner chassis.

The tuner seems to be working well now, tuning in all channels nicely from my VA62. Great news, as I really hate working on wafer switch tuners.

UPS says the resistors will be in by midweek. Then I can start trying to work on the vertical sweep and sync issues. Vertical height takes several minutes to build up sufficient amplitude, and there seems to be pretty severe foldover....

[N2IXK]

Started going through the entire set checking resistors and replacing any out of tolerance. Finding a fair number of drifted ones, mostly higher resistance ones that have gone even higher.

Found a strange situation with R168 and R169 in the focus circuit. Both are 680 ohm, 1W, 10% carbon comps. Connected directly in parallel and located right next to each other. Appear to be identical manufacturer parts, both look fine visually. One reads spot on 680 ohm, one near 1K. Go figure...

[N2IXK]

Got through most of the chassis and replaced a couple dozen resistors that were outside their marked tolerance. Have a few more to do, mostly in the sound IF stages.

Decided to power the set back up, and see what effect all this work has had. Sync seems a lot more stable, and the foldover is gone, but vertical sweep still takes several minutes to achieve decent height from a cold start. Tried a NOS 6SN7 in the vertical osc/output socket, but no improvement. Will try running the chassis on its side tomorrow, and start applying freeze mist to the vertical circuit after the set warms up. Vertical linearity is still pretty bad, and the pot is all the way at one end of travel to get it this good. Maybe a bad mica cap? How reliable are the caps inside the yoke? Is it possible that the linearity issue is an artifact of the 5" test tube vs a 10" tube? Other possibility is the cathode bypass cap on the vert output tube. Used a 150 uF, rather than the 130 uF called for on the schematic. Anyone think this could be a problem?

Got the first real pictures on the set today by feeding the external modulation input on my VA62 from a DVD player. Contrast range seems pretty limited, and a lot of interaction between contrast and vertical sync. But nice to see a real live picture on this thing as opposed to a test pattern. Multiburst looking a little better now, too.

[Penthode]

Originally Posted by N2IXK

... Multiburst pattern only shows resolution up to 3.5 MHz, so may need to go through an IF alignment eventually.

The 721TS only has three IF stages. You are not going to get the resolution you will see on a 630TS.

3.5MHz is pretty good and what you would expect from this set.

[N2IXK]

Replaced the last few off-spec resistors, including a 6.8 meg (R171 from the AGC line to +225V) that was completely open. Made a MAJOR difference. I think the video portion of the set is good to go at this point. The point about reduced performance vs the 630TS is a good one.

Pictures look great now, and with careful adjustment of height, centering, and linearity pots, I can get a nice, reasonably circular test pattern. Contrast range much improved now, and it makes a really nice picture. Will try with the 10BP4 this weekend. to see how it looks with the real tube in it.

Sound still WAY out of tuning range. Can barely start to hear it at one extreme of the fine tuning adjustment. I assume this is a simple oscillator slug adjustment? Not too familiar with split IF strips like this set has.

Am considering the addition of direct AV inputs so I can easily run this thing without need for a modulator. RCA described this for the 630TS chassis in an article in "Broadcast News", and the relevant circuitry looks pretty similar to the 721 chassis. Anyone done this and have any suggestions?

[Jon F]

Originally Posted by N2IXK

Sound still WAY out of tuning range. Can barely start to hear it at one extreme of the fine tuning adjustment. I assume this is a simple oscillator slug adjustment? Not too familiar with split IF strips like this set has.

Am considering the addition of direct AV inputs so I can easily run this thing without need for a modulator. RCA described this for the 630TS chassis in an article in "Broadcast News", and the relevant circuitry looks pretty similar to the 721 chassis. Anyone done this and have any suggestions?

My freshly capped 721TS had the same audio issue. I haven't gotten back to working on it so please post if adjustment fixed the issue.

Also interested if the composite jacks work out

[Penthode]

All RCA TV's prior to 1952 used a split sound or non-intercarrier sound system. This means that the fine tuning is to be adjusted for best sound. Typically with these sets, the fine tuning is adjusted to the best sound much like any FM radio with a ratio detector or FM discriminator circuit. Typically as you adjust the fine tuning control from one end to the other, you will start with little or no sound, then weak sound through a null for best sound. As you continue turning the fine tuning knob, you will find a second null and then weak sound fading away as you reach the other end.

This means that the video and audio IF alignment can be a little tricky. The 721TS is a little simpler. But what you need to do is to adjust the local oscillator slug for best sound on the channel and then look at the picture.

Proper alignment tools are better than using the VA62 for this, but it can be done with a VA62 with a little understanding and care. Just remember to optimize the sound first with the tuner local oscillator then address the video IF. I suggest it is best not to touch the sound IF and discriminator and see if you can tune to get good sound first then check the picture.

I would like to add that RCA sets, especially the ones with four IF stages using the split sound design, although it is trickier to set up, will ultimately yield better pictures. This is because of the heavy sound attenuation at the at the video detector allowing wide bandwidth video with no sound patterning.

[N2IXK]

Had a chance to play around with the set a bit today. Decided to put the sound issue aside for now, and see what kind of picture I can get through a composite input. Following the RCA suggestion, I injected the video at the output of the video detector. I lifted the plate end of C103, and connected a 75 ohm terminating resistor from the free end to chassis ground. I injected a standard 1Vpp composite video signal across the added resistor. Picture looks acceptable, although the video level is just a touch too low. Cranking the signal up to ~1.5Vpp (or increasing the value of the 75 ohm terminator) gives full contrast range. Video frequency response is somewhat better without going through the IF strip, with details visible into the 4 MHz bar.

Also gave the original 10BP4 a try, to see what it looked like. Image was a bit on the dim side, but I guess that is to be expected with the fairly low accelerating voltage and non-aluminized tube. Certainly watchable in a darkened room. Sweep more than adequate to fill the 10" screen. Electromagnetic focusing coil and ion trap both working properly. Have a total of 4 10BP4s here to choose from, so will go through them all with my CR31 and choose the best one for final assembly.

[N2IXK]

After testing the composite video and line audio input circuitry, I went ahead and did a permanent installation. I drilled 3 holes in the lower left area of the rear chassis skirt, to accommodate an RCA jack for audio input, a BNC jack for video input, and a small DPDT toggle switch to select between line input or off-air operation. The external audio gets injected right into the top end of the volume control., and the external video gets injected into the grid of the 1st video amp tube, via C103. Will label the added jacks and switch before final assembly.

I didn't see any need to have the input selector switch remove B+ from the IF stages in line input mode as the "Broadcast News" article shows. There doesn't seem to be any appreciable signal bleedthrough from a strong RF input signal when the line input is selected. This saves the need for a third pole on the switch, as well as the need to install a large wirewound resistor to replace the load normally drawn by the IF tubes and tuner.

Next up will be to try tweaking the off-air audio in so it tracks better with the video. Got started on the cabinet work this week, as well. Cabinet was already stripped and veneer repaired/replaced, so the set essentially just needs a new finish put on, a missing safety glass replaced, and a few bits of metal trim polished up.

Set is completely missing the "jewel" for the pilot light below the speaker. It wasn't in the box of knobs and loose parts that came with the set. Does anyone have a good picture of what it looks like, or, better yet, a spare unit they would part with? Looks to be about a 5/16" hole that it would mount in.

[N2IXK]

Finished up the cabinet work today. Disassembled the cabinet front completely, to repair a crack that split the grille into 2 separate pieces. Reglued a few areas of loose veneer and delaminating plywood, and gave everything a good sanding. Stained the cabinet sides, top, and front frame with Minwax dark walnut oil stain, with the speaker grille and CRT bezel in a lighter "gunstock" shade. The inner surface of the bezel and the mask behind the safety glass were done in a gold metallic paint, as per original. Stain covered with 5-6 coats of Minwax aerosol lacquer. I think it came out pretty well, considering my lack of experience doing this kind of work.

Unfortunately, as it had already been stripped before I got it, the cabinet is missing the control label decals. I looked at the Radio Daze website, but couldn't find any suitable reproductions. Does anyone have a source for these?

The missing "RCA Victor" decal under the screen was replaced with a brass logo plate of the same size which I salvaged from a 1948 model 8T241 which was too far gone to be anything but a parts source. Looks classier than a simple decal, IMO. The missing safety glass was replaced with a sheet of 1/4" polycarbonate plastic. All the cabinet needs to complete it is the jewel for the pilot lamp. Will leave the cabinet here in the shop for a while to allow the solvent smells to dissipate a bit before bringing it back home to reunite with the chassis and CRT.

On the electronics front, I got on eBay and found a suitable vintage sweep generator so I can check and touch up the IF alignment as needed. Have never done an IF alignment before, but decided now would be a good time to learn. Once the generator comes in, It will get its own cleaning and probable recapping, then hook it all up and see what gives with the sound issues. Chassis probably has 8-10 hours on it now playing through the added composite input, with no problems other than the failure of a 5U4G tube last night. Appears to be a bad solder joint on one of the filament pins. Have plenty of 5U4GBs to use, but the ST-bulbed "G" version just "looks right" in this set...

[Jon F]

Very nice love the machine age cabinet design