Rca 721ts - Videokarma.org TV - Video

Adam · #1 07-26-2007, 04:10 AM

This is the next set I'm starting to work on. Pictures are after I cleaned up the chassis a bit, it was full of lint. The insulation is bad on the wires going to the yoke and the volume on/off switch. The plastic piece which goes inbetween the neck of the crt and the focus coil was totally disintegrated. Otherwise everything looks ok. I do have the cover to the hv cage and all the knobs.

John Folsom · #2 07-26-2007, 07:45 PM

That looks like a nice clean chassis, no corrosion. The wires on the early RCAs always gets stiff as a board and cracks when bent. Nice thing about 721s is that the glass front keeps the front view of the set looking good, even if the sides and top are not.

Adam · #3 07-27-2007, 04:00 AM

A note from someone who once worked on this set, the red piece of paper in the picture below. It says "wrong part", it's tied around a resistor going to the vert. lin. control. It's supposed to be 560ohms, and it reads green-blue-brown-silver, which I think is 560ohms 10%, so I don't see how it can be a "wrong part", but I tested it and it tested about 1.6k so it is bad. I wonder how long ago that note was put in there.

ohohyodafarted · #4 07-27-2007, 04:45 PM

Color code is correct, but is it possible that it was replaced with a resistor of insuficient wattage at some time? Maybe it was overworked. That may explain why it is reading 1.6K.

Does the schematic call for the wattage which is currently installed?

kbmuri · #5 07-29-2007, 03:26 PM

Adam,

I picked up a 721 last spring at the ETF Convention. I'm really anxious to get to it, it's next up in the in-basket after the Haunted Roundie Admiral from Hell. I thought I might do another step-by-step series here with it, but you beat me to it

I'd be happy to doublecheck anything on mine if you want, and compare notes. Datasheet is SAMS 70-7, I have one handy.

- Kirk

Adam · #6 07-29-2007, 04:38 PM

The schematic does call for a 1/2w resistor, which is what's in there.

The I have the sams 70-7 and it is actually the schematic for the 730tv which is very close but it doesn't have the audio amp on the tv chassis, and has a few other small differences. Just last night I found a copy of the original service information for the 721ts here: http://www.njarc.org/manuals.htm I ordered all my capacitors based on the parts list in the sams and then later found out I didn't have all the ones I needed.

The power supply is odd in this set with that -85v supply and not just tying all the filter caps to ground, it makes figuring out how to put the new electrolytics under the chassis a bit more difficult.

I've been closely following that Admiral thread, strange tv problem, I've learned some interesting stuff reading that.

kbmuri · #7 07-29-2007, 06:48 PM

GREAT catch, finding the RCA manuals. Thanks.

All of my Capeharts have the filter caps isolated from the chassis, at roughly the same -90V. Jordan did something really brilliant with an isolated terminal strip to securely mount his electrolytics. Thread is here:
http://audiokarma.org/forums/showthread.php?t=96894

But I haven't opened up my 721 yet to know how much room there is under there.

Yeah, I didn't really figure I wanted to learn how to do alignment work but that Admiral has made me stretch my brain cells. It's been a tough one.

Adam · #8 08-07-2007, 12:45 PM

I fired this set up probably for the first time in 50 years this morning, at first I got a very dim picture / no sound, but I just had the ion trap in backwards. Now I get a good picture / no sound. I get some buzzing if I turn the fine tuning control. But it does get sound on channel 1 - only it's picking up 92.3 FM on channel 1 - very strange, some kind of allignment trouble? It's also picking up FM radio on ch 3, but I don't know where because that one didn't announce the station number. No FM radio on any of the other channels.

kbmuri · #9 08-07-2007, 01:39 PM

Adam -

NICE!

If you could tune an FM radio to hear the same audio you'd know what the frequency is you're getting on Ch3.

Keep notes on how you troubleshoot your lack of sound. I'm obviously interested.

Great job so far!

Steve K · #10 08-07-2007, 02:39 PM

Adam:

You do really nice work. I'm sure that you'll get the sound working as nice as that picture. You're getting closer to that Mercury set!

Steve

Adam · #11 08-09-2007, 05:51 AM

I want to try the alignment for the sound on this set, I've been reading through the allignment instructions, but I didn't have all the necessary equipment, so I picked up a signal generator and a sweep signal generator. The signal generator actually works as is, I was able to pick it up on the frequency it said it was on on over an AM radio station (640 Khz), and I plugged it into a scope to see what the output looked like and everything looks ok, some of the controls stick and are dirty, but that's all. As for that sweep signal generator, it looks like someone started to recap it and didn't finish.

Steve McVoy · #12 08-09-2007, 07:08 AM

Before you do a sound alignment, you should make sure the local oscillator is on the right frequency. These sets are tricky, since they don't have intercarrier sound. You can have an excellent picture and no sound if the local oscillator is off by as much as 1 mHz. The best way to tell is with a frequency counter.

Adam · #13 08-09-2007, 07:09 PM

I went and picked up a frequency counter today, I tested it with that method with the signal generator and the AM radio outlined over in the Admiral no sound thread and it seems to work fine. The only problem with it seems to be that the number 2 doesn't light up in the leftmost nixie tube.

Dave S · #14 08-11-2007, 09:42 PM

Quote:

Originally Posted by Steve McVoy

Before you do a sound alignment, you should make sure the local oscillator is on the right frequency. These sets are tricky, since they don't have intercarrier sound. You can have an excellent picture and no sound if the local oscillator is off by as much as 1 mHz. The best way to tell is with a frequency counter.

I have an excellent picture and no sound on my 721. It happened right after I moved it from my living room to our club's museum.

A frequency counter is one toy I don't have. Any recommendations / specifications for these purposes?

Thanks,
--Dave

kbmuri · #15 08-11-2007, 11:20 PM

Quote:

Originally Posted by Steve McVoy

Before you do a sound alignment, you should make sure the local oscillator is on the right frequency. These sets are tricky, since they don't have intercarrier sound. You can have an excellent picture and no sound if the local oscillator is off by as much as 1 mHz. The best way to tell is with a frequency counter.

I measured mine (I think) on the Admiral. Given I have an accurate frequency counter and semi-accurate o'scope, what's the best procedure for measuring it? I was able to get a clip-on probe into the tuner area thru a service port, but touching anything in there is pretty disturbing to the picture reception. Is there a less invasive method?

I asked the same question on my thread, sorry to hijack Adam's. It's just that here is where Steve M's quote is.

Dave S, I got my counter on eBay for 130 bucks and I'm thrilled with it. It's utterly accurate at 15 Mhz and 1190 KHz and at all the TV RF and IF frequencies. Presumably everywhere else. It apparently goes into the GHz too if I ever need it. The eBay link is still there and is on the Admiral no-sound thread. The heathkit I got is also accurate, but only up to 20MHz -- not enough for TV work. I guess make sure it goes to 100MHz at least.