RCA 630TS sound problems

[Phil Nelson]

The sound is way off on my 630TS. On stations with a good picture (i.e., 4 & 5), there's no sound at all when fine-tuned for best picture. There's no channel 6 here, and when tuned to channel 6 on the TV, I can bring in FM radio 89.5 with good fidelity and volume.

I did some voltage checks, and found 0 volts on the plates and screens of the sound IF tubes (V104, V105, V106), which seems puzzling. They are on the 135-volt line, and the power supply is indeed putting out ~135 volts.

I posted the schematic at these URLs for anyone who wants to follow along at home.

http://antiqueradio.org/art/temp/RCA630TSSchematic1.gif
http://antiqueradio.org/art/temp/RCA630TSSchematic2.gif

Anyhow -- the sound section *can* make sound, but where to start? For example, should I investigate T2 converter transformer!

Thanks for any hints!

[old_tv_nut]

I dunno - hard to believe you're getting anything with zero volts on all three audio IF plates and screens. Are you sure your meter probe (plus or ground) isn't loose or broken? If not, chase down that 135v on the high side of R213, 234, 218. Fix the obvious first (or find out it's really OK), then look for the mistuning.

[John Folsom]

I gotta agree with old tv nut.... maybe your meter was on AC volts?

The '630 is a split sound set (as you may well know). This means the allignment of the video IF and audio IF are completely independant, and is why sound and picture may not coincide. You should realign the sound IFs to 21.25MHZ. And of course, it would not hurt to realign the vidoe IFs as well.But without test equipment support, it is very difficult to get the sound and video alignment to coincide. You can try a little careful hand-tweaking of the sound IF coils for best sound with best picture, but make notes of any adjustments you make so you can put it back if need be. Allignemt by ear is a good way to wind up worse off then when you started!

[Phil Nelson]

Originally Posted by old_tv_nut

I dunno - hard to believe you're getting anything with zero volts on all three audio IF plates and screens. Are you sure your meter probe (plus or ground) isn't loose or broken? If not, chase down that 135v on the high side of R213, 234, 218. Fix the obvious first (or find out it's really OK), then look for the mistuning.

Thanks, I get normal readings from my meter on other tubes, including other 7-pin tubes. I'm using Pomona tube extenders to take the readings with the chassis sitting normally on the workbench. At one point, I thought the extender might have loose pins, but testing other tubes ruled that out.

I will do my best to check out the 135v line. On this set, the CRT is not supported by the chassis, so you can't just flip the chassis sideways and power it up. I carved a piece of stiff foam to support the heavy end while installing the CRT on the workbench. The service manual recommends building a CRT support bracket so you can set the chassis on its back panel and do powered-up tests. Ha-ha . . . maybe later!

I may be able to test some things by sliding one corner off the workbench and crouching underneath with a probe and a flashlight. In a couple of past projects, I went so far as to tack-solder a lead to a test point, turn the chassis back down and reinstall the CRT, then make a test. But that's a lot of hassle just for one reading.

Meanwhile, maybe I can learn something by checking passive components. I'm wishing, however, for one of those wonderful military boatanchor manuals that gives you a complete resistance chart as well as a voltage chart (can you spell SX-28? :-)

[Phil Nelson]

I eliminated the 135v line and B+ to the sound strip as the cause. Running test leads from both ends of R213 gave me normal voltages from the 135v line. Checking again (after cleaning the pins/socket on my tube extender!) gave me normal screen and plate voltages for the tubes in the sound IF strip.

Maybe a workman is only as good as his tools :-)

We'll keep lookin' . . . .

[Jonathan]

Now that you figured out the plate voltage problem, I think that the sound IF sections need to be aligned. John is right, one of the beauties of this set is the fact that video and audio are seperate. This way you can align the sound section indepentant of the video section. To align the sound section you need an RF generator that can generate a 21.25MHz with some audio modulated like a tone or something. This set is one of my personal favorites and I'm quite eager to see a writup on your site about it.

Jonathan

[wa2ise]

The sound IF looks like those in older FM radios that use a discriminator detector circit. Aside from the IF frequency being different from 10.7MHz, the alignment procedure would be the same, peak everything except the discriminator coil part of the last IF transformer. The discriminator adjustment you'd do just like in Fm radios. Be sure to use instructions for an FM set where one of the detector diodes goes to ground.

[Phil Nelson]

Trying not to get discouraged about this problem.

I hauled out and recapped my old never-tried EICO 360 sweep generator and got it to light up, but don't have any way to tell whether it works properly. I tried going through the sound alignment drill, but couldn't get anything resembling a useful display on my oscilloscope.

This was feeding a 21.25 mhz marker signal into the sweep gen with my old but working EICO 324 signal generator. I can verify the setting on the sig gen with a digital SW radio.

Cautious twiddling of the sound IF and discriminator transformers didn't make any noticeable change, so I set each one back to where it had been.

Just as before, I can tune in FM radio with decent fidelity and volume on some stations. But the TV is silent when fine-tuned for best picture. And I don't get TV sound on any channel.

The solution is to buy a new sweep generator, I suppose. But it's frustrating to be totally blocked in the meantime. I wonder if there's any way I could do an approximate alignment of the sound IF using only a signal generator and scope (or multimeter, for that matter).

[blue_lateral]

I would do the video part of the adjustment procedure (without actually adjusting anything) to check the sweep geneator. I dont have the procedure to look at, but probably if you inject sweep with a marker at the mixer grid, and pick up the signal for the scope coming off the video detector with a dierct probe, or just a piece of coax with a 47k resistor in series you shold be able to come up with something looking like an IF curve to verify operation of the sweep generator.

John

[Phil Nelson]

That makes sense. Can it be done with the CRT out? This CRT is not supported by the chassis, so I took it out to be able to turn the chassis on its side and monkey with the sound stuff under power.

[John Folsom]

Phil, you can operate the set without the CRT, no problem. I like to tuck the HV lead inside a small glass or jar to keep it out of trouble. You can disable the horizontal sweep, but this will remove a considerable load from the power supply, and cause it to run high, which can change the operating point of you IFs and change the alignment.

[Phil Nelson]

OK, maybe y'all can help me interpret what I'm seeing.

The manual section "Retouching Picture IF" required less setup than other video alignment sections, so I started with that. I followed directions, setting the picture IF grid bias to -3 volts and connecting the sweep generator output to the antenna terminals. I connected my signal generator, set to the picture IF marker frequency of 25.75 mhz, to the external marker input of the sweep generator. They also want you to insert a second marker at 22.3 mhz, but I don't have that many signal generators in the house.

The instructions said to connect the oscilloscope across the picture detector load resistor. Perhaps they mean R136, which appears to be wrapped in a little coil (peaking coil?). Anyhow, I didn't get a coherent display with that connection, so I connected the scope probe to the far side of that resistor/coil (i.e., the junction of L187, R136 and C138), which did give a display.

The first photo shows the display with the sweep generator connected and sweep width set to 0. (The signal generator is not connected at this point.)

http://antiqueradio.org/art/temp/RCA630TS24.jpg

The second photo shows that I can get a sort-of shouldery waveform by turning the sweep width way up (from 0 to around 24).

http://antiqueradio.org/art/temp/RCA630TS25.jpg

The third photo shows the waveform after I connect the signal generator (set to picture IF frequency) to the sweep generator, with the signal generator output set pretty low.

http://antiqueradio.org/art/temp/RCA630TS26.jpg

The last photo shows that I can increase the size of the marker blob (I wouldn't call it a pip!) by increasing the output of the signal generator.

http://antiqueradio.org/art/temp/RCA630TS27.jpg

So . . . is it worth trying to align the sound IF with this gear? Or should I just throw in the towel and buy a new sweep generator?

[blue_lateral]

It looks like the sweep generator works. I think the marker will have to get sharper. Where are you injecting the marker? If you are running it all the way through the IF like the sweep, you have to keep the level low. They are usually bloblike, but yours is so bloblike it's almost unusable. Maybe it would sharpen up a little if you put a resistor in series with the marker output?

One part that confuses me, youre injecting sweep and marker at IF frequencies, but at the antenna terminals? I would think that if you injected at the antenna, you would have to use a TV channel frequency, and that the fine tuning would affect the results.

I wouldnt run off and buy a sweep generator just yet

John

Oh, one final thought... I dont really know, but with split sound it would seem to be a 50-50 chance whether the sound or picture is on the wrong frequency.

[Phil Nelson]

Thanks, I'll play around with the marker output a little more. As I recall, it disappeared completely when I turned the sig gen output to minimum.

I am injecting the marker by connecting the signal generator output to the external marker input of the sweep generator. I suppose I could try connecting it independently.

Yes, the instructions did say to connect the sweep gen at the antenna. In other alignment procedures they specified other connection points. This section was called "retouching picture IF" and you did it after completing all the rest.

I tried connecting the sweep gen at the converter tube rather than the antenna, and the display was much the same.

I don't have much experience using an oscilloscope, so I don't have loads of confidence that what I'm seeing is meaningful. Maybe I should set this project aside and practice viewing waveforms on one of my other restored TVs.

Originally Posted by blue_lateral

Oh, one final thought... I dont really know, but with split sound it would seem to be a 50-50 chance whether the sound or picture is on the wrong frequency.

Now you are just bummin' me out, man :-)

[bgadow]

Something that is real handy for working on these is an "outboard" test crt. I have one, a Sel-Sun, and they show up on ebay for not much money. They have a built-in yoke and you can clip that in to the circuit. Anyway, they make bench-testing these sets a dream.