#16
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Yesterday I recapped the underside of the HV and HOT area and the
vertical convergence amp. The latter was a disaster zone. All resistors were way off value except a 270K in the first stage plate. And that one appears to be the wrong value, despite being listed as 270K on the RCA schematics! The tube has 9.3 volts listed on the cathode with a 2.7K cathode resistor. That's 930 volts across 270K ... not right. Sam's has 27K there. It looks like I need to change it to 27K. And the cathode resistor in the second stage was 47 ohms ... both RCA and Sam's have 4.7K with 20V and a 100 uF cap across it ... very reasonable. Thus, this set much have been seriously worked on in the past. I'm going to have to check each part value with both RCA and Sam's, a nightmare. There are resistors and caps everywhere that have been replaced with 1950's parts by cutting leads and twisting the new leads around them. Oh did I mention that one lead of a cap in the vertical convergence amp area was already broken, and when I touched the cap, the lug on the tube socket attached to it fell off? I was able to kludge that by soldering a #32 wire to the remaining tube pin part since it is a 7 min miniature socket. But the bottom does LOOK perfectly neat. The previous owner (who was a TV person and probably did it himself) did a good looking job. |
#17
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The 1954 RCA book "Practical Color Television For the Service Industry" shows a 27K resistor to pin 6 of the 12AU7 (Vertical convergence amp) and a 4.7 K from pin 2 to ground.
jr |
#18
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Thanks for the pointer. In other words, RCA corrected itself.
I'm half way through the three color amps. Good resistors, but there was a replacement coupling cap that had one end NOT SOLDERED, just LOOSELY wrapped. Could this set have worked? At all? |
#19
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Clearly not a Heathkit version, which would have had a checkbox for the soldering step!
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#20
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I'm now about 60% through the recap. With the chassis vertical,
power transformer at lower left, the left half and the bottom 1/4 is done except the 4 big power supply caps. Interesting tidbit: there are many 0.1 uF "paper" caps in what looks like ceramic tubes. I'm replacing everything, and the replacement Panasonic mylar ones (listed 5%) are typically within 1% ... but so are most of the ceramic case ones removed, and they all would be in tolerance at 5%. Could these be early plastic caps? Removed waxed paper tube caps are all over the place in value. I found another cathode resistor that was low (by a factor of ten), i.e. it is marked that, and measures it too, and the voltage listings again say (and so does a reasonable current) that the schematic value must be right. This one clearly is original ... from RCA! Bizarre. Were the assembly line workers color blind? (That's actually meaningless in this case, since it was black band versus brown, unlike black versus red in the previous case). Was it traditional to use 50-50 solder in those days? I have older radios and TVs that had 60-40 (tin-lead) solder. And the solder is in very very big blobs, much bigger than necessary. There's probably a pound of excess solder (maybe a slight exaggeration.) |
Audiokarma |
#21
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The white ceramic tubes with values that were also available in cardboard cased paper were typically also paper caps, but with a fancier shell. They usually test bad at working voltage even in sets a decade+ newer. Don't hesitate to replace them.
__________________
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 |
#22
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The recap of the paper and tubular electrolytic caps is done. I still have the chassis
mount caps in the power supply and at the front of the set opposite the tuner to do. I'm going to do the latter set today, then I have to make mounts for the power supply ones. I'm not restuffing them. After seeing the chassis bottom after the recap, I'm quite happy with its look. Its not original, but it sure looks neater and not so cramped. I'm not happy with the look of the blue rectangular caps I had to buy for some values, like 0.001 or 0.0022 at 1600 or 2000 volts. I'm going to look for better looking ones, either Panasonic brown or axial lead. I assume that mica or polypropylene would work. Is that correct? The blue ones I have are rated for RF current use, which their spots are used for (well, overtones of the sweep frequency.) Any suggestions? |
#23
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Sunday (the 27th) I completed the recap except for the B+ supply.
Monday I designed and built a B+ supply replacement for the SE diodes and the four big chassis filter caps. A picture is attached. One of the four replacement caps is visible, the other three are behind it and not clearly visible. After finishing, I inspected the chassis for about 20 minutes for shorts and solder blobs. At this point there were no tubes or yoke. I connected it to a Variac and slowly raised to B+ to 50 V [sic] with no problems and let it sit for 30 minutes. The voltage was monitored on a scope. Then I slowly raised the voltage and at 140 I heard a "pop" from the horizontal sync section. I turned off the power and spent another 30 minutes inspecting, finding one solder blob and one pair of tube pin connections that looked a little too close, and fixed them. After that the B+ went to 240 with no problem. I didn't want to go too high with no load since the nominal +385 and +285 were actually the same. I then connected the yoke and purity coil and tried again, with no problems at 220 v B+. I then put in all the tubes but removed the ballast. Turning it on produced filaments and no problems. I reinserted the ballast. At this point I had a scope on the 400v B+. I set the Variac to 105 volts with it off and threw the switch (did I mention the one on the set is broken, stuck on.) Nothing awful happened: the voltage went up to about 370 and then down a bit. I checked the other B+ supply voltages (385, the two 285s, the 200, and the -30). All were as expected a bit low. Turning the Variac to 117 produced reasonable values. Focus voltage was very roughly 8 kV and HV was somewhat low at 15 kV. I turned off all lights in the room to look for red plates and saw none, including the HV rectifiers and regulator tube. Also, no bad smells. At this point I hooked up my Ch. 10 transmitter and installed the channel selector knob. It was not on 10. At this point a minor disaster struck ... turning the channel selector knob produced no clicks. Turning hard I got it to 10 ... just as the knob broke. The crack is probably fixable. I tried looking at the signal with a scope to the green signal to the CRT and saw only hum. I then tried looking at the grid of the audio output with the scope (the speaker is not connected). All I saw was horizontal sync pulses. At this point I decided to try a sweep generator. I connected the scope to the test point at the video amplifier grid. Turning to Ch. 10 produced a passable result. So the tuner did change to Ch. 10. I reattached the RF signal (of Letterman) and was rewarded with .... AUDIO??? Yes, audio. It was clearly Letterman, since the waveform followed the sound from my flat screen. Perhaps bad fine tuning? I installed the fine tuning knob and was rewarded by a good video waveform. This is of course a big milestone! At this point I checked for sweep from the yoke (clearly there was horizontal there, since the scope probe picked it up.) To do this I set the scope for X-Y with no sweep or signal , just a dot in the center. I set the yoke on the scope (which had the screen facing upwards) with the set off and turned the set on. Soon I was rewarded with a raster about 5/8 (H) x 3/16 (V) inch. Another good milestone. Finally I looked for sync. I put the scope on dual trace, Ch. 1 to the test point, ch. 2 to a scope probe connected to nothing (picking up horizontal sweep). Scope sync was to Ch, 1; I could not get the TV to sync to the signal. I measured the sync pulse timing: as expected 6.4 divisions. I switched sync to Ch. 2 and measured the period: 6.0 to 6.2 division was the whole range of the horizontal sync control: no wonder it did not sync. A repair will be needed. Then I moved the Ch. 2 to a test lead wrapped around the vertical sweep tube and repeated the above, but synced to the power line. The vertical was easily adjusted to be in sync, though the range was narrow. A scope trace is attached. That's enough for yesterday (its now 1 AM). Doug McDonald Last edited by dtvmcdonald; 07-29-2014 at 01:27 AM. |
#24
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The horizontal was not broken, the oscillator transformer just
needed adjustment. The video was getting all the way to the (unconnected) CRT, but there was no color. This turned out to be another invisible broken wire, in the reactance tube circuit. Next is trying to get the tuner to tune. |
#25
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Nice progress reports. It is good to know that you have had no major problems so far.
__________________
Chris Quote from another forum: "(Antique TV collecting) always seemed to me to be a fringe hobby that only weirdos did." |
Audiokarma |
#26
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I'm moving this to the color TV forum ... IT LIVES!
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#27
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Its back here, for a few more technical notes.
While it worked, it became obvious that the CRT cathodes were at too-high a voltage, requiring too much drive (contrast). This was eventually traced to the 1.8K resistor in the CRT divider chain, which was 18K when cold, lower when warm. I happen to have an 5W replacement part. Also, the purity pot (20 ohms) in the B+ cage is bad. It probably will work OK as its only broken near one end. I ordered a replacement (25 ohm at 12.5 watts) from Mouser, but would like to find an exact replacement. Where does one find such a thing? |
#28
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Well, the resistor is replaced, adjustments that can be done without
a picture and needing bottom access have been successfully completed, its back in the cabinet and I went through the purity and B&W setup things and they seem to work Ok. The B&W picture is a nice 6000K. Purity is excellent. I can get a good ordinary color picture. But I don't know how to adjust the I gain exactly, as the instructions don't tell how to do it without I-Q bars, which I don't have. Does anybody have a download that I can use to make an I-Q bar DVD? Oh! I can do it with Photoshop as I have a player that can play stills. But can it output composite from stills? If anybody has one, I'd like a copy. I carefully examined the frequency response of all the video and color sections. All were exactly to spec except the Q output. Its supposed to be only a little down at 0.5 Mhz (80% response there) and it was in fact at 80% at about 0.15 Mhz and at 20% at 0.5 MHz. I tried the two new 6.8 mH coils I had purchased in place of the white one (which, of all the white ones, showed the least blue copper corrosion). The old one had a very low Q. The new ones were much higher out of the circuit, and still too high if clipped in with clip leads and them sitting on my (glass) workbench. But soldered in, near the metal chassis, they were both "just right" with a peak at 0.43 Mhz at 85% the response at DC and a small dip at .25 MHz, and excellent transient response. The I response is stunningly good ... actually a little flatter than in the RCA publications, as is the Y. Now the bad news, as Phil Nelson suggested in the other thread in the Color TV forum. As I said, purity is perfect. Convergence and focus is abysmal. Well, there is one setting where focus is really excellent in the center. But that's with the focus control all the way up and the convergence control all the way down. The latter converges the blue and green and leaves the red 3/16 inch off. Moving the convergence control all the way up converges red and green with blue off, and ruins the focus. Any suggestions, other than dinking with the divider chain? I did set the HV to 19.5 kV (with the CRT disconnected) and measured the focus and convergence, and they were at least ballpark OK (I had to use a scope due to the nature of my HV probe ... I can make an adapter to use my Simpson meter, and will. Its got an odd resistance ... 2.165 Gohm which corresponds to 13.6 uA, so my multimeters aren't very accurate. I think I need help with this problem. And there's another one ... the adjustment magnets on the purity assembly don't do anything ... at least moving the little knurled knobs doesn't do anything. And one is missing. I might have misunderstood how these work ... I was screwing the knobs ... are you supposed to screw the threaded rods and then tighten the knobs to hold them? What to do about the missing one? |
#29
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those are for Adjusting center convergence First, the screws themselves are magnetic and will sit very close to each pole piece of the Electron Guns, you adjust these First before other Convergence adjustments. the Nuts just keep them from moving after you adjust the Screws
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#30
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Focus problem found: a previously good 8.2 meg 2 watt carbon
composition resistor which should have 2800 volts across it (0.95 watt) measures 27 megohms. Luckily I have a 10 Meg HV resistor I can use, which likely will put the focus voltage in range. Also. its in the HV cage which can be serviced without removing the chassis from the cabinet. |
Audiokarma |
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