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Work begins on the Penncrest CTC-15
As soon as I got home from work, I pulled the chassis out of the set, and started cleaning it. I pulled all the tubes, and noted them down. I have the schematic for the set (Sam's 810, folder 3), and compared the tubes to the tube layout on the schematic. There are some differences:
The first video IF tube was a 6BZ6. The schematic shows a 6JH6 Chroma Bandpass Amp/Color Killer is a 6EA8 (schematic: 6GH8A) Z Demod is a 6HZ6 (schematic: 6GY6) Chroma Sync Phase Det./Color Killer Det. is an a Rayetheon tube with the number rubbed off, schematic lists it as a 6JU8. When I pulled out the 6BK4, it jingled. It jingles quite a bit when tilted back and forth, although I see nothing loose inside the glass. It still has vacuum and does not look very old. The set came with several used tubes - the previous owner got a whole bunch of tubes real cheap when a TV shop closed, and replaced all the tubes he could with new ones, just in case. So I have the used 6BK4. The vertical output transformer is a Stancor. Likely a replacement. Looking under the chassis, I looked closely under the flyback cage - the previous owner said that he saw smoke from under there when the set failed. R204 and R115 are large ceramic block resistors, and look to have run pretty hot, but that's normal. C99, a .01 @ 1400v ceramic disc cap is singed on the top. I don't know if it was due to the heat from the resistors, or it's own failure. Electrolytics are all original, and I would like to replace them, although I don't think I have all of the proper values in my stock. How close do I have to be? For example, 80uf @ 450 is an odd value. Can I use 100uf? Or what about going from 160uf up to 220? I've done similar subs in other sets before, but not in a complicated color set. I know I can use 47uf in place of 50uf - since the difference of 3uf isn't enough to matter, especially with the tolerance of those old parts. I don't have a ceramic cap to replace C99. But I do have some modern .01uf @ 1600v plastic block type capacitors. Are these suitable for this kind of circuit? Any pointers about this chassis before I fire up the soldering iron? Thanks! -Ian |
#2
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Last edited by andy; 12-07-2021 at 01:58 PM. |
#3
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I replaced the crispy looking capacitor, as well as an inch or so of wire that was cooked from being too close to the power resistors. Lacking a test jig, I lugged the thing up to the cabinet and hooked it back up to everything. Plugged it in... nothing. Tube light, it's not drawing too much current, but no sound and no picture.
So, I lugged it back to the bench to do some voltage checks, I pulled the horizontal tube, propped it up on it's back on some scraps of wood, hooked it up to the bench speaker, and plugged it into the isolation transformer. Tubes light up, start checking voltages. No B+. No high voltage of any sort. Nothing coming off the rectifier diodes. Huh? I unplugged it and traced wiring. The HV lead of the power transformer runs to the circuit breaker... check it, good, goes to the degauss coil plug... wait. Degauss coil! I never plugged it in. I searched the chassis for the degauss thermistor indicated on the schematic. Nothing. Ran back upstairs - guess what? The deguass thermistor and limiting resistor are on the degaussing coil frame. And without that... no HV. So, I bypassed the degauss coil connector with a substitute component with the approximate hot value of a termistor (a clip lead). Fire it up... I have sound! TV static comes in on the speaker... followed by sparking from underneath the damper socket. There is some signifigant arcing coming from a terminal strip. Closer inspection reveals the defective component - the terminal strip itself. This is a small terminal strip, and it carries high voltage, with a couple of large rated (4kv and 1kv) ceramic caps on it. It has carbonized along it's length from a high voltage terminal to the grounding lug in the center. The caps connected to it look pretty crummy from the sparking this has been putting off, and there is a burnt mark on the metal chassis just above it - hidden by a big resistor. So, I'm going to stop by my local electronic parts store tomorrow, and see about getting a couple replacement caps, and see if I can get a new terminal strip. I have a handfull of used/salvaged ones. But I'd like to use a new one in this particular application. Anyone ever seen this before? -Ian |
#4
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Well, I replaced the carbonized terminal strip, as well as a 180pf 1kv cap. This stopped the arcing, but when I hooked it all up, I still get no HV.
-Ian |
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Last edited by andy; 12-07-2021 at 01:58 PM. |
Audiokarma |
#6
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You're getting close! Fun to read along...just like doing it myself, but without the work.
When replacing a 160uf cap I usually will parallel a 100 & a 68. (not everyone carries the 68) I always use 100uf to replace the 80's. (seems like I bought some 82uf caps once but I haven't seem them listed lately)
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Bryan |
#7
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Well, I did some more poking around. I replaced a small electrolytic in the vertical section, just because I knew it was going to be replaced eventually, and I had one on hand.
I did some more checking in the HV section. There are two 1.5 meg 1 W resistors that the Sam's indicates should be a matched pair. Both read a bit high, so I replaced them with new metal film 1 W resistors (2% tolerance). I checked a couple other parts, and tested a big 7W 13K resistor. It read... uh.. I do have these leads connected... wiggle. wiggle. poke.... it's open. So, another drive to my friendly neighborhood electronics store, and $5 later, I had a brand new, old stock, 13K 8W non-inductive resistor. I installed it, put the set back together, and... it's alive! I did some quick tweaking and hooked up the cable. It took me a while to get a good color picture from the cable - I think the tuner might be a little out of alignment. I hooked up a DVD player with an RF modulator and was able to get a pretty decent color picture. It doesn't fill the entire screen - the bottom inch or so isn't used. Convergence could be better. It's not terribly rich or bright, I had to turn the color and tint pretty much all the way up to get normal color. There are some retrace lines at the top - probably macrovision from the DVD though. And I'm sure it would look a lot better if I wasn't next to a window. But... it's alive. -Ian |
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Last edited by andy; 12-07-2021 at 01:58 PM. |
#9
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Oh, suweet!
Nice job! veg
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#10
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Steady, steady...You're almost there !
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Benevolent Despot |
Audiokarma |
#11
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Great job!!
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Stereo Rules!!! |
#12
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Well... so much for working pretty well.
Since getting high voltage back, I've been watching the set for a while. I watched a movie today, and it worked very well. Tonight, I showed it to a friend, and we watched The Simpsons for an hour, and then, while it was sitting at the DVD menu screen, I noticed everything looked... fuzzy... Starting another episode, the blue screen at the beginning of the show bloomed up and got real, real fuzzy. And kept getting bigger. I shut off the set. After letting it sit for a few hours, I powered it up again. I turned the DVD player and the TV on at the same time, and when the HV came up, the DVD player (a cheap Cyber Home) was still at it's main "loading" screen, which is blue. It bloomed and fuzzed away, and the screen went black. I shut off the set, and let it sit for a minute. Then I turned it back on. This time, it came up with a fuzzy menu screen for the DVD, which is darker, with black at the bottom, not an intense blue. So... something is wrong. I'll have to pull the chassis again. I'm still on the original electrolytics - I should have checked to see if they were warm when the problem first arose, but they weren't warm before after long periods. The picture never quite filled the whole screen, and the linearity and color were always a little crummy, but it worked. So... back to the workbench. Don't know where to start here. -Ian |
#13
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OK, well, maybe I posted too soon. I just went down and tried it again - fuzzy, blooming out. I changed the 6BK4, and it worked again. So, it must have just been that tube on it's last legs.
I was using the used 6BK4 that came with the set, not the one that came installed in the set - because the one that came in the set made a lot of jingling when I pulled it out when I first started cleaning the chassis. The used one jingled, but not as much. Apparently it works, because it's in there now. Anyone else had "jingleing" 6BK4's before? I don't see anything loose inside either of them. -Ian |
#14
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had the same issue with my 15. my 6bk4 was jingling too. change it out and change the resistor between the vert lin and height controls. it is always bad causing the screen to be a little short. shawn (freakafter8), helped me with that one. good score and post more pics!!!
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#15
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I am trying to remember what happened with my CTC-11...it did the same thing, pic went blurry then dark then dead. I turned it off...I only recall the fix was fairly easy...probably a tube, too.
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Bryan |
Audiokarma |
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