#166
|
||||
|
||||
Quote:
really? You must have the one chassis the was built right then, none I have ever seen made more than about 22kv. You can copy the circuit from any of the later sets to add the HV adjust pot, then you should see some improvement in brightness. Set it so the chassis puts out around 23kv with a blank raster, see if that does the trick. Tube swapping the 6BK4 won't help, you need to change the tube bias with the HV pot.
__________________
Evolution... |
#167
|
|||
|
|||
yea I was kinda shocked by all I have seen with other reports, that's is why I had not even bothered to try disconnecting the shunt tube cap before as I just assumed it would not make any difference.
I hate the idea of drilling a hole in the chassis to add a pot, so if a tube swap does not work I will try upsetting the voltage divider of the two 1.8 meg resistors that are used to establish the bias with a fixed resistor bridged on the lower half of the divider. Will start with a 10 meg just to see. A better way would be to just replace one of the resistors with a 1.5 meg fixed and then a 500k pot is series as done on the 7's. The schematic shows a 100meg cap in the cathode to grid, 100meg? yikes, anyway if that has gone low it would foul things up to. I did replace the .0033 cap that is there as I have read that leaky caps there have been responsible for regulation problems due to bias issues on that tube. |
#168
|
|||
|
|||
After talking it over, I think I will just check to make sure that all the existing values are correct and not try to up the HV more than the design calls for.
My concern is HV breaking down the insulation and arcing over in the HV tire. I would rather have a dimmer CRT than no HV at all with a fly that is so hard to come by. I did try a new shunt, same HV 20kv. the old one was completely dark, which I presume is from Xray bombardment of the glass. |
#169
|
||||
|
||||
I've been following this thread, I'm about to tackle my Wingate chassis in a few weeks (finally! ). After studying a lot of early color set designs, I found it really weird that the schematic has 19.5Kv going to the anode while all others are 21-25Kv. I guess RCA was experimenting with different methods and I'm glad this one was only done on the CTC5. I agree with you, Dave. I think when I get into my chassis, I'll just make sure everything is as it should be and live with it. The only thing I'm thinking about doing is that contrast mod just to see what happens. It seems that the mod works better on some sets than it does on others.
__________________
My TV page and YouTube channel Kyocera R-661, Yamaha RX-V2200 National Panasonic SA-5800 Sansui 1000a, 1000, SAX-200, 5050, 9090DB, 881, SR-636, SC-3000, AT-20 Pioneer SX-939, ER-420, SM-B201 Motorola SK77W-2Z tube console McIntosh MC2205, C26 |
#170
|
|||
|
|||
Well the good thing about my set anyway is the HV is rock solid (with 5kv of head room it should be). I did not try it without the mod, but with it there is no sign of blooming at all.
Make sure you check the plate load resistor on the horz osc borad, its the 39k. |
Audiokarma |
#171
|
||||
|
||||
Quote:
Won't happen, starting with the 5 there was a little bit of potting and a rubber insulator around the HV winding. Previous designs (CTC-4/21-CT-55) only had some wax dribbled around the bare windings, and they operated at higher voltage than the 5 does. CT-100 was different as well, it has some sort of plastic around the winding instead of wax. It already makes 20kv, another 3-4 isn't going to hurt it. You don't have to drill a hole in the chassis either, you can use the pot temporarily to figure out what value you need. When you know, replace the pot with a fixed resistor and be done with it. The benefits are worth the potential risk, IMO. If I could get 23kv out of my set, I definitely would do it. Also, chassis for the 5 seem to be popping up lately. So if a flyback did blow, chances are you could find another one. It wouldn't be as hard as trying to locate one for a CTC-4, those are made of unobtanium.
__________________
Evolution... |
#172
|
|||
|
|||
while futzing with it I noticed the green was a bit weak, after screwing with drive controls, I finally decided to check tubes. Normally if they test a bit low on my mercury tube tester I don't sweat it, and one of them was a bit low. swapped in and color looks a lot better. per my tester SHB 3000, was about 1500, new tube was 2000 for what ever that is worth.
another odd thing was when I tried to use my vector scope the pattern was a mess on color bars. found out that if i disconnected the grid leads (so the vectorscope was not ready the chroma) the color bars on the screen look good, with in connected it was a disaster. I have never had a problem with the vector scope loading down the signal like that before. oh the tubes where the 12BH7 difference amp tubes. Last edited by DaveWM; 02-01-2013 at 02:59 PM. |
#173
|
||||
|
||||
Quote:
Dave |
#174
|
||||
|
||||
Quote:
Since the 6BK4 is a "shunt" regulator, I realize it shunts current to maintain a constant load current, hence supply voltage. Also to regulate, the 6BK4 does not sense current, it senses voltage. The boost B+ voltage is sensed which should be directly proportional to the HV. What I was suggesting is that if unregulated HV voltage is at or below the point the 6BK4 begins to conduct, the 6BK4 will not shunt any current, hence will not regulate the voltage. I measured about 23kV with the 6BK4 cap disconnected. (The cap removed effectively removes all HV regulation). When the cap is connected, the voltage remains about the same. This means the 6BK4 is not conducting, probably because the unregulated HV is too low. The question I have is if the focus voltage remain constant at the point the HV begins to drop (HV out of regulation), this means the problem is the unregulated voltage at the output of the HV rectifier is too low. This in turn implies that the HV pulse arriving at the HV rectifier is of insufficient amplitude. Further, because the focus voltage remains the same (constant CRT load), the HV transformer core is not being over saturated with the HV supply current demand. In other words, the transformer is okay. On the other hand, if the focus voltage drops with the HV, the transformer core is perhaps becoming over saturated and I would suspect a fault with the transformer. It is interesting to note that the schematic suggests the HO Tube cathode voltage is 7.5 VDC. This means the cathode current should be 7.5/24 = 312mA. I do not think the current should be appreciably lower than this because I believe the flyback pulse would be reduced which in turn reduces the HV. |
#175
|
|||
|
|||
while checking some other stuff, mainly I wanted to see if the two 5600 ohm resistors in the chroma matrix used to drive the green were ok (they were I noticed the 5600 ohm resistor in the grid circuit of the green diff amp was about 50% out so I replace it. while check other stuff I found R702 (6800) was reading over 100k. The odd thing was it was not wired as in the schematic. In the schematic it show it across C and D of the chroma take off transformer T701. In mine its connected from pin 7 of V701a to D with no direct wire from pin 7 to D as shown (which is why it was easy to check in circuit).
I replaced it as well. Turn the set on and there is too much green in the highlights now, so a minor tweek and all good again. for fun I tried putting a 22meg in P with the ground side of the voltage divider on the shunt, increased about 1kv to 21.5. The overall bias with this setup is about 12v but of course it would change with brightness settings. the schematic shows a 15v (375/390) so its all pretty good. adding the 22meg in p is within the tolerance level of the resistor so I will just leave it at that. I don't know what happened but the HV started going up to over 25kv and I could see the shunt was not glowing green suddenly. I tried wiggling the tube and even a new tube, still not regulating. finally I pulled the chassis and check the filaments for voltage, all the resistors, put it all back in the cabinet and now its working again, I have no idea what was going on with that. I am going to try changing the voltage divider resistor to 120k vs 83k (as done in later production runs) just to see if there is any effect on the cathode current in HO tube (the voltage divider resistor is on the horz osc board). |
Audiokarma |
#176
|
|||
|
|||
the 82k measured at 94k I went with a 120k. No drive bars, horz lin looks good,
21kv (with a 22meg on the ground side of the shunt) and the cathode is less that 180, about 178, which is about as low as I have seen on ANY of my color sets. there is a 'tiny' hint of a drive line that I can see on a blank raster, so I may go back to the 100k but will leave it be for now, and will check the fly temps as well. The color looks good, but I still need to go over the purity and convergence again. duh after looking a bit closer I think I am at 22.5kv the divisions are not 1kv I should have looked closer. Last edited by DaveWM; 02-02-2013 at 08:06 PM. |
#177
|
|||
|
|||
some progress pics
|
#178
|
|||
|
|||
couple more OOT resistors, a common cathode resistor on the 2nd bandpass amp and blanking amp 270 reading 750, and I doubt it will make any difference but will replace anyway.
|
#179
|
|||
|
|||
normally when I am replacing parts on old wafer style tube sockets, I just clip the lead and pig tail, this is to prevent breaking a tube socket pin connector and ruining my day.
In this case it was just the 270 ohm resistor and I could see it was not really wrapped around the hole, so I got out the solder wick and safely removed the resistor. Replaced the resistor all looked good, the only other connection to the tube socket was the wire that went to the blanker tube socket, but I did not disturb that. I always confirm my work so if its a pig tail I make sure I have good continuity across the pig tail connection (if the wire is not clean its easy to have a good looking connection but no actual contact INSIDE the pig tail. in this case I did not have to worry about that as I could see I had good flow and had filled the hole in the tab with solder. I did check to make sure the wire that goes to the blanker tube cathode was still good and surprise, it was open. a closer look and I could see the wire had broken right at the pin that I had just soldered to. it was partially corroded and I guess the minor movement done in the install of the resistor must have broken it. I had to fish the wire out and strip it to get a good clean piece, lucky there was enough slack (just). solder wick out the connection again and redo thru the hole and wrap, solder back. Needless to say I was very careful as the socket tabs break easy. when done I confirmed that I had continuity back from the pin thru the wire to the other tube socket, AND the 270 ohm resistor was still 270 and connected. Watch out for those very fragile hookup wires used, in this case I think it was 26 gauge solid so not much to break it. |
#180
|
|||
|
|||
after fighting poor greens I decided to try the vectorscope again. this time I clipped on the other side of the grid coupling caps for RGB on the CRT. Much better, did not effect the CRT display at all.
While there were color bars, the blue and reds were fine but the magenta was weak as was the green. looking at the scope you could see the fingers but they were much more of an elipse that an a circle. My interpretation of this is the phase of the CW from the osc was not correct to one of the demodulators, and the transformer that changes the phase needed to be adjusted. Since green is derived from the X and Y demodulators, then a defect there would really mess up green. What I like about the vectorscope is the instant visual response (not just that its changing but that its changing in the right direction. as it turned out about 1/4 turn of that phase adj transformer was able to open up the elipse and give a nice circle pattern, when I looked up at the CRT it now had a very nice color pattern. This is the second time the vectorscope came to the rescue for me, I highly recommend one. Besides they also have nice dot and cross hatch settings. funny thing is how good you can get flesh tones even with green so useless, I guess its just the flesh tone does not need much green unless your looking as Spock. |
Audiokarma |
Thread Tools | |
Display Modes | |
|
|