|
|
|
#91
08-05-2022, 02:47 PM
|
||||
|
||||
|
Quote:
I tried changing the 22 pF cap to ground to 10 pF and that upped the 3 dB frequency but resulted in a visible and actually irritating timing mismatch: the Q was too early. I experimented and found that the best overall was about a 3.3 mH peaking coil and an 18 pF plate to ground cap. This has the transition a bit early and a bit of orange line at the green to violet transition but the brightness error is about at the right spot. In my personal opinion they really needed just a bit shorter delays for Y and I. The only problem is the very rare violet to green transition. Green to violet is OK as is either violet or green to or from gray. See the I-Q only test pattern. IF you turn off the color entirely on this pattern you see only the white and black areas ... the rest is constant gray. http://www.videokarma.org/attachment...1&d=1659728707 http://www.videokarma.org/attachment...1&d=1659728096 Last edited by dtvmcdonald; 08-05-2022 at 02:50 PM. 
|
|
#92
08-05-2022, 05:17 PM
|
||||
|
||||
|
1) Agree that 200 kHz was not right, but unless the 6 mH is a misprint, there ought to be some other problem causing it
2) The bit of orange at the green-violet transition is most likely due to quadrature distortion of Q into I, especially if you are using a test pattern DVD and going in RF. It is not due to something wrong in the set itself. The distortion mechanism is that the original pattern was generated as RGB full bandwidth, then the MPEG 2 chroma channels cut this down to equiband (R-Y) and (B-Y) chroma with perhaps 1/2 luma bandwidth, so the Q channel in the signal is wider than 500 kHz and actually made by the proper proportions of (R-Y) and (B-Y). Then the IF/video detector in the set cuts off the upper sideband due to the sound trap(s), leaving some unbalanced lower Q sideband. Also note that there is no noticeable ringing in the +/- I transitions, so the ringing in the Q transitions must be at least partly due to the Q channel filter. I think it might be possible to improve the Q transients. The way to work on this visually is to disable the I demod so you eliminate I quadrature effects; or use a scope to look at the I and Q signals separately. Last edited by old_tv_nut; 08-05-2022 at 05:23 PM.
|
|
#93
08-05-2022, 05:41 PM
|
||||
|
||||
|
dtvmcdonald, did you verify the value of L47, 1.9 mH?
I believe it resonates with the input capacitance of V10, Q phase inverter, and those two values could be critical in the high frequency response. Peaking in tube circuits is a pain because tube circuits are generally high impedance and variation in stray capacitance is an issue.
|
|
#94
08-05-2022, 05:47 PM
|
||||
|
||||
|
A further thought: the self-resonant frequency of L47 could have been intended to be a big part of the peaking effect, so if it was replaced by a coil with a higher self-resonant frequency, that could cause a problem.
If that's the case, a small cap across L47 might increase the peaking.
|
|
#95
08-05-2022, 06:33 PM
|
||||
|
||||
|
The images I posted were with equal bandwidth I and Q from the source,
files on a USB key played on my Sony Blu-ray player. I have files that are bandwidth-limited in Q and the orange artifact is much less visible. So far I can't find the screenshots of them. It takes at least 50 pF across L47 to resonate it at about 500 kHz ... and results in intolerable ringing. Both screen appearance and measured waveforms concur with what a carefully done simulation, including the tubes and stray capacitances (in many cases actually measured while changing parts) shows. In any case, what I show is what I use at the moment, and results in the best overall pictures from real DVDs (not Blu-rays) and the output of my digi-converter box fed into a top end Blonder-tongue modulator, tested on channels 3, 8, and 39 (I have a channel 39 UHF strip). Last edited by dtvmcdonald; 08-05-2022 at 06:56 PM.
|
| Audiokarma |
|
#96
08-05-2022, 07:03 PM
|
||||
|
||||
|
Replies in bold
Quote:
Last edited by old_tv_nut; 08-05-2022 at 07:11 PM.
|
|
#97
08-05-2022, 07:12 PM
|
||||
|
||||
|
Changed comments from "check" to "okay - good" because I didn't mean to tell you to check anything.
|
|
#98
08-06-2022, 11:59 PM
|
||||
|
||||
|
Have been reading your interesting comments on the peaking coils. Working on the peaking coils tonight: the two 1mH coils in the I channel. A previous repair replacement for one was 400uH and the other was open and unfixable. The 6.7mH and 1.9mH peaking coils have been installed in the Q channel.
Have been doing some further power up tests (with scope and multimeter). Found an AGC fault: picture overload and inability to adjust with AGC control. Found two resistors at V17 AGC Keyer 4.7M and 2.2M at the control had drifted upward. RCA notes said later chassis's substituted 3.3M for the 4.7M. Replacements made and now can set AGC to the point sync starts to clip. Getting good horizontal and vertical sync and traced good luma to first Video amplifier. Ballast Resistor maximum temperature after an hour run reached and sat at 75 degrees Celsius. Happy with this as the temperature is close to my target max and the heat is properly deflected away. After the peaking coils with the chassis essentially complete will next focus on constructing the jig to inject the IF alignment generator to the tuner. RCA obliged with the details and I have my fabricating tools ready. Still a few weeks away from testing the I and Q channel response and look forward to comparing results.
|
|
#99
08-10-2022, 10:54 PM
|
||||
|
||||
Still fussing over the power supply.
To accommodate the inherent forward resistance of the selenium rectifiers when replacing them with silicon diodes, I initially inserted a 10 ohm 15W resistor in series with the AC feed to both diodes. I had been powering the chassis and didn't initially pay much attention until I noticed the resistor leads appeared to be blueing! The ac voltage drop across the resistor measured 20 volts. At 2 amperes, the resistor was dissipating a whopping 40 watts!! And it survived!! So rethinking this I initially determined the forward resistance of the selenium rectifier was 10 ohms at 600mA. That is 600mA per diode whereas my 2 amperes above was a culmulative measurement of both silicon diodes as the circuit is a full wave doubler where each diode conducts only 180 degrees. I did find that the B voltages were low at an input of 118vac. The higher selenium resistance may be due to aging of the rectifiers? Just to note I left the selenium rectifier running under test at 1 ampere for half and hour and it was only moderately warm. My guess is the fins may help to better radiate the heat? Nevertheless, I next tried attaching a chassis mount 50W 8 ohm resistor to the chassis upright with a little bit of silicon grease to reduce the thermal resistance. Boy did that heat the chassis! The surrounding metal after five minutes was too hot to touch and after 10 minutes or so the whole upright section was pretty warm. As the B voltages were a tad low, it made sense to reduce the series resistance. with about 4 ohms and an AC input of between 115 and 120 vac at the set input, the B voltages were close. It is interesting to note that as this beast consumes 475 watts to load down the mains supply! I had an old pack of 1.5 ohm Ohmite ceramic 11 watt resistors. With three in series, I measured a voltage drop of 8 vac which means that the resistors are dissipating each about 10 to 12 watts. I have mounted them suspended on a tag strip and left the set running half an hour. The surface temperature at the center of the resistors as measured with an infrared thermometer shows 205 degrees celcius! they are mounted directly under the old selenium rectifier (left in place for appearance) and the chassis remains relatively cool (45 degrees celcius and the adjacent electrolytics remain cool: one electrolytic has a cardboard shroud and the other measured 35 degrees celcius after half an hour, I am going to leave it here and it will be a good test to see how these Ohmite resistors last while running at 100% of rated capacity. (If they fail, they will simply go open and they are in seies with the 4.5 ampere fuse). You will see below photos of the three Ohmite resistor location and the silicon diodes on the opposite side of the chassis upright. I have included a photo of my ballast resistor concoction with the surface mount resistors on the heatsink. The Infrared Thermometer reads about 80 degrees celcius maximum when pointing directly at the resistors from below as mounted. I am happy with this solution as it plugs in to the original ballast socket with no modification. Last edited by Penthode; 08-11-2022 at 12:19 AM.
|
|
#100
08-11-2022, 02:57 PM
|
||||
|
||||
|
The chassis has been running fine on the bench (minus CRT) with it tuned to local programming thru a Zenith DTV box. Listening to just audio all day.
Next is to construct the input head and an IF test block to begin the sweep alignment. I shall be using my HP8601a Sweep Generator and HP8600a Digital Marker Counter. Last edited by Penthode; 08-11-2022 at 03:08 PM.
|
| Audiokarma |
|
#101
08-12-2022, 12:11 AM
|
||||
|
||||
|
Quote:
So keep in mind that the usual power, voltage, and current calculations only work for DC current or sinusoidal AC current (with RMS value for the sine wave). Once you go to pulsing current in a rectifier circuit, things are quite different! The instantaneous power, voltage, and current still follow the usual relationship, but the relationship between instantaneous current and average current is no longer so simple.
|
|
#102
08-12-2022, 11:21 PM
|
||||
|
||||
|
Quote:
I have left the trio of 1.5 ohm 11W resistors in series with the silicon diode rectifiers I installed and they are pretty hot. I loomed at the Ohmite recommendations and the temperature is well within the range Ohmite specifies. And the B plus voltages are near correct. I will leave as us for now.
|
|
#103
08-14-2022, 08:51 AM
|
||||
|
||||
|
Saturday afternoon made the input block attached to the KRK12C tuner using the dimensions from the CT-100 manual. The RF sweep signal is applied to the mixer diode in the tuner and using and alligator clip there is awkward. The input block position is reinforced with a sprung clamp around a transformer can a well as the two phenolic pins with straddle two holes on top of the tuner.
This will be used to perform the IF alignment and chroma channel as well as facilitating full sweep tests as outlined in the RCA Service manual. Now on to the testing and alignment this week.
|
|
#104
08-14-2022, 10:23 AM
|
||||
|
||||
|
Quote:
|
|
#105
08-14-2022, 10:56 AM
|
||||
|
||||
|
Quote:
I immediately searched EBay for a replacement part or even generator for parts and found a brilliant and enterprising engineer redesigned the 8601a output amplifier module and offered them for sale. I bought one and the 8601a now has a flatter output response than the original amplifier. Had to be patient on the CTC5 waiting to fix first the HP generator but it was worth it. I am grateful fortunate that gains do often come after adversity. The photo below shows the contact spring against stage mixer diode pin. Last edited by Penthode; 08-14-2022 at 11:25 AM.
|
| Audiokarma |
 |
|
|
Linear Mode
