noisey pic in Roundie philco
 

figured I would share my attempt to resolve this with the group.

the pic has excellent color, but there is always some background noise. Not snow, more like light streaking through out the pic.

I put the B&K analyst on it, the streaking is AFTER the tuner. I tried injecting at various IF stages, but with the transistor IF I had a hard time getting to the small leads to clip in.

I now plan to try signal tracing, again this time it will be the B&K analyst, with and with out a slide (what ever works best for finding the noise). I already put in a tube spacer at the video out tube. figured I would start there at the grid and plate and see if I can start to narrow it down.

it has been sugested that I check the cathode bypass cap on the video out tube (its a can cap, orig to the set) which is a good idea, but I really want to see if I can use my equipment to help me understand and trouble shoot.

I assume its a noisy plate load resistor, or a decoupler if its no in the bypass cap. Of course it could be up the chain so I am starting near the end. there is a video driver transistor and a cathode follower tube before that.

I just hope its not in the IF somewhere, if so I will have to see how well my demodulator prob works (something I picked up but have never used).

well I tried the scope from the video out tube all the way back to the grid of the cathode follower tube (the 1st amp after the detector diode) I think I am still seeing the noise. I am going to try disconnecting the feed line from the detector that goes to that pin of the tube (its on a different PCB so I hope I can just find the wire that connects it) if its noise from the detector, then I assume disconnecting it will clear the screen. then it on the the video board, yuk.

lots of times noise can be generated from loose of missing ground or shielding. Also a noisy horizontal output tube can cause barkhousen (noisy tube). Most times these problems look more serious than they actually are to fix.

Maybe the video detector diode is failing, my CTC-7 did funky things when it's detector was going out.

maybe. if grounding the grid of the 1st video amp tube confirms the prob is before it, I will focus on video board, that will be the 1st thing I check.

well I decided to fool with the IF AGC (a pot on the video IF board). It seems to have cleared the problem.

I need to go thru the hole AGC setup (assuming the sams is correct) I dont think its right. the AGC control on the on the back of the chassis will not cause the pic to tear when fully CW. Maybe I should just leave it be, the noise in the pic seems to be gone, but its late at night, so I better look again after some sleep.

A tip worth bookmarking.. with any kind of "noisy pic" or a 'overload' symptom, try the AGC control first, and also look for an RF AGC adjustment if a set is SS or hybrid.

roger old coot. I should have just scoped the AGC line, I have chased things before that had to do with AGC one of which was hash on it from open caps, etc...

ok noise is worked out but the pic was dim. I could make it ok by adj the CRT bias, but should not need that as I was able to get all the guns to show in the setup mode at the lowest setting. I checked the CRT pins and sure enough the cathodes were at least 50v too high (was 240v shb 190v).

So looks like a video out tube. I checked and yes the plate voltage was too high, checking the Cathode and again too high (about 5v not the 2.5 it should have been). So if the video out tube plate is high, and since its directly coupled to the CRT cathodes, that explains the bias problem and the dim pic. I did some resistance checks and it looks like a 100ohm resistor on the bottom of the brightness control may have drifted. Its mount up on the pot on the control panel so I will need to pull it out to check. Hopefully this will resolve the dim pic.

I also have a intermittant blue screen issue, Its like the G2 drops down about 10% all by it self. I will be monitoring the G2 voltage and see if that is whats going on. there are 100k resistors in series with the G2 pots so maybe the blue one is getting intermittant. I will prob just pull the chassis and replace all 3 of them while I am at it. Lastly there was a .1 cap in the grid circuit of the video out coupling the delay line to the grid. I will prob replace it as well since I seem to be having bias issues in that tube.

I will also check the PCB track for any break around that G2 lead but I am just guessing that is what is causing the intermittant drop down of blue in the crt (it like someone just turns it down a few degrees, but its quick not gradual, and it will come back and then do it again). I dont feel like breaking out the cold juice just yet.

pulled the tuner out, the 100ohm was 300ohm off that pot. I need to go ahead an pull the entire chassis, will replace those other parts (the screen resistors and that coupling cap. there are some .001 disc caps across the G2 pots that I am tempted to replace while I have it out, just in case one of those is acting up causing that intermittant problem. The only thing is I never really captured it on the VTVM to confirm it was the G2 voltage that was doing it. could have been the G1 or the cathode I suppose. Just getting tired of yanking the chassis as you have to pull the tuner with it.

ah now that is much better. the grey scale setup worked as it should, where as before the incorrect bias of the video out tube resulted in too high a plate voltage, that when connected thru the serice switch in the normal mode would tend to cut off the CRT. Now with the CRT bias set at the min, I can put it in service mode, just bring up each gun, and then go to normal mode and get a nice bright pic with correct grey scale. while at it I put the old focus coil back in and now have a best focus point, before in never quite got there.

I still need to double check all my HV setup for current and voltage, but it now has a very good bright pic, great color, and NO noise in the pic.

I will play with it some more later and see if the intermittant blue in the grey scale was fixed by replacing the G2 100k resistors. I only replace the one .001 disc cap on the blue since I only had Y5U caps and not the Z5P that it had originally. Since the other guns were ok I left them be (still replaced all the 100ks just so they were balanced, the were between 108k and 120k, but I was thinking they could have been noisy).

HV is down to around 23.5 holding pretty steady. HOT current is a bit high on some bright scenes, but If I switch to serv to cut the beam curent its right at about 200ma, up to about 240ma for normal viewing. I may try some different HOT's just to see if I can get that down some. I forgot to check the resistors in the HOT feed back circuit, darn it....

opps forgot to check the line voltage, will set that to 117v and check again. I always forget that...

Yep, all the readings I took the other day were at full line voltage (120). No doubt it would run cooler if I hook it up to my RCA TV Isotap transformer. I generally run old sets at 117/115.

if you get a chance would you pls snap a pic of your CRT neck. I don' think the blue lateral and the purity rings are in the right posistion on mine.

Wouldn't do you any good, what I have for blue lat is completely different than yours. The one mine has is metal...might not even be the right one. I also learned today that my set is missing the degaussing coil! Guess that explains the vacant 4-pin plug on the chassis. Anyone have one????

wow, wonder how that came to be (no coil),

ok blue intermittant still there, I poked around on one of the 6BL8's and you can make it flicker, but that just may be the shock going thru the board. I will try cleaning the sockets (again) but I need to try and isolate exactly where the problem is. Heck could be a tube with a short I guess (that would certainly account for a thermal issue). I am tired or working on it, so will button it up and give it some time off for pretty good behavior. One thng for sure, it has EXCELLANT color demodulation, no bleeding what so ever, passes the homer simpson test with ease.

I fooled around with the scope, I get about 180pp on the horz drive, which is close to the 190 called out in the sams. I need to break out the VTVM to check but I am guessing based on the scope the drive is fine. I also checked the screen but did not use my VTVM and I dont trust a DMM on that reading since there is a lot of pulsing going on, need to use the VTVM like it says to, I just dont think they react the same to pulsed DC. On the DMM I had 175v (should be 210 per sams).

I set the horz bias so the HV is 25.5 per the sams setup (brighness all the way down). It has a great pic, brighness range looks right, but the HOT current goes up to near 300ma in some scenes (all white screen) most of the time its around 230ma. The HV with the brightness set to normal is about 22-23kv. My only issue is the fly gets pretty darn hot after about 30-60 min.

I tried bridging another VDR and was able to adj the bias to get it down to about 200ma with normal scenes, but then the HV was around 19 and the horz sweep was falling short about 1/4 inch on both sides.

I tried several 6KD6's all about the same. I wonder if a HV rectifier or Damper could affect the fly load?

Dave, in cases of borderline width and you've exhausted all avenues, there is a little trick that often helps. If you happen to have some 6KV rated disc ceramic caps of around 68pf, 82pf and 120pf, what you do is take the smallest one first (the 68pf) and put it directly across the damper, cathode to plate. This will kick the width out a quarter inch or so on both sides. If it's not enough, sub the 82pf, and if still not enough, try the 120pf.
Each stage will also drop the HOT current by 5 milliamps or so, which you can compensate for with the bias or by upping the screen grid voltage.
You can only take this mod so far, because each stage also drops the HV a bit (as in the old saying "there is no free lunch"). But there is usually a compromise where you have adequate width with reasonable HOT current and decent HV regulation. This situation also often crops up in Admirals using the 6KD6.
Bill(oc)