Finally getting a real start on my T-120. Have some questions though . . .

[benman94]

Well I found a rebuilt 12KP4 to replace my problematic 12LP4. Though I still plan on performing the repair to the 12LP4, I'm going to hold off for a while seeing as my new tube actually costs less than the supplies needed to create 2 new leads and seal the base back onto the neck. $35 is a lot less than I was planning on shelling out! Anyway, I have a few questions . . .

#1: The Sams photo-facts for the T120 has a complete listing of the set's capacitors. Unfortunately, it doesn't say what type each capacitor is. Papers and electrolytics are given in uF, and micas and ceramics are given in pF. Just looking at the list however, I can't figure out which are paper, which are mica, and which are ceramic (electrolytics are easy to tell due to the can: C1-A, C1-B, C1-C . . .) . I was thinking about replacing them by simply going through the chassis, marking them off, writing down the values, and ordering them (like I've done with all of my radios). This won't work though. There are WAY too many to do that by hand and not screw up. I also can't read through the darkened wax. When I try to scrape it off, I end up taking all the ink with it. The list in the Sams does have a voltage rating column though. Some of the capacitors have ratings given, others don't. Am I correct in assuming that the caps with the ratings are papers or electrolytics?

#2: What is the difference between grid-fed and cathode-fed video?

#3: What resistors, if any, should I be concerned about checking and replacing?

Thanks,
Benny

[vts1134]

Originally Posted by benman94

Well I found a rebuilt 12KP4 to replace my problematic 12LP4. Though I still plan on performing the repair to the 12LP4, I'm going to hold off for a while seeing as my new tube actually costs less than the supplies needed to create 2 new leads and seal the base back onto the neck. $35 is a lot less than I was planning on shelling out! Anyway, I have a few questions . . .

#1: The Sams photo-facts for the T120 has a complete listing of the set's capacitors. Unfortunately, it doesn't say what type each capacitor is. Papers and electrolytics are given in uF, and micas and ceramics are given in pF. Just looking at the list however, I can't figure out which are paper, which are mica, and which are ceramic (electrolytics are easy to tell due to the can: C1-A, C1-B, C1-C . . .) . I was thinking about replacing them by simply going through the chassis, marking them off, writing down the values, and ordering them (like I've done with all of my radios). This won't work though. There are WAY too many to do that by hand and not screw up. I also can't read through the darkened wax. When I try to scrape it off, I end up taking all the ink with it. The list in the Sams does have a voltage rating column though. Some of the capacitors have ratings given, others don't. Am I correct in assuming that the caps with the ratings are papers or electrolytics?

#2: What is the difference between grid-fed and cathode-fed video?

#3: What resistors, if any, should I be concerned about checking and replacing?

Thanks,
Benny

From my extremely limited knowledge so far I've found that the big electrolytic caps are listed first. The mica and ceramic caps will have "high" ratings on the parts list. The values will of course be low but the numbers will be higher because they are listed in pf. The paper and electrolytics will have a "low" rating, which will be higher in actuality because they are listed in uf. I found it helpful to have the bottom of the chassis, the capacitor parts list, and the chassis bottom view capacitor alignment identification page all in front of me. Then start on the part list with a capacitor, find it on the alignment page, and locate it in the set itself. When I found it I would write down what value I need and order them.

Just the thoughts of another beginner.

[kramden66]

Benny
I am looking at the sams right now and in the parts list the capaitors to change are listed with voltages of 200 volts , 400 volts , and 600 volts , those are the paper wax , the others not marked or shown as 500 volts and 1000 volts are micas or ceramic , you can usualy tell by the value listed , the paper/wax have values like .01 , .22 . .002 . etc etc , the micas are listed like 82 , 1500 , 270 , etc etc , there are two 1000 volt caps that are not mica or ceramic and are paper caps , C94 is .039 at 1000 volts and C95 is .018 at 1000 volts , if you can't find those exact values something close will do like .015 or .02 for .018 , .04 for .039 or .035 .... this might help - if you copy the page that shows where each capacitior is you can write on it the value and voltage so when you go to put it in the set you have a guide to do them one at a time and check off on that paper the ones replaced ...... as far as resistors you might want to check the big long one mounted to the back , it has 3 sections , if you are not getting proper readings check back here because they can be tricky to read sometimes ... if you want to be sure you could go over ever resistor in the set to see if it is the right value , some may read wrong and will have to have one end disconnected to be sure it is the right value , or just check the big power one on the back which i already mentioned but if you do that without reading the others if there is a problem one little 1/2 watt resistor thats no good or way off in value could cause a problem , however some just replace all the capacitors / electrolytics along with tubes and then bring the set up on a variac slowly up to about 105 volts to see what goes on , or just turn it on..... any problems comeback here and it will be discussed.

Mike

[ChrisW6ATV]

Benny-

Rather than copy from the Sams part list, I recommend making your own list from what is actually on the chassis itself. This is for several reasons:

-Many TVs went through production changes, so the Photofact list may not match exactly. You want to replace the parts in YOUR particular set. (Also, some Sams manuals have had mistakes; you should match what you have, not what the list says typically.
-Actually checking every part in your set is good hands-on experience with reading values and understanding different part types.
-Physically checking every part of the chassis will ensure you don't miss any parts.
-The close inspection will also help you to find any parts that have been replaced in the past; the more hands-on you are, the easier it will be to identify repaired areas or parts that don't match up well with the rest of the chassis. Repaired areas should be compared to the schematic; if you find things that don't match, you can always ask here for help from other members.

Regarding resistors, the high-value ones are the most likely to change, so check the yellow-third-stripe and green-third-stripe ones (or blue if you have any), since those are the ones over 100,000 ohms.

Grid-fed video means the output of the video amplifier circuits goes to the first grid on the picture tube; cathode-fed means the video amp feeds the cathode of the CRT. You should be able to tell which you have by looking at the schematic diagram. I do not remember learning that either was better than the other.

[benman94]

I'm used to working on old regen radios that may have, at the most, 20 caps. I should have recognized the difference between the "low" and "high" values. Feel kind of silly now . . .

[Electronic M]

If you have a junk soldering iron you can melt the top layer of wax on paper cased caps to make the lables ledgible (do this in a ventilated area it can stink).

Tom C.

[bandersen]

Originally Posted by benman94

#2: What is the difference between grid-fed and cathode-fed video?

I don't believe there is any performance difference. It's a matter of how many IF and video amp stages there are as each stage will invert the signal. So if it's an odd/even number of stages you feed the video to the cathode/grid.

[Eric H]

Originally Posted by bandersen

I don't believe there is any performance difference. It's a matter of how many IF and video amp stages there are as each stage will invert the signal. So if it's an odd/even number of stages you feed the video to the cathode/grid.

And if you reverse them you'll get a negative image, right??

[bandersen]

Originally Posted by Eric H

And if you reverse them you'll get a negative image, right??

You might need to play around with the brightness/contrast controls and retrace blanking to get a decent negative image. I suppose you could try bypassing one of the IF stages too.

BTW - The B&K 1077 TV Analyst has a polarity switch on the video output to handle either case.