"Old Sparky" (CTC-31)

[bgadow]

Earlier in the year I worked on this CTC-31, finally giving up due to intermittent arcing in the area of the burst amp/color crystal. The last suggestion made here was to check for bad solder joints, especially the larger grounds. Well, restless, I pulled it back out Sunday & went to checking those joints. Nothing real bad, some sort of looked marginal so I fixed 'em up. The burst phase coil had previously gone open and I replaced it; I suspect this arcing took it out, and while I have a couple spares I don't want to kill one for nothing. There is a big ground trace beneath it so for good measure I ran an extra wire off it to the chassis. Everything looks to be in order. So I fired it up and as soon as the switch was turned on-LIGHT SHOW! I narrowed this down to the area underneath and between the blanking amp & the video output. I pulled off the rear panel for a better view, and have not ran it for more than a second-hard to get a closer look. Once when I turned it on I got no spark, just smoke, smelled like wax, but I could not find anything hot. (no paper caps in this set) The next time I flipped it on, fireworks again. From seeing/hearing it, this is a pretty good voltage arcing. There are some things I can try (seeing what it will do with hv disabled, with normal/service switched to service, pullling tubes, etc.) but does anyone here have any good ideas? One thing I notice on this set that I don't recall running into before are spark gaps, lots of them, either alone or as part of disc caps, but I can't say thats related to anything here.

[andy]

Is there any way you can operate the chassis with it out of the cabinet so you can see the bottom of the board where it's arcing? Does it arc before it's warmed up, or only after? Try disconnecting the CRT socekt. There aren't that many voltages in a TV that can arc unless the board has carbon tracking. Have you checked for arcing in the flyback?

[Chad Hauris]

I know the boost voltage in this set that is fed to the screen controls is more than 1,000 volts...maybe the arcing is somehow related to a PC board trace with this voltage?

[bgadow]

FYI, the arcing occurs the instant power is turned on. Odd, I have a switched socket with (plugs into the wall, tv plugs into it, cord about 6' with a switch on the end) so I used that so I could kneel down & be looking straight at it when I flipped the switch. With cord switch off, I turned on the tv switch, it started arcing! But no AC? Or miswired cord? Light bulb plugged in same way didn't light! I want to ignore that odd problem for now! I'll rig up another such way to flip it on while I'm kneeling. It will scare the devil out of me when I turn it on, even though I know what is going to happen, but gotta do it! I don't see any signs of the arcing, though-no carbon or burnt spots. It will be a night or two before I can work on it again, think I'll try cleaning the traces good both top & bottom. There are spark gaps connected between the screen control shell & chassis ground, I suppose so that in the event of a short that shell doesn't go live with 800-1000 volts. I'll also see if I can get a look into the HV cage while this is going down.

[Bill R]

Check the focus rectifier socket for carbon traces. Look for any tracks of carbon in the HV area. There are several spark gaps, unless the leads are bent and shorting they usually are not the problem. In other words you would only get a spark if there is a problem causing the voltage to go way higher than normal. There are gaps on c143, and c142 both in the 800volt boost. One of these is located near the screen controls, the other is under the HV cage. Also check the controls on the rear panel. The vert lin control has 800v boost on it, and can arc.
Good Luck

Bill R.

[Chad Hauris]

If it is arcing as soon as it is plugged in it has to be the primary b+ supply voltage arcing somewhere...could not be HV or not likely boost voltage as the horizontal output tube has not warmed up yet. You may want to disconnect the leads from the different branches of the B+ supply and see if you can pin down the arcing to a specific area.

[bgadow]

I had a few minutes to work with this tonight. The chassis is on the bench, no connections to the crt or yoke, horiz. output tube removed.

I quickly switched the set on & off a few times with no adverse reaction. About the 4th time I got an arc, & repeated it. It was between 2 legs of the burst phase coil, the ground leg & the leg hooked to the plate of the 6GH8 burst amp. (355v per Sams) What was in between the two traces was, of course, carbon. I had noticed it was suspect here but thought I had cleaned the traces enough. This time I cleaned it with gojo & a toothbrush, then wiped down, then cleaned good with alcohol. Everything looks like new around there now. But I'm not ready to try it again. I know this was not the only place arcing, unless I was seeing things. There is a chicken & egg thing going on here-I assume it arced, taking out the coil at some point, leaving carbon, continuing the arcing, but I could swear other areas are arcing instead, on occasion. I have to figure out why. The plate lead also connects to a .82pf cap into the color oscillator and ACC circuits. This is where my lack of any 'real' training in electronics shows, as I can get to this point but have trouble grasping where the problem lies because I don't fully understand theory. This is how I learn, though. And I've learned more off these forums than anywhere else. Thanks for the advice so far, which I will continue to follow up on, & thanks for any advice to come!

[andy]

Sometimes circuit boards break down for no apparent reason. The correct way to fix this is to physically scrape away all the blackened material with a knife. Cleaning it won't be a reliable fix.

[bgadow]

I've worked just a little more on this. Got that trace cleaned up some more, and cleaned it out with a blade. Further testing shows no arcing, though I did get a slight wisp of smoke from an undetermined location on the chroma board with a waxy smell. This was with just flipping the switch on & off fairly rapidly. I don't know how safe on the chassis it is to bench test it like this-hv is disabled, of course, but this has got to put B+ too high, right?

Studying the schematic, I was eyeing the 80mfd filter cap on the 355v line. Could that cause problems like I'm having? I might have changed it out already but I don't have anything that high in value in stock (gotta place another order after Christmas) although I could parallel a 47 & 33.

(I might have played with it more but I decided I need to run each of the roundies for at least a short while, as several have not been running since early summer. Once I got the CTC-5 to straighten up and fly right I, of course, could not take my eyes off of it for the rest of the afternoon!)

[bgadow]

Last night I changed the electrolytic cap in the last post. After giving things a once over I hooked everything back up, but left the horiz.output out of circuit. After a few seconds I noticed smoke, this time from R174 (see diagram), color killer emitter to ground. I hooked up the meter & tried again; Sams says to expect 2.6v here, but I got 115. I replaced this resistor (with a 1w) & also replaced or at least checked R173, R170, R177, R172. I replaced C131, a .82pf cap in the burst amp circuit, where I had problems earlier. (I was in no hurry to replace such a low value cap but I figure things are pretty screwed up anyway!) After all that I tried again; the new R174 is overheating & now reads 80v. Slowly, ever so slowly, I learn. Any ideas welcome-thanks!

BTW, if anyone is following along, this is Sams 987-2.

[Chad Hauris]

Shorted transistor q1?

[bgadow]

I had my suspiciouns about Q1. (I'm suspicious of anything solid state!) Back in the spring when I was trying to fix the no color problem I substituted a NOS transistor here, though the old one tested fine. Tonight I put the old one back; no change. Also changed another off value resistor, this one between 2 of the filter caps. (Had gone from 680 to 1.2k) No change there, either.

[Bill R]

What are the other voltage readings on q1? Of the resistors you checked or replaced, which ones were replaced? Did you check R176? It is part of the voltage divider that provides collector voltage to q1 from the Horizontal blanking amp. Is Q1 shorted?

[bgadow]

In answer to Bill's question: C=113,B=114,E=102. These readings were taken with it on the bench, no hv, no yoke, etc. I also checked R176 & it was fine, much to my dismay! Am I smart enough to tell you without a doubt that Q1 is okay? No! Seems alright by my little Sencore tester; I bet those voltage readings mean something to someone smarter than me-I hope!

[wa2ise]

In answer to Bill's question: C=113,B=114,E=102

Usually transistors will have between their bases and emitters a
voltage difference of 0.7 (if silicon) or 0.2 (for geranimum). Unless
the transistor is biased off (passing no current). Where is that
color killer trimpot set? Looks like, depending on the setting, the
base voltage could be anywhere between 0 and 300V. Adjust the
trimpot so the base voltage is what the Sam's diagram says, and
then check to see if the other voltages around the transistor
are close to what Sam's says they should be.