slyvania roundie

[DaveWM]

scoped the TP for agc, def some horz noise on it. there is a .1 cap from AGC to ground that I assume is there to filter it. Looks like I will end up pulling the chassis again, I dont recal checking that cap. Could be some stray horz getting into the wiring from the sweep board to the IF board. With out pulling a working set out I dont know how much is acceptable.

[DaveWM]

ok both the good set and the bad set have a horz pulse in the AGC BUT on the bad set there is a very strong vert pulse as well, I dont know why I did not pick up on this before.

I am going to check over the vert bypass cap again.

[DaveWM]

this this is really kicking my rear. The AGC just does not seem to work right, full CCW just dims the pic some and full CW just a bit more contrast. I am used to going from full snow to overloaded pic (dark screen loss of sync).

AND its back to its old tricks of having to turn the channel to get the pic to come in (the AGC voltage skyrockets, over 10v+ flipping the channel returns pic and drops down AGC to near zero.

[miniman82]

Sounds like your Automatic gain isn't being very automatic.

[DaveWM]

well I did a few more tube swaps, figured it could not hurt (famous last words). this time IF tubes, I had some NOS the last one was brand new and shorted, oh well I got it out before it did any damage. At least the AGC issue seems working somewhat again (do not need to flip the channel selector).

I realize IF tube swaps "could" mess up the alignment due to gain differences but then again I figure if things got bad I would just put the old ones back in.

Also the line pairing seems less. hard to say as it largely depends on the screen content as to how well I can pick it up.

Anyway time for a break again, will let everything cool done and will check again.

Oh and checking the video feed to the AGC, it was much lower than what sams said, so more cheking and I find the outpuf from the video detector to be about 5v PP sams says it should be 15 (this is at the grid of the 1st video).

So before I go nutz I check it to my CTC-16 hmm say PP, so not sure if the IF tubes were all just weak (this was done before the swap) on both the sylvania and the RCA or if the Sams is just wrong, 15v seems like a lot at the video detector output.

Anyhow I am chipping away at it. I sure am glad I dont get paid to do this, considering how many hours I fiddle away, I would never have a customer willing to pay. I really just dont know how TV guys back in the day made money. Of course with time I guess I would get better at diagnostics and not spend so much time second guessing and wonder what looks 'right".

[old_coot88]

Originally Posted by DaveWM

...Anyhow I am chipping away at it. I sure am glad I dont get paid to do this, considering how many hours I fiddle away, I would never have a customer willing to pay. I really just dont know how TV guys back in the day made money. Of course with time I guess I would get better at diagnostics and not spend so much time second guessing and wonder what looks 'right".

Heh.. back in the day, there seemed to be three fairly defined classes of TV guys. There were the tube jockeys who did the house calls and pulled the sets in when tubes or fuses wouldn't fix the problem. And there were the shop techies who dug in and did the actual troubleshooting and circuit diagnostics. Of course those two classes did tend to overlap somewhat. Then there were the 'dog shooters', the supergeeks who took on the real brain-buster repairs, who usually worked by piece work (as opposed to by the hour).
oc

[DaveWM]

while comtemplating my next move on this set, I will go ahead and replace a funky Left blue horz pot, its very touchy.

I am short on 6KT8's and 6AW8's (sync/AGC and 1st/2nd video driver, will pick up a handfull of those from my tube guy. Have some pulls but they all tested kinda weak. I am hoping those may shead some light on the odd AGC, esp the tendancy for it to overload when 1st turned on.

[DaveWM]

it was not a 6KT8 but a 6KA8, an replaced it with an older used one and it seems to work better. I have a thread on this over at ARF as well for those who dont frequent both sites. I think I am getting close.

[John Folsom]

Dave, If your remove the RF input to the set so and adjust the AGC so there is clean blank raster, does the set still line pair? If so, then the problem is likely in the vert. osc/output. If not, the problem is in the video/sync, or maybe even an IF issue. Something to consider.

[DaveWM]

I have a B&K 1076, I can try a few things with it. Its out in the garage, but will be coming in soon, should be easier to work on then. the plan will be to try injecting a composite signal and see if that works better. I also plan to scope the output from the detector diode and compare that to the composiit signal from the digital TV tuner. I have done that in the past just to see how well the RF/IF/detector are working (by comparing the dvd comp signal on one trace with the comp from the set on the other). that gives a real good indication of how well the tuner and IF alignment are working.

I have gone thru and replaced or checked just about eveything in the vert osc section, the only thing left I I have not checked (since I dont really know how) is the VDR, but I have one coming soon so I will just replace that. its the one right after the .0082 cap from the plate of the vert out.

[DaveWM]

finally got it in the house, vert pairing does not seem to be an issue anymore, but now the pic seems noisy, dont recall if that is a new problems, like a weak RF amp tube. could also be related to agc, so I will try a new RF amp tube AND a bias supply on the RF just to see.

There is a tuner subber locally for sale, I am thinking about getting that, seems like a fast way to clear a tuner.

I nver did get the scope out and go over the video signal again. too lazy I guess.

oh and even thru the noise I can see the same kind of ghost as the maggie has (both are CTC-15/16 clones). So either they both have a design flaw, or they both have the same problem, or my signal is bad. Not something I am overly worried about.

[Chad Hauris]

If you have another known good set with a transformer power supply (or a hot chassis set with an isolation transformer) this can work good as a tuner check out device. It doesn't matter if it's color or black and white. If you suspect tuner problems in the suspect set, sub in the test set's tuner using the 45 mhz IF output and if you suspect IF problems in the suspect set, hook its tuner IF output to the test set's IF input.

I used a Zenith B/W set with a solid state tuner this way to help find problems in an RCA CTC-28. The picture was very weak and snowy but the RCA set's tuner produced a good picture on the Zenith set. It turned out to have one bad IF tube and and bad 1st video tube.