New Project - RCA 8PT - Page 5 - Videokarma.org TV - Video

Phil Nelson · #61 03-07-2011, 09:24 PM

Here ya go.

If you send me an email via http://antiqueradio.org/contact.htm , I can email you the whole thing.

Phil Nelson

***Eric H*** · #62 03-07-2011, 10:41 PM

They all leak a lot of wax out of the Fly, it would have made more sense to put it on the bottom and the tubes on top!

A resistance check may not do much good, a shorted turn or two wouldn't show up that way.

maxhifi · #63 03-07-2011, 11:58 PM

Phil:

Thank you, will send email soon! Mine is the KCS100K, so the flyback is different than that one - I started comparing resistances, and then realized that diagram has C78 = 0.047uF, which corresponds to the earlier KCS100B chassis.

Eric:

Yep, I thought it looked upside down too - when I took apart the chassis to clean it, the first thing I thought when I encountered the wax covered damper and HOT was hmmm heat rises, what's up with this idea?

I spent about an hour or two with the TV again tonight, without any real progress. I did however get a much cleaner photo of the horizontal waveform, as below.

Phil Nelson · #64 03-08-2011, 12:34 AM

Schematic sent (336-14, chassis KCS100B, the only match I found in the index).

Which waveform is that last photo showing?

If you want to go beyond resistance testing, this thread from the "other forum" has an article describing how to test for ringing with an oscilloscope (haven't tried it myself).

http://antiqueradios.com/forums/view...er=asc&start=0

Phil Nelson

maxhifi · #65 03-08-2011, 01:07 AM

Hi Phil,

Thank you for the sams! Been reading through it, and it has some perspective which the RCA manual does not. It also has a much better drawn schematic, from an understanding how the circuit operates perspective, even if the sweep circuit has some subtle differences.

Last waveform is the a few turns of wire around the horizontal output tube's plate lead, into my scope's probe.

Thanks for the article - Looks like I've got something to do tomorrow night now!

daddyspike · #66 03-09-2011, 10:07 AM

heres one that i might need some guidance on i,am looking to get and old b/w tv and convert it to radar the tx and rx of it will be around 300mhz now i know that the tubes won,t be able to handle the high freq so should i stay with ic,s or go with tube?

Phil Nelson · #67 03-09-2011, 11:02 AM

Your radar question seems completely unrelated to this topic. You can click the New Thread button to start a separate discussion in this forum.

Phil Nelson

maxhifi · #68 03-10-2011, 12:03 PM

Ok, had some time this morning to look at the TV. I think I want to try a new fly, and maybe width coil. The only waveform which looks wrong, according to the RCA service manual, is the horizontal.

The testing method for ringing flybacks in the above article counts on having a "known good" to compare with.

One thing about this problem, is I am sure getting well acquainted with the inside of this wonderful little tv - it's actually more fun to work on than my early 70s Zenith whose performance was my previous interest, with its massive chassis, hot chassis and overwhelming number of parts.

On another "note" I think I figured out the problem with audio buzz too.... this is a design defect caused by the 6AW8 being nonlinear when the contrast is set too high. Once I get the TV working perfectly, I am going to think about ways to have the audio not go through the 6AW8

maxhifi · #69 03-14-2011, 11:48 AM

Noticed a new 'defect', or possibly characteristic. When contrast is cranked up, text can get fuzzy. Plate voltage on video amp is off spec, quite a bit too high. Since contrast control works by varying bias on 6aw8 pentode video amplifier, and current is too low, i'm guessing either tube is bad or resistors have drifted. This design doesn't appear to have a lot of headroom built in.... Effect not terrible, it just means one has to balance contrast with sharpness. Will work on this one once new flyback installed. In the mean time have been using this as my only tv... Works great... Wonder how long until i start to dream in black and white?

compucat · #70 03-14-2011, 08:45 PM

Quote:

Originally Posted by maxhifi

Noticed a new 'defect', or possibly characteristic. When contrast is cranked up, text can get fuzzy. Plate voltage on video amp is off spec, quite a bit too high. Since contrast control works by varying bias on 6aw8 pentode video amplifier, and current is too low, i'm guessing either tube is bad or resistors have drifted. This design doesn't appear to have a lot of headroom built in.... Effect not terrible, it just means one has to balance contrast with sharpness. Will work on this one once new flyback installed. In the mean time have been using this as my only tv... Works great... Wonder how long until i start to dream in black and white?

When I turn the contrast all the way up on mine it almost looks like overloaded agc. I think what you are seeing is just a characteristic of the design and not a defect. The picture and fine tuning controls need to be adjusted for the best overall performance which on a set like this is invariably a compromise. I expect this was typical of 1950s sets. The one thing these little RCA sets would have benefited from is an aluminized tube.

maxhifi · #71 03-15-2011, 12:51 AM

Quote:

Originally Posted by compucat

When I turn the contrast all the way up on mine it almost looks like overloaded agc. I think what you are seeing is just a characteristic of the design and not a defect. The picture and fine tuning controls need to be adjusted for the best overall performance which on a set like this is invariably a compromise. I expect this was typical of 1950s sets. The one thing these little RCA sets would have benefited from is an aluminized tube.

I agree completely, just for the sake of improved contrast. If attached advertising (radio & tv news, July 1954) is credible, it would have been a pretty big improvement. I wonder if any aluminized replacement CRTs were made for this set.