Video inputs Tube TV

[ARC Tech-109]

Didn't know about the industrial monitors, they're built tough enough I'd say.

So the video input amplifier was a bust, everything was built to the schematic including the 1N34 but the Magnavox isn't happy with the addition. The issue is it seems not enough of the chroma signal is passing thru and this is causing most of the color to wash out. The amplifier bandwidth seems to be good as I have tested it with a sweep signal and it rolls off only slightly just over 5.5 MHz. Colorburst is strong but the color itself is lost except in the saturated areas, 75% colorbars seem to have a lag to the leading edge and they bleed over into the next bar, I forget the technical name for this. Running the set with RF has good color.

I've tried uploading images with zero luck, the best I can do is a 50k image file and that doesn't show the needed details, if I can't get the server to accept them I will do a quick vid and post it on YouTube as words can't accurately describe the issues, honestly I need a fresh set of eyes and some ideas here and any help would be appreciated.

Thanks to all
Brett

[old_tv_nut]

Sounds to me like the DC bias may be wrong for the TV's circuit, if you can measure good response in the amplifier alone, but something is happening to the chroma when feeding the set.

Especially if the burst comes through OK, but chroma up in the active video voltage area is being mangled somehow.

[old_tv_nut]

Conrac made 21 inch broadcast color monitors that were direct competition for RCA.
When SMPTE standardized the rare-earth phosphor colors for broadcast monitors, they settled on a unique batch that came from Conrac and became known as "SMPTE-C" standard. Three barrels of R,G,B were set aside and RCA made special runs of tubes to supply the broadcast monitor market.

[ARC Tech-109]

I need some fresh ideas here as the video input amp is really kicking my backside. The top trace is the output of my Tricaster with a chirp pattern going into the video amp, the lower trace is what I'm finding on the grid of the 6MU8 which has been disconnected from the set video detector and has a 470k resistor to ground. On the amp board the heaters and +285V are bypassed to ground via 0.01uf ceramics but it seems like something is ringing. Here are the images of the scope, these are about the best I can capture.

Thanks in advance for any insight, I'm close but missing something I'm not completely sure of.
Brett

Deleted links from this post, images on post below.

[old_tv_nut]

I'm getting 404 file not found on these.

[ARC Tech-109]

Lets try this again using imgur.com

Input board
https://imgur.com/JJnWFXd

Colorburst signal expanded on Tek 465 scope, top is source
https://imgur.com/Fag4K42

Chirp signal, top is source
https://imgur.com/2Agk7k8

[old_tv_nut]

Well, I see the thick traces on the output, but can't tell from these if it's some kind of high frequency oscillation, just lack of DC coupling, or hum.

[old_tv_nut]

Are you using the circuit in broadcast news or something else?

[ARC Tech-109]

Running the same circuit in the Broadcast News article on page 18 using the 12AT7. Looked at it much closer again on the scope and it's clearly oscillating so now I get to figure out how to best neutralize this without compromising the response.

[kf4rca]

Where is the termination? On the board or at the end of the loopthru (bnc tee)?
Try other cables. Is your scope on 20MHz bandwidth?
Ground loop between scope, bar generator, and/or TV?

[ARC Tech-109]

Termination at the scope with another Tee and the scope at the full 100 MHz bandwidth (tek 465 10:1 350 MHz probe), I looked at the output with my R-2670 spectrum analyzer and it appears to be oscillating around 10-15 MHz depending on the input signal so I'm going to drop a shield around the tube itself and see where it goes.

[kf4rca]

Push that 20 MHz BW switch in and see if it doesn't clear up. I never ran my scopes at full BW at the TV station for obvious reasons. WHY do you need 100MHz BW for only a 4.5 MHz signal???

[ARC Tech-109]

Actually my career was 2-way radio and that demanded a high bandwidth scope, even with the 20 MHz limit I can still see the ripple but as it stands now I have a good handle on the issue with a shield around the 12AT7. As it turns out the reason the color was washed out was because the 1st video amp in the set was being pushed to saturation so I'm going to add a gain control pot to its grid. The amp can pass 5+ MHz to the 1st video and everything I own is previous generation broadcast be it DigiBeta, DVCPRO and Type-C so bandwidth here isn't an issue, the picture tube begins to blur at the 350 horiz line point of the resolution chart so this is about the limit of the delta gun CRT but by no means the limit of the set itself when unleashed from the 4.5 MHz trap.

[ARC Tech-109]

So after a few long nights with the soldering iron and sifting thru the parts bins... I think the picture speaks for itself. 1970 Magnavox T-940 AstroSonic on composite video direct from my Sony BVH-2000 Type-C with the four card internal TBC. The audio is very strong and clear, I'm still having some instability issues from the lack of video AGC so my next project is to integrate the existing AGC into the input video amp and adding an outboard sync separator like the LM-1881 to directly feed the horiz & vert, I have a dedicated house BB gen so this would be a bonus.

Here's the screen shot of INXS direct from Type-C. Nice to actually watch something on a real CRT.

https://imgur.com/7CNslBz

[old_tv_nut]

Nice!