That Darn DuMont Again

[bgadow]

This is the DuMont color set which has made the rounds of AK TV Heads & is now residing here. Just as I fixed a short vertical problem something else developed & I havn't figured it out yet. That should come as no surprise, I still have lots of learning to do!

I have a video problem (well, the sound is messed up, too) that acts sort of like its overloaded. Sometimes I can just about hold a picture though its not nearly correct. Its going negative, the sync isn't too good; I have been looking at the video amp & agc circuits. The agc control is basically just lightening/darkening the picture. Earlier it was working more normally, with an almost correct picture locking in at times with the control cranked all the way up. Now I pretty much have a blank white raster with occasional video getting in there. I get the same results tuning the local uhf station directly or through a vcr on ch 3. I have been searching for a bad solder joint or bad resistor but everything looks okay. I have subbed all the tubes that I suspect. Remember, this is basically a NOS tv set with just a matter of hours on the tubes, so I am not too quick to suspect them. I measured voltages on the 6KA8 agc/sync tube & the 6LF8 video amp tube. They are all over the place:

6KA8 pin 1 should be 130v, is 75; pin 2 should be 5, is 19; pin 8 calls for 130 but reads 68; 9 should read 30 but I show -11.

6LF8 lists 5v on pin 2, has -1; pin 8 should be 110, is 73; 9 is listed as 185 but reads 18v.

I thought about swapping the video detector diode. I don't know anything about them & what their failure would cause. I have a NOS ECG109 but can't tell which end is which. No markings, not banded like the old one, one end is grooved like the end of an electrolytic cap. I have an RCA 3088 that I think should work but haven't dug it out yet.

This is in Sams 1114-1. Another learning experience!

[Celt]

"That Darn DuMont" Wasn't that a 50's TV sitcom?

[Chad Hauris]

This sounds like maybe an IF problem...had the same kind of problem with a ctc-39, turned out the power supply resistor was open to the transistor IF stages...is the Dumont transistor or tube IF? You may want to check the supply voltages to the IF stages and tuner.

Also, based on research I have done in period service magazines and books, bad peaking coils were reported a lot. There was one account of a shop delivering a brand new ctc-25 to the customer and there was no video, turned out a peaking coil had burned open.

There may be an open winding to a peaking coil somewhere in the video circuits. One test is to inject video from a vcr directly into the output of the video detector...if you can get some video on the screen then, you probably have trouble in the IF/tuner stages...if video still poor trouble likely in luminance signal chain.

That 18 volt reading where there should be 185...is that the plate? check for the peaking coils or other bad coils in that circuit.

[RetroHacker]

Originally Posted by CELT

"That Darn DuMont" Wasn't that a 50's TV sitcom?

Yes. It was a very short-lived show, and was on following the similarly unpopular "My Mother the Philco".

-Ian

[bgadow]

My Mother the Philco? Man, now that was a show! Remember the episode where her flyback shorted out! Hilarious!

Well, Chad, looks like you hit the nail right on the head. Pin 9 of the 6LF8 is the plate. I started checking peaking coils in that circuit, the second one I looked for was L101, hmmm, where is it, oh, hard wired under the chassis-EUREKA! Right next to that resistor I pulled out of circuit to check, back when I was hunting down the height problem. And you know the rest...I hit the coil with the soldering gun (or something) & it didn't look so bad but it was enough. Rats.

So, looks like its time for my long overdue lesson in peaking coils. I pulled out the 3-wheeled 2-sided resistor/capacitor/inductor decipherer & set it up for 390uh (what sams says) and it says orange-white-black. The 2 dots I can see on the damaged part are blue & white. Okay. So, where do I buy a new one? I could just go hunting through scrap modules up in the attic but not so sure with this color code question. I have an Allied catalog where I found under "molded inductors" a 390uh ferrite core rated at 105 mA. Would that work? I bet the ratshack doesn't carry them! I'm sure Moyers does but this makes for a pretty small order.

[Chad Hauris]

You may just try what you have on hand from a scrap set...you may be able to use a different value and have it work...any coil you put in there will probably get your plate voltage back, just make sure there is not a short or overload on the load side of the peaking coil like from a bad cap which may have caused the original to open.

[bgadow]

Well, I found one on an old Admiral module. My only concern was the old part was one of the ball shaped ones while the "new" part is the square pc mount type. Wasn't sure if it mattered; the DuMont is full of those square ones but I guess for a hard wired part they don't work so well. Anyway, I put it in there & it worked! Its not 100% but I am almost positive this is because all of the IF shields are off. Tonight I'll button things back up & see how it does. From there I can move on to the convergence board; most of the pots there are frozen. When Veg had "Allen" he tried, I'm pretty sure, spraying them with PB Blaster. I know I have some old convergence boards upstairs so I'll probably just swap the pots.

[Chad Hauris]

Glad to hear it worked!

[veg-o-matic]

Way to go, Bryan! Can't wait to see some pictures. Yeah, I tried the PB Blaster stuff on the convergence board and you can see how much good it did. It's crazy.

Might be picking up a Sylvania this weekend. I'll work on it some but if it turns into another Allen, well, you know where I live. Bring your truck

veg

[bgadow]

Here it is! All back to normal now. I stole the convergence controls from what looks like a CTC-55 board. Some of the values were a little different but close enough. The old ones had major rust, some were really rusted in two! 2 of them were okay, they look like aluminum cases, but all the steel ones were shot. I was able to get decent enough convergence. The degaussing thermistor had been bypassed before it got to me so I went looking in the drawer for a replacement. At the bottom of the pile was a NOS Oneida part which listed various mfg part #'s it replaced. Included was an Emerson # so I checked the sams & sure enough, it was the exact replacement part! The set isn't perfect but is very close to what it must have been in 1970.

[veg-o-matic]

WAY TO GO BRYAN!!!!

I can't believe how good Allen looks now!


(Can I have him back? )

veg