Probably just be overly cautious, but . . . - Videokarma.org TV - Video

bigaudioal · #1 03-25-2014, 09:18 AM

Put my Admiral 21K1 chassis on the bench last night for a few little "nits" that I was waiting on parts for. I had a 8.2 meg resistor to replace and wanted to test the one 3Kohm candohm chassis-mounted resistor. The candohm tested fine, so left that in. 8.2 meg replaced. Found one .01 uf flat molded paper cap (thought it was a mica at first), so replaced that too.

So the set has been totally recapped (except micas) and all resistors replaced that needed it. Then I remembered the two resistors in the HV cage. In the base of the 1B3 socket. So looked at the SAMS and here is what SHOULD be in there:

R89 - 2.7 ohm, 1/2 watt (HV Rectifier Filament)
R90 - 470k ohm, 1 watt (HV Filter)

Popped the socket open and could tell immediately that someone had already been in there at some point. Socket popped open way too easy and the solder joints had all been un-soldered and re-soldered. So the 2.7 ohm resistor tested over 6 ohms. Should replace that. Now for the uncertain part . . .

Instead of a 470k ohm resistor, there was a 150k ohm resistor in there.

Actually, had written on it "0.15 MEG, 1 WATT."

Does anyone know why this would be???? Now, I have the Admiral Tele-bar, which uses the 21L1 chassis. 21L1 uses the exact same SAMS and the TV chassis is almost identical to the 21K1 except for the plugs and circuitry for the combo phono/radio addition. So I brought that chassis out, popped open the 1B3 socket and whadda ya know???? That has the correct resistors in it. 2.7 ohm and 470k ohm. I see no service notes in the SAMS of any kind regarding this 470k ohm resistor indicating that it should be a different value. I also have the original service manual (which comes with a schematic) and that also indicates a 470k ohm resistor.

Any reason anyone can think of that I should NOT remove the 150k ohm resistor and replace it with a 470K one???

Thanks.

bigaudioal · #2 03-25-2014, 10:09 AM

As a side note, the set has been a great performer for the past year. Have put many hours on it. Only issue really is a dark picture. If I try to turn up brightness, picture gets blurry - especially if there is white text on the screen. When I say a dark picture, it is not bad. Can watch in a room with lights on, just not as bright as I would like. Certainly watchable. Example with lights off . . .

https://www.youtube.com/watch?v=fgCbTkjPYIE

init4fun · #3 03-25-2014, 10:24 AM

Hi Al , You mention a dim picture , and a lower than specified value resistor under the HV rectifier . Could this have been some previous service in attempt to address the low brightness , by jacking up the HV a bit ?

bigaudioal · #4 03-25-2014, 10:29 AM

I was kinda thinking that. But not sure. That is why I mentioned the low brightness. Guess I really need to know what that 470K ohm resistor does on the HV filter circuit and how altering to a 150K resistor effects it? I am a good mechanic and still need to work on my theory!!!!

bigaudioal · #5 03-25-2014, 10:54 AM

Below photo is a portion of the schematic with the 470K resistor circled in red. Can right-click on it - save it and then zoom in on it pretty good to see . . .

Actually when I loaded it it lost resolution - so too hard to read, sorry.

init4fun · #6 03-25-2014, 10:59 AM

You were thinking the same thing I was . It's likely that a previous service attempt had been the lowering of that resistor's value as that would raise the HV . Thing is , the lack of brightness would be far more likely to be the 2.5 ohm resistor that is now up over 6 ! If the HV rectifier's filament doesn't receive the power it needs no amount of lowering the resistance on the high side is going to get any more HV out of the tube . I'll bet you see a great improvement when you get an actual 2.5 on the filament and go back to the 470 K on the HV filter .

bigaudioal · #7 03-25-2014, 11:02 AM

Quote:

Originally Posted by init4fun

You were thinking the same thing I was . It's likely that a previous service attempt had been the lowering of that resistor's value as that would raise the HV . Thing is , the lack of brightness would be far more likely to be the 2.5 ohm resistor that is now up over 6 ! If the HV rectifier's filament doesn't receive the power it needs no amount of lowering the resistance on the high side is going to get any more HV out of the tube . I'll bet you see a great improvement when you get an actual 2.5 on the filament and go back to the 470 K on the HV filter .

So you think that once I get that new 2.7 ohm resistor in there I should go back to the 470K? Good deal!!! I am sure others will chime in if they think we are way off base.

init4fun · #8 03-25-2014, 11:06 AM

And yes , Thank You for the schematic , that resister is exactly the one that someone would lower if they were trying to raise the HV . Thing is , that is a mistaken , "Band Aid" type of repair likely because they were unable to locate the actual source of trouble . MANY things can cause a lack of brightness , but the somewhat inexperienced may quickly jump from a plug in brightener not fixing it to "lets just crank up the HV and see iffin THAT gets it brighter" when confronted with a tough repair . I really think the 2.5 being over 6 is the problem they mistakenly overlooked , and the 150 for 470 swap was the knee jerk reaction to that .

And hell yea , go back to the 470 when you get the 2.5 sorted . I really do think that is gonna produce great results

Kevin Kuehn · #9 03-25-2014, 11:21 AM

Somewhere I read that the series 470K is there for current limiting. 1 meg was also a real common value. It's value isn't that critical under normal operation, but without it the CRT can be damaged from excessive beam current if something goes wrong. Agree the 2.7 ohm drifted high on the HV rectifier filament is more likely to be holding back the HV.

bigaudioal · #10 03-25-2014, 11:25 AM

That seems to make sense now and thanks so much for explaining it. So the 1B3 filament is not getting the power it needs b/c that 2.7 ohm resistor is way out of tolerance. Theory is someone thought they could fix that condition by dropping the resistance of the HV filter resistor (470k down to 150k) to increase HV? But that would not work b/c tube still not getting filament juice it needs. Do I have it?

Kevin Kuehn · #11 03-25-2014, 11:29 AM

Quote:

Originally Posted by bigaudioal

Do I have it?

I think so.

bigaudioal · #12 03-25-2014, 11:40 AM

THANKS fellas!!! I will let you all know how it goes. Funny thing is, I was not actively trying to troubleshoot the dim pic. I just figured I wanted to check these last two resistors in the set out of thoroughness. Will be a nice bonus if the picture improves b/c of this. I was happy to watch a slightly dim picture on a 62 year old set.

init4fun · #13 03-25-2014, 11:41 AM

Exactly what I was thinking , Thank you Kevin for confirming my thoughts on this .

And yea , pictures of properly working old TVs are always welcome !

bigaudioal · #14 03-30-2014, 04:37 PM

Well, put in the 2.7 ohm HV filament resistor and the 470k HV filter resistor. Fired up the set and the picture was pretty bad. Out of focus, could not adjust it in, etc. So I switched to a 100k resistor on the HV filter. That yielded buzzing in sound and unstable, staticky picture. Shut down very quickly!!! Put the 150k HV filter resistor back in, which the set seems to like even though all literature calls for a 470k.

Been watching for over an hour now and all seems stable. So I am now calling this set done until something quits. I think it looks good . . .

http://youtu.be/7kg7lyJU8Xs

***Eric H*** · #15 03-30-2014, 05:41 PM

Just curious, have you tried a new 1B3 (or whatever it uses for a HV rectifier)