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  #31  
Old 02-15-2019, 10:34 PM
Jon1967us Jon1967us is offline
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I have decided to tackle the board and try to disengage it from everything that's connected to it, so I can not only get at the diode, but take the opportunity to replace any remaining wonky components and check the traces, etc.

I don't have a video signal generator, so I'm going to have to rely on a conventional video source into the antenna input for now, but will try feeding into circuit in an alternate way, if I need to get to that point.


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  #32  
Old 02-16-2019, 09:30 AM
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Electronic M Electronic M is offline
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It looks like if you can get that bolt and soldered on under board shield off you can work on the board without removing the board.

Be aware if you plan to power it up before fully reassembling it that you will need to jumper any desoldered board ground to chassis. PCBs of this period usually did not have a contiguous ground plane, but instead had a few separate ground traces that each went to one of the board solder mounts...The board solder mount grounds effectively tied together thru the chassis. If any solder mount is disconnected or has a bad joint the portion of the board it grounds will float and the circuits that use those ground points won't work (until reconnected to chassis ground).
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  #33  
Old 02-16-2019, 03:25 PM
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A few notes.
How do you service it ? Almost impossible ! My boss couldnt say
no to anything. There were still some floating around in the early 70's
and he refused to work on them. I dont know how RCA & others
built sets like this. Manuals said to cut out or crush then they had little
spring like things to make the connection.

Video detector diodes were troublesome up till the mid 60's. Often gave
odd symptoms so if any question just change the damn thing. When
built into a can often the can had a 2 piece shield & the top pulled off
so look for that.

BTW tube testers are almost a bigger liar than a politician. A known good tube
is the only way to be sure. We had a TOTL B&K & never used it. It was
for the customers & the biggest obscene profit maker in the shop.

73 Zeno
LFOD !
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  #34  
Old 02-16-2019, 06:03 PM
Jon1967us Jon1967us is offline
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I was finally able to work free the can hiding the detector diode. I desoldered one leg of the diode and tested it. It tested "normally" with a drop of .275 and no reverse current. I may replace it anyway, as there may be other problems with it undetectable by my multimeter and after 60 years of use.





Another idea that ham friend, who is also very experienced in TV repair suggested, was to inject a 1Khz signal (using a .1 uF) along the signal path and see where there's a hard stop. The 1Khz signal should display black bars (according to him) on the screen. Injecting at a point where black bars don't display on the CRT is supposed to indicate the location of failed component.

I may see if I can get my hands on a tested/NOS 8AW8A to swap out.

Roger on the separated grounds on the board and making sure to clip those to chassis prior to testing. Great call.
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  #35  
Old 02-17-2019, 01:33 PM
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The 1KHz signal may not work that well, unless you traced the signal with a scope. There would be no sync, so seeing bars on the screen would be very unlikely. You don't know how far off your sweep frequencies are free running.
That brings up a point, if you had no sync and the sweep frequencies were off, then a video signal getting to the CRT could have the image jumbled so much it would not be recognizable.
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  #36  
Old 02-17-2019, 02:02 PM
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Notimetolooz Notimetolooz is offline
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Quote:
Originally Posted by Electronic M View Post

One thing you can do if you need an adapter fast and have a spare socket of the right type and a dud tube that fits it, is make your own adapter...Carefully bust all glass but the base, cut all leads inside the dead tube as far from the base as feasible and solder wires (3" is probably a good length) from them to the socket keeping the lead order correct.
A lazier way if you have several good spare tubes of the right type that you don't mind killing is to solder thin wires to the top of the pins for test points...If you heat too long the glass will crack and the tube will die of vacuum loss.
Have you had good luck doing this? I tried soldering to the leads inside a miniature tube and the solder didn't take. I believe the leads are nickle alloy or something like that which lead-tin solder doesn't work with. Maybe with a special solder, flux and a torch.
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  #37  
Old 02-17-2019, 02:12 PM
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Quote:
Originally Posted by Jon1967us View Post
Another idea that ham friend, who is also very experienced in TV repair suggested, was to inject a 1Khz signal (using a .1 uF) along the signal path and see where there's a hard stop. The 1Khz signal should display black bars (according to him) on the screen. Injecting at a point where black bars don't display on the CRT is supposed to indicate the location of failed component.
Works best with a square wave or pulses which are rich with harmonics... read here about the “mosquito” signal injector, a very useful device.

https://stevenjohnson.com/donbosco.htm

jr
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  #38  
Old 02-17-2019, 04:47 PM
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Quote:
Originally Posted by Notimetolooz View Post
Have you had good luck doing this? I tried soldering to the leads inside a miniature tube and the solder didn't take. I believe the leads are nickle alloy or something like that which lead-tin solder doesn't work with. Maybe with a special solder, flux and a torch.
I have had luck with this using normal 40/60 lead solder...Last year I pulled the 21AXP22 out of a CTC-5 (to stick in a rarer Moto that needed one) and replaced it with a glass tube...The change in CRT blocked the installation of the front/center tubes on the chassis. The easiest solution was to make an adapter with a 90-degree bend to lay the tube down on it's side on the chassis...I used the exact method quoted only bending the wires 90 degrees. It has worked fine for me so far.

Quote:
Originally Posted by Notimetolooz View Post
The 1KHz signal may not work that well, unless you traced the signal with a scope. There would be no sync, so seeing bars on the screen would be very unlikely. You don't know how far off your sweep frequencies are free running.
That brings up a point, if you had no sync and the sweep frequencies were off, then a video signal getting to the CRT could have the image jumbled so much it would not be recognizable.
Years ago (on my first resto) when I did not own a test pattern generator, but had an HP generator that covered 1Hz-10MHz I tried using that to inject into the video stages. I found that as I changed the frequency the number of bars changed from and they would vary between horizontal and vertical...If the amplitude is high enough and the video stages work you can get very contrasty patterns easily distinguishable from snow or blank raster...Also, sync is of little consequence unless your generator is fixed frequency. The sweep free runs fairly stable in most sets without sync so as long as you tune your gen to SOME multiple of one or both free run sweep frequencies you will get bars. If you don't tune it you get fingerprint-like patterns.
At some point in the ~10 years since I did that, I repeated that experiment on a different set (think I didn't want to dig the right generator out cause I'm lazy) and got the same results.
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