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 Post subject: Horizontal AFC Diodes- Philco Predicta Princess
PostPosted: Mar Tue 31, 2020 1:23 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 728
Location: Indian Land, SC, USA, 29707
I am working on a "plain jane" Philco 21" "box" shaped table model from 1960 that more or less has the same chassis as the Predicta Princess, with the power transformer and same main circuit board, or very close. The horizontal sync is a bit weak, causing the horizontal hold control to be "touchy" to get the picture to sync, and as a result, just a very slight, what seems to be normal drift from warming up, will cause the picture to start rolling sideways. The vertical sync is strong, so I doubt the syn sep tube or circuit. The entire set was recapped, and the couplates (networks) rebuilt. I have heard that the AFC diodes might be notorious for going bad. There is a plug-in module with 2 diodes in it, with a notch in one corner that indicates orientation. What kind of diodes are best to use? Would standard germanium detector diodes like 1N64 be OK to use in this circuit, or are there too high of voltage spikes that could damage them? Let me know what types I should try, and if you think this could be causing this problem.

Dave


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 Post subject: Re: Horizontal AFC Diodes- Philco Predicta Princess
PostPosted: Mar Tue 31, 2020 2:22 am 
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Joined: Mar Sun 01, 2009 10:27 pm
Posts: 5282
Location: Ann Arbor, MI
Most any small signal silicon diode such as the 1N914 or 1N4148 will work. Those are the small red diodes. I Think that Radio Shack used to sell them as something like 100 for $5. These are rated to take 100 volts reverse .

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Last edited by Tom Schulz on Mar Tue 31, 2020 2:32 am, edited 1 time in total.

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 Post subject: Re: Horizontal AFC Diodes- Philco Predicta Princess
PostPosted: Mar Tue 31, 2020 2:28 am 
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Location: Detroit, MI USA
Those little silicon diodes are what was used in the next generation of TV's.

The numbers Tom mentioned should be a great improvement over the original selenium dual diode.

I wouldn't use 1N400x types although people have reported they work.

You can verify the original part is bad by comparing front/back ratio between the two halves. They were a very high failure part and some sets even had a socket for the dual diode to plug in so it could quickly and easily be replaced in the field.

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Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: Horizontal AFC Diodes- Philco Predicta Princess
PostPosted: Mar Tue 31, 2020 3:27 am 
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Location: Belrose, NSW, Australia
Yes 1N4148s work fine in hor AFC circuits.

Actually in pretty much any signal diode position in a TV and that includes video and ratio detector circuits. They are fast enough as well as being tougher.

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 Post subject: Re: Horizontal AFC Diodes- Philco Predicta Princess
PostPosted: Mar Tue 31, 2020 9:49 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 728
Location: Indian Land, SC, USA, 29707
Hi guys, thank you for the responses, I think I will order some 1N4148s for this and perhaps several other future projects including an actual Princess set I hope to get to eventually. That's right, I forgot that the original plug-in part was actually selenium, not germanium.

Dave


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 Post subject: Re: Horizontal AFC Diodes- Philco Predicta Princess
PostPosted: Apr Wed 01, 2020 10:11 am 
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Joined: Jan Thu 16, 2020 12:29 am
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DaveD wrote:
Hi guys, thank you for the responses, I think I will order some 1N4148s for this and perhaps several other future projects including an actual Princess set I hope to get to eventually. That's right, I forgot that the original plug-in part was actually selenium, not germanium.

Dave


I am inclined to agree with the other responders about using the 1N4148/1N914 diode, as a replacement .

However, to better replicate the properties of the original Selenium part, the AA112 gold bonded germanium diode would be about as close as you could get (look up the specs). It is probably unlikely the benefits of the lower fwd voltage drop selenium, or the AA112 diodes would be that helpful, in this circuit configuration, it would depend on the exact circuit design.

The other time you come across small selenium signal rectifiers is in vintage analog multi-meters used on the AC ranges. When replacing these, it is more critical that a very low fwd voltage drop diode is used, but most folks know this, so its all a bit old hat.


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 Post subject: Re: Horizontal AFC Diodes- Philco Predicta Princess
PostPosted: Apr Sat 11, 2020 2:47 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 728
Location: Indian Land, SC, USA, 29707
Hey I installed two 1N4148s in place of the plug-in original selenium module, and what a world of difference, the horizontal sync is now rock solid, and I can turn the horizontal hold control all the way up and down and it never goes out of sync! Now to just iron out the other minor glitches in the set (mostly just need to rotate the yoke so that the picture is not tilted), and it will be ready to go! I checked the plug in double diode, and it was reading about 140 ohms in both directions for one diode, and about 89 ohms in both directions for the other diode, so I guess you could say for all practical purposes, they were both shorted! Dave


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