Motorola 14P3 restoration

[snelson903]

Originally Posted by bandersen

I finished a quick recap and tried to power up the set - nothing. I traced through the filament string and eventually found a tube that wasn't seated fully.


Bingo! I have a nice bright raster Now to try feeding in some signals.

you work on lot of these set's ,have you ever replaced the selenium. rec before with any modern diodes before , and if you did how did turn out.

[Phil Nelson]

I replace selenium rectifiers with type 1N4007 diodes, which are more than adequate and work fine. They cost a few cents. Take the old rectifier completely out of the circuit (don't bridge the diode across it).

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[zenithfan1]

Yep, It's that easy. I leave it in there for looks but totally out of circuit.

[snelson903]

Originally Posted by Phil Nelson

I replace selenium rectifiers with type 1N4007 diodes, which are more than adequate and work fine. They cost a few cents. Take the old rectifier completely out of the circuit (don't bridge the diode across it).

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

oh no i never leave any old comp. behind if it bad it goes, is there in some application a resistance issue ,like there was more of a voltage lost across the old rect. than in the modern diodes so that you would have to add a resister so the output would match the old rectifier. i see that 1n4007 has 50a surge but opperates at 1amp is that all that circuit draws is under a amp of current.

[bandersen]

Originally Posted by snelson903

you work on lot of these set's ,have you ever replaced the selenium. rec before with any modern diodes before , and if you did how did turn out.

Yes, I always replace the selenium rectifiers with 1N4007 didoes. I then check the B+ and add a dropping resistor if it's too high.

I also like to replace the series filament dropping resistor with a diode or capacitor when possible.

[snelson903]

Originally Posted by bandersen

Yes, I always replace the selenium rectifiers with 1N4007 didoes. I then check the B+ and add a dropping resistor if it's too high.

I also like to replace the series filament dropping resistor with a diode or capacitor when possible.

i was wondering about that being half wave rec. with the resistance the early ones have ,the full wave ones are touchy when they get older and if there mountings loosen or become rusty they will cook them selves slowly building resistance internaly ,i build large 100amp or more bulk charger's when you use modern rectifiers it makes the older transformers so much better ,i have a picture of the last selenium rect i replaced, or removed to build my own type of bulk charger, all i really wanted was the step transformer out of the old unit. this is a 100amp prob. more but i cant id it i just wanted the trans. probley changing that rect. in that tv to a modern diode makes it alot better less distortion in the d.c. doesnt it .thanks

[bandersen]

Wow, that's huge!

I've been experimenting with replacing the selenium rectifier. Through trial and error I determined that using a 1N4007 silicon diode plus a 12 ohm, 5W resistor produces the desired +125VDC with a 120 VAC AC line voltage.

That got me thinking that if I could bump the existing fusistor up from 3 to 15 ohms, it would make a cleaner modification.

Well, I didn't have any on hand, but I do have some modern 4.7 ohm fusible resistors so I combined that with a regular power resistor.
While I was at it, I figured why why not add the diode too ?
It works fine, but the diode is getting pretty hot so I think it might be best to relocate it away from the resistors.

[snelson903]

how many watt is the small resister ,in the picture it looks like a 1/2 watt one, if it is i wonder if that is contributing to the diode heating up . how much is the current draw is on that circuit ,i think the 330v supply on my zenith iv been rebuilding is about 450ma you might just need to up the small resister to 1 watt and see if the diode stays cooler . does the diode heat up fast or does it take alittle time. its looks like a neat idea does it plug into a base socket that easy to get at, if it does you could put everthing on a small breadboard and have the two prongs sticking out the end and it could be a fusible filter setup that can kwick change out.

[bandersen]

The smaller one is a 2W fusible resistor. It's made out of ceramic and is designed to take the heat. After a while though, I discovered that the solder was melting!
Probably should use a high temp solder like silver solder.

It's drawing about 400ma so I figure that little 4.7 ohm resistor is putting out less than 1 W.

I decided to just use one of those old 7.5 ohm globar fuse resistors and locate the diode near where the older selenium rectifier had been.

[snelson903]

is normal for that to run that hot that it would start to melt the solder ,i think it would burn out before that you know ,i like how they named that resister [ glo-bar ] i have seen them after they have been hot and darken the outside in older stuff , i hope they dont glo before they blow l o l

[bandersen]

I'm curious what others have used to replace fusistors with. Power resistor + fuse ?

[bandersen]

I performed an alignment and tweaked the linearity. The response curve was lopsided when I started and it took a while to get symmetrical with a bandwidth of about 3.5 MHz. There's a bit of pincushion, but otherwise it's not bad.

[snelson903]

i dont think you can make that anymore perfect than it is,they probley wasn't that good new ,nice sharp picture, good work .

[bandersen]

Thanks!
I just found a great photo of this set in excellent condition: http://farm8.staticflickr.com/7014/6...aca97d8d_b.jpg
Time to go knob hunting

[Komet]

WOW!! Really a nice work!!!