[Winky Dink]

I tried replacing the resistor with a choke The smallest thing I had was the original audio output transformer from this set. I had replaced the original speaker/transformer assembly with one from a model 46-250. That didn't change the hum, but I may be on the wrong track. I need to learn something about hums. The only time I was successful in subduing a hum was a Philco Model 87, which had a hum adjuster on the upper chassis.

This hum starts almost the instant the power is on--no lag for warm-up--and it's unaffected by the volume control.

Regarding the K3KU mystery, I assumed that in code it would show some symmetry or pattern. Maybe it has some meaning backwards. I'll send it over to the NSA Cryptography Institute.

- Henry --I won't tell you how old I am, but we are three brothers, less than 3 years apart, and I'm the baby. Art was the smart one.

[Winky Dink]

Starting from zero knowledge I started working on a 1950 Motorola TV with a Sams Photofact folder.  That's how I learned to read a schematic, so I'm comfortable with it.  Rider schematics are just as good for me, but I can't make heads or tails of a modern schematic.

I tried boosting the first cap by paralleling a 10uF cap, but that didn't help.  As I said above, I really don't know what might cause this particular hum.  If you have 25 seconds to spare, you can listen to the hum here:

https://picasaweb.google.com/lh/photo/8a...directlink

[Radioroslyn]

 I would try a couple of simple things. Unplug the power cord and plug it in the other way round. Sometimes the phasing on the AC line going into the set can make a bit of difference. If the have replacement tubes try swapping those. May have some leakage from heater to cathode or cathode the grid which will cause hum. The rectifier and output tubes are suspect here as they have much larger heater than the others. Connect a earth ground wire to the chassis THOUGH A .05 @600v. Be careful there is the possibility(50/50 depending on which way the plug is inserted in to the outlet) of having 120vac across that cap.
Bottom line is that ac/dc sets are at the bottom of the line when they where designed. Most of them have small spkrs and small coupling cap in the audio stages to attenuate the low frequency response (read 60cy here). This is done to minimize the reproduction of hum. Chassis layout can be a cause if the 1st audio amp tube is near the rectifier  it can pickup hum. Wire layout if there are ac wires (read line cord or heater wiring) near the 1st audio stage. Having the chassis floating above earth ground(using AC neutral as ground) can be an issue.
So bottom line is if you have a little bit of hum with the volume control turned all the way down which is easily over come by the audio signal if the volume is turned up a bit I would lose any sleep over it. It's pretty much a design/profit problem.

Terry
Did you get your Motorola tv up and running?? I've got a few 1948/49 Motorolas the 7" jobs.

[Winky Dink]

I modified the vintage plug to make it polar, but I figured out a way to reverse it anyway (didn't help). I will look at all the other issues.

Regarding the earth ground: Of course I don't have metal plumbing and the house's main ground is too far away to practically access. This year I put a single 8-foot copper pipe into the ground under my window and have 14 gauge stranded wire running to the work bench. This earth ground was to use for these testing situations--not for any long-term use. However, I was warned by experts ("I worked for the electric company...", "That doesn't meet code...") that I would die and burn my house down. I'm gonna use it anyway.

I won't lose sleep over it. I'm just a bit obsessive about making everything as good as I can.

[Winky Dink]

I did the alignment on the set, but had some trouble with the one of the tuner trimmers.  As a result, the stations at the higher end of the band are received well, but not at the right place, and I think the highest frequencies are missing altogether.  Here are the trimmers before and after cleaning, and the condition of the mica on the suspect trimmer.


[Image: https://lh3.googleusercontent.com/-C2Uot...20comp.jpg]

I was wondering if I could use the mica from a model 46-250 Transitone.  It’s not the right shape, but it fits and covers all of the area of the round dielectric.  The screw from the older model also fits, and it would be much easier to do the adjustments because it has an actual slot.  
 
[Image: https://lh3.googleusercontent.com/-bPIWD...252520.JPG]

What do you think; is it worth trying the older dielectric and screw?

[Radioroslyn]

Sorry have been away for a bit.
The only thing that with it that matters much is the thickness. It's just there to stop the two metal strips from shorting. The closer the strips are together the higher the capacity. This means more you tighten the adjustment screw  it will lower the frequency (more C)
If it where missing or broken and the metal strips shorted then it would make the antenna or osc coil act like it was shorted.

Terry

[Winky Dink]

Intuitively, I had mistakenly thought that tightening would reduce the capacitance, but then I got down to determining the range of the tuner capacitors and trimmers.  Both trimmers were 0-25 pF.  The troublesome trimmer wasn’t defective, but it acted funny because it was fully tightened (max capacitance) at the conclusion of alignment.  Then I realized that the mysterious 7.5 pF cap under the chassis (not in the schematic) was parallel to the trimmer--to jack up the tuner capacitance a tad.  I replaced the 7.5pF cap with 20 pF, which put the total capacitance back into an adjustable range, and I got a good alignment.


So, everything is good EXCEPT that there’s an unacceptably loud buzz.  I have tried all previous suggestions, although this is different from the original hum.  I switched out all the tubes except the 14B6.  I tried moving the radio to a different room.  And of course I’ve been over the circuitry numerous times looking for errors, shorts, bad connections, etc.  
 Here’s a 15-second listen:

https://picasaweb.google.com/lh/photo/Uf...directlink

The only variation from the schematic are that the electrolytics are 33-22-22 uF instead of 30-25-20.  The buzz starts instantly when the set is turned on, and once the tubes are warmed up it increases with the volume.  The only thing that reduces the noise is taking off my hearing aids.

I’m hoping that an experienced person can give me some direction for determining the origin of the noise.  

[Winky Dink]

Hum/buzz resolved. Finally got down to locating the source. Pulling tubes didn't help. I was advised that if C200 was disconnected and the hum persisted, that problem would be in that section. That localized it to the audio section. By luck, the first thing I did was disconnect C203. That stopped the hum completely.

After many unsuccessful trials with more and less capacitance, I finally realized what the problem was. Originally the 120VAC neutral went through the on/off switch; then to B-. C203 connected to B- at the switch. When I wired the hot side through the switch, I didn't change the C203 connection. C203 was connected to the rectifier filament/plate/dial light.