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Hi All,
A few of us have the old Knight Kit AM broadcaster and I thought they would be interested in this.
Hi All,
A few of us have the old Knight Kit AM broadcaster and I thought they would be interested in this.
Mine is ancient, being built and put on the air around 1959. Other than the usual worn out tubes and a problem with one of the sockets it has served me well for many years. Tonight, I saw it on the shelf and fired it up and it worked as it always has.
Out of curiosity and just for grins I decided to measure the “final amplifier input power” which is actually the power into the oscillator. This was easy since the screen grid is connected to the control grid and they are both self biased near ground so the screen power need not be measured. The plate current is 1.71 mA and the plate voltage is 95.5 volts DC. So the power in is 163 mW. I have read some claims of watts of power from this device but mine certainly falls short of this.
Most here won’t care but I thought those who still have one would be interested.
Neil
Total posts : 45366Hi Neil,
I have two versions of this knight kit transmitter, the one with the enclosed case works perfect, the other one the earlier version, will produce a carrier but is loaded with hum, you cant use it so much hum, no input on audio, Hums all the time tons of it. Just wondering what your thougts are on what might be the problem. I tried new tubes, and jumpered the power supply caps with good caps. I am wondering about the seleium rectifier if thats bad, would those go bad? I have the orginal schamtic, things looke good but I didnt wire it so maybe there is still a problem.. Any case histories you may know of?
Total posts : 45366Joe,
I am pleased that someone who has this unit responded. Hum has always been a trademark of the Knight Kit broadcaster, but not as bad as you described. My unit has a grease pencil mark that I put on the plug to remind me which way to plug it into the outlet for minimum hum. Trouble is, I don’t remember (after 40 some years) whether the mark goes to the neutral or the hot (wide or narrow slot) on the receptacle. Try reversing the plug.
Anyway, last night while doing my experiment there was no hum. Hmmmmm?? It really sounded good. My unit has the original filter cap and the selenium rectifier.
Here are a few suggestions from an old tube guy regarding your hum. If there is a filament to cathode short in one of the tubes this will cause hum. I note you said you tried new tubes but try exchanging the 50C5s and see if the hum changes. If it does, then one of them has a problem. The only way to test the 12AX7 is either on a tube tester or by replacement but if it hums with the volume down then this tube is probably not at fault. Try the one from your good unit and see what happens.
Selenium rectifiers have a relatively large reverse leakage so that can contribute to the hum, yet mine sounded fine last night. You might try taking it out of the circuit and using a 1N4007 as a replacement. Also make sure the fins on the selenium rectifier are not touching the chassis. It is very close on mine and in my youth I put a piece of electrical tape between the fins and the chassis. I also put on on the bottom cover under the rectifier, but your hummy unit doesn’t have a bottom cover.
Instead of just shunting the filter cap, take it out of the circuit and try the new caps. If the old caps are leaky they will cause hum even when shunted since the load R on the supply is too low.
Way back when, I obtained a crystal for 1320 kHz and modified my unit for crystal control. All I had to do was put it in series with C7 in the circuit and tune C9 for maximum output as measured with a receiver with a S meter. That was really great since I always knew where my station was on the dial and it also reduced the heterodyne growl from other stations. That crystal got lost when I loaned the unit to my son but now I have it operating with a 1400 kHz crystal. Just a hint for you if you get inspired to play around some more.
Neil
Total posts : 45366Hi Guys:
Sounds like I’ve got to get a Knight Kit AM Broadcaster
some day.
Neil, you mentioned rigging it for crystal control. I had
a Ramsey AM-1 that I modified for crystal control years
ago. That mod made it work a lot better.
Best regards!
Total posts : 45366Hi Neil,
The early version of the Knight Kit broadcaster, with the very bad hum… some time ago I took all the tubes from the good” late version” transmitter put them in the transmitter with the hum and no luck, still had all the hum, very bad.
I thought about the heater to cathode short possibility but replacing the tubes proved that wasn’t it. So then I checked all the wiring to make sure nothing was touching where it shouldn’t been, things looked good there. I have a 50K per volt ohm VOM and checked B+ that seemed to be on par with my schematic, which I have for the early version with voltages listed. This is rather simple circuit but complex problem. I haven’t worked on it now for about three years time goes fast, but it’s on the workbench and I want to fix it. This isn’t a reverse plug hum; I know where you’re coming from there! The hum is so bad, it’s like if you were modulating 100% hum…I wonder if there is problem with the Mod transformer… I also tried to isolate the preamp section from the rest of the circuit by breaking the audio path, seems the hum is in the oscillator or modulator. It’s been a while and I don’t have the schematic in front of me right now to make sure I know what I’m talking about as far as what I have checked during troubleshooting.
Actually I have seen three versions of this transmitter, there must have been a very early version at some point, and I saw it on eBay few months ago. That was not the Lafayette brand phono oscillator.( I had one of those too.) The one I had transmitted to a radio if you had the antenna from the transmitter wrapped around the radio but at least the audio was nice and clear.
As with most of these treasures I can’t remember what I did with it but should have kept it.
For those who haven’t seen one of the Knight Kit broadcasters, here is mine, this is the late version. ((1958 or so vintage)
http://i425.photobucket.com/albums/pp339/JoeChezk/100_2702.jpg
http://i425.photobucket.com/albums/pp339/JoeChezk/100_2700.jpg
Total posts : 45366Joe,
Nice pictures and thanks for sharing.
The schematic I am going from is from my original manual. It is the same as found on page 23 of this PDF:
http://www.we0h.us/Amateur_Radio_stuff/Knight/Wireless-Broadcaster/Knight_Kit_Radio_BC___Amp.pdf
I think in the earlier model T-1 was a choke rather than a transformer. I don’t know of any other differences or of a third model.
You have eliminated almost all causes of hum. It seems that a replacement of the rectifier and filter cap is in order. Again, don’t just shunt the caps, take them out of the circuit. I don’t know what else could be causing the hum since you have eliminated the tubes as the cause. You said you checked, but it could be miswired.
Micro1700,
They are a lot of fun to play with, especially if you are a tube guy. They are also very rare. If you see one for sale on an auction site, grab it! I have no idea what a fair price is but they originally sold in the late ’50s for about $13. My dad bought mine as a Christmas gift to me and it cost him about 5 hours wages. He was very good to me.
Neil
Total posts : 45366Hi Neil:
Thanks for the info. It sounds like your dad was like
mine. He gave me all kinds of great electronic stuff.
My mother would get mad at him and say there wasn’t
enough money but he would do it anyway!
Thanks to all you guys. I have really enjoyed being on
here.
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