Total posts : 45366
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)