My new KG-650 RF Signal Generator

I decided that restoring old tube radios was to be my new hobby (Thanks, Nuts & Volts…) and I quickly realized that I could use an RF signal generator. Rather than do the sensible thing and buy something new, modern, tested, and functional, I decided to grab a 1959 Knight KG-650 RF Generator from ebay:

Here’s a picture of the RF generator, after a little bit of restoring:

Knight KG-650 RF Signal Generator

Before it even arrived, I realized that the test leads I have use BNC connectors, whereas the piece of ancient history that I ordered had some kind of funky 1950s RF connector that no longer exists:

Old RF Jack from KG-650 RF Generator

I found out online that I could cut the center pin in a PL259-BNC adapter down to about 1/4″ and that would fit the funky knight connector. So I did that. I actually cut it down a little too far, but some solder deposited on the stub of a center pin managed to make up the difference. The adapter worked.

Adapter from KG-650 RF Jack to BNC jack

However, it turned out to be a really stupid idea. I spent ~ $7 making this ugly adapter when I could have just bought some BNC jacks from digikey (P/N A97548-ND) and replaced the jacks in the signal generator with modern BNC jacks. I did that, at $1.74 per jack. They fit perfectly, and now I can attach modern test leads to the old RF generator without any issue.

BNC Jack installed in KG-650

While I had it torn apart, I replaced the filter capacitor, because old filter capacitors are prone to failure.

I also ditched the old non-polarized AC cable and replaced it with a receptacle for a modern AC cable, the type typically used by computers an test equipment. I like my test equipment to use standard power cables, and I like the safety of things being grounded, especially things that might have a hundred and twenty volts wandering around in them. It took some case surgery to make this happen:

KG-650 AC receptacle, outside view

AC Receptacle, inside view

This modification is a bit controversial, as it now means the KG-650’s BNC jacks are grounded. Some radios are hot chassis and non-polarized, which means one could encounter a radio with 120 VAC on the chassis. It would be really bad to connect a test lead ground to that. The answer is to use an isolation transformer on the radio under test — you really ought to be doing that anyway. However, it’s conceivable that one day a guy could forget, and turn some perfectly good test leads into a short-lived welder.

Next it was time to take a look at the output of this wonderful RF signal generator. Here are some screen shots using my Rigol DS-1052E. The yellow trace is connected to pin 3 of the 12AT7 tube (i.e. upstream of the RF attenuator) and the blue trace is connected to the pin 6 of the 12AT7 tube. The yellow trace approximates what you’ll see on the output of the instrument.

Band A, 200Khz, yellow -1.0V to 1.7V, blue 5.4V to 15V

Band B, 600 Khz, yellow -1.5V to 3.8V, blue 5.2V to 17.4V

Band C, 2 Mhz, yellow -0.8V to 2.3V, blue 8.4V to 14.5V

Band D, 7 Mhz, yellow -0.38V to 0.53V, blue 1.9V to 3.3V

Band D, 19 Mhz, yellow -0.22V to 0.36V, blue 3.9V to 4.9V

Holy Crap! What’s going on here? Those first two plots at Bands A and B are atrocious. The output vaguely resembles a sine wave, but it all truncated and crappy. At lease band C and D look a little better. NSE Labs at reports similar problems with the KG-650, and resolved the issue by inserting a 2M pot between C8 and pin 6 of the 12AT7. The idea is that mixer stage of the 12AT7 is being over-driven. I have a 2M pot on order and am going to give this a shot. I expect similar results on Bands A and B, but I do wonder what the effect will be on bands C and D, whether the signal will be too low to be usable at those frequencies.

Band E could not be resolved with the ‘scope.  Maybe something is broken.

Comments (4)

  1. Rich Ferraro says:

    Hi Scott,
    I read with interest your mods to the kg-650, as I have recently purchased one also. I know very little about hollow state repair and nothing about solid state, and am just a hobbyist newbie at this point. I am still collecting my diagnostic equipment at this stage. I can’t find a manual for the 650, not even BAMA. They are out of print and should be public domain, but people are selling them for about $25. I was able to get the schematic from BAMA. Don’t know if you could help me with a few questions? 1)Is the filter cap the 470 MMFD? 2)When you ‘scoped the 650 via the 3 pin (12AX7, yellow trace) and 6 pin (12AT7, blue trace) would this have been done using 2 separate channels? I have a Tekronix 2246A scope but haven’t used it yet, so I’m trying to figure how to do the same test you did. And finally 3)what is a 2M pot? I’m curious if that helped with the 12AX7 being over driven, and perhaps a little more detail of the installation.

    Thanks so much for your posting this information!

    Rich Ferraro

  2. admin says:

    Hey Rich,

    The filter capacitor is a pair of 20uF capacitors with a 2.7K resistor between them. It’s marked C3 on the schematic. While these are technically two separate capacitors, old radios will often locate them in the same metal can. It’s been a while since I tore into my KG-650, but I think this was the case. Yes, I did use two channels on my scope (I have a rigol DS 1052 digital scope). “pot” stands for potentiometer, and 2M means 2 megohm (2,000,000 ohms). I haven’t modified mine with the pot, though I do plan on trying it out. Basically the idea came from the NSElabs blog post that I linked in my blog post. It seems like a good idea, though while I think it’ll correct the overdrive, it might lead to an underdrive situation in other bands.

    Unfortunately, these old tube-based RF generators can be pretty mediocre when compared to a modern solid state design. I’m currently building a solid state RF generator described in the June 2014 issue of nuts and volts, and so far it seems a lot cleaner and more versatile than the KG-650.

  3. Michael Baseman says:

    Do you have a copy of the KG-650 manual that you could email me?

  4. admin says:

    Hey Michael, I don’t know if I have a link to the manual, but I do have one for the schematic:

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