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Old 02-16-2016, 03:13 PM
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Kinight KG-686 Transistors

I have an unrestored Knight KG-686 RF generator which I’d like to put into service. The only problem I have is that it has TRANSISTORS and GERMANIUM DIODES. Having no experience with these critters, I need to know how much can I rely on these 1970’s transistors and diodes?

In addition to the rest of the routine electronic restoration, should I:
Replace all the transistors?
Remove and test each transistor?
Leave them alone and see how the set works?
And the same questions for the germanium diodes.

The set has ten Motorola NPN transistors in three flavors. This is the information which I have gleaned from various sources:

MPS-6513 (marked 0131/744) osc, E.F., 100KHz osc, audio osc.
MPS-6541 (marked 0142/744) modulator
? ? ? ? ? (marked 0134/712) meter amp, 1 Mhz osc, mixer, audio amp.

The first two are described as general purpose amplifiers and I have data sheets for them. For the third one, all that I have determined is that it’s an NPN and is probably also a general purpose amplifier.

Any advice will be welcome.
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Old 02-16-2016, 04:37 PM
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I would try and see if it works as is, if anything it's probably got some weak electrolytic caps and dirty switch contacts. I wouldn't change transistors unless one is known to be bad
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Old 02-16-2016, 05:29 PM
Bill R Bill R is offline
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I wouldn't change any transistors until I did the recap and tested it. The Germanium diodes may be hard to find, but they are probably fine.
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Old 02-16-2016, 09:06 PM
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Thanks much. I'll follow your advice.
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Old 03-20-2016, 12:42 AM
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Stymied Again

I finished refurbishing the KG-686 and hooked it up to a frequency counter. The RF Signal for the AM band was perfect--no alignment needed. BUT, the AM is band "B" on the dial and I had to set the band switch knob at band "E." I don't have the assembly instructions, and it would take me quite a long time figure it out from the schematic.

So I put it away the KG-686 for now and pulled out a Knight (no model) RF Generator. This is an older one--a proper one with vacuum tubes. There's no model name or number anywhere on the inside or outside, but it seems to be identical to the schematic for the KG-650.

My first problem with this one is that the on-off-fine attenuator switch is absolutely frozen. I tried greasy tuner cleaner and penetrating oil without success. It feels like it's a mechanical obstruction rather than corrosion. I thought the problem might be in the on-off part of the switch, but when I took it apart the on-off worked fine and the potentiometer shaft still wouldn't turn.



The schematic says that this is a 1K pot, but between the end terminals I get 30K ohms, and 2.3K ohms from the sweep to either end.

Is this thing repairable? If I try to disassemble it further I may not be able to get it back together again (Humpty Dumpty Syndrome).

Can I replace the switch with a toggle and a 1K pot? I have 1K ohm/500v pot and some 125v/3 amp toggles.
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Old 03-20-2016, 01:33 AM
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Get some PB-Blaster Penetrating oil and spray down the front and back parts of the shaft, and let it sit. If that don't get it loose let it soak in it. And if all that fails replace it.
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Old 03-20-2016, 10:59 AM
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Thanks for advising.
As I mentioned, I've already tried the penetrating oil and I believe that it's a mechanical problem, but don't think I can disassemble further.
The question is, can I replace the 1K attenuator section with a garden variety 1K 500V 1/2W pot?
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Old 03-20-2016, 11:26 AM
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Sure, if you have a 1k pot that will fit give it a try, should work fine.
A switch located on the opposite side of the frequency scale symmetrical to the pilot light would likely look ok and function well. It's not as if this generator is some priceless antique that must be restored to "as new" condition...just needs to be functional.
just my 2 cents,

jr

ps: should be fairly easy to sort out the problems with the other generator, if the coils are just wired to the wrong switch positions, note which bands appear at each position, and rewire the 2 leads from each coil to the proper position.

.

Last edited by jr_tech; 03-20-2016 at 11:44 AM. Reason: add ps
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Old 03-20-2016, 01:39 PM
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The shaft had about 1mm of free rotational movement. Using the big pliers I got the shaft to move vertically (not rotation), then with little effort got it turning again. Applied the meter and confirmed that it was, indeed, 1.2K to 0 ohms. Put it back together and it worked for a few tries, then locked up again. I'll spend some time trying to resolve that. If I can't fix it, I'll go to plan B.
Thanks much.
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