[AMRadio] GK500 OTA almost


Collin n4tua at aol.com
Wed Jan 4 21:45:23 EST 2017


OK, Jim, I think I am posting with reply all. Here goes. 

I had a few minutes to work on the GK500B tonight and checked the schematic again around the buffer amp plate circuit. One capacitor is not connected as it is shown on the schematic. Incidentally, the same capacitor on the crystal oscillator is misconnected the same way. Although, I don't think it makes any difference because that capacitor blocks DC from going through the coil in either position. I have the luxury of having a junker to compare with also and it is wire the same way as my keeper, different from what the schematic shows. 

That is not the only problem I have found with this GK500B documentation. There are two resistors with different values that what the parts list shows. Both of these check out on my junker example also.

So, I did find what might be a poor ground on the bypass capacitor at the DC input to the plate choke. I have cleaned it up and reassembled everything. 

Check these,
 buffer amp parasitic choke and blocking capacitor
final amp DC input bypass capacitors (2) and plate choke
All look and test good. 

Tomorrow evening I will fire it up again and note any changes in results. I hate to do to many things and fix the problem and then not know what it actually was that caused the problem. 

Best 73rds, Collin



> On Jan 3, 2017, at 9:21 PM, Collin <n4tua at aol.com> wrote:
> 
> Now wait a minute. I may be sleepy but I think that there is a wiring problem in the buffer amp plate circuit. It is not wired like the schematic. 
> 
> Anyway, I am out of time right now. Will be back in front of it tomorrow evening. More to come. You guys are giving me all sorts of things to look at and I am so slow. I may be on to something in this buffer amp plate circuit though. I took the parasitic choke out and there was a capacitor attached with it, they both checked good but while looking at the schematic verses what was hooked up something looks wrong. 
> 
> Let me tell you about this jewel. I bought it for a song but it was in sad shape when I got it. It had been sitting in a barn and then in a carport and had been wet and did I say rusty? Really rusty. I tore it down and sandblasted the chassis and painted them. Then cleaned and checked this as I reassembled. Painted transformers back to black. It looks nice. I have not done the front panels yet. I want RF out if this rig first. Anyway, you might be able to tell, at this point that I am in a personal relationship with this mess. Have been for years. Told you I was slow. 
> 
> So for now goodnight, see you at 0415. 
> 
> 
> Good Day, Collin
> 
> "You can live to be a hundred if you give up all the things that make you want to live to be a hundred."  Woody Allen
> 
> 
> 
>> On Jan 3, 2017, at 4:46 PM, Jim Candela <jcandela at prodigy.net> wrote:
>> 
>> Collin,
>> 
>>    One thing I remember on my GK-500B was the single screen bypass capacitor for the 4-400 tube. It was something like a 0.002 uf Mica-mold condenser. I replaced mine with four 500PF HV caps, two on each G2 pin, and then going to each grounding to a socket hold down screw using a solder type terminal lug. I also bypassed the filament leads in a similar way, but the exact details escape me now. I do recall that those changes made the King load up closer to ideal....max out and MIN plate current dip, and when the screen current was at a peak...In fact I liked to use the loading control to get no more than ~ 40ma screen current. 
>> 
>>   The GK 500 has a high impedance Screen supply, therefore the screen voltage rises as the screen current drops. It might be something like 600V at 20ma screen, and 400v at 100ma screen. The screen current on a tetrode drops as the tube is heavily loaded, and therefore the screen voltage rises. Perhaps your in a situation where loading to > 300ma plate current causes the screen voltage to go too high, and then the power sensitivity will be so high that a parasitic is more likely?? The screen bypassing, and a whole host of issues might be at play here, but the point however is that low screen current from a heavily loaded plate current might make parasitic oscillation more possible. It just depends how much regeneration you got going on..You don't want to solid state that screen supply.
>> 
>> Jim
>> Wd5JKO
>> 
>> 
>> On Tuesday, January 3, 2017 2:39 PM, Collin <n4tua at aol.com> wrote:
>> 
>> 
>> I will check out the buffer amp parasitic choke when I get home. The RF plate choke looks good and I see no burn or scorched areas. I will pull the HV bypass capacitors and check them. 
>> Good Day, Collin
>> 
>> "You can live to be a hundred if you give up all the things that make you want to live to be a hundred."  Woody Allen
>> 
>> 
>> 
>>> On Jan 3, 2017, at 12:23 PM, Jim Candela <jcandela at prodigy.net> wrote:
>>> 
>>> Jim
>>> W5JO
>> 
>> 
> 


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