[heathkit-yahoo] Re: DX-60 oscillator circuit

[Jim F.]

Brian, Ken,
   
  Current wisdom is that mica capacitors seldom fail
but watch out for mis-marked colored dots.
   
  A nice looking Heathkit AT-1 transmitter, with the usual 
extra faceplate hole, was purchased recently at a hamfest.
  
Every crystal tried in it had a low grid current 
and overheated after a few minutes. Some would not oscillate
at all. A favorite 80 meter crystal from my Novice days 
overheated and "popped", never to be excited again.
  
The 6AG7 oscillator tube was changed along with some of the wiring. 
Every part was examined, probed and poked, all voltages measured, 
RF chokes changed, all to no avail. 
  
Frustrated, I decided to vary the ratio of the two little mica capacitors 
(the ones that never fail) located in the oscillator grid circuit with little 
colored dots to indicate value.
 
A 15pF mica was unsoldered and a 20pf tacked into it's place. 
  
 This brought the oscillator grid current up nicely with no crystal overheating.  
The problem was somehow solved.
   
  The removed 15pF capacitor with the correctly colored dots 
actually measured 458pF and been this way from day one. 
  
No wonder the little transmitter showed very little use.
(Darn that extra hole in the faceplate !)
  
A good lesson here....
   
  Jim Fitton - W1FMR

Brian Carling <bcarling at cfl.rr.com> wrote:
  Thnaks Ken - I will try that. I think part of the problem was that I 
had the "DRIVE TUNE" control mis-adjusted.
When I went back and retuned that control using the GRID 
CURRENT of the final to peak it, then the keying improved 
considerably.
I just don't like an oscillator to be slow to kick in.

I measure the screen grid resistor as almost 18,000 ohms.
It should be 15,000 ohms. I will need to the go get an RFC to 
measure screen voltage. 

I have just removed the erroneous "33 ohm resistor" (actually 37 
ohms) that was in the place of the 33,000 ohm resistor at R26
and realized I had mis-read the 100 uuFD as 100 uF and thought it 
missing! That was just a 33 pF disk and it IS there.


On 30 Dec 2005 at 20:23, Kenneth G. Gordon wrote:

> I just took a look at the schematic for the DX-60s oscillator and buffer 
> stages.
> 
> C-2, a 100 pfd capacitor, is connected between the oscillator tube and 
> ground...or at least it SHOULD be. You might want to mess with the 
> value of that cap a bit. This capacitor adjusts feedback to the oscillator.
> 
> However, FIRST I would measure the voltage at the screen of the 
> oscillator tube at key down. Connect an RF choke between the screen 
> connection and the DVM probe to keep RF from getting into the DVM 
> and causing erroneous readings.
> 
> If the voltage isn't at least 140 VDC or so, check the screen dropping 
> resistor. It is probably high in that case.
> 
> Ken W7EKB
> 




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