[Central-Electronics] 20A VFO drive requirements

[Jim Candela]

Perhaps try a 40m FT-243 crystal to transmit on 40m. This changes the 6BA7 mixer to a crystal oscillator. It should provide full drive to the 6AG7's. Using an in-band crystal bypasses the balanced modulator, so this is a CW only test. Is the output still low? If so, scope it out. You could also try a crystal offset by 9 Mhz.
JimWd5JKO
 

    On Monday, March 6, 2017 6:09 PM, Kihwal Lee via Central-Electronics <central-electronics at mailman.qth.net> wrote:
 

  blockquote, div.yahoo_quoted { margin-left: 0 !important; border-left:1px #715FFA solid !important; padding-left:1ex !important; background-color:white !important; } I have a similar experience. My 20A is also lower on the output on 40m compared to other bands.  I've tested it using 458 VFO and si570 + a buffer amp. I didn't investigate further. 
Kihwal, K9SUL


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On Monday, March 6, 2017, 3:21 PM, Robert Nickels <ranickel at comcast.net> wrote:

On 3/5/2017 8:31 AM, Jim Candela wrote:
> It would be useful to know how you are driving that VFO input. Do you 
> have a sine wave at 8-10v peak? With the BC-458 CE Mod'd VFO, 40m is a 
> bad band because the VFO has to be tripled to make 7Mhz when mixed 
> with 9 Mhz. The result is often low VFO drive.

Thanks Jim.

For test purposes I'm using an HP 3336B frequency source which is not 
capable of providing the required 8 volts.  However it does allow direct 
input on all mixing frequencies (so no tripling) and a calibrated 
variable output.  Here's an example of the variation from band-to-band 
I'm seeing with the output set to 2.7 volts P-P at the 20A VFO input:

80 meters  VFO freq = 5100 Khz:  5.5 watts

20 meters,  VFO freq = 5100 Khz:  5.0 watts

40 meters,  VFO freq = 16200 Khz:  3 watts

When testing with a VFO capable of 8 volt output I saw this same 
variation so it seems to be proportional to VFO level.  For practical 
purposes 80 and 20 are the same and quite usable but 40 meters output is 
about half for the same VFO drive, more than I figure it should be.    
All trimmers are peaked in the 20A and I'll double-check the traps.

73, Bob W9RAN

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