Central Electronics BC-458 VFO low drive

[kn4hh]

I have a Central Electronics 20A and a BC-458 VFO.  The VFO only has enough drive to get 6 watts of RF out of the 20A.  If I use a 5 mhz crystal, I get 18 watts output.  I have replaced the 1625 and 1626 tubes and adjusted the BC-458 coupling capacitor and can not increase the drive enough for full output on the 20A.
I would appreciate any suggestions.

Thanks
Bob, KN4HH

[WQ9E]

Bob,

Have you tried a different 6BA7 mixer tube in your CE-20?  If this tube is marginal, conversion gain will be too low even if the BC-458 has sufficient output.  A tube tester is USELESS for testing tube performance at RF so don't rely upon one, try a different tube.

And be careful when testing aligning your CE-20A because some 6AG7 tubes don't like operating with a horizontal orientation leading to sagging elements and internal shorts.  I restored a CE-20A many years ago and it taught me to ALWAYS check the fuse as part of initial checkout.  I was aligning my 20A with it on its side when I noticed the rectifier plates were starting to blush. One of the 6AG7 tubes developed a short with the 20A on its side and the prior owner had replaced the fuse with a 20 amp part.  I was very lucky not to lose the power transformer!

Rodger WQ9E

[kn4hh]

Thank you, Rodger, for your comments.  I have ordered a set of tubes for the 20A minus the 6AG7's since they are putting out full power when I use a 5mhz crystal. I am aware of the finicky 6AG7's.  I always keep the chassis vertical above and below when doing troubleshooting.  Speaking of troubleshooting, i am getting poor audio reports when using a D-104 microphone.  I have tried three different D-104 elements and get the same report.  Distortion and no lows.  I have followed the 20A manual as far as setting the proper carrier/audio levels for best quality.  Still poor audio.

I will report back when I get the tubes installed.
Thanks again,
Bob, KN4HH

[kn4hh]

Rodger, My tubes came in today.  I replaced all but the 6AG7's.  The 20A output increased to 17 watts with the BC-458 on 75 meters.  However, The most output I can get on 40 is 7 watts after re-peaked the BC-458 tuning cap.  I also tried both 20 and 15 meters and get less than a watt output. I have never attempted a re-alignment of the 20A.  Would there be any benefit to attempting a re-alignment.
Thanks for any comments.

Bob, KN4HH

[WQ9E]

Bob,

Definitely go through the 20A alignment procedure, after tube replacements and just years of aging it is due.

If you go through the sideband and carrier suppression section of the alignment, note that using a scope as suggested would be a last choice.  I use a spectrum analyzer plug-in for my Tektronix 7000 series scopes for adjusting phasing type transmitters which makes the process very fast and simple, second choice for me would be a modern highly selective selectable sideband receiver.

Even using a good quality scope, several different issues along with alignment faults will cause a very similar pattern.  I can't imagine the frustration of the process back when these were new and most people were using radio/TV shop level "bench" scopes.

Rodger WQ9E

[kn4hh]

Thank you, Rodger.  I have an IFR 1200 Super S service monitor which has a decent spectrum analyzer.  I will use the procedure in the manual and report back.
Thanks again,
Bob, KN4HH

[WD5JKO]

Some good on topic reference material;

http://amfone.net/Amforum/index.php?topic=37143.0

Jim
Wd5JKO

[n8fvj]

I decided I need two 1 watt resistors in parallel.