[SBE] Continental 816

[Cris Alexander]

Barry:

As I recall (and boy has it been a long time), driver screen current is
related to PA grid current and the third harmonic resonator across the PA
cathode. Ideally the PA grid current should peak at the same point that the
driver screen current peaks (or dips -- dang, I can't remember!), and both
of these parameters are affected by the resonator. I recall juggling the
resonator control with the driver plate tuning to get a happy coincidence.
Driver screen current is a good indication of stage tuning/loading.

If I recall correctly, driver cathode currents should be close to 200 mA
each. Driver screen voltage should be close to 300 volts and screen current
should be under 50 mA. PA grid current should be in the 100 mA range and
fully saturated (i.e. a small change in exciter power should produce very
little change in PA output).

If the PA is not saturated and PA grid current is low, I would run the
driver plate tuning through the PA grid current peak and observe what's
happening with the driver screen current. If the grid current peak is not
pretty close (within 20 degrees or so of rotation on the driver plate knob)
to the same point where the driver screen peaks (or dips -- which is it?!?),
make a small adjustment in the filament resonator cap on the PA, then check
it again. Seems like this is more critical on one end of the band than the
other but I can't remember which. 

I'm forwarding this to Stephen Poole, who has an 816R-4 he can look at and
some very recent experience tuning one up. He probably remembers all this
stuff a lot better than I do. 

______________________________
W.C. Alexander, CPBE, AMD, DRB
Crawford Broadcasting Company
Denver, CO
(303) 433-0104 Tel.
(303) 433-0905 Fax.

-----Original Message-----
From: SBE [mailto:sbe-bounces at sbe.org] On Behalf Of Barry Thomas
Sent: Wednesday, August 27, 2014 10:03 PM
To: SBE Member Discussion List
Subject: [SBE] Continental 816

I haven't seen much activity on this list in a long time. I hope the brain
trust is still active. 

It has been soooo long since I've troubleshot a Continental 816 with a tube
driver stage i really don't remember the old skill set.  Some advice on the
first things to look at would be great. 

The problem:  I am not getting enough power out of the transmitter .... Only
80% with it running at full blast.  The final is new and good and tunes
well. 

I'm fairly certain it's driver-related. 
At first I thought it was just bad 4cx250b tubes...They were old.  I
Replaced them but still have the same results. Both tubes are operating and
seem balanced.  I've got a good match from the exciter into the driver stage
and the driver plate tunes into the final, although PA grid current is low
(of course). 

Driver and PA Plate and screen voltages are fine. Cathode currents of the
two drivers are balanced but low. In fact All driver current readings are
low by 50-70% EXCEPT driver screen current reading which is PEGGED.  Backing
off the power to 40% final output un-pegs the meter but it is still at the
max scale. I know I should remember exactly what this means but it has just
been too long.  Or maybe the meds have killed too many brain cells. 

I am hoping with your help I can short circuit some of my re-education on
this old box. 

Any suggestions on the FIRST things I should check in the driver stage?  

Thanks for your suggestions. 

Barry Thomas, CPBE DRB CBNE

Sent from my iPhone 5


Barry Thomas, CPBE DRB CBNE
barryt at sbe.org
(323) 841-0682

Sent from my iPhone 5


_______________________________________________
The SBE Roundtable, SBE at sbe.org
To unsubscribe, go to http://seven.pairlist.net/mailman/options/sbe

http://seven.pairlist.net/mailman/listinfo/sbe