Wade -
The circuitry is quite different between TUNE and CW modes.
In TUNE. the Mic Amp is converted to an audio oscillator, and the tone
produced is fed to the Balanced Modulator where it is treated the same
as any other modulation. Also transmit ALC is active, which is why the
output "levels off" in TUNE mode as the GAIN is increased.
In CW, a DC bias is applied to the Balanced Modulator to unbalance it.
The Carrier Oscillator is then shifted approximately 1 kHz to put it
within the passband of the LSB filter so it can be passed on through to
the rest of the transmitter.
This DC bias is controlled by the GAIN control through R104. A common
problem is R104 going up in value with age, decreasing the available
bias current.
Other possible problems are a defective C177, R105, misadjusted C6 or
defective LSB filter. If you have a receiver that covers 5.645 MHz,
listen to the Carrier Oscillator and see if it changes approximately 1
kHz when switching from SSB to CW.
Let us know what you find.!
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
wmacleod wrote:
Hi Drakelist
During a recent alignment of a T-4XC I encountered an interesting
problem which I am still trying to troubleshoot. I am hoping that
someone out there may have already experienced this problem and can
point me in the right direction.
Description of Problem:
When placed in Tune Mode and loaded into a dummy load, the transmitter
easily outputs 140W on all bands from 80-10 meters. Gain setting is
approximately 5 (mid scale) and plate current is approximately 230 ma.
However, when placed in CW mode, the gain setting must be increased to
10 (full scale) just to achieve 90W at 200 ma. There appears to be
insufficient gain to drive the PA to the same power level as achieved
under Tune conditions. Readjusting Plate, Load and RF Tune does not
provide any additional output power.
Analysis to Date:
The Drake manual indicates that there is always some additional gain
adjustment and sometimes repeaking required when going from Tune to CW
modes but I sense the wide open gain setting indicates there is
insufficient gain to fully drive the PA stage in CW mode. I tried
replacing the 12BY7A driver and PAs but the problem remains the same.
Now I need to understand how the Gain really works in CW vs. the Tune
mode and compare the expected signal levels which the driver should be
seeing to determine what is causing the difference between the two
modes. I am starting to read through Gary’s maintenance CDs but any
suggestions on where to focus or what to measure would be greatly
appreciated.
Regards
Wade
VA3HM
_______________________________________________
Drakelist mailing list
[email protected]
http://mailman.zerobeat.net/mailman/listinfo/drakelist
