Re: [HCDX] Drake R8 bandpass adjustment

[Djaci Franklin <djaci@xxxxxxxxxxxxx>]

At 14:25 15/07/00, you wrote:
> I find in checking further that the tuning is right on in LSB, USB, CW,
> and RTTY. It is only off in the AM mode.

More information from the service manual:

The front panel PTB control is a user adjustment which provides approximately
+- 3kHz shift at 45.05 kHz. The uP controlled DAC provides programmable 
offsets which can be different for each mode.
The uP controlled DAC for PBT tuning is formed by U21 and associated output
resistors connected into the non-inverting input of the operational amplifier
U25B. U21, under uP control through the papallel data bus, switches a
combination of 8 precision 1% resistors to either ground or +5 VDC, thereby
changing the reference voltage of the operational amplifier circuit 
composed of U25B and associated components. Each mode (AM, CW, USB, etc.) 
can be programmed with a
unique combination, allowing a different factory programmed PBT frequency for
each mode. Additional tuning is available to the user through the front 
panel PBTR control.
The front panel PBT pot is read by the uP (U17 pin 54) and the 
digital-to-analog
converter is adjusted accordingly. Both factory programmed offsets and 
front panel
PBT tuning are accomplished through the digital-to-analog converter.
Q30 and Q31 disable the 45.05 MHz signal when in the FM mode. Tuned 
circuits around
L39, L40, L42, L50 and L51 are tuned to 45.05 MHz thus enhancing the 2nd 
harmonic.
L41 allows the signal to be split into two paths. Q37 amplifies the 45.05 
MHz signal
which is then coupled to J8. Connector J8 is the 2nd LO output port to the 
front endboard.
Q36 ampfifies the 45.05 mHz signal which is coupled into the digital mixer 
circuit.


73s

Djaci

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