Re: What is the purpose of Diode D511 ?

Helmut Wabnig

On Sat, 21 Mar 2020 17:26:53 +0000 (UTC), you wrote:

D511 is a Toshiba RF PIN diode. The circuit switches capacitor C510 (10 pF) in parallel with C517 (12 pF)capacitor. This changes the characteristics of the low pass filter formed by C514, L505 and C517.   
D511 is biased through R518 (1K)  and source driver Q988 (not an op amp).
The Q988 driver is controlled by the Q7 output of IC981 which is a 4094 8-bit shift and store register. Control data is shifted serially in to the register to control the Q0 - Q7 outputs. 
Source power to driver Q988 is supplied by 'P2R8'. My best guess is this indicates Power, 2 meter Receive, 8 volt. 
I would look at Q7 with a scope or DVM and see what it does under various conditions of the transceiver.
Good luck figuring it out.
Steve,73 de KD6NQA
Ah, I did not get that shift and hold register.
I was seeing it as a counter, sending pulses.
But your description is fine.
So it tunes the passband filter to various frequency ranges.
Will have to check that.
After the lightning I had to replace this D511 diode together with the
preamp FET and now sensitivity is down. Possibly another diode fault
or does it just need the receiver adjustment procedure, will see.
D511 was physically cracked in two parts, easy to spot.
ATTEN and PREAMP switching works obviously.

Must get that 7000 sensitivity back again.

Thank you for your help.


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