[DSP-10] SR105 diode question

[dsp-10@mailman.qth.net]

Hi Bole,

I just finished building my DSP-10 and had the same problem the
documentation does not show the location of the diode. I decided that it
is best for mechanical stability as the leads of the diode are heavy to
solder the cathode to the center lead of Q106 gate (which is connected to
Q104 collector) and the anode to the right side of R131 that is connected
to R130 this connects for Q103 Base.

The information below was obtained from
http://www.proaxis.com/~boblark/dsp10.htm

"SR105" Diode When the DSP program is first started and before the PC
program is run, the DSP-10 can come up with BOTH transmit and receive
turned on. This sets up a loop that oscillates at full power! Very bad.
My DSP-10 shows about +16 or +17 dBm out. The happens because the shift
registers (U107, U108) come up uninitialized. The eventual solution will
be to program an EPROM that includes the setting of the shift registers
at DSP startup. Here is a work-around until you have an EPROM version:
Acquire a Schottky diode and solder the cathode (bar end) to the junction
of R132 and Q104-collector. Solder the anode (non-bar) end to the
junction of R130 & R131 (base of Q103). It must be a low barrier voltage
diode like the Schottky, but it can be either a rectifier or small-signal
type. I used a SR105 from Digi-Key, but I notice that Radio Shack has
1N5817-1N5819 or 1N5820-1N5822 on special order status. W7LHL suggested
that a 1N34 type of germanium could work also. This work-around prevents
the 'transmit' from being enabled when 'receive' is enabled. (Added 5
Sept 
01, based on 27 Jan 00 reflector posting.) 

73,
GL

Bob Johansen WB2SRF

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