[SFDXA] Troubleshooting Pin diode circuits

[Alan Harp]

This message is kinda long.  It contains information on how to troubleshoot
problems with Alpha Amp QSK circuits.  It this doesn't interest you don't
read it.

Two weeks ago I blew my QSK T/R circuit in my Alpha 86 amplifier.  There is
a fuse in the circuit.  It blew.  This is in a pin diode circuit where the
signal from the antenna to your tranciever is coupled through capacitors and
forward biased diodes.  This switches very fast allowing you full QSK.  If I
replaced the fuse, the QSK circuit would work (It connected the antenna
through the amp to the receiver) but as soon as I transmitted with the amp
the fuse blew.

I looked at the schematic.  I assumed the pin diodes were bad and called
Crosslink for replacement diodes.  There were four of them in the circuit.
I ordered four.

I didn't tear into it until the replacement diodes arrived.  When I did I
checked the diodes with a multi-meter and decided that the diodes I thought
were bad were really OK.  I did find one large diode on a heat sink that was
indeed bad.  I then ordered this diode from Crosslink.  When it arrived I
replaced it and tried to run the amp.  With a new fuse I could hear through
the circuit until I transmitted with the amp.  Then the fuse blew.

I called Crosslink again.  I had been talking to Molly in technical support.
She told me that I needed to read some info on pin diodes.  She sent it to
me in an E-mail.  Here it says to check them with one lead out of the
circuit in the forward resistance mode of your meter.  (Not the diode check
mode).  Using this method I found that three of the small pin diodes were
actually bad.  I replaced them and now the amp is OK.  I can transmit
without blowing the fuse and  the QSK circuit works.

Here is the message I got from Crosslink about pin diodes:

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  To check the PIN diodes one end must be lifted from the circuit.  The
multimeter must be in the NORMAL resistance measuring range, NOT the
"diode check" function.

  The forward resistance will vary a bit depending on the voltage that
the meter puts across the leads in the resistance measuring function.
We usually use Fluke 77 meters, and they use 0.750vdc when measuring
resistance, and 2.5vdc in the diode check range.  I have another meter
here that uses only 0.25vdc in the resistance function, and it reads a
good diode ever higher (about 1.5 megohm) in forward bias. Measure your
meter in the normal resistance check function with another in the volts
range to check the bias voltage that is used (leads connected together).

  The tan/yellow diodes are KS1001 types, and with one end lifted and
.75vdc from the meter used to forward bias it, they read 1.5 megohm.  If
yours are reading 10k and your meter uses a similar voltage on the test
leads, then it is leaking too much.



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Molly Hardman, W�MOM Manager, Alpha Products & Technical Support