From: "VE3AX" 
Subject: [drakelist] TR7 PIN  Diode Replacement


HI all:

Thought I'd pass on my recent experience in replacing the PIN diodes.  This
applies particularly to those who are using the MA4P1200's from DownEast
Microwave as replacements for the UM9401's.  Nothing wrong with these 50
watt PIN diodes for replacing the three on the rear of the two HPF boards,
but I.D.'ing the cathode end is not intuitive.

I have used these before some time ago but when I went to do this classic
repair on a  friend's TR7, I just assumed the diodes were the basic black
body and the cathode stripe was the blue end.  WRONG!  They are a blue body
with a black stripe!  Trouble is the blue and the black parts are 50/50 on
the body of the diode and even under a magnifier could not make out what was
"original body color" and what was "stripe".  After
installing them, and
finding the rig not working yet, I checked other MA4P1200's I had with an
ohmmeter and determined the black end is the cathode.  A call this a.m. to
Steve and Sandra at DEM confirmed, after even they had to do some thinking
about it, the BLACK end is the cathode.  To repeat....the diode has a blue
body with a black stripe.

Might as well add some other notes here for those not familiar with this
process while I am at it....

With the the rig upright and facing you, and the HPF boards on your left,
the MA4P1200's (UM9401's) on the rear of the two HPF boards are positioned
as follows:
Single one in tx line is cathode down. It's just to the right and slightly
above the 1 watt 82 ohm resistor.  The other two in the rx line are cathode
facing left.  On later TR7's, and I don't know where the serial # cutoff is,
they are horizontally positioned in a line, below and right of the 1 watt
resistor.  I have 3 service manuals here that all show these positioned
vertically with  a cap soldered above the board to one of the leads.  I have
yet to come across a TR7 wired like this.  Maybe someone else can shed some
light on the early serial # PIN diode placement.  The picture of this board
in my service manuals are quite dark and it is not easy to see the PIN
diodes.

The front HPF board has the two "transistor looking" (but with only two
leads) PIN diodes.  Flat side of these are both on the left side as you face
the front of the rig - this is cathode down.

Interestingly after replacing all of the PIN diodes in this rig, and
correcting the polarity on the rear ones after goofing up, it still did not
work.  Turned out the .05 rx coupling cap, C1504, was open somehow!  No
signals would pass through it, or at least none to speak of.  Someone else
had worked on this rig before me and there were several extra caps not on
the schematic, some caps that were on the board had been removed and not
replaced, small signal diodes had been tack soldered above the board in
place of the PIN diodes, and there were "extra" diodes that
"went nowhere"!
One of the front HPF board PIN diodes fell out on its own when I went to
unsolder it, before the soldering iron got near it!  I am happy to report it
now works, is essentially back to stock, and it just needs a good tune-up to
finish it off.

I find that a dental pick helps clear the holes for re-installing the
diodes, and a hemostat helps hold the parts for insertion.  It isn't easy to
solder the parts back in without the rather involved process of removing the
boards entirely but it can be done if care (and minimal solder) is used.  A
dental mirror and flashlight make inspecting the board for solder bridges
easy.

Lastly, your mileage may vary.  I freely admit I am a
"technician-in-training" - if I have made any errors in this post, please
tell me before I send others the wrong direction. That said, there is
nothing quite like fixing your own.  My wife is addicted to crossword
puzzles - I detest the things, but I think solving a rig's problems step by
step yield a similar sort of satisfaction.

73

Peter
VE3AX





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