"VE3AX" <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Tom;
The 1st clue that one or more of the high power PIN diodes (there are 3 )is
that power output on 15 and 10m is low - maybe 30 watts on 15 and 10 w or
less on 10m while 20m and below is just fine.
The next test involves removing the lid and checking for voltage at two pins
on the rear HPF board near the top edge during tx and rx. Watch you don't
bump the blue pots on the regulator board at the right side of the radio.
With the radio facing you, the voltage at the LEFT pin near the top edge of
the board (red wire attached) should be essentially 0 volts in RX and +13
VDC in TX. The pin almost in the middle of the top edge (brown wire
attached)is opposite; +13 VDC in RX and essentially O VDC in TX. If either
of the "0 volt" measurements is greater than 0.5 VDC I would change all 3
diodes. If one or more of the PIN diodes are bad, I typically see about 4-6
VDC when it should be ZERO.
Note - the service manual picture showing the PIN diodes will NOT look your
rear HPF board. That was a really early picture of what I believe was a
developmental TR-7. There is an extra disk cap on the picture that is not
in the schematic or production models. Mike Elliot (now SK) told me this
was a remnant of a break-in keying scheme he was experimenting with.
The cathode ends point to the LEFT with the rig right side up and facing you
for the two horizontal PIN diodes 1/2 way down the board. The vertically
oriented one is cathode end DOWN.
The high power PIN diodes are available from Down East Microwave at about $4
each. Order the MA4P 1200's and get some MPN 3404's while you are it for
the two xtal calibrator PIN diodes on the front HPF board. They are only a
buck each.
Now the trick with the MA4P 1200's is telling which end is the stripe
(cathode) end. Trouble is they are 50-50 blue and black on the body. Most
diodes are black, right? So that must mean the blue part is the stripe,
right? WRONG!!!! These babies are BLUE BODY with a BLACK STRIPE. Install
accordingly!
Check the two xtal calibrator diodes on the front HPF board with an ohmeter.
If you aren't sure, pop the top end out after desoldering it and re-measure.
They are often shorted and it's often difficult to tell operationally if
there is anything wrong with them. I have seen (but not always!) an
increase in birdies and weak AM Broadcast band signals showing up in the 20m
band with bad calibrator diodes. They are easily replaced.
The high power diodes on the rear HPF board are not as easy to replace as
the front HPF ones as two in particular are located 1/2 way down the board.
I have found a dental pick to be helpful in ensuring the holes are free and
a small dental mirror is helpful in checking for shorted PC traces after
installing the new ones. It can be done, if you are careful, without
removing the board.
Hope that helps Lloyd (and others) out.
And the usual disclaimer - think I have told it right here, but if I have
goofed on something, somebody say something!
73
Peter
VE3AX
----- Original Message -----
From: "Laird Tom N" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Friday, February 16, 2007 7:55 AM
Subject: [drakelist] PIN diode replacement
"Laird Tom N" <[EMAIL PROTECTED]> made an utterance to the
drakelist gang
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On the Collins net last night K4HWB asked me about there being an easy
way to check/replace the pin diodes in his TR-7 (I checked into their
net with the TR-7/L-7). Thought I would asked the Drake group guru's.
I'm assuming he meant on the HPF board (anyway he say the board that's
buried). I haven't been that deep into mine yet.
Tom W9QI
Moline, IL.
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