Care and Feeding an Icom PW1
W6DE
Copyright 2017, All rights reserved.
Regarding: The MRF-150s on one of My PW1 final boards are bad. Should I buy a matched pair of
MRF-150 devices or just buy two from stock?
You can replace a set of finals on one board of the four PA boards in a PW1. That is, you only have to
replace the finals on the bad board. You should replace both finals on a board as a set.
A matched pair of MRF-150 devices is cheaper than two non-matched devices. Get the matched pair at
mouser.com (part number: 937-MRF150MP). When shopping at Mouser get some Thermal Paste. You
can use some computer CPU chip thermal paste if you have it. It is also available at a computer
shop/store. Or, go ahead and order some white thermal paste when you order the MRF-150 devices. I
had some white Wakefield thermal paste lying around.
Without specialized equipment it is difficult to separate the MRF-150 devices from the PC board without
damaging the PC board.
1) While the board is still mounted on the PA heat sink, with an X-ACTO knife carefully cut all the fins on
both MRF-150 devices in order to protect the board.
2) Remove the board from the heat sink and
3) Then remove the remnants of the two MRF-150 devices from the heat sink.
4) Remove all traces of thermal paste from the heat sink.
5) With the now removed PA board clean up the board by unsoldering and removing the fin remnants.
6) one-at-a-time, apply a very thin paste of thermal conducting material to the base of each MRF-150
device and test fit it to the heat sink. Gently and firmly push the MRF-150 onto the heat sink. Remove
the MRF-150 device and inspect both the device and heatsink. If you see peaks in the thermal paste,
you have used too much. Wipe off the MRF-150 device, leave the paste on the heat sink. Then repeat
the mounting process and removal again. When you have a slight wave like appearance on the device
and heat sink, you have the correct amount of paste—do not mount the device yet! Carefully set aside
the devices, protect them from dust.
6) Re-install the board onto the heat sink. Mounting the board before the MRF-150 will allow the MRF-
150 fins to lie on top of the board where they are supposed to be.
7) Then install the MRF-150 devices.
8) Finally solder the MRF-150 fins to the pc board.
There are two potentiometers to set the bias on each board, one for each MRF-150. These pots are
used to balance the pair of devices on each board.
I’ve found the Icom factory settings for the MRF-150 bias settings to be different from what the Service
Manual calls for. You must make a choice to either: 1) measure what the other PA boards have their
idle current set to and then match the replaced MRF-150 to be the same value; or 2) to set all four of
the PA boards as per the Service Manual. I found enough variance between all the boards to cause me
to set the bias on all four boards, eight MRF-150 devices, to the 100ma per MRF-150 (which equals
200ma per board) as stated in the service manual. Note: There are four PA boards total, two boards on
each side of the heat sink.
The instructions in the service manual seem incomplete to me. Icom has you set bias on one side first
by biasing the 2
nd
side to cut off and then set the 2
nd
side by turning both of them on and doubling the
value you set the first one to. I found this didn’t always get each side to a close match. So, I did my bias
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