[Icom] Fw: IC751A caps mod

[Matt & Carrie Trott]

Daniel,
I know you have a 751A and I don't know how similar it is to the 765, but
here is the way to do it on the 765 as per KA8OKH (via VA7OJ's 765 website).
My call is K7BG and I found this on the web. I haven't actually done this
yet. I was having some trouble as the rig warmed up with the lower bands
cutting out (everything below 8MHz). I did check the caps as mentioned at
the end of this article and the low band adjustment was 8v instead of the
correct 6.5v. I made the adjustment back to 6.5v and no problems so far. I
also took the power supply out and am using an outboard supply so it doesnt
get so warm in there. I don't know what the correct voltages are for the
751A but it's 6.5v for the 765. Also, I don't know where the PS is in the
751A, but in the 765 it sits right on top of the PLL board where the
trimmers are. Maybe someone on the list here can tell you what the correct
voltages for the 751A are.

If I find anything else out I'll let you know.

All the best,
Matt--K7BG

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Icom 765 PLL Trimmer Repair - A Case History
Updated 11-10-01
KA8OKH/KB4NPI Web


I have had my Icom IC-765 HF transceiver for close to 7 years now. It has
logged many (perhaps hundreds) of hours of use. This past spring, however,
it began to develop some quirks. One day while it was receiving on 30 meters
during a particularly busy contest weekend, I started noticing contest
activity showing up on the band. The callsigns were recognized as leaders in
the sport of contesting, and I found it odd that they would be active on
this traditionally non-contest band. Another oddity - the signals were
weak - weaker than I would expect from these high power stations. No
matter... the contest ended that Sunday evening and things appeared to
return to normal. A few weeks later, while operating on 40 meters CW, the
receiver suddenly drifted 500 hz up over the course of 5 seconds or so, and
stayed there. A couple of days after that, everything received below 10 mhz
would become garbled and unintelligable after 30-40 minutes of warmup.

At this point I decided that my trusty 765 was in need of some TLC. A few
years ago I had printed out and read the 765 troubleshooting notes on the
Icom web site, and I recalled something about raspy audio ocurring after
warmup, and that it sometimes affected certain frequency ranges. Sure
enough, there it was - the dreaded plastic trimmer problem. Being a
technician by trade, and not wanting to sacrifice my radio to the shipping
gods for who knows how long, I decided to attack the problem myself. Here
I'll describe for you how I got everything back into shape for just a few
dollars and a few hours of time. The repair is not difficult. I feel that
anyone with proper soldering technique and a decent digital voltmeter can do
it. But if you feel squeemish about it, contact Icom Tech Support, and they
will do the proper repairs for you.
What Happened?
What happens is this - the plastic trimmer capacitors in the PLL (phase lock
loop) section of the radio are enclosed in a shielded can. This can also
contains several coils that are covered with a wax to prevent (I presume)
vibration. Also in the 765, the PLL board is located directly below the
internal power supply. Now you would think that the heat would radiate up
and away from the PLL unit, but enough heat reaches the board to slowly,
over years, cause the wax to flow down into the trimmer caps. It then seems
to eat away at the plastic dielectric of the trimmers, causing the HPL lock
voltage(s) to drift out of acceptable range, which in turn causes all sorts
of nasty things to occur. The raspy audio might just affect a certain range
of frequencies, or several ranges, or all. In my case, it just affected
everything below 10 mhz.
How do I Fix it?
Only one way - remove the plastic trimmers from the PLL board, and install
ceramic replacement units. We'll go step by step, removing that nasty wax
substance (or most of it anyway), and re-align the vco's according to the
service manual. While we're in there, we'll check out a few other things
that may need attention. Time's wasting, so let's get started!
What You'll Need
�  Soldering iron - 15-25 watt
�  Solder wick - to remove solder from traces and shield cans
�  Soldering gun - here we want something on the order of 100 watt
�  60/40 rosin core solder (I use Kester Rosin Core 44)
�  #1 and #2 phillips screwdrivers
�  Small pair of "nippy" cutters (wire cutters). Smaller the better
�  Small "jewelers" type screwdrivers, preferrably plastic handles, or
similar adjustment tool
�  Digital voltmeter

Of course you'll also need the replacement trimmer capacitors. The
capacitors we'll be replacing are designated C11, C20, C29, and C37. C11 and
20 are 10pf (picofarad), while C29 and 37 are 7pf. Now... here's where I
begin to deviate from the book a bit. First, let me say without
equivocation - I have nothing against Icom service, or ordering replacement
parts from them. However, in this situation I had the parts in hand to do
this repair, and they work fine. But - if you feel that you should get these
parts from Icom, by all means do so. Here are the Icom part numbers to ask
for -

Icom part#
�  4610001130 - trimmers C11 & 20 (order two)
�  4610001000 - trimmers C29 & 37 (order two)

Now, having said that, you can also get suitable replacements from Digikey.
They have to have leads spaced 4.5 mm apart, be ceramic, and of course be
the proper capacitance. I found these Sprague GKG series ceramic units to be
more than adequate -

Digikey part#
�  SG1029-ND - 3.5-10pf ceramic (white - Sprague pt# GKG10024)

I used this 10pf unit for all 4 caps. Why? Well, C11 and 20 were 10pf
anyway, but Icom recommends in their service notes that C29 be changed from
7pf to 12pf for ease of adjustment. C37 is(was) 7pf as well, so I figured
why not try a 10 there as well? No worries - the 10pf works just fine for
all 4 replacements.
Let's Start
Begin by laying your 765 on a CLEAN surface - a clean towel to keep from
scratching your radio. So as not to cause any confusion here, throughout
this repair, keep the front of the radio facing you. Remove the little door
that covers the adjustments under the top cover, and set it aside. Remove
the screws that hold down the top cover. Remove the top cover by gently
lifting from the rear, then pulling back. Detach the internal speaker lead,
noting the direction of the plug, and set the cover and screws aside. Now
turn the radio over, and remove the bottom cover, and set it and the screws
aside. Now flip the radio face up again. Ok, see that big black thing on the
left side with cooling fins? That's your 765's power supply. Our bad caps
are below it, so we have to get the power supply out of our way. It's held
in by 4 screws - 2 of them go through the left side of the chassis, the
other 2 go down on the right side of the power supply into two little
chassis tabs. Remove these screws and unplug the two white interconnect
cables going to the supply. Lift the power supply out, taking care to not
damage the cooling fins, and set it and the 4 screws aside.

Now what you see on the left side of the radio is the PLL unit. It can be
recognized by a few shield cans, one with 4 access holes in the top. We'll
remove this board next. First, notice that there are a few cables that need
to be disconnected. There is one grey coax that cannot be disconnected at
the PLL board. It runs under the chassis to the RF unit. Turn the radio
upside down. The RF unit is on the right side, under a shield cover. Remove
all the screws holding the shield cover down and remove it. Notice the grey
coax in front of the RF board that goes through a grounding clamp - this is
the coax we need to disconnect. Remove the screw holding the grounding
clamp. Trace the coax over to the RF board, and unplug it from the board.
Pull the coax back through to the top of the radio. Turn the radio right
side up again. Now remove the screws holding the PLL board down, and remove
the board. Set the radio aside for now.

With the PLL board in front of you, again take note of the shield can with
the 4 adjustment holes. Our 4 bad caps are under that cover (you can see
them through the access holes), so this cover has to be removed. Using a
soldering gun, apply heat to the corners of the shield cover, and use wick
to remove the solder from these areas. It sometimes helps to gently pry on
the corners with a small screwdriver while heating. Take your time, and
don't apply too much force on anything.

Now flip the board over, and remove the lower shield from the area
underneath the shield can. Using a solder gun, apply just enoough heat to
free each tab on the lower shield. Locate the mounting holes for the 4 caps,
and desolder them using a 15-25 watt iron and solder wick.

Now working from the top of the board, pull the old trimmers out (if they
didn't fall out already). See all that tan gummy stuff inside the can?
That's the wax that has caused our grief. Using a jewelers screwdriver,
gently dig out the wax from the vicinity of the trimmer mounting holes. If
there are signs of wax that may later migrate down into the area of the caps
later on, go ahead and gently remove it, taking care to not distort any
coils or break any other components. If the wax is fairly solid, it
sometimes helps to apply heat from a blowdryer while scraping.

After the areas have been cleaned, install the new trimmers in their proper
positions and solder them in. Be sure to clip the leads flush with the board
underneath so they don't touch the lower shield. At this point inspect all
the traces underneath the shield can for possible cold joints and touch-up
with a little solder as nessesary. Icom recommends doing this before
re-attaching the lower shield, so I suspect they have seen problems with
solder joints here - so it's best to give it a good look-over.
Reassembly
Re-attach the lower shield to the underside of the PLL unit. Re-attach the
top shield can cover (the one with 4 holes), taking care to orient the cover
so you can see the trimmers through the adjustment holes, and solder the
corners. Mount the PLL unit back into the chassis, taking care to not pinch
any connecting cables. Re-attach all connectors. Re-route the grey coax to
the RF unit, attaching the grounding clamp. Attach the coax to the jack on
the RF unit. Replace RF unit shield cover.

With the radio right side up, connect the plugs to the power supply, but do
not put the supply back in the radio yet. You can lean it up against the
left side of the chassis for now. Plug the AC cord in, and attach some sort
of antenna (dipole, piece of wire, whatever). Turn the radio on. Don't worry
if you can't receive anything. You probably won't right now, as the new
trimmers need to be adjusted.
Alignment
Remember - radio is right side up, facing you.
Now the Icom service manual says to use an oscilloscope to make these
adjustments, but I obtained excellent results with a good digital voltmeter
(DVM). Attach the dvm ground lead to the chassis. Attach the positive lead
to resistor R6 on the PLL board. R6 can be found immediately to the left of
the 4-holed shield can that we worked on - it's marked on the board. Next
we'll make a series of adjustments to our new trimmers, working from the
front trimmer (C11) to the back (C37).
�  Set frequency to display 7.99999 Mhz, and select LSB mode. Adjust C11
(the front adjustment hole) for a reading of 6.5 volts DC on the dvm.
�  Set frequency to display 14.99999 Mhz, LSB mode. Adjust C20 (2nd from
front) for a reading of 6.5 volts DC on the dvm.
�  Set frequency to display 21.99999 Mhz, LSB mode. Adjust C29 (3rd hole
back) for a reading of 6.5 volts DC on the dvm.
�  Set frequency to display 29.99999 Mhz, LSB mode. Adjust C37 (rear) for a
reading of 6.5 volts DC on the dvm.
�  Now check the voltage at the following 4 frequencies to ensure that it is
more than 2 volts DC - 0.03000 LSB
8.00000 LSB
15.00000 LSB
22.00000 LSB
Together...Again
Re-install the power supply, taking care to not pinch any of the leads or
damage the heat sink fins. Now would also be a good time to inspect the big
white power supply plug for any signs of damage. Several 765 owners have had
trouble with this plug, again due to heat. The plug will turn brownish in
color, and become brittle. Contact Icom Service for a proper replacement.

Attach the top and bottom covers, speaker plug, and top access door. Enjoy!

Rich, KA8OKH

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