[Icom] Fw: IC751A caps mod

[Daniel I. Nieto]

Matt:
Thank you so much for your info, I wonder if someone out there has the
voltage info for the 4 caps in the PLL of the 751A, it would surely help.
Best regards.
Daniel Nieto.
CE3HRX
----- Original Message -----
From: "Matt & Carrie Trott" <aa7bg@3rivers.net>
To: <icom@mailman.qth.net>
Sent: Wednesday, October 30, 2002 3:08 PM
Subject: RE: [Icom] Fw: IC751A caps mod


> 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
>
> -----------------------------------------
>
>
> 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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