From: "The Firefly" 
Subject: th-77A Mod
Date: Sun, 03 Dec 2000 22:07:43 -0600

After receiving the service manual and completing the 'chip resistor' mods
for my TH-77A, I have another mod to share.

Some of you may have been annoyed at the constant beeping when the PLL is
not in lock even though you are able to listen at a given frequency.  Yet
adjusting the VFOs is quite a task.  They are both shielded and soldered to
the board.  There is no external access to the VFO can for any adjustments.
In fact, there are no adjustable coils or trimmers in the VFO!

I have found my VFO range to be quite adequate, however, just the beeps were
bothersome.  But not anymore!  Here is a simple mod which defeats the PLL
unlock signal from reaching the CPU.  There are some pros and cons 
associated
with this however :

*** Pro ***
- The receiver never beeps anymore even though you have a marginal battery
  (which affects VFO/PLL lock) and are capable of receiving the given freq.
- Since the receiver no longer beeps, you can dial thru frequencies quickly
  without waiting a full half-second for the display to change each time
  the radio beeps.
- When an out of band transmit is attempted, the transmitter no longer shuts
  off after 1 second due to marginal PLL lock.
*** Con ***
- You will not know, w/o some sort of monitor receiver, if you are xmitting
  when far out of band.  That is because the meter will show full scale
  even if the PLL is out of lock and not transmitting on frequency.
- At the extreme edges of PLL lock, the VFO will vary widely in 'sling-shot'
  fashion until lock occurs (sometimes after 1 full second).  This means
  your transmitted signal may be all over the band until lock occurs.

If you feel the pro outweigh the con, you may be interested in performing
this mod.  Be aware, that YOU are responsible for the purity, accuracy and
stability of any transmissions you make from your TH-77A.  THIS MODIFICATION
WILL DEFINITELY VOID YOUR WARRANTY AND PERMIT OUT-OF-BAND TRANSMISSIONS OF
QUESTIONABLE QUALITY AT CERTAIN FREQUENCIES.  You should not perform any
out-of-band transmissions with this modification in place.  THE INTENT OF
THIS MODIFICATION IS TO ALLEVIATE PLL LOCK PROBLEMS IN *RECEIVE* ONLY!!

Now that the legalities are done, on with the mod.  (Because I am too
lazy to make drawings and directions, I have copied the one below from
an earlier mod posted about the TH-77A - with the required changes)

MODIFICATIONS FOR IGNORING PLL UNLOCK

Open up the radio.  There are 3 screws visible on the outside and 4 more
underneath where the battery pack goes.

Locate the control board which is fixed to the front section of the radio.
Look for the 100uF electrolytic capacitor (it's yellow in mine) that is at
the dead center of the board (there are two IC's, one above it and one
below it).  It is most likely a PC mount (NOT an axial type capacitor with
one lead on each end) capacitor lying on its side.  You will be attaching a
single wire to the negative lead of this 100uF capacitor.  To avoid tearing
the flex board foil traces, do not bend the leads of the capacitor.

Orient the radio so that the volume controls and BNC are at 12 O'Clock on
your table (farthest from you) and the bottom (where the battery connects)
is at 6 O'Clock (nearest you).  Looking at the lower IC which should be a
rectangular NEC 75116GF-67x-3BE, there are 19 pins running along the lower
edge closest to where the battery connects.  There should be an embossed
dimple or dot on the lower left corner of the IC to indicate pin 1.  As you
count from the left, locate pins 12 and 13.  These two pins are defined as
follows:

pin 12 - VHF Unlock Input
pin 13 - UHF Unlock Input

Normally these pins are low to indicate the PLL is in lock.  When you change
frequency, they *momentarily* go high (at worst about 250 milliseconds) 
while
the VFO comes into lock.  If the pulse stays high for longer than this 
period
the CPU interprets this as a marginal lock and begins to beep.  What we are
going to do is permanently ground these pins (or just one if you prefer) to
trick the CPU into thinking the PLL's are always in lock.  There is no need
to worry about shorting the output of the PLL's unlock pin since there is a
4.7K resistor between it and the CPU pins.

The best way to do this is with some fine gauge wire (I used #30 wire-wrap).
If you are going to disable both VHF and UHF unlock, you can just short pins
12 and 13 together.  Then connect the other end of your wire to the negative
lead of that 100uF capacitor you found earlier.  Here is a little pictorial
to clear things up:


                  --       --       --
                  ||       ||       ||
                 /  \     /  \     /  \
                --------------------------
                |                        |
                |                        \-\
                |                          |
                |           +---------+    |
                |           |         |    |
                |           | Hitachi |    |
                |           |         |    |
                |           \---------+    |
                |                          |
                |       X      | | |       |
                |  +----X                  |
                |  |                      /
                |  |                     /
                |  | /---------------\   |
                |  | | NEC           |   |
                |  | |               |   |
                |  | |               |   |
                |  | | .             |   |
                |  | \---------------/   |
                |  |  1      11     1    |
                |  |         23     9    |
                |  ----------++          |
                --------------------------

    The X's are where the 100uF capacitor is soldered to the board.  Just
    tack your wire onto the capacitor's negative lead or onto it's circuit
    pad.

That's it!  Of course this mod does nothing for you unless you have already
completed the 'chip resistor' mods for allowing out of band reception.  Try
dialing up a frequency that used to beep every time your rotated the tuning
knob (most likely a 800 Mhz frequency).  It should no longer beep.  In fact
you could probably dialup 512 Mhz and key up the transmitter and get a full
scale reading.  But remember, your VFO probably won't get that high anyway,
and you are most likely transmitting at some frequency where the VFO tops
out at and begins to ripple in frequency as it fruitlessly attempts to lock.

SOME PERSONAL OBSERVATIONS

After retuning my UHF front end, I can now listen to some public service
frequencies in peace, without the annoying beep.  However, keep in mind
that if you are listening to something at 490 Mhz, your VFO is running way
down at about 432 Mhz.  I think a number of people feel that if they can
receive at a given frequency (even marginally) they should be able to key
up solidly.

If you like living dangerously and transmitting out-of-band please be
considerate and know the limitations of your TH-77A.  Although being able
to transmit out-of-band with this non FCC type-accepted is risky enough, I
must reiterate that YOU must now be watchful about your TH-77A operation.
With this mod in place, the TH-77A will NO LONGER protect you from
transmitting with a marginal output.  The transmitter will key up whenever
you ask it to, even while the PLL is still hunting for lock.  However, I'm
sure most of you will perform this mod simply to make receiving out-of-band
more enjoyable as I've found.

Well, I hope this information helps you get more from you new toy!  Happy
monitoring!







One more feature worth mentioning for humour value:

If you hold down M and PTT and power up, the HT enters "game mode".  You get
"0000" on the left of the screen. Press "0" and digits start changing
rapidly in hex (consequently all appearing as "8"). By pressing 1,2,3 or F
you stop the appropriate column. If they all come up the same, er, well I
guess you win... maybe something good happens then. Maybe it enables
transmit on, cellphone, air, and 30cm :-). At 1 chance in 65536 I'm not
about to try to find out, though...

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