TH-77A PLL Unlock Override Mod
            ==============================

This mod stops that annoying "beep" while receiving out-of-band, but has
some drawbacks.

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. Adjusting the VFOs
is quite a task as 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 found my VFO range to be adequate but the beeps were bothersome.

It is a simple mod to defeat the PLL unlock signal from reaching the CPU but
there are some points for and against associated with this mod.

*** For ***
- 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 through 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.
*** Against ***
- You will not know, w/o some sort of monitor receiver, if you are xmitting
  when out of band. 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 spread all over the band until lock occurs.

If you perform this mod, you must avoid making any out-of-band transmissions
as they are illegal, can jeopardise other services and could cause you to be
prosecuted and lose your licence.  THE INTENT OF THIS MODIFICATION IS TO
ALLEVIATE PLL LOCK PROBLEMS IN *RECEIVE* ONLY!!

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 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.

Turn the radio so that the controls and BNC are farthest from you and the
bottom (where the battery connects) is 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. Note the 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. The mod
permanently grounds one or both of these pins to trick the CPU into thinking
the PLL's are always in lock. Don't worry about grounding the PLL's unlock
in since there is a 4.7K resistor between it and the CPU pins.

Use fine wire and strap pins 12/13 to the negative lead of the 100uF
capacitor described earlier.

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

The X's are where the 100uF capacitor is soldered to the board.

Note that this change does not work unless the mod described in TH-77A.PT1,
removing chip resistors to enable frequency expansion, has been completed.

(Information downloaded from Compunet by John, G1YYH @ GB7NWP)

(Downloaded from GB7SYP)