Modifications for the Kenwood TH-77

Expanded RX/TX for TH-77A

From the factory, it can receive 138-174MHz and seems to do it quite More? y

well with the supplied rubber duck anten»Ď»┘can receive 438-450MHz and, likewise, does this quite well too! One very interesting feature of the TH77A is that it can do *DUAL* UHF receive! To do this it uses the VHF receive section and you have less sensitivity, but I really haven't noticed much signal degredation at all.

NOW! On to the *EXTRA* capabilities (there are other BASIC functions that I didn't mention, those were just the highlights).

After making a few modifications to this radio, you can get it to do the regular stuff that the IC24AT will do, such as AM aircraft, expanded UHF (400-512 depending on PLL lock) and 800-950 (again, depending on PLL lock) in addition to cross band repeat.


Easy (if you've got a steady had for removing a chip resistor!).

Open it up. There are 3 screws visible on the outside and 4 more More? y

underneath the battery pack. Unlike most radios, this one doesn't have a spring for the battery release switch. It gets spring action from the metal plate that you will remove when you take out the four screws.

Anyway, the area you will be working in is on the control board which is fixed to the front section of the radio. In particular, look for the yellow electrolytic capacitor (its yellow in mine) that is at the dead center of the board (there are two IC's, one above this and 1 below this). You will have to remove the electrolytic capacitor (C124) and set it aside for a few minutes. You will be putting it back afterward. The reason for removing this is so that you can remove a chip resistor that is underneath it. To avoid tearing the flex board foil traces, do not bend the leads of the capacitor.
There is a green wire that is connected to one side of this capacitor. If you disconnect one side of this green wire, you will get RX from 400-512 (PLL lock depends on the radio, but you can DIAL from 400 to 512). 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'Cloc┘÷he area where you More? y

removed the capacitor, you will see a blank space where there could have been a chip resistor followed by two chip resistors side-by-side and in the next row below that, you will see 3 more that are side-by-side.
There is one to the left of these two rows that is kind of off center. Now, there are three ways to configure this radio at this point:
  1. With one side of the green wire pulled, you will have 136-174 RX, 400-512 RX. If that's all you want, you are done.
  2. By removing the rightmost chip resistor (R129), in addition to keeping one side of the green wire disconnected, you will have 136-174 RX, 400-512 RX, 118-136 AM RX, 800-950 RX.
  3. By removing both of the chip resistors, in addition to putting the green wire BACK where it belongs, you will have 136-174 RX/TX, 400-512 RX/TX, 118-136 AM RX and 800-950 RX.
All three of these options include, of course, putting the electrolytic capacitor back in place afterwards.

       I  I  I
  __    !
     !  !  !    !
     !  !__!    !
     !          !
     !  =rr <------- remove the right one for AM/800-950 RX
     ! r        !    remove both to also get extended TX
     !   rrr   /
     !  ___    !
     !  !  !   !
     !  !__!   !

    Layout shown with electrolytic capacitor C124 removed
The best way to do this is to heat up both sides of the resistor and More? y

push it out of the way with something small. At least this was the best way for me! Once it's out, put the capacitor back in place and close it up. (Put the green wire back also, if you are making the out of band TX mod too!)
receive AM aircraft

To receive AM aircraft, get the VHF side in the main band and hit the ENT key twice to go to the VFO. Once you are in the VFO, hit the UxU key twice. Once for dual UHF receive and the 2nd time for AM (118-136).

To receive 800-950, get the UHF side in the main band and hit the ENT key twice to go to the VFO. Once you are in the VFO, hit the UxU key once.
Crossband repeat

To enable crossband repeat, hold down the SUB BAND UP ARROW key while turning on power. To disable, do the same thing again. Kenwood says that both bands can contain shift information but only one band can include an encode/decode tone.
TH-77A PLL Unlock Override Mod

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 ***
*** Con ***
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)


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.


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!

Clifford K. Yamamoto - KA6JRG Email:
Pushing the TH-77A UHF receiver higher in frequency

Have you ever wondered if the TH-77A UHF receiver could be "squeezed" out further beyond the end of the amateur band at 450MHz? If you've already made the chip resistor mods that allow you to go beyond 450MHz then you're half way there! When I made the chip resistor mod I was hoping to hear activity up in the 480MHz area, and hopefully in the 506MHz area also. Well, the 480MHz region was quite noisy and I always had to find a hot spot to listen. Needless to say that the 506MHz region was non-existent. (NOTE: Even after these adjustments the 506MHz region was not very good in my radio. However, I have seen some radios that work rather well at 506MHz without this adjustment!). So, I decided to "take a look" inside and see if there was anything in there that I could "turn" to improve reception. It turns out that there is a bandpass filter in the front end of the radio that can be re-tuned to your liking.

Finding the bandpass filter on the schematic

Grab your schematic diagram entitled, "TH-77A/77E Schematic Diagram" and flip it over to the back side. Locate the antenna input at the upper right of the schematic. Follow the trace from the antenna to the left towards the "UHF Pwr Amp Unit". Just before this unit follow the trace down through L207, Q207 and then stop at L226 just before Q206. If you look at the "TH-77A/77E Block Diagram" and follow the same flow you will see the bandpass filter between Q207 and Q206.

Finding the bandpass filter in the radio

It's time to open the radio up! Since you've already made the chip resistor mods we'll assume that you know how to carefully open up the radio. Once open, you'll be working with the front half of the radio (opposite that where you made the chip resistor mods). This half of the radio is where all the RF is done (both TX and RX). There is a VHF board and a UHF board. Unfortunately, the VHF board is on top so it must be removed to gain access to the UHF board. Observe the screws that are holding down the board. Carefully remove them and put them in a place so that you know which holes they go back into. Once the screws are removed you'll have to "work" the board out of the header connector towards the bottom of the radio. It goes without saying, but BE CAREFUL!! Once you've removed the board you'll see a metal plate that separates this VHF board from the UHF board. Fortunately, Kenwood made three holes in the metal plate to access the bandpass filter that you've just found!

Tuning the bandpass filter

I was a bit anxious to "play" with the bandpass filter so when I did mine it was around 2am at home so I didn't have a RF generator to do it properly. I would suggest that you use a RF generator, if possible, so that you can keep track of the sensitivity of the receiver in the amateur band. Without a generator you'll have to tune to a high frequency that you are interested in receiving (hoping that the channel is active) and then tweak the bandpass filter to improve reception. Then you'll have to switch back to the amateur band and key up a repeater to see what the sensitivity is like.

Since I did not use a RF generator I can only give my view of how it should be done. I would connect the RF generator to the radio and dial up a frequency that I am interested in being able to receive. Then, start with the generator output very low (modulated with a 1KHz tone at 2 or 3KHz deviation) and bring it up slowly until you begin to hear the tone in the receiver. At this point, adjust the bandpass filter a little at a time, making note of where the coil screw is before you begin to move it so you can go back if things are not working out. I wouldn't adjust the bandpass filter to get too much improvement because any improvement at this frequency means a degredation in the amateur band. You don't want to be too far away from the 0.18uV sensitivity specification for the amateur band. You'll have to play with it to find the spot where the tradeoffs are acceptable. One thing I can't remember, though, is whether you have to put the VHF board back in each time you make an adjustment so the radio will work (in order to listen to the speaker). That may have been necessary to "transmit" since in my case I had to kerchunk a UHF repeater to test my sensitivity in the amateur band.

I was interested in getting the 506MHz region to work better, but I had to give up on that. It is much better than it was without making these adjustments, but I still need to be in the primary coverage area for the 506MHz transmitter to hear it "ok".

Once you're satisfied with the performance, put the VHF board back in, screw everything down and close the radio up. Another good test of how your radio is performing in the amateur band is to switch the VHF side into UHF operation. Dial up your favorite UHF repeater on both the "VHF doing UHF" side and the normal UHF side. Then kerchunk the repeater and do a quick comparision of the S-Meter's. (This might be a good comparision to try before hand also to get an idea of how the two compare before making these adjustments).

That's about it! Enjoy the extra capabilties of your radio! Now, since we can hear all these other frequencies i'm anxious to hear from anyone who can find out how to modify the radio to get more memory channels, HI HI!

Gerald J. Walsh

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