Subject: TH-45 diodes Here is the function of the diodes and pull up or pull down resistors (for both the TH-25AT and the TH-45AT unless otherwise specified): D4 selects the display type. installed: channel display removed: normal frequency display You probably don't want to install this. R6 is the pullup resistor for D4. Do not remove it, so that if D4 is removed, pin 44 of IC2 will be properly pulled up. D3 selects VHF or UHF. installed: VHF removed: UHF You definitely don't want to change this! R7 is the pullup resistor for D3. Do not remove it, so that if D3 is removed, pin 45 of IC2 will be properly pulled up. R25 selects automatic offset selection. installed: automatic offset selection enabled. removed: automatic offset selection disabled. R25 seems to have no effect on the TH-45AT. R23,27 frequency selection step size. Frequency selection step sizes is only known for TH-45AT. R23 R27 frequency selection step sizes: --- --- --------- --------- ---- ------ out out 5 and 10 kHz in out 5 and 25 kHz (TH-45AT default) out in 5 and 12.5 kHz (author's preference) in in 10 and 20 kHz R23 probably has no effect on the TH-25AT. I never checked. I was quite satisfied with the 5 and 10 kHz step sizes for the TH-25AT. I never tried changing R27 to see what would happen. R24 ? something with the CTCSS tone ? installed: When a TX offset is selected, the CTCSS tone is always transmitted. The TONE button does not work correctly. When the TONE button is depressed, the display goes to 430.000 c, and nothing seems to work. (A "c" is displayed in the channel number position.) The action of the TH-25AT is unknown. removed: CTCSS tone selection works correctly. Both the TH-25AT and the TH-45AT are delivered this way. You probably don't want to install this. R19 function unknown. R19 is not installed in the European version. R20 function unknown. R20 is not installed in the European version. R21 function unknown. R21 is not installed in the European version. R19, R20, and R21 are all in parallel. Thus, if any of them is installed, it is like all of them are installed. R26 function unknown. R26 is installed in the European version. On the TH-25AT: R22,28 select RX and TX frequency range. R22 R28 RX range TX range --- --- -- ----- -- ----- in out 144-148 141-163 (USA default) in in 144-148 144-148 out out 100-200 100-200 (only where your PLL locks up.) out in 142-151 ???-??? R18 exact function unknown. installed: as delivered in USA. removed: TX disabled from 146-148. The RX and TX frequency range based on R22 and R28 are unknown if R18 is removed. On the TH-45AT: R18,28 select RX and TX frequency range. R18 R28 RX range TX range --- --- -- ----- -- ----- in out 440-450 440-450 in in 438-450 438-450 (USA default) out out 200-500 200-500 (only where your PLL locks up.) out in 215-230 215-230? (PLL won't lock up.) R22 TX offset selection installed: 1.6 MHz offset (perhaps for 220, or Europe) removed: 5 MHz offset (USA default) --------------------------------------------------------------------- PLL adjustment --- ---------- I have found a quick and easy way to retune your PLL (in the TH45-AT) with a minimum of test equipment. All you need is a scope and a small tuning tool. First, take off the battery pack holder plate. Then, remove the silvery sticker covering the tuning pot access holes. If the radio is positioned on its back, with the top folded over so that the touch tone pad is also facing down, the test point you want (TP1) is on the bottom half of the radio, near the center (left to right), and close to the battery; the tuning pot you want (TC1) is on the bottom, and closest to the PTT switch. Under no circumstances change the tuning of TC51. This is used to calibrate the output of the radio with the display the radio is giving; you don't want to mess with it. Once again, the Service Manual makes it very clear where these points are, if you are having trouble with my descriptions. On with retuning the PLL. With the radio on, and receiving, monitor the voltage and the waveform on test point TC1. Tune the radio DOWNWARDS in frequency until the PLL unlocks. Note that the radio will beep when this happens, and the waveform on TP1 will change. Tune the radio about 1 MHz higher so that the PLL locks up again, and note the voltage on the testpoint, TP1. Now, tune the radio to the LOWEST frequency that you want to be able to receive. (Be very careful when you adjust TC1. If you are not careful, you could break TC1 loose. This will cause your radio to have microphonics. Mechanical vibrations will change the value of this capacitor, and thus change the tuning on the PLL. If you encounter this problem, you can repair it by dripping wax through the small hole onto TC1, or by replacing TC1. It is of course better not to break it in the first place.) Adjust TC1 until the voltage on the test point TP1 is the same as what was noted earlier. Button the radio back up, and you're done. You will not be able to tune the PLL to any range you want. There are limits. On my radio, I have been able to retune the radio so that I can recieve from 439.2-468.6 MHz with a set of batteries fresh out of the charger. The tuning range will probably diminish as the battery voltage decreases. I have not retuned the PLL on my 2m HT, but I'd imagine the same technique will prove fruitful. --------------------------------------------------------------------- In the rare case that original ideas Kenneth J. Hendrickson N8DGN are found here, I am responsible. Owen W328, E. Lansing, MI 48825 Internet: kjh@pollux.usc.edu UUCP: ...!uunet!pollux!kjh