Hello, I have been working on a Zachary T off and on for awhile with a no transmit or receive problem. I'm not a real tech, but I have enjoyed fixing a few cb radios over the last few years. This was listed as a tech special on Ebay, and it has been a bigger challenge than I thought it would be. Looking at the solder, no one has touched this radio since it was built, until I got it. The first thing to do on a 50 year old radio was to clean it up and I used deoxit on the control pots on the front of the radio. I also re-capped the radio, because I found a few capacitors that were way out of spec. This radio has the PCB PC-198AB. I can't find any info on this board, so I assume it has different parts installed from the factory than the PC-198AA board. I did have to replace the VCO L20 can because the old wax had messed up the plastic threads and I couldn't adjust the VCO voltage. Also I replaced the L24 can because when I did have recieve, I couldn't adjust the 37.880 mhz signal (on ch19) at TP 6. Eventually I decided It must be IC5 VCO uhic-004 modual. i can't find any info on this 10 pin modual other than it has 2 substitute numbers of NTE 1203 and ECG 1203. I orderd the NTE 1203 off Ebay, and it didn't work after i Installed it. Eventually the RX came to life while checking other components. I noticed the voltage at TP 5 was high around 3.6 volts, I should have left it alone, because when I adjusted the voltage to 3 volts per the sams manual, I lost the RX and TX again. I decided to order the ECG 1203 modual and it hasn't worked at all.
Basically where I'm at with my trouble shooting is that lock pin 1 on the upd858c pll is 2.2 volts. It should be close to 0 volts to lock in. Here's the mystery part, pin 1 of IC5 VCO should be around 3 volts, but its 200-300 mv. This pin also feeds the collector of TR17 PLL stop transistor (2sc945 which I replaced), with 200-300 mv dc instead of the 3 volts. TR17 has .7 volts on the base. I forgot to mention all the voltages on VCO IC5 are correct, except for pin 1. It's been a mystery to me as to why the 3 volts are missing on pin 1. I've tested the L21 can and it test okay, it also worked when the radio had TX and RX, I could adjust the signal to max using the O'scope. I first thought the 3v for pin 1 was maybe being provided the Pin 1 on IC5, but then I realized it's probably being feed by R117. On R117 (560 ohm) it has a Vcc supply of 4.7 volts, but on the other lead of R117 the voltage is on 300 mv that goes through L21 to Pin 1 of IC5. I did test and replace C143 also, but no difference. Even with Pin 1 of IC5 desoldered, it only shows around 300 mv on that trace from L21. One other possability that maybe clamping down the 3 volts, is the channel selector. It looks like the channel selector is tied to the base of TR17 by a yellow wire, and also to D16 to LD pin 1 of PLL iC3. The channel selector works fine and all the digits are readable, so I assume its okay??
One last thing to mention. With a s9 signal (-67db) to the radios antenna jack, I can inject a 37.880 mhz 200 mv signal from my Owon signal generator to TP6 and the receiver comes to life. Also it shows around an s-4 signal on the radios meter. This proves to me that all the components in the receivers section is fine. Sorry for the long post, but I was hoping that maybe someone has seen this problem before either with this chassis, or with a simular one. I should also mention that i've been using my 13.8 variable bench power supply instead of the 120v radios power supply. Just to rule out any strange voltage behavior. I've spent a lot of time researching this, and it seems like any utube videos with a Zachary T no TX or RX problem was fixed by doing a full re-cap of the radio. Thanks.. Warren.
Basically where I'm at with my trouble shooting is that lock pin 1 on the upd858c pll is 2.2 volts. It should be close to 0 volts to lock in. Here's the mystery part, pin 1 of IC5 VCO should be around 3 volts, but its 200-300 mv. This pin also feeds the collector of TR17 PLL stop transistor (2sc945 which I replaced), with 200-300 mv dc instead of the 3 volts. TR17 has .7 volts on the base. I forgot to mention all the voltages on VCO IC5 are correct, except for pin 1. It's been a mystery to me as to why the 3 volts are missing on pin 1. I've tested the L21 can and it test okay, it also worked when the radio had TX and RX, I could adjust the signal to max using the O'scope. I first thought the 3v for pin 1 was maybe being provided the Pin 1 on IC5, but then I realized it's probably being feed by R117. On R117 (560 ohm) it has a Vcc supply of 4.7 volts, but on the other lead of R117 the voltage is on 300 mv that goes through L21 to Pin 1 of IC5. I did test and replace C143 also, but no difference. Even with Pin 1 of IC5 desoldered, it only shows around 300 mv on that trace from L21. One other possability that maybe clamping down the 3 volts, is the channel selector. It looks like the channel selector is tied to the base of TR17 by a yellow wire, and also to D16 to LD pin 1 of PLL iC3. The channel selector works fine and all the digits are readable, so I assume its okay??
One last thing to mention. With a s9 signal (-67db) to the radios antenna jack, I can inject a 37.880 mhz 200 mv signal from my Owon signal generator to TP6 and the receiver comes to life. Also it shows around an s-4 signal on the radios meter. This proves to me that all the components in the receivers section is fine. Sorry for the long post, but I was hoping that maybe someone has seen this problem before either with this chassis, or with a simular one. I should also mention that i've been using my 13.8 variable bench power supply instead of the 120v radios power supply. Just to rule out any strange voltage behavior. I've spent a lot of time researching this, and it seems like any utube videos with a Zachary T no TX or RX problem was fixed by doing a full re-cap of the radio. Thanks.. Warren.
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