Showing posts from February, 2024

Kenwood TS-790A Light Replacement

 I noticed a while back that my TS-790 meter displays had gone dim.  While I had the radio opened up to adjust the master clock. I decided to replace the burnt out incandescent lights with new white led lights. I neglected to take any pictures until after the work was done, but this will show where the lights are located, and which led bulbs I used to replace them. To access the 3 bulbs for replacement, I removed the top cover.  This and the bottom cover had already been removed in a prior repair in my case.  So I don't know if the bottom cover needs to come off or not. There are two screws on each side of the front faceplate.  I removed the top screw and loosened the bottom screw to allow the faceplate to tilt forward off the edge of the bench.   Faceplate Screws   Be careful when tipping the faceplate forward.  I managed to break one wire out of a connector on the bottom right side.  I was able to reattach the wire with the faceplate open.  It had just gotten hung up on the chass

Kenwood TM-D710GA Low Deviation_Troubleshooting

 I went out to Quartzfest in late January to support the ANSR balloon launch, and spend some time camping with friends of ours. While there, I was presented with the opportunity to purchase a Kenwood TM-D710GA APRS 2-meter/70-cm dual band transceiver for a price considerably lower than market value. The ham selling the radio stated that it had low audio, but otherwise worked well.  I had been looking for one of these on the used market for some time, as it is widely considered to be the cream of the crop in APRS transceivers. I picked up the radio at a convenient meeting place the next week.  I learned then that the actual reported problem was low transmit deviation, no low receive audio.  No big deal, just a different troubleshooting path. When I got the radio home, I powered it up and confirmed the problem as low deviation.  It didn't seem to matter if I was using the microphone input or the audio in through the accessory jack (intended for echolink, according to the service manu

TS-790 TCXO Adjustment

 In the past year or so, I have noticed that the displayed frequency for both the 144 Mhz and 430 Mhz operations of the TS-790 have been considerably off. The frequency for both units is derived from the 144Mhz PLL unit.  It has a 10.24Mhz TCXO that is multiplied by 10 or 12 (depending on band) and mixed with various other frequencies to arrive at the final target frequency. The Service Manual suggests monitoring the x10 frequency of 102.4 Mhz at TP5 on the 144Mhz PLL Unit, and adjusting the TCXO there.   10.24 Mhz Probe Point   I have an HP5334 Counter that is tied to my external GPS locked 10 Mhz standard.  This is a very stable and accurate counter, but it is not able to count above 100Mhz without an option that I do not have installed. Still, it is the best option I have for setting the PLL frequency of the TS-790.  So rather than adjusting the PLL at the 102.4 Mhz test point, I adjusted it at the 10.24 Mhz output of the unit.   10.24 Mhz Oscillator Off Frequency   The unit was off

TS-790 9600 Baud Mod

I was working on a 2-meter FM radio that I have recently acquired. It had a low deviation problem, which requires a method of measuring deviation in order to fix it and adjust it to specs. While I do not have a service monitor or other direct way of measuring deviation,  I happened to recall that I had made a 9600 baud modification to my old Kenwood TS-790 that I have had since the late '90s. The modification brought out connections that allow direct FM modulation (no pre/de-emphisis), and a direct discriminator output. If I could connect an instrument to the discriminator output, I could use my HP 8640b RF Generator to feed an FM signal to the radio at various deviation settings, and record AC voltage measurements of the audio output from the discriminator.  These could be plotted on a chart and used as a reference to measure the deviation of other radios when they transmit an FM signal. Digging through my old documentation, I found a schematic on which I noted the modification. F