This beautiful Kenwood TS-590 came in with the owner reporting that it was deaf on 10m.
A quick check showed that it was exceptionally deaf on 10M but also quite deaf on all of the other bands too. On 10M no reading was possible because the radio was so far off frequency. 30 Hz are OK in SSB but this was 150 Hz off. In the table below we can see the transmitter power (measured in dBm) required to give a SINAD reading of 12 dB. This is a fairly popular method of measuring receiver sensitivity and a lower signal level (larger minus value) is better.
Some of you may be more familiar with uV as the input signal level. In this table, you can see the relationship between uV and dBm for a 50 Ohm system.
Now adobe and a computer screen are fine for looking at schematics but sometimes you need the big picture to see the whole chain and narrow in where the issue may lie. That's when scissors and some glue come in handy to paste the pages side by side so the signal path can be followed with ease.
Kenwood have done a fab job with this schematic, including bold marking of the signal path and easy to read D.C. voltages for components that get switched in and out in the Tx and Rx path.
So at the top right of this schematic, are the A and B receivers, one above each other. As both receivers had the same issue, the fault must lie before that point. By a process of elimination you can narrow down the fault to a very close area. In this case it was before diode D108, marked yellow in this picture above. All other sections were eliminated considering what they do and how you might exclude them. For example, the second page in the pic above, from left to right, shows the band pass filter. As we were now getting bad values across the bands, and not just in one band, these could be excluded too. This left something between the input and the Diode D108.
In my previous post about a repair of a Yaesu FT-7900, the mic mute transistor was at fault. In this circuit portion we had a similar Rx mute transistor Q103. Could it be that?
A quick test with my voltmeter on R107, showed that on one side I had my 13.5v but on the other side I only had 9v. It seems the transistor was slightly on and thus pulling down the Rx signal to ground via C117. As this component lay on the underside of the board, I had to remove it to get at it with my hot air gun.
If I zoom in your can see where the transistor was situated.
These components can be resoldered back in with the help of a good microscope and a steady hand. As the owner had these tools, he will refit the new component.
After the component was removed, the measurements were repeated again on all bands.
Much better. Finally the radio was tested by listening to a 10M a nearby beacon sending out in morse code. An S8 signal, showed the radio was back in form and ready to provide enjoyment to its owner for many years to come.
