Sent to me by the author: Nelson Ricardo - PY1NR
Dedicated to the Detailists
Are you a TS-940S Rx lover? You should know that it can be even better if you do some corrections.
Two errors that I found, both also showed in the Service Manual, Revised Edition.
See the FETs Q10 (RF PreampRF Preamp 2SK125, N-FET, 25V, 0.1A, 0.3W)
and Q4 Q5 (2nd balanced mixer, also 2SK125).
The picture above was scanned from the TS-940S Service Manual, Revised Edition.
In these pictures the FETs are in the wrong position.
I can assume that thousands of other TS-940S are also waiting to be corrected and perform
Rx even better (mistakes "hidden" for almost 20 years!)
Check yourself with instruments, it can be probably more than 10 dB of gain and a better
AGC performance.
Remark: Itīs also very important to check any bad connection, any intermittent problem before
these two corrections. Other problems may mask the effects of the two corrections described in this site.
See the Level Diagram in the Service Manual. With the FETs on the right position,
we would have 12.5 dB (17 - 4.5) gain in the RF Preamp, and 17 dB (44 - 27) in the 2nd balanced mixer at the
indicated points in the picture above. These gain values are related to the 0 dB input.
It would be interesting to measure these gains before the correction. With this measurement we would have
an idea about the sensitivity losses due to the two errors.
.
I laugh a lot when I confirmed both errors! Almost two decades waiting to be corrected!
As I never heard about this problem, I decided to reveal it to the whole world.
My radio was working apparently well. This is the 1st TS-940S that I could observe and analyze in detail
its circuits. First I found the RF Preamp FET in a odd position, but only few days after I could focus and
see that the cascode circuit was correct in the schematic diagram and wrong in the PC board. It was also
wrong in the PC board view pictures in the Service Manual.
I also noticed that a FET was hotter than the other. It was as simple as de-soldered and turn the FET 180°
and re-solder it all over again.
I did some tests with the 2SK125 using the Source and Drain swapped and I noticed some differences,
therefore I don't beleive that this FET is simmetrical regarding Source and Drain.
This picture shows the FET in the correct position.
The RF gain increased and the noise decreased. Very fine! But, in the middle of the night, excited with this
amazing discovery, I kept on thinking if this error was the only one.
I got back to the Service Manual, and following the circuit diagram, checking the PC boards, I found a second
mistake: in the 2nd balanced mixer, in the IF unit, another inverted FET (Q4), harming the mixing/conversion,
the gain, and the AGC performance.
This picture shows the FET in the correct position.
By eliminating these "losses", my famous TS-940S "won" 10 dB more! Because of that, I had to made
S-Meter adjustments.
I suggest you to revise the famous TS-940S receiving performance to be even better, of course!
Our next step will be identifying different N-FET's for replacement. The 2SK125 is discontinued
since 1996. Probably by using modern FETs, the receiving performance will increase. Who knows?
Also, we would like to ask everyone who has some 2SK125 available. If you have some, please send us.
We don't have this FET in Brazil. We will start experiences immediately in order to clarify the issue regarding
the differences between this FET with Source and Drain swapped and in the correct position.
I recommend voltage measurements before and after the correction.
The modifications were verified and tested by myself, so if you try this mod on your own equipment, it's on
your own risk.
Have fun, revise your TS-940S! Smile!
Nelson Ricardo - PY1NR
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