TS-930 (and TS-940…) Power amplifier repair
(or how to replace bipolars by FET’s…)
Introduction:
Kenwood made a big mistake by putting two MRF485 transistors rated 25 V DC Vce max in the driver stage of these great rigs running on 28 V. It is still a mystery for me how it ever worked but there is a huge collection of broken TS-930 and TS-940 around this globe waiting for repair. Usually the 28 Volt regulator transistors fail due to heat (use 240 Volts setting instead of 220 V) and these broken series regulators put 36 Volts on the final module. Boom!!!! The powerstage MRF422 survives, as well as the predriver, but the MRF485 goes into smoke after milliseconds with a short circuit between C and B.
Start by replacing the two shorted regulator transistors 2N5885 by the far better and cheaper 2N3771. Check for 28 Volts.
Don’t try to find spare MRF485 transistors. They are no longer made and the ones you can still find at an indecent price are NOT SUITABLE due to a far too high HFE causing instability and oscillation. The otherwise excellent article by W6NL Dave Leeson suggests using the NTE236 but this ‘ersatz’ one is also having the same low Vce of 25 V DC. Unless you want start by blowing up three pairs of these 15 $ transistors like I did, you better believe me, this is a bad replacement part….
Halleluia….There is a far better AND CHEAPER solution, putting in FET’s instead… I started on my broken rig number one (I collect them J …) by using IRF730’s. IRF710 would be a better choice but no longer available in Europe after Radio Shack closed down. After replacing some other parts I got 100 watts out on all bands from 160-30 meter, but only 50 watts on 20 and QRPp on 10 meter… although better than no output…but not a perfect solution. Boah…
One day (a sleepless night and one bottle of 1998 French wine) later…however, broken rig number two ( J …) was modified using the even cheaper but well available P4NA80 by ST. The lower gate capacity of this 0.25 $ device gave 100 watts on all bands except 80 watts on 10 meter YES !!! (oops, bottle again empty….)
The mod:
start by rotating both bias pots fully CCW. Replace zener D5 by a 1k load resistor. Replace R13 3.9k by a 1k resistor. Take out the (burned?) R9 and R10 both 22r. Take out the ferrite bridge between these two resistors and mount a 8.2 V 400 mW zener between a ground hole from one of both now missing resistors (anode) and the other hole from the now missing bridge (cathode). This zener replaces the original series of D2 and D3 no longer used. Fets don’t require diode heat sensors. Remove both MRF485’s and put the rig back on. Check if you can adjust the driver bias voltage somewhere between 2 and 6 volts approx. If not the Q6 2SC496 is gone (not in my case but could be…) Turn back the bias pot CCW. Now take a quick nip of the freshly opened bottle Bordeaux 1998…
Perhaps you don’t believe me but all feed-through platings ARE BAD !!! Kenwood really messed up things those days… Cure: put a 0.3 mm silver wire through all of them, especially at the two groundings at the former MRF485 emitter place and at the base and collector of the MRF422’s. Put the P4NA80’s in the place of the MRF485’s after bending the gates completely upward and cutting off the middle pin (drain). Solder source to the groundlug (with the wire in it). Solder one end of a 10 ohm resistor in each base connection (both sides of the PCB) of the former MRF485. (all resistors ¼ W) The other end is going to the ‘flying’ gate of the P4NA80 preventing oscillation. That’s it… Put an Ammeter in the power amp red wire supply. Initial current during TX with NO DRIVE is 220 mA. Increase the bias of the FET predriver to 100 ma = 320 mA total consumption by adjusting the bias pot VR2 (about 3,7 V gate voltage). Adjust the final bias to 500 mA = 820 mA total consumption by pot VR1.
Finish by readjusting the IF/RF unit VR8 to 110 watts max after setting the current limiter VR10 to 11 amps while slightly overdriving the amp. Close the rig and empty the bottle Bordeaux.
Who ever said technical articles have to be boring, hips….
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