Tuesday, 31 March 2015

Trio TS-120v repair - part 1

I recently picked up a used Trio TS-120v that was listed as 'spares or repair' on eBay .
The TS-120v is a low power version of the TS-120s HF transceiver and it provides 10w of USB/LSB/CW on the 80/40/20/15/10M bands.

On receipt of the TS-120v I powered it up and connected it to my HF antenna - it received well on all the HF bands which was promising , next step was to try the transmitter.  The radio did not come with a microphone , so I wired up an old Shure 527B microphone using the wiring information available in the TS120v operators manual. When I tried to transmit into a dummy load there was no output on any of the bands on either SSB or CW.

The radio was opened up to perform a visual inspection - it was covered in dust and dirt - not surprising considering it is more than 30 years old.  An aero-duster removed most of this and some servisol switch cleaner was used to clean the wafer switches used for band switching.

Once the dust was removed I could see that there was an electrolytic capacitor (C63) missing from the RF board - looks like the capacitor died violently as its legs were still left in the PCB.

missing C63

No sign of it loose in the radio though. The TS-120v service manual was obtained from the very useful Kenwood TS120 and TS130 Yahoo group.  This indicated that c63 was a 10uF 16v electrolytic  - more importantly it showed that it was possible to gain access to the underside of the RF unit PCB without having to remove it and the band switching hardware from the chassis .  This is accomplished by removing the IF unit and a shield plate.

Access to rear of RF unit under IF board

Once I had access to the rear of the board I de-soldered the old capacitor legs and replaced the missing capacitor. Shining a strong light through the PCB from the top down allowed me to see which solder joints belonged to C63.  Unfortunately the replacement of C63 did not resolve the TX problem , so further investigation was required.

Next step in the investigation was to remove the final board from the radio and check the output transistors. On removing the final board I noticed several problems -

Burnt R3 and R5

- Resistors R3 and R5 on the final board were burnt looking and R3 had actually split in half.

- There was a 3 pin connector at the end of a cable from the filter board that was not connected to anything.

- The final board appeared to be installed back to front , the coaxial cable from the RF Unit was connected to the output of the Final board and the cable from the filter unit was connected to the input of the Final board.

unconnected cable from filter board J44

It looks like someone had attempted to repair this radio and had re-assembled it incorrectly .

First step in attempting to repair the final board was to replace the burnt out 15 ohm resistors R3 and R4. At this stage I also tested the final transistors and found that these were both faulty.

Final transistors Q2 and Q3 are 2SC2509 NPN RF power transistors - I wasn't able to find exact replacements , but a search on eBay turned up a matched pair of 2SC1945 transistors which are a direct replacement. Having a matched pair ( same gain) is important in an amplifier such as this that only has a single bias adjustment for both devices.

matched pair of replacement final transistors

I replaced the final transistors and applied some fresh heatsink compound.

Next I needed to find out where the 3 pin cable was supposed to go .  From the service manual I can see that there is a Q5 transistor on the final board that is connected by a cable J41/44 to the filter board .

Schematic of final and filter board TS-120v
This is a 2SD235Y switching transistor that is mounted beside the finals board so that it can share the heat sink. Another trip to eBay found a suitable replacement.

Final board showing mounting of Q5 on right.

At this point it was time to set the bias on the PA .  The manual for the TS-120v only had a bias adjustment procedure for the TS-120s which has a different PA  - 100w instead of 10w, however the TS-120v shares the same final board with the later TS-130v and the service manual for the TS-130v does contain a bias adjustment procedure.

ts-130 bias adjustment procedure 

rather than remove the bias link on the board I just disconnected the supply line from pin 14A and inserted an ammeter in line.  To provide some degree of protection against the possibility of blowing my new PA transistors I placed a 2A fuse in the supply line to the radio.

On switching to TX I could not measure any bias current flowing , so I probed the base voltage of the 2 final transistors and measured only 0.05v irrespective of the setting of VR1 . It should have been somewhere around 0.7v .  Further investigation found that the bias transistor Q4 was also faulty. Back to eBay to buy a replacement 2SC496Y transistor.
I replaced this and went back to setting the bias current , this time I was able to adjust VR1 for 100mA as per the manual .  Things were looking up !

After re-assembling the final board into the radio I tried to transmit again - now I am getting between 8 and 10w out on 80/40/20m on SSB.  Not quite there yet though, only seeing about 2w on 15M and nothing out on 10M.  Also not getting any output on CW but it is still a lot better than when I started .

More to come .....