Ts-930/940 repair

In January I got to repair the TS-930S from my club ( Garden State Amateur Radio Club ).After opening the case I found that the problem is in the power supply ( 28Volts ).

After inspection and measurment following parts need to be replaced:

- both power transistors MJ802 ( used MJ15003 as replacement)

- transistor Q1 - 2SB861 - ( used NTE332 as replacement )

- zener diode MTZ24JA

- zener diode -RD33F80-B1 (33V/1W ) - I used 1N5364BG ( same Voltage but 5W )

- resistors - 2.2 ohm /500mW

One of the problem with this power supply is when power switch is turned off the 28V output voltage increses for a while and then decreases. This is not good for PA module ( drivers transistor ).This could be fixed by adding 3 resistors in parallel 3.3k/1W from collector of Q1 to ground .

Above power supply module after repair.

After connecting everything back and adjusting the output voltage ( 28.5 volts) power supply began to operate properly but no transmiting - no output power.

After taking the measurements I found that one of the zener diode ( 35V/1W) is bad and both drivers transistor are bad too.Orginal transistors are MRF485 with low beta.Those transistor are not available now. I used NTE236.Don't forget to re-adjust both final and drivers bias. For final ~1.3A and for drivers ~60mA.Also check IC chip on the PA module ( 723 stabilizer ).In my case the 723 was bad and burned out the NTE drivers. After replacing MA723 and NTE236 again and re-adjusted bias the power come back to 100W on all bands.

After few hours of operation power drop to 50 watts on 160m and 80 watts on rest of the bands.In this case i had to do full aligment according to service manual .

NOTE !!! Please find service bulletin from keenwod webpage and adjust PLL with new voltages.

Final photos with TRX ready to go.