WB4TUR 2/12/2000


  1. The TX RF PA final tube is VERY fragile. Seems to be more SHOCK sensitive these days due to aging. Do NOT drop the mobile radio, transmitter strip or tube(s). I've seen final tubes go bad just by the radio falling over on its side.
  2. Do not shock vibrate the TX strip in any way while or after tuning it up. I've seen tubes go bad by tapping the tx rf pa area with a screw driver. I've seen the rf pa tube go bad by placing the lid back onto the tx strip.
  3. I would do any major vibrational work AFTER the tx strip and filament has cooled down. This only seems to be sensitive wth the final PA ceramic tube. The other two tubes dont seem to be as sensitive, and I've rarily seen one of those go bad at all.
  4. If the receiver or transmitter first tuning points dont work, make sure you've got the xtal in the right position (1 of 4 possible). Also, you must apply 10VDC to the RX and Ground the TX to select F1. (see pinout on the ac ps)
  5. There is an interlock screw terminal on the AC power supply. Make sure the interlock is bypassed if you are working on the radio.
  6. Check the 12.6 VDC and 10 VDC regulator voltages, make sure they are correct. Carefully measure the
  7. 300 VDC and 660 VDC to see that they are proper.
  8. Older AC EP38s had large paper electrolytics. Replace these with never caps.
  9. Never try to run a repeater with the mobile DC-DC power supply. It will fry even under intermidiate repeater duty cycle use.
  10. The 12.6 VDC supply fried on one of my EP38's. I redesiged this circuit using newer linear adjustable regulators. The 10 VDC regulator can be made out of an 8 VDC 3 terminal 7808 with two 1N4001's in the ground leg of the regulator .
  11. Some tubes when new are capable of MORE than the rated output power. DO NOT do this. They are expensive and will burn out very quickly.
  12. Tube transmitters are cleaner than solid state transmitters. You may get away with split antennas or less duplexer isolation using them. The newer solid state radios such as the GE master II generate more side noise and require more isolation for repeater service.
  13. The TX power output will vary more with the load than with a solid state transmitter. You need to keep a closer eye on the output power of the Master Pro.
  14. An RF PA tube going bad may start to generate nasty noisy outside the ham bands. We once had a VHF 145 MHZ ham repeater get into an FAA control frequency at a big city site. The tube had been getter weaker on its output and we hadn't replaced it. The station was immediately shut down and proper tube replacement was done.
  15. I would NOT recommend going out and spending $125 for a new RF PA tube, unless your club has a very good budget. Instead, go buy a complete used mobile radio for $25-$50. Almost all of these still work. One ham I bought some tubes from had bought a stack of mobiles for cheap. He then learned they had been serviced and retired without ever returning to service. A stack of brand new tubes!
  16. The COS output of the RX is right NEXT to the 117 VAC line in on the AC power supply strip. NOT a very bright idea in my opinion. I've fried more than one RX audio board when a jumper clip came off at the wrong moment. (2N3904's seem to sub nicely for the transistors that die when this happens).
  17. For some reason, I've had little luck with the GE PL decoders & encoders used with the master pro.
  18. Instead, I wire in a TS 32 inside the RX. I bring out the PL encode line out the main RX strip connector on an unused pin and route this to the TX for PL encode. The TS 32 can be wired in such a way to work precisely like the original circuit, but with the added capability of complete dip switch programming of the tone.
  19. You need to pay close attention to the tuning in the TX PA area. It can be mistuned and drastically shorten the tubes life. I would say the TX RF PA is the most challenging to tune in the radio.
  20. There is a nice GE EP39 rx only power supply available. This was used for satellite voter rx sites. It is a 19" unit about 5-6" high. The rx strip bolts into it. Some have a built in AC power supply which provides the 12.6 VDC and 10 VDC required by the RX.
  21. I have been told from multiple sources that the RX strip really doesnt need the 10 VDC and will run on a well regulated 12 VDC. Thus only needing one power supply voltage. The 12 VDC only ran the audio output stages. Due to mobile use, the 12 VDC wasnt regulated, so they used a 10 VDC regulator for the more critical circuits.
  22. If you are canibalizing the mobile radio just to get the RX strip. Save the PA tube(s). Some ham out there would sure love to have them. Also, the radio has a nice little RF antenna relay that would come in handy for a furture project. Its easy to separate from the mobile. Be sure to keep the black rectrangular connectors which connect to the pins on the rx & tx strips. Cut these loose and leave on the tx/rx strips.
  23. GE used to sell just the 20 pin connector blocks which were used on the tx/rx for interfacing to the outside world. These are filtered connectors and are great for getting into and out of shielded boxes for other repeater projects.
  24. There are conversions available for the following:

(These instructions may be posted to my website at a future date. )


If you have some tips you'd like to post here, please email me and I will be glad to add them.


73's WB4TUR