Re: UHF self oscillation

Steve W3AHL

I've had some intermittent problems in the UHF section after working on it (especially older units pre-2006), but not self-oscillation that I can remember.  Here's a few things you might try, in no particular order:
  1. The coax cables that connect the DDS to Main and Main to PA have connectors that can become intermittent due to poor contact of the shield braid to the outer shell of the connector.  Try flexing the cables near the connector in several directions to see if that makes the problem occur.  if so, replace the cable.  Order several -- they're cheap.
  2. The ribbon/flex cables that connect the boards don't age well in my experience.  I've had intermittent problems with them.  If the plastic spacer on the back of the cable that centers the contacts on the cable traces comes loose the control signals can be intermittent.  Again, try moving the cables a little from side to side near the connectors.  Unfortunately it is usually the ones on the PA board that you can't reach that are problem for me....  If you tried to reglue any of the spacers, that's probably a problem.
  3. Make sure all of the grounding fingers on the circuit boards are bent up so they make good contact with the case.   Also check the grounding clips on the Main board coax and the DDS to Fan mounting screw.
  4. Verify the PCB mounting screws are tight.
  5. If the TX Gain adjustments are too high you might have self-oscillation.  This could be due to the input to the driver from the Main board being too low due to a cable problem or a problem with the UHF mixer circuit on the Main board.  The older units' mixer circuit is a little touchy, with the symptom being varying power output on UHF occasionally.  There were design and PCB layout changes on newer units.  I don't know of any changes released to retrofit older units.  The point is:  if you adjusted gain when the output was low, the gain may be too high.  Dial back the gains a little, one at a time, to see if it helps.
  6. The latest RD60HUF1 parts I got last year were higher gain than the older ones.  If you have a damaged decoupling cap in the UHF PA circuit it may have worked with the old FET, but not the new one.  Visual inspection with a 10X magnifier and good lighting may show a poor solder joint or cracked ceramic case on a cap.  It's a long shot....
  7. If you will record you values for the Final Idle Current(430M), TX Total Gain (430M), TX Output Power (430 MHz) --all 5 power setting values, and the ALC and Drive Min (430) values, I'll compare them with typical ones I've logged to see if they are in reasonable ranges.  Sometimes if one is way out it points to a circuit problem.
  8. In FM mode what is the ALC meter reading with no audio input?
That's a start.  Maybe you'll get lucky....  

Steve, W3AHL

---In ic7000@..., <iw0hex@...> wrote :

Hi guys
I installed a new RD60HUF1 in my ICOM-7000, buyed by RF-Parts.
I followed the service manual adjustment correctly (bias, total gain, power, alc, drive in UHF) but my IC-7000 tends to oscillate at 430 MHz (UHF).
I can see it on my CMT-52 communication test radio (by Rhode & Schwarz); this also happens to little power (10%), with full power I have 35 watt without problem.
This self-oscillation is random, sometimes that happens or not.
Any ideas ?
Thanks a lot.

73, Pasquale IW0HEX

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