Re: UHF FM Transmit Power

Steve W3AHL

On Fri, Mar 23, 2018 at 01:04 pm, <k6msm@...> wrote:
Thank you, both.  I will look into this.  Is it something I can replace with my unsteady hands or should I send to Icom?

Also, the ALC meter doesn't seem affected by changing mic gain on FM.  On 2m it's always too high (beyond the scale) and at 440 it's always 0.  Not sure if this is ok and/or related.

FYI at 100% power it only puts out 8W on 440.  I probably should dial back the RF % until I get it fixed to avoid further damage.

The ALC meter acts differently on constant carrier modes like FM and RTTY.  A high reading on 2M is OK, being 0 on 440 means that since TX output power is too low the ALC circuit is not trying to reduce the IF gain to maintain the desired power.  

Replacing the UHF PA is relatively simple.  But getting to it takes a little work.  The driver board is soldered to the PA board, so it has to be unsoldered.  Make sure each pin wiggles before trying to remove the PA board, otherwise you may pull a plated-through hole out, which will have to be repaired.  The PA board has ribbon cables that attach it to the Main board.  Carefully pull them out and make sure the plastic stiffener on the back side remains  attached.  On older cables it often comes loose, in which case the cable should be replaced.  It centers the cable in the connector and assures the contacts are properly aligned.  

After the unit is back together you will need to do the TX Adjustments in section 4-4 of the Service Manual.  Specifically you must do:

  1. DRIVE/FINAL IDLING CURRENT -- Step 1 &11 Final Idle Current (430M)
  2. TX TOTAL GAIN -- Step 1 & 7 TX Total Gain (430M)
  3. TX OUTPUT POWER (430MHZ) -- Steps 1-5.
For all of the above, read the section 4-1 and make sure you have the required equipment for the above steps and understand what to do.  If you are unsure about doing this type of work, send the unit to an authorized Service Center, such as the one in Michigan.  

The UHF PA is sensitive to voltage spikes greater than about 35 volts peak, which are often found on a vehicle's 14 VDC battery supply, due to the starter motor counter EMF when starting the engine.  Since I started using a Littelfuse TVS (transient voltage suppressor) in my mobile RigRunner Power Pole distribution units, I have had no further UHF PA failures in over 10 years.  There is a document in the Files section that describes how to add a TVS diode to a RigRunner to clamp the power spikes to a safe level.

Steve, W3AHL

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