High power XPR4550 maxed out at ~20 watts

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Norman
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Posts: 58
Joined: Sat Nov 15, 2008 9:44 pm

High power XPR4550 maxed out at ~20 watts

Post by Norman »

Hi. I have a few questions about my 403-470 XPR4550. It's a 20-40 watt model, but I've never been able to adjust it beyond 20-22 watts. The model number as seen in CPS and on the sticker indicate it's the 20-40 watt model.

The PA bias current is set correctly. One thing I find suspicious is that TUNER won't accept the correct measured wattage readings I enter in the softpots. For example, a given softpot value might be set for 20.xx watts, but I measure only 17.xx watts. I am able to lower the values but only by a few hundred milliwatts -- not enough to be correct. After taking and entering measurements, attempting to WRITE to the radio using TUNER displays a message that my entered value(s) are out of range. I have no choice but to re-enter original values.

The radio otherwise performs as it should. On-frequency with good deviation. On a side note, the radio had a much older firmware version. I figured it couldn't hurt to upgrade it to the latest (R1.12.x), and though the upgrade went smoothly, it didn't have any effect on the transmit power issue. I'm out of ideas as to what might be wrong, so I'm wondering if my P.A. is defective. If so, does anyone have any experience or tips replacing the P.A. transistor on these radios?

Any suggestions will be appreciated.

Thanks,

Norman
W6OG
com501
Posts: 1063
Joined: Fri Nov 02, 2001 4:00 pm
What radios do you own?: Over 50 - All Motorola

Re: High power XPR4550 maxed out at ~20 watts

Post by com501 »

Make sure that in the radio programming, low power watts and high power watts are both programmed at their factory min-max values. 20w for low and whatever max is for high. The will sometimes affect tuner. You should run the radio through autotune on a service monitor. The tuner help file never worked for me. Autotune ALWAYS works.
Norman
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Posts: 58
Joined: Sat Nov 15, 2008 9:44 pm

Re: High power XPR4550 maxed out at ~20 watts

Post by Norman »

Thanks for your tip. Low was set to 20.00 and high set to 28.50, so I set high to 48.00. I backed out of CPS, switched to TUNER, but it still won't do more than 20-25 watts. I took measurements using a Cushman (no-autotune capability) as well as two stand-alone watt meters.
n1gtl
Posts: 113
Joined: Tue Mar 25, 2008 12:43 pm
What radios do you own?: XPR5550, XPR7550e

Re: High power XPR4550 maxed out at ~20 watts

Post by n1gtl »

I assume you're using it in analog? Digital output reads lower because it switches on and off rapidly and analog meters can't accurately keep up.
Norman
New User
Posts: 58
Joined: Sat Nov 15, 2008 9:44 pm

Re: High power XPR4550 maxed out at ~20 watts

Post by Norman »

Yes -- I took measurements on analog frequencies with the mic PTT as well as via the KEY TRANSMITTER button in TUNER. I believe TUNER uses analog transmission for softpot adjustments. According to the service manual, the final P.A. (Q0702) is an MRF1570 and is described as 'consisting of two parallel high-power devices within one plastic package'. I'm wondering if one of the two devices might be defective causing the low TX output power.
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