You sure that thing's driving into a solid 50 ohm load that isn't heating up and changing characteristics? I won't fire up a MII PA into anything other than my 500W Bird load... seen too many cheap 100W "brick" style loads that aren't 50 ohm, aren't consistent, etc. I'm also concerned about your input voltage. Is it sagging significantly? That's another simple thing to check... what's the current draw while it's doing that "ramp up" thing?
Two other posts covered the solder joints. Vast majority of the problems I've seen are those after these things are 20+ years old... with the help of Scott W0KU, I've started tearing all my "rebuilds" all the way down to the heatsink, scraping off and cleaning off all the old heatsink compound, retorquing all the screws to the specifications in the LBI when re-assembling, etc. Basically complete "rebuilds", including those pesky solder joints. The PA's that have had this "treatment" survive. The others, not so much. I also won't run them without isolators anymore. But it sure sounds like the protection circuitry/power control circuitry is involved if heat and time allow it to come up. Some component changing value. See any burn marks, discolored components, or otherwise? What happens if you crank on the power control while it's "coming up to full power", does it have any effect? Is it "linear" or does it jump around a lot? Sure "sounds like" a capacitor charging up very slowly in the circuit somewhere, doesn't it? You can easily "swap" the major sections if you're doing the "full teardown/rebuild" method to see which of the three major sections the problem is in... the "driver" section (on the left where the RF input comes in from the exciter, the "power" section (the middle with all the wilkinson divider "stuff" and power transistors, or the LPF board (usually not a cause of problems, but... mentioned here for "completeness), just by swapping those three components around between PAs/heatsinks. The power control on the UHF's is beyond the "driver" section, in the "power" section (if I remember correctly). The driver just runs flat out with whatever it's been "handed" from the exciter, and power control is done on the "power" board. The VHF's are more persnickety and have a feedback loop and an adjustment for that circuit on the "power" board, and the power control IC over on the "driver" board, monitors an SWR bridge over on the "power" board, I think. Been a year or two since I've had to go through any of them... I had one where there was VERY subtle damage that was causing all my headaches. The black rectangular power control IC chip had taken some kind of surge (probably lightning) and blown up internally, and the only outward sign was a slight bulge and the case of the IC was SPLIT down the seam line at the top. VERY hard to spot. But that PA never acted right! :-) Nate WY0X On Nov 23, 2009, at 4:27 PM, Adam Feuer wrote: > Hello All, > > I have on the bench into a dummy load an exceptionally clean Mastr II > 100 watt UHF PA. It appears to be a "late model" version because it has > the Z-Match board right above the RF output connector. > > At 13.6 volts and 21 amps, it's doing 100 watts easily. I can vary it > down all the way and vary it up to about 128 watts. So, to me it seems > like it's in good shape. I left it key'd for 30 minutes and the current > and power output stayed the same, 21 amps and 100 watts. > > After letting it cool, I key'd it up again. It immediately went up to > only 20 watts, then after about 30 seconds it hit 75 watts and finally > after close to a minute and a half, it hit 100 watts. I let it cool > again, and same thing. This appears to be the way this amp is as > nothing I can do can get it to change. I checked the exciter and > immediately upon being key'd it puts out 220 mw. > > Any thoughts or suggestions would be greatly appreciated. > > Thanks!! > > Adam N2ACF > Nate Duehr [email protected] facebook.com/denverpilot twitter.com/denverpilot ------------------------------------ Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/Repeater-Builder/join (Yahoo! ID required) <*> To change settings via email: [email protected] [email protected] <*> To unsubscribe from this group, send an email to: [email protected] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
