Batboard

I have been working on a spectra hi power vhf drawer unit. I was checking the output power today. At the 146 test freq it does 100 watts, at the mid level test freq it drops to 60 and at the 170ish hi level test freq it drops down to about 30 if that.

I resoldered the transistors on the PA and still the same issue.

Anyone have any suggestions.

Jason

Is this measured in the vehicle or on the bench.

If on the bench, do you have a pretty decent power supply?

Amplifiers are less inefficient as you go up in frequency. That means they need more current at higher frequencies to produce the same RF output as it would at lower frequencies.

Your power supply may be just fine making 15Amps which is good enough for 100watts at 146MHz, but the radio may need 23Amps to make 100watts at 170MHz. (these numbers are just examples, I'm not sure what what the exact specs are for a high power VHF Spectra)

Of course, if you have a 35Amp Astron or are running this test in a vehicle with the engine running, then ignore all I just said.

This is in the vehicle.

Jason

KB9KST wrote:This is in the vehicle.

Jason

On a known good 50 ohm dummy load?

Or, as some of us think, using the vehicle antenna?

On a good know 50 ohm dummy load and a bird wattmeter. Tested about 5 other radios during this test session and those worked fine. Only problem is this one radio.

Jason

OK, that narrows it down to the radio. It could be a bad PA, bad power control or even a bad VCO.

Next step is to measure the control voltage at the three points durring TX. The control voltage appears at pin 2 on J1 on the PA board.

Next step is to check the TX current limit in the service section of the RSS. If set too low it can give you the same lower power at the higher frequencies. As Motofanic mentions it takes more current on the higher frequencies.

Yup I upped the TX current level on the bench earlier today, just gotta toss it in the truck and check it.

Thanks

Jason

Ok here is what I have in the service mode of the rss

TX on 146 - 100 watts hi, 50 low
TX on 160 - 80 watts hi, 50 low
TX on 173 - 30 watts hi, 30 low

I have looked at the VCO. It is missing the shield over the hybrid on the bottom side of the VCO. Could someone have modified this for ham use and would that explain the low power on the hi end?

Jason

Are there any differences in the low-split 136-162 PA? Perhaps it's a low split PA with a standard-split codeplug in the MLM?

/shot in the dark

As far as someone messing around with the vco in the drawer, were these units previously owned, or were they bought new for the application they are used for?

If they were previously owned, it certainly is a possibility that it could've been modified for ham, though. But if they were bought new, then I would say it's a different problem than the ham speculation.

I've check already to make sure the right range parts are in the radio. Its a butcher job that one of the board members bought and I am working on getting it up to par. I've had to replace the command board and MLM and bitbang it to get it to the stage its at now. Its all good except for the power output. I am guessing the VCO has been modified.

Jason

Well I found another VCO put it in, same exact result, would the command board or RF board have anything to do with it? Or is the PA just shot.

Jason