Harris 25K output meter

[brandonb959]

Hello all,

The Harris 25K that I had worked on for a while and have posted about on here has been on the air since July with really no issues. Recently we began having problems with the output meter. The Forward/Reflected Switch had dirty contacts and so I replaced it with a basic toggle switch (better than sending Harris a ton of money) the schematic just showed a basic dpst switch. The meter worked intermittently even with the old switch, which you could tap on and it began working again, but now with the brand new switch the meter works sometimes, but not others? I have measured around 8vdc across the meter when the switch was set to forward power and about 400mv when it was set to reflected power, even thought the meter was not working. These measurements seem to be about right. It sounds to me like the meter is going bad, but just wanted a second opinion.

[BroadcastDoc]

Certainly sounds like a bad meter to me!

[radio_guru]

It sounds like the meter is going bad. Yes, even meters do periodically fail.

Get the check book out however as that meter is likely >$500.

Also, keep in mind that since you will be changing the calibrated power measuring display instrument, you will need to recertify the accuracy of the transmitter's power. That is unless you have a calibrated external direct read power meter such as a Bird or Coaxial Dynamics in-line meter as your primary power monitoring.

Before simply entering in the station's Maintenance Log that you verified the accuracy against the old meter, keep in mind the old meter is not working. So what are you calibrating against if you don't have another meter of known working quality?

In as much as the power meter provide an indication of the TPO relative to the license, it's also an important diagnostic tool in general operation. If you can get your hands on a calorimeter dumy load, you would do well to go through the exercise and verify the new meter's calibration against a good standard. That's good engineering practice.

RR

[jammerdave]

Hello all,

The Harris 25K that I had worked on for a while and have posted about on here has been on the air since July with really no issues. Recently we began having problems with the output meter. The Forward/Reflected Switch had dirty contacts and so I replaced it with a basic toggle switch (better than sending Harris a ton of money) the schematic just showed a basic dpst switch. The meter worked intermittently even with the old switch, which you could tap on and it began working again, but now with the brand new switch the meter works sometimes, but not others? I have measured around 8vdc across the meter when the switch was set to forward power and about 400mv when it was set to reflected power, even thought the meter was not working. These measurements seem to be about right. It sounds to me like the meter is going bad, but just wanted a second opinion.

Might be worth checking, but I used to have lots of problems with static on the 25K meters causing them to not move, or hang. A bit of static spray on a rag solved it..

df

[awsherrill]

I had a PA-V meter on a Gates-One go intermittent like that...it would quit working and tapping on it would eventually make it work. I carefully pulled the transparent bezel off the front, and found that the internal screw terminals had vibrated loose. Tightened those and problem solved.

[brandonb959]

I called Harris this afternoon and they said the evidence pointed towards a bad meter due to constant voltage on the meter terminals even when the meter was not working.....I ordered a new one for about $120

@awsherrill-I didn't see your post in time...I will certainly take a look at the old meter when it comes out

[w9wi]

Might be worth checking, but I used to have lots of problems with static on the 25K meters causing them to not move, or hang. A bit of static spray on a rag solved it..

We had this problem with a Harris TV transmitter in Wisconsin.

[Kelly]

Maybe I'm completely behind the times, out of the FM loop, or out to lunch period, but unless one reads power via the Direct Method using a calibrated sample (which typically is never the output meter on the transmitter), you use the Indirect Method of power calculation: Ep X Ip X published efficiency Factor. Isn't the meter on the transmitter pretty much just used as a reference? At least with a tube-final transmitter, assuming your transmitter is tuned and operating correctly, isn't measuring power via the Indirect Method actually preferred?

[RGORJANCE]

Years ago when still selling for H, I recall that the output meter calibration from final test at the factory was legal for direct msmt of TPO for one year. After that period of time, you switched over to indirect for the purpose of calibrating the output meter.

If you had a dummy load with wattmeter, you could use it for direct msmts for one year, and after that you were required to send the slug/wattmeter back to be re-calibrated the at the factory.

Fossil

[ChuckG]

Isn't the meter on the transmitter pretty much just used as a reference? At least with a tube-final transmitter, assuming your transmitter is tuned and operating correctly, isn't measuring power via the Indirect Method actually preferred?

That's the way I've always done it. The Efficiency factor off the final test sheet has always been acceptable to visiting inspectors, but without some sort of calibration docs for the power meter, using that for a final determination may not be.

I DO use the wattmeter output for remote power readings, but it's calibrated to the figure calculated indirectly.

[Deep Thought]

you use the Indirect Method of power calculation: Ep X Ip X published efficiency Factor. Isn't the meter on the transmitter pretty much just used as a reference?

This is pretty much how everyone does it except for some low power (<1 KW) transmitters which do not reveal Ep or Ip or eqivalent directly. In those cases you can use the power meter if the manufacturer provides calibration info.

There is no prescribed schedule for recalibration.

[brandonb959]

New meter works great! I tried taking the old meter apart but it started to crack on me, so I gave up.

We have an external bird wattmeter that I use as a point of reference for calibrating the power output meter on the transmitter. TPO on bird = 100%, simple enough?

[Deep Thought]

The Bird is not a primary reference. The actual procedure involves some specialized test equipment used to calibrate the Bird via caloric output (basically a calibrated dummy load which measures the heat produced) which you can then use as long as nothing else changes, and then you can use that documentation as a paper trail to the primary reference.

In practice, I know of nobody who has actually done this for over 30 years...and I doubt any FCC field agent gives two squats as long as what you have makes sense. The transmitter's output meter, however, is at the bottom of the acceptable list for a primary power measurement. Indirect power is preferred since any error will cause the power to be too low, not too high. Use that to calibrate the transmitter meter and document the process.

[brandonb959]

I have setup a sine systems for indirect power measurement before when the transmitter's power output reading was not working properly....but that was just for my own personal reference.

Are you legally required to have a watt meter on every transmitter? or just required to have the transmitter output meter properly calibrated?

[ChuckG]

Are you legally required to have a watt meter on every transmitter? or just required to have the transmitter output meter properly calibrated?

Neither, although grossly uncalibrated output metering is not territory I'm comfortable in either, even if the meter is disregarded in day to day operation.
For AM's relying on the base current/common point current meter is appropriate, for FM's the indirect method is just fine. Trying to keep the transmitter output meter calibrated well enough to use as primary reference on some boxes is an exercise in futility. And calibrated to what? Either a calorimiter (who has one?), or you are back to the indirect calculations anyway. Might as well just use them as primary reference and close that door to unnecessary questions.
On Am's the transmitter output is not as relevant as the antenna input.