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Retuning a Harris 2.5K FM Transmitter
- Thread starter jboyd
- Start date
K
kyscott
Guest
Just tune for maximum smoke. That short of a jump, it should not need much.
I'd call Harris and ask them if they've ever set one up on that frequency, which they probably have. If not, the closest to it. Get the test settings (dial settings, strap dimensions, etc. etc.) from them and use it as a starting point. Then accomplish the book tuning procedure. They keep a copy of everything, it's the same info as the tune n test data spplied the customer when they shipped the box. Make sure all the RF portion of it is squeaky clean before you start.
As an alternative, if it isn't in or close to a market in which my employers have stations, for the very modest honorarium of $850 a day plus travel I'll come tune it for ya. And demonstrate factory spec or exciter measured performance, which ever is the lesser, into a nonreactive load.
As an alternative, if it isn't in or close to a market in which my employers have stations, for the very modest honorarium of $850 a day plus travel I'll come tune it for ya. And demonstrate factory spec or exciter measured performance, which ever is the lesser, into a nonreactive load.
For those who don't want to click on the link, my previous post... :
"Just took one off the air, was on 89.5 when we put it in but only had to move it to 92.3. Just went looking for that Harris bulletin, but must be on the computer at work.
As I remember, on the grid of the final there is a little link which you shorten as you go up in frequency. It's soldered in place. Also, in the cavity, there is a plate that can be moved as a coarse tuning adjustment on the final.
If you can, find an exciter that will put out about 30 watts or better; and connect it directly into the final. The IPA is connected to the final by a short jumper, on the right hand side of the cavity (looking from the back) you'll see the BNC jack. On ours, the IPA is little more than a buffer amp--an MX15 will drive the final directly to about 1200 watts, I used an 802A to get the full 2500 watts, with around 25 watts of drive. Final grid tuning becomes critical to match the exciter, but once set it just runs.
Also--crawl in the cavity and check the plate loading capacitor. It's a little air variable with three plates; it can come loose over the years and start arcing. Once it does, you are dead. Don't know of a replacement source.
An in-line watt meter is almost necessary. These things were built back in the time of hand-picking finals for best efficiency. On my box the listed efficiency is 74%, never could do much better than 69%, and it REALLY did not want to make full output--usually settled around 2300~2350 watts. My licensed tpo was 2450, so no big problem."
Addendum, dum, dum, de, dum...
If you drive the final directly, use a 1/4 wave shorted stub on the exciter feed to protect it against a shorted final sending 4 kv back down the line. We replaced our "H" with a Nautel V2, which just runs. Considerably louder than the old Harris, though, and you need a deep rack--36" not enough, 42" might just make it. Am now working on a "K" as backup for our other station--filament contactor coil is dying, fun trying to find a new 4 pole 30 amp contactor with a 240 volt coil. Local supplier found a new Furnas 4 pole, but it only had a 110 volt coil. Waiting to hear back from him to see if he can order something.
"Just took one off the air, was on 89.5 when we put it in but only had to move it to 92.3. Just went looking for that Harris bulletin, but must be on the computer at work.
As I remember, on the grid of the final there is a little link which you shorten as you go up in frequency. It's soldered in place. Also, in the cavity, there is a plate that can be moved as a coarse tuning adjustment on the final.
If you can, find an exciter that will put out about 30 watts or better; and connect it directly into the final. The IPA is connected to the final by a short jumper, on the right hand side of the cavity (looking from the back) you'll see the BNC jack. On ours, the IPA is little more than a buffer amp--an MX15 will drive the final directly to about 1200 watts, I used an 802A to get the full 2500 watts, with around 25 watts of drive. Final grid tuning becomes critical to match the exciter, but once set it just runs.
Also--crawl in the cavity and check the plate loading capacitor. It's a little air variable with three plates; it can come loose over the years and start arcing. Once it does, you are dead. Don't know of a replacement source.
An in-line watt meter is almost necessary. These things were built back in the time of hand-picking finals for best efficiency. On my box the listed efficiency is 74%, never could do much better than 69%, and it REALLY did not want to make full output--usually settled around 2300~2350 watts. My licensed tpo was 2450, so no big problem."
Addendum, dum, dum, de, dum...
If you drive the final directly, use a 1/4 wave shorted stub on the exciter feed to protect it against a shorted final sending 4 kv back down the line. We replaced our "H" with a Nautel V2, which just runs. Considerably louder than the old Harris, though, and you need a deep rack--36" not enough, 42" might just make it. Am now working on a "K" as backup for our other station--filament contactor coil is dying, fun trying to find a new 4 pole 30 amp contactor with a 240 volt coil. Local supplier found a new Furnas 4 pole, but it only had a 110 volt coil. Waiting to hear back from him to see if he can order something.
Not for 2500 watts. Although you can get a Nautel for around 18K less exciter, a BE for less; my 2.5k I bought a few years ago as a back up was under 4K.
Nice thing about this series is that you can bypass the IPA stage and drive the final directly (it's 50 ohm in), requires about 25 watts drive for full 2500 watts out.
Nice thing about this series is that you can bypass the IPA stage and drive the final directly (it's 50 ohm in), requires about 25 watts drive for full 2500 watts out.
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