ECO-4 question

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Joe Friday
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Joined: Tue Dec 16, 2014 3:47 am

ECO-4 question

Post by Joe Friday » Sat Jan 07, 2017 11:27 am

Need some help with a ECO-4 issue. Last week, I lost the IPA in this transmitter. I returned the next day with a 250W Crown that I have used previously as a sub for the IPA section in this same transmitter. I patched the Crown directly to the input connector on the bottom of the PA cabinet. I turned the output Crown down to zero, and fired up the main TX. Slowly adjusting the power on the Crown, I started seeing output from the ECO-4 at about 150W out of the Crown. I advanced the power to about 175W out, and then looked at the VSWR back from the transmitter, It was showing about 1.3:1, so I adjusted the grid tuning and loading for a minimum, and it smoothly came down to 1.06 measured on the built in metering.

Shortly after, I noticed something burning, and the SWR on the crown was at 1.9 and climbing, and forward power was falling off on the ECO-4. I shut down, and did a careful visual examination, and it appears that R1 on the PC-85A (RF Detector) board has smoked. I am missing the interconnect diagram from my manual, and the one I found online is too blurry to be of any help. Can someone help with what TB10-3 on the PC-85A board is connected to?

With a direct connection between the Crown and the PA, I don't understand how this board comes into play. This configuration has worked several times for me in the past. The Crown is currently heating the antenna and running properly at 250W. All of the IPA leads are taped off and tied up so nothing is shorting, and I had the IPA breaker off.

Any thoughts or suggestions welcome. Thanks!

Kevin

TPT
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Re: ECO-4 question

Post by TPT » Sun Jan 08, 2017 8:19 am

The built in IPA has it's own reflectometer, so with the IPA out of the circuit that's not the problem. I suspect the shorting resistor in the power sample detuned the output stage, causing your VSWR problem on the Crown. Removing the burnt resistor may cure the problem with reflected power.

If no-one else admits to having an ECO-4 I'll bring my book home Monday & try to decipher the schematics.

Kelly
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Re: ECO-4 question

Post by Kelly » Sun Jan 08, 2017 3:24 pm

I'm not familiar with the ECO-4, but are you sure the PA drive input is a true 50 ohms with no matching network between? I remember from working on a QEI 30kW box years ago, that after trying to replace a burned up QEI IPA with a generic solid state driver, discovered the actual impedance as presented to the PA was something goofy, like 80 ohms. I later determined that QEI used twin interconnect lines and a 1/4 stub coiled into a matching balun to provide somewhat of a match. Problem was, the PL259 connectors QEI used to connect to the IPA burned up over time. Yep, 750 watts VHF through twin PL259 connectors! Ultimately, I had to build my own balun to get the match over a single line and replaced the PL259's with an N-connector on each end of the balun.
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Joe Friday
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Joined: Tue Dec 16, 2014 3:47 am

Re: ECO-4 question

Post by Joe Friday » Sun Jan 08, 2017 7:07 pm

I have used this same transmitter as a driver with this box, and other ECO series transmitters in the past without issue, and have always been able to easily achieve a low VSWR match.

I know some transmitters present something other than a 50 ohm impedance to the IPA, but in this case, I believe that it is a 50 ohm circuit.

Thanks!

Kevin

ChuckG
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Re: ECO-4 question

Post by ChuckG » Sun Jan 08, 2017 9:05 pm

TPT wrote:The built in IPA has it's own reflectometer, so with the IPA out of the circuit that's not the problem. I suspect the shorting resistor in the power sample detuned the output stage, causing your VSWR problem on the Crown. Removing the burnt resistor may cure the problem with reflected power.

If no-one else admits to having an ECO-4 I'll bring my book home Monday & try to decipher the schematics.
That triggered a repressed memory regarding an ECO-10 I used to maintain. Bernie had me bypass the RF Detector board entirely as a work around to a similar problem where IPA drive was not reaching the grid.
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Chuck Gennaro
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TPT
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Re: ECO-4 question

Post by TPT » Mon Jan 09, 2017 7:25 am

There's two tuning controls on the grid of the 3cx3000A7, so yes, you can kinda match to 50 ohms with a delicate touch. Then it depends on how tolerant an amp you use to drive the final. Had to use an RVR one time, could only get about half power because it would foldback.

Also, the match varies by temperature. My ECO-4 is in a former commercial garage, 25X15 room used as the office & it's heated/air conditioned. Still, I check everytime I'm up there & notice the match change with the weather.

Joe Friday
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Re: ECO-4 question

Post by Joe Friday » Mon Jan 09, 2017 6:55 pm

Well...I guess that is where I am stumped. If I am driving the PA with a direct connection to the N connector on the bottom of the chassis, how does the detector board come into to play? R1 is connected to TB10-3 on the PC-85A board, and that is what I am trying to figure out. The IPA has a refelectometer on it that is completely out of the circuit. I don't know where the signal comes from that drives the board.

Thanks again to all!

Kevin

TPT
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Re: ECO-4 question

Post by TPT » Tue Jan 10, 2017 2:56 pm

Well, now you've gone and done it--forced me to read the manual! :lol:

It looks to me that the exciter input RF is connected to terminal 3 of the PC-85 (as well as to the input of the IPA). The PC-85 board has a two pole relay.
One pole appears connected to the control ladder--thus preventing the transmitter from coming on if no drive. The other pole enables the automatic power level circuit--when left in auto.

In my ECO-4, since there isn't a convenient way to interlock the transmitter, I use this exciter failsafe to kill the transmitter so I can transfer the antenna to our back-up kilowatt transmitter. Arming the transfer switch mutes the exciter, No drive==plates dump. The one time I had to bypass the IPA, I used an RVR 100 watt transmitter directly into the final. Hence I may still have had the exciter feeding the IPA circuit--looks like R-1 provides a termination to the exciter since they talk about half the drive going to this board, half to the IPA input. That might explain the burnt resistor--if the IPA was out of the circuit all of your exciter's RF would be dumped into this board. Resistor opens up-transmitter goes off. With the final "cold" the match changes to the cathode (the grid is grounded, so the IPA feeds the cathode, not the grid)--and you see VSWR.

Be aware of another circuit you may or may not be using-there is an exciter mute circuit (BNC connector on the top of the transmitter) that was used with the exciter they sold with the transmitter. It connected to the mute terminals on the exciter--believed it shorted out that circuit until the time delay passed then it allowed the exciter to come up. I use an 802A and don't use that circuit--in fact I need a short to turn on the exciter, opening the short mutes the RF.

If you need a better copy of the 11X17 sheet, PM me your mailing address. I can get it copied for about 50 cents at Office Depot.

Joe Friday
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Joined: Tue Dec 16, 2014 3:47 am

Re: ECO-4 question

Post by Joe Friday » Wed Jan 11, 2017 6:27 am

Well...that was the part I was missing. Thanks for helping with the connection info to TB10-3 on the PC85 board. It appears that it must still be in the circuit somehow. I'll head back out to the site and look things over and see if I can figure out what error I made.

Thanks for looking at the diagram...the one I have from the on line scan shows that the connection goes to two square blocks with very blurry printing...certainly not the typical way a coax connector would normally be shown on a schematic.

Thanks again!

Kevin

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