Question regarding CCA FM 20,000E

[CDJCD5]

Hey everyone, just got a few questions concerning a 1977 Vintage CCA 20,000E Transmitter. I am a newbie and just learning all I can about transmitters and the whole broadcasting process We have a new Chief Engineer and I make sure to learn as much as I can from him. So here is the story, our CCA has been dropping power over the past few months, had the whole tuning thing done and was able to get it back up to power, which is only about 8kw into our combiner.. About two weeks ago, the transmitter dropped power by half.. about 3.8kw is all it would put out. So we switched to the backup and just a few days ago the road was opened so we can access the transmitter site, heading up Monday. The tubes are fairly new, the readings looked normal, plate voltage and amps, as well as the reflected power were all good, just that it wouldn't go full power. So has anyone dealt with these transmitters much and have any idea what could cause that? The unit has been a great transmitter and well maintained over the years, spotless inside etc... What are the good and bad points of those vintage CCA's??

Thanks much for any info!!

Chris

 

[bradgoehl]

I don't know the CCA directly, but i just started a new gig myself and came in to a transmitter that burned up a tube in about 6 months. Turned out that the previous person never turned the filament voltage down after the burn in period. And if all your readings look normal the tube is what i would suspect. You can check by simply turning up/down the filament voltage and see what you get; also the filament voltage history would be a good reference tool.

 

[TomZ]

This might help..<tell me if you can't open it w/o a password>

https://www.dropbox.com/s/ebhpwj0k26vyicg/ccatips.doc

 

[frankberry]

If the plate voltage is normal and the plate current is normal, there are a couple of things that you should look at.
Since the tube is drawing normal current without putting out normal power, the difference is being produced as heat.
Re-check the PA plate tuning and loading. The PA tube may have changed characteristics for some reason.
Don't overlook the fact that the forward power meter might not be correct either. The transmitter may be putting out more power than the meter is indicating.

 

[BobOnTheJob]

I must preface this with the advisory that these things can kill you in a heartbeat if you touch the wrong thing inside, By all means, hang with your Chief while he plays with it, but do NOT mess with it yourself. This is how people safely learn to play with these RF breathing monsters and we do need new blood that understands them. That said...

What is the current and voltage? What someone else said is 'normal' should be confirmed before accepting 'that's what it's always been'. If you have a Bird Wattcher or similar (pretty common where a combiner is in use), confirm that it's accurate. I once went to a site with the Wattcher showing the expected 3KW output, but with 3600 volts and 0.25 amperes. Obviously you can't get 3000 watts output with 900 watts DC input. Verify the accuracy of all metering involved. Needless to say, the owner was ecstatic at this 'find'. No idea who the previous engineer was or why this had been allowed to go on for years.

Look at the manual as far as power determination goes. The D series allows you to read the final current directly. The G series requires that you take the Cathode Current and subtract the Grid Current from it to get the final current. I'm not sure which method the E series used. But depending on which is used, the formula for indirect power determination will need to be adjusted accordingly. The 2 formulas are Final Current x Final Voltage x Efficiency Factor (found in the manual or on the test data) for the transmitter with the recessed meter (D for example) or (Cathode Current - Grid Current) x Final Voltage x Efficiency (for the transmitters with the non-recessed final current meter). In the case of the recessed meter, the entire final voltage and current (up to 8000 volts/3 amperes) flows through the meter itself. The newer ones got around that, at the expense of having to do the pain in the neck formula to calculate power.

I cut my teeth on CCA's in the mid 1960's...their exciters are not the world's finest but the rest of the transmitter is an iron clad battleship. Depending on the model, feed it a new tube or tubes every 12-30 months (I realize that's wide time frame, but learned from experience) and open it up and give it a bath inside and out annually, don't run any higher final voltage than necessary (you should be able to produce 8KW with about 5000 volts on that rig) and you should have a transmitter that does nothing but run and run--especially in the case of a 20KW running 8KW--can you say Easy Street?. If you think I'm a big fan of CCA's, you would be correct.

Please keep us updated on where this goes...I (along with others on this board) have just about seen it all where these venerable transmitters are concerned. I think I can speak for more than myself when I say that we'd like to see you end up as a fan of that transmitter.

 

[frankberry]

BobOnTheJob ....

Thanks for posting the high voltage warning. I should have done that too.
I've had experience with the 20,000E. I agree with everything that you have posted.
The Plate Current meter on this transmitter is actually in the plate circuit (as I recall).
One more thing about this particular model. The IPA Plate is a little difficult to tune. The "dip" is very sharp.
A very slight change in the position of the knob will make a huge change in the drive to the PA.

It could be nearly anything. The transmitter is very old. The last time I worked on one of these was 1978. The transmitter was old at that time.

 

[K4WRF]

Consistent with past post, these things can KILL you quite *DEAD*. Bare in mind that recently a fellow met his death on one of these near Greenville Mississippi so I can't advise you to do anything but seek a well trained professional to address these issues, there is a lot that can go wrong. Couple points: There should be a meat thermometer in the output PA tube vent and in normal operation it should be no higher that about 175 degrees or so. This can be a huge help in getting the PA tuning and loading to the "sweet spot". Too much heat dissipated in the anode is just about as bad as operating the filament voltage too high. Adjusting the filament should have only small impact on the power output. If it is a lot, the tube emission is gone. The other Achilles heel on these transmitters is the 5CX1500 Pentode IPA tube, life is not long and the other common failure is the screen grid voltage rheostat opening as a result of age or perhaps the screen by-pass capacitor that is part of the tube base/socket shorting out. A couple other IMPORTANT things to remember; if for some reason your highly trained and qualified engineer ever has to replace the 3CX15,000 PA tube socket, be sure to remove the screen/shield material on the bottom of the old socket and attach it to the new one first or the transmitter WILL NOT WORK. Also and VERY important, if your engineer ever has to change out the PA tube, be absolutely certain that the 3CX15,000 is fully seated into the socket or you will have erratic operation or fireworks. This may require the use of a 2 X 4 levered against the open front inspection plate to force it to bottom out. Again, your trained, professional and highly qualified broadcast engineer should be aware of this.

NEVER operate this (or any)transmitter with the interlocks or safety features defeated or people could die....

All the best....

 

[CDJCD5]

Thanks everyone for the replies!! I completely understand the dangers of those things for sure. I work on High Voltage rectifiers as well as igniters daily (Scary, nasty stuff). Our engineer is new to this transmitter and these tips will help I am sure. As for readings it usually runs around 7.51 Kilovolts, 1.38 Amps and that is at 98% percent. 10k feet altitude. We are going up tomorrow, I will be sure to pass along what went down. Thanks again Folks!!

Chris

 

[BobOnTheJob]

Chris,

With numbers like that, you can do better efficiency wise. Here's my take on it : When the transmitter is running with no drive, it draws about 0.3 amperes. That 0.3 amperes of final current has 0% efficiency. Reducing the final voltage to about 4800 or so and running the current up above 2 amperes will do two positive things :

1) The 0.3 amperes of 'idling current' will drop to a smaller figure...let's say 0.2 amperes. That reduces the amount of 0% efficient final current.

2) The final current will increase. As it is now, your 'idling current' is about 22% of your total current. Say your new final current is 2.25 amperes. The new idling current may be as little as 9% of the total current. More of your current will be producing power...and that is the goal.

I've ran 4800 volts on 2 different CCA transmitters. A 20KW (FM20000D) running 7KW. The efficiency chart suggested I'd get 7KW..I got 7.8KW on the Bird Watt Meter (almost 78% efficiency). In the other case (I admit this was extreme, but I had to try it), I reduced the final voltage on a 25KW CCA (FM25000D) and ran the current up to 4.5 amperes (the meter went to 5 amperes) to produce 18KW on the Bird Wattcher into the dummy or the antenna. The efficiency was an unheard of 83%! I've proven beyond much doubt that CCA FM's run better on lower voltage and higher current. If your Chief is adventurous, I encourage him to try it. In the case of both the 20 and 25KW transmitters, there was a High Low switch and in Low, both landed at 4800 volts. In each case, I went inside and hard wired the switch to Low to avoid accidentally going back to full voltage.

Another thing that the lower voltage helps with is the plate blocker. In one case back in the early 80's, the plate blocker was failing every year or two ( and judging from the several shorted blockers on site, I'd have to say that it was a long term problem even though the transmitter was only 10 years old at the time). In 1983, I dropped the voltage and increased the current. That transmitter was retired in 2011 and it still has the same plate blocker in it that I installed in 1983.

Have fun, be safe and as Watt pointed out, be ready to replace that 5CX1500B about once a year with a rebuilt tube. Crazy thing is that the same tube in a Harris FM2.5K lasts a couple or three years and is being run harder...go figure!

Bob

 

[Dale H. Cook]

these things can kill you in a heartbeat if you touch the wrong thing inside

I once had a 20,000 of about the same vintage as backup to an 816R-3, and its lack of a ground stick is what prompted me to buy a ground stick from Continental (small enough to fit in my main tool box) which I used in the CCA. You only need to draw a HV arc with a ground stick once to make you a firm believer.

Dale H. Cook, Market Chief Engineer, Centennial Broadcasting, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html

 

[Kent T]

And we engineers of old call these grounding sticks Jesus Sticks for great reasons. Fail to use one when you poke into the high voltage innards and ye could meet Jesus much sooner than planned. Don't disable the interlocks. Don't work on the CCA when tired or any tube rig especially. Be careful and methodical and practice safety. Every transmitter in my care has a Jesus Stick close by.

 

[Nostalgia]

And we engineers of old call these grounding sticks Jesus Sticks for great reasons. Fail to use one when you poke into the high voltage innards and ye could meet Jesus much sooner than planned. Don't disable the interlocks. Don't work on the CCA when tired or any tube rig especially. Be careful and methodical and practice safety. Every transmitter in my care has a Jesus Stick close by.

Thanks Kent T for bringing a smile to my face. I had not heard that term in many years. You echoed the warnings of the old Chief that I interned under many years ago and I never forgot! (still alive aren't I )

 

[LA_Guy]

These are grounded grid rigs. As such, the drive power is CRITICAL. Starve the final tube with too little drive and your efficiency goes into the toilet. Why? because the driver power passes through to the output. I'd be looking at the first driver (the little tube) first.

 

[WNTIRadio]

Ahhhh the old Jesus Stick!! I called it that to one of my clients who is a Christian broadcaster... he looked at me funny and then I said "Yep, you pray to Jesus that the HV caps were discharged when you touch them with the stick. " He then laughed and said "good point!"

 

[stephend2]

as another note on the "jesus stick", I tend to check resistance to ground, especially on a transmitter I've never met before... Haven't found one open yet but just making sure...

I have a small CCA, FM2500G, 2000-2001 vintage, in my care. Best I've been able to get is about 74% efficiency out of it at 4000v, TPO 2300w.

It eats a 3cx3000 in about 18-24 months, 3cx800 driver has been in there over 4 years and still going strong.

 

[BobOnTheJob]

as another note on the "jesus stick", I tend to check resistance to ground, especially on a transmitter I've never met before... Haven't found one open yet but just making sure...

I have a small CCA, FM2500G, 2000-2001 vintage, in my care. Best I've been able to get is about 74% efficiency out of it at 4000v, TPO 2300w.

It eats a 3cx3000 in about 18-24 months, 3cx800 driver has been in there over 4 years and still going strong.

Thanks for mentioning this one stephend2...I have a FM4000G (1991 or so vintage) that uses the same tubes as yours and the driver tube is only lasting a little over a year (and looks like new when removed). Do you happen to know what Voltage and Current that the driver tube is running in yours? And if it uses the 20W or 60W exciter? Mine runs roughly 2600W TPO. My recollection of the 3cx800 in a FM10000G was that it ran for what seemed like an eternity. The owner says he's rarely gotten more than a year out of it. Any thoughts?

 

[stephend2]

We have a 30w Energy-Onix (RVR) exciter, was there when I got there. If I recall, exciter is running about 14w.

I will be at that site this weekend, I'll check on the parameters of the driver and report back. I don't have any ideas on the short life, I'm rather new to engineering, not enough experience to "just know", best I know to do is report back the operating parameters of the tube thats over 4 years in....

 

[BobOnTheJob]

We have a 30w Energy-Onix (RVR) exciter, was there when I got there. If I recall, exciter is running about 14w.

I will be at that site this weekend, I'll check on the parameters of the driver and report back. I don't have any ideas on the short life, I'm rather new to engineering, not enough experience to "just know", best I know to do is report back the operating parameters of the tube thats over 4 years in....

Thanks! And welcome to radio engineering...we need more new blood!

 

[WNTIRadio]

Your best bet for those CCAs is to replace the tube IPA with a solid state 300w unit. You'll be a lot happier, and your transmitter will have a better incidental AM noise figure. It will pay for itself when you're not swapping a new tube in all the time.

Someone, I forget exactly who, makes a "lunchbox" 300W unit for them. Heck, you could get a 300W Bext amp and do the same thing, and it becomes an emergency backup transmitter should the main go south.

 

[stephend2]

What is involved in the solid state IPA conversion? What matching components would be needed to present a proper 50 ohm load to the solid state amp ?