Got a good one for all the Brains here. I have a CCA 35kw that is giving me fits! My IPA input load and tune keep shifting, causing my exciter to VSWR out. I have changed the input Coax, the input doorknob, the 2 air variable caps and the doorknob between the variables, the tube socket and all the components associated, the tube, twice! I have tapped the cavity back and put screws all the way around as I know if the Deuz clips are not solid, it will cause drift. TX will run fine for about a week or two and Wham, VSWR trip on exciter. I have even changed the Exciter thinking it could be a hair trigger protect circuit. I can find peak and null the VSWR out and it is good for another week or so. When TX is tuned, Everything is in parameter according to the factory test data sheet. Thoughts?
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CCA FM 35KW HELP!!!!
- Thread starter RFGuy
- Start date
Is this a G series CCA?
What IPA tube does it use?
If it's a 5cx1500a, make sure the tube's fins are all in their proper sockets. This tube goes in from the bottom, and twists into place...sometimes a fin on the tube will miss it's clip...but since you've changed the tube and socket, I doubt this is the case.
What exciter are you using?
How long is the coax between the exciter and transmitter? What kind of coax is it? I assume you've looked at the solder connection from the bottom of the panel mount type N connector on top of the IPA cavity.
How's your main tube? How's your VSWR to your antenna? Is this transmitter on the factory set frequency, or has it been changed at some point?
I have dealt with several similar problems, but need some more info to offer advice.
What IPA tube does it use?
If it's a 5cx1500a, make sure the tube's fins are all in their proper sockets. This tube goes in from the bottom, and twists into place...sometimes a fin on the tube will miss it's clip...but since you've changed the tube and socket, I doubt this is the case.
What exciter are you using?
How long is the coax between the exciter and transmitter? What kind of coax is it? I assume you've looked at the solder connection from the bottom of the panel mount type N connector on top of the IPA cavity.
How's your main tube? How's your VSWR to your antenna? Is this transmitter on the factory set frequency, or has it been changed at some point?
I have dealt with several similar problems, but need some more info to offer advice.
RFGuy said:Got a good one for all the Brains here. I have a CCA 35kw that is giving me fits! My IPA input load and tune keep shifting, causing my exciter to VSWR out. Thoughts?
In the past I have had similar issues with a Continental 20kw and replaced a sealed variable cap that adjusts the load to the IPA. (Expensive) We have a solid state ipa relacing the old dual 4cx250.
Alos ran into this with a QEI 10 kw and found an issue with the ground in the socket area that had a fix of a platic sheet protector between the output (rf) and ground) by way of the tube enclosure. I sat at the site and through multiple explosions you don't see if you aren't there found the black arcing on the enclosure.
I know these run and run but they also get hit by lightning. Can you trace the start date to an event? Do you find black anything? This is a modified cabinet that formerly had a final for the low power transmitter and then a final for the amp? Did it have a previous tube prior to the output of the low power cabinet final? The old ones had an exciter, tube 1, tube 2, then the final tube 3 just in teh low power cabinet pre big amp.. How many tubes ya still using?
Where the solid state turns to tube look in that socket. Replace it if you don't find a cracked ceramic doorknob. If you find one bad doorknob shotgun them all. They won't appear bad ven if they are unless they are cracked or melted. Sometimes just replacing teh doorknobs will fix this. Give us more particulars.
Thanks for the reply's I am using about 6 feet of line between the Exciter and TX. 5CX1500 for the IPA 3CX20000 final. I did ensure that the fins all line up with the socket flanges. It is a 35000G. We had problems with the Plate Voltage and Van at V&J outside of Atlanta came up to help. He has been here twice. Great guy, Very knowlegable about the TX. Anyway, He tapped the plate transformer to get the B+ to 8500v. TX ran for 2 years with no trouble. Had to change the PA Blower 3 weeks ago. did not touch the IPA. Before this trouble I could see the VSWR drift a little, .3 to.5 watts reflected on Exciter. After the Blower replacement, the problem magnified. I lost the second blower last friday. I am thinking now that the blower could have been vibrating the cavity, causing the drift. I am using a Digit for the main Exciter, a THE1 for the secondary and have even plugged a BE FX30 in to see if there was exciter trouble although it cannot provide the 45watts of drive the TX wants. In the prior post I stated I had replaced all the doorknobs with no change. PA is working properly. I will know when the replacement blower arrives. I will post and let you all know how it went. Anymore Ideas would be helpful and thanks to all who posted with help!!!!
I know it sounds odd...but try changing the feedline length...drastically, just as an experiment.
I haven't seen a 35000G in a long time...but...I believe there is a small capacitor that may or may not be part of the tube socket in the IPA. I am blanking on what these capacitors are called...but down south we call them "sandwich" capacitors. I know the CCA's with the 3CX800A7 IPA have these...and I believe the larger ones do also. When this cap fails, it will cause the exact symptoms you are describing. There is no good way to check this cap...since it seems to fail only under load...just replace it. Another situation I ran into was when an assistant was changing a component in the main tube socket, he disconnected the two copper straps from the doorknob caps right under the main tube. When he put them back, he didn't put them in the same holes...(transmitter had been retuned at some point, so the straps had multiple holes in them), and the transmitter would run, but it ran much hotter than usual, and the exciter would not tune up properly. That took a few minutes to figure out.
One last thing to think about...the dial knobs on the front of the transmitter that control the IPA tuning can lose their gearing...they will spin, but they are not actually turning the tuning controls 100% of the time....maybe only 50% of the time, or not at all. Confirm that these are turning the rods connected to the variable caps 100% of the time.
Do you have a network analyzer? Hooking that to the IPA input should be able to give you a good idea what's going on.
Good luck.
I haven't seen a 35000G in a long time...but...I believe there is a small capacitor that may or may not be part of the tube socket in the IPA. I am blanking on what these capacitors are called...but down south we call them "sandwich" capacitors. I know the CCA's with the 3CX800A7 IPA have these...and I believe the larger ones do also. When this cap fails, it will cause the exact symptoms you are describing. There is no good way to check this cap...since it seems to fail only under load...just replace it. Another situation I ran into was when an assistant was changing a component in the main tube socket, he disconnected the two copper straps from the doorknob caps right under the main tube. When he put them back, he didn't put them in the same holes...(transmitter had been retuned at some point, so the straps had multiple holes in them), and the transmitter would run, but it ran much hotter than usual, and the exciter would not tune up properly. That took a few minutes to figure out.
One last thing to think about...the dial knobs on the front of the transmitter that control the IPA tuning can lose their gearing...they will spin, but they are not actually turning the tuning controls 100% of the time....maybe only 50% of the time, or not at all. Confirm that these are turning the rods connected to the variable caps 100% of the time.
Do you have a network analyzer? Hooking that to the IPA input should be able to give you a good idea what's going on.
Good luck.
Could the exciter be putting out spurious off frequency RF? I've seen this happen with the BE FM250 exciter and it drove me nuts until I noticed that when the temperature of the exciter changed, so did the problem. The symptom of the exciter SWR going up & the tuning seeming to change pretty much mirror what you are seeing.
very interested to see the outcome. Let me see though...2 blowers in a few weeks. This can't be random complimentary blower failure.
Since van changed the taps on the transformer to increase b+ and this was 2 years before the problem started look at the blower failures. This just doesn't happen together unless there is a related issue. Lightning is one. The rotron blowers do fail but not together.
Have you had lightning?
BTW - by pass the tube IPA. the cost of 3 tubes buys a nice solids tate amp. Van can provide the parts to totally bypass this puppy and then you are driving the tube directly.
Since van changed the taps on the transformer to increase b+ and this was 2 years before the problem started look at the blower failures. This just doesn't happen together unless there is a related issue. Lightning is one. The rotron blowers do fail but not together.
Have you had lightning?
BTW - by pass the tube IPA. the cost of 3 tubes buys a nice solids tate amp. Van can provide the parts to totally bypass this puppy and then you are driving the tube directly.
These are not Rotron Blowers, This model has regular squirrel cage blowers. These are 17 years old and the bearings had just worn out. I should have changed them both at the same time as both have the same hours, but, to spread expenitures I changed them one at a time. I have been through every doorknob cap in the IPA and PA, Checked the shorting bars, ect. I will get the new motor sometime today, I should have an update soon,,,,,,, Stay tuned! (boy that was a bad joke concidering my problem with this box)
Haven't seen a recent post. Did changing the caps resolve the problem?
OK I replaced the blower motor today and found the old blower threw oil with shavings into the HV section. Needless to say I now have a HV ovld. Sorry for not posting, I had to move studios over the weekend and had a real nice time with Century Link and Pulsecom PCAU cards as well as poorly engineered lines. Had to deal with Kansas for the lines and I kept thinking "click my heels together 3 times and think of home" still not done I have to go in after midnight.
Sorry for not getting back, had a ton of work to do. The Problem seems to be vibration of the old blower motors throwing the IPA input load and tune out of whack! It has been on the air now for a while and have not had to re-tune. VSWR is at 0 and I am getting full power out! CCA is an OK Transmitter, But, I would prefer a new Nautel solid state rig and put this box out to pasture. My old Harris 20k would stay as an aux,Although Harris (the company)has gone to crap the old Transmitter works better than the CCA even in a bad day! ED
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