Harris 25K

[brandonb959]

Today I spend 6 hours replacing the L4 grid tuning inductor. I must say that it was successful as I now have the best matchup that I have ever seen between this transmitter and the external Silicon Valley IPA, but most importantly I am now making plenty of power. But........I now have a choke in the screen power supply lighting up like a christmas tree when I get over about 500V on the screen. Its a Triad C-45AL 10 Henry @ 500MADC...Anybody know where I can get a good deal on one....

[RGORJANCE]

Not sure how physically big that one is, but you might give Harbach Electronics a try. He took over the Peter Dahl transformer rewinding business, and has a large listing of iron for broadcast and ham equipment.

Glad to hear you made good solid progress. As a thought, is that choke arcing internally, or to ground??? If it is arcing to ground, sometimes you can lift them off ground, insulate it and keep on truckin!

Fossil

[Deep Thought]

Its a Triad C-45AL 10 Henry @ 500MADC...Anybody know where I can get a good deal on one....

Onlinecomponents.com shows two new ones in stock, $242.49 plus shipping.

Someone recently sold one on eBay for about $50 shipped. I stole this photo from that listing. I don't see big chokes like these come up very often though.

[RGORJANCE]

Also try Surplus Sales of Nebraska. They have a 15hy at 600 ma for $150.00.

Fossil

[Deep Thought]

Just make sure the insulation voltage is at least 5 KV. IMHO, for the extra 90 bucks I'd stick with the original new old stock part since it is available.

[brandonb959]

I did look at onlinecomponents.com and that is a real possibility...I also looked at Nebraska Surplus but they didn't have exactly what I was looking for. What do you guys think of the Hammond 193Q? http://www.hammondmfg.com/pdf/5c0033.pdf Its 10 Henrys/500MADC/1000 Volts Its only $100 and seems to be fairly common. Since the MAX screen voltage is 1000 than would this be acceptable since the average screen voltage is probably closer to 500-700 volts?

[Deep Thought]

The voltage (insulation) rating takes into account the inductor's environment and potential operation modes. As a practical matter, the peak voltages in a reactive circuit at startup and shutdown can be significantly higher than the DC operating voltage...and remember that it is connected to a circuit which is mere fractions of an inch from a much higher voltage supply in the tube. You might be able to fudge the rating 10-15% but I certainly would not use a component which sports only 20% of the part it is replacing.

[Kelly]

I know it's tough sometimes when you're trying to get the rig up and running, but try to be methodical when doing troubleshooting on this transmitter. What I mean is to check voltages, currents, forward and reflected power from one end to the other. Start with exciter drive, making sure you have drive going to your IPA, then that your IPA isn't folded back with reflected power from the grid. If the IPA does have excessive reflected power or lack of drive then look to your tube, socket assembly, externally mounted caps, ect.

Generally a screen cap that blows through the Capton is caused by excessive screen voltage, a tube with a screen to plate short, really bad tuning, or dirt. The challenge is that once it arcs through the capton film, a carbon trail has been created in the copper. Just replacing the capton usually won't work because chances are good it will arc right at the carbon points. Unless you can completely eliminate the burned spots, this problem will keep happening, even at lower voltages.

[brandonb959]

Update: The screen choke has been replaced (very expensively from Harris might I add). I also had to replace the outer filament finger stock in the tube socket due to broken fingers and arcing. I also had to the replace the cheap little plastic chain between the gears for the plate tuning. The transmitter has a brand new tube in it and is now making about 23-25KW and seems fairly happy.

Has anybody had any experience replacing the "captive nuts" that hold the P.A. cavity door on? I have two that won't tighten, probably not a huge deal but would be nice to replace for the long term...