Replacing filament contactor in 2.5k

[TomT]

There are a number of these Harris FM boxes out there, so this info should be useful to someone else. Our backup 2.5k had a bad filament contactor--coil would not hold the contacts in, just chattered. Original is a Furnas 41 series 4 pole 30 amp with 240 volt coil which, of course, is no longer made.

Siemans/Furnas do make a similar contactor; a 42 BF 25 AF. The coil is a 75D70545G, (and you can switch coils in-series, our supplier used a 4 pole with a 110 volt coil and a 3 pole with a 240 volt coil to make up our new contactor). And it will mount in the same place. The larger electrical distributors should have these items in stock.

Problem is that the coil and contact terminals will be vertical rather than horizontal as with the old contactor. More to the point, you can't just switch the wires to the new screw terminals. The 42 series provides a deeply recessed #10 screw terminal, plus two exposed studs for the push-on terminals. Also the old contactor was used as a tie point: "L2" terminal has 5 wires on it. This is not a good job to start at midnight.

So you will need several "female disconnect" 22/18 gauge .250/.032 crimp terminals; plus a few ring or spade terminals for #10 bolt. Plus a good crimp tool/wire stripper. Cut ALL of the tie wraps off the wire bundle to the old contactor, all the way back to the wire raceway. I mounted the new contactor coil down, not sure if it makes much difference, as I think there will still be some wires that a pulled fairly tight no matter which way you mount it.

In my box the wires are all numbered, though sometimes hard to read. Best to make up a cheat sheet before you start. You can relieve some of the congestion on "L2" by moving wire #31 to mate with #33 on K3; but I had to tie two other wires together into one crimp to get them on that line. Another problem is finding crimps for #10 stud that are happy with the small gauge wire used. I wouldn't recommend trying to solder to the contacts, but I only need about 4 of the larger crimps, you could solder those to prevent the wires from pulling out.

Note that the plate contactor is the same as the filament contactor in these transmitters. Also note that the Harris 2.5H3 is a very similar transmitter, although I don't think there are as many of these boxes still on the air.

 

[more kilowatts]

This a "definite purpose" contactor, used in countless equipment throughout many industries. Although most Siemens/Furnas suppliers probably won't stock it, they can order you a 240 volt coil version of this 4-pole contactor from Siemens. Part number is 42BF25AG. It is listed only in the Siemens Definite Purpose Contactors catalog. Since this item costs less than $100 in most cases, it might pay to order one and have it on hand just in case.

 

[TomT]

Indeed. I paid $52 for mine from our local electrical supply house. We're in a smallish market (128,000 12+); but it is an industrial area so we have several good electrical suppliers including a Fastenal store and a W.W.Graingers.

Given the PITA factor of changing these things out, best to have the contactor, crimp lugs, etc. gathered together & take a nice calm Sunday afternoon to replace the old contactor once it starts getting flaky. But, of course, some Christmas day a power line spike will come along and burn out the coil, or weld the contacts together on the old contactor. So $52 is cheap insurance.

In my case, this is a back-up transmitter for my 10 kw main. However, just retired a 2.5H on another station where it was the one-and-only transmitter. Have a Nautel V-2 in there now, with two separate amplifiers will take some doing to completely shut it down to where I can't patch around somehow to get back on the air with a decent power level.

 

[TomT]

: Wouldn't ya just know it! The plate contactor has the same weak coil problem as the filament contactor. Rest of the transmitter works--got about 500 watts without even tuning...even with the contactor buzzing like a hive of angry bees. As I mentioned, the plate contactor and filament contactor are identical: 4 pole, 30 amp contacts, with 240 volt coils.

The plate contactor is mounted on a panel in the front of the transmitter, so it is a lot easier to change. Unfortunately, it is mounted differently than the filament contactor, using two bolts and nuts. And the spacing is different than for the new Siemens contactor. You will have to drill out a new hole--use the existing one on the left and drill the new hole on the right. And you will find that some wires are just too short to reach the far side of the new contactor.

In addition to the items I listed in my first post, you will need some #18 wire in two colors (red and brown matches the Harris color scheme), shrink tube or butt splices, a variable speed drill (cordless one that will go real slow is best), and some patience.

Pull the panel out of the transmitter. You can remove the old contactor and let it hang on the wiring harness while you mount the new contactor. Then cut all the tie wraps back on the wire bundle up to where the wire goes off to the fuse on the left. It's easy just to pull off the wires on the left hand side of the old contactor, replace the crimps with the push-on kind and attach them to the new contactor. Unlike the filament contactor, none of the poles on the plate contactor have more than two wires. Note that two poles have heavy grey wires, leave these for last, on the bottom of the new contactor, and start with the smaller wires on the top, and just work your way down.

Here's where the variable speed drill comes in--the grey wires have yellow-plastic ring connectors, which are the right size for the wire (looks like a #10), but not the screw on the new contactor, which appears to be a #10 bolt. Take the drill and slowly and carefully ream out the ring connector until it is big enough to fit the screw on the new contactor. You will have just enough slack to make the connections on the right hand side of the new contactor if you re-use the existing crimp lug.

On my box, the remaining red and brown wires on the right hand side of the contactor were just too tight. So I spliced in extra lengths of #18. I ran out of time today--have to go back and neaten up with some tie wraps.

As you can see, this is not a good project to start on a holiday weekend. As I mentioned, this transmitter is my back-up, so I've been working on it when time allows. So, once the contactor in your "K" or "H" starts giving you trouble, order the new one in. The complete contactor turned out not to be a stocked item by my supplier--but they stock 4 pole 30 amp contactors with 110 volt coils, and 3 pole contactors with 240 volt coils. They ordered one of each for me, and we swapped coils. (They figured that they could easily sell the 110 volt/ 3 pole contactor they ended up with). Cost: $48.00. The push-one crimps (red) were $8.00 at Fastenal (bag of 50). But be sure and get a good crimp tool--mine is a Klein. You don't want to have to come back later because a wire vibrated out of a crimp--and it is difficult to get a good crimp on the filament contactor wires because of the angle you have to work at.