Friday, December 30, 2011

Rebuilding the R7000

Given the number of times that Cushcraft has re-designed it's multi-band, no radials vertical, I'm not sure it has been their most successful product. The R5 / R7 designs had problems with trap stability and mechanical complexity, so Cushcraft introduced the R7000, using enclosed traps very similar to designs they used on their very successful trapped tribander series. Later this became the R6000 / R8, where some traps were exchanged for multiple vertical elements. 

The R7000 also went through a trap re-design in mid-1998. There are three different manual available, the original from October 1996, one from June 1998, and the last from May 1999. The latter two are hardly different -- only a slight difference in assembly hardware. However, the 1996 and 1998 versions don't even share the same dimensions.

For those of us using the original version, there's at least one change from the 1998 version that might be worth adopting. When adding the R80 80m add-on kit, there are two tubes BH and BI that are inserted into the ends of the CT1 trap. This strengthened the tubes on the trap, which can help avoid fold-overs like this. The 1998 changes incorporated these tubes as part of the standard product. Since this only requires two pieces of 5/8 inch tubing about 6 inches long, it should be recommended for any rebuild. (Provided you can find 5/8 inch tubing)

First step on any rebuild is disassembly. My unit had seen 15 years out in the weather, but was actually in pretty good shape considering. No parts missing, and only a very slight bend in the bottom of the CT3 trap tube.

When I opened the MN7000, I found several spiderwebs and debris. Getting the circuit board out of the box requires unsoldering the SO239 center contact -- this requires a large soldering gun. Once I lifted the board out, I was surprised to find a very live spider who was not happy I disturbed his home. Sorry, fellow, but the eviction has been posted.

MN7000 after re-assembly.
I found the MN7000 in pretty good shape. None of the components seemed charred, cracked or damaged. Some of the connections on the circuit board showed a bit of corrosion, and I re-soldered some of the connections. 

Unfortunately, in trying to shorten the leads on the 27 pF capacitor, I ended up destroying it. In the picture, you can see my substitute -- four 100 pF 50V capacitors in series. As soon as I get the correct part, I'll swap these out.

Pay attention when disassembling the MN7000. I found some of the hardware to be slightly corroded, along the brackets. Remove as much of this as practical.

The general advice is that high SWR on all bands means an MN7000 failure of some kind -- but I couldn't find anything wrong, other than a few questionable connections.

Not sure if the MN7000 was the problem, I turned my attention to the traps. After a short debate, I decided to disassemble all the traps. 

CT3 trap disassembled. The 20m trap is on the bottom
(right) of the assembly, and the 30m trap on the top (left).
The traps consist of two nylon bobbins that are held inside the aluminum tubes by four self-tapping screws and four dimples. You have to drill out the dimples with a 1/8 inch drill bit. The top rubberized weatherproofing was removed by slitting it part way up the non-connected side (the smaller lump) and pulling it off. Try to cut as little of the weather proofing as possible. Remove the screws and pull the bobbins out, or you can tap them through (but be sure to move the aluminum wire out of the way).

CT2 trap half-way assembled. 15m trap is inside the tube,
17m trap waiting to be installed. 
The traps were in pretty decent shape. The top trap bobbins were clean, with the bottom trap showing a bit of insect debris. The bottom traps showed a bit of yellowing as well. A few checks with an ohmmeter showed the connections between the wire and inner trap tube were good. 

Re-assembly was easier than disassembly. Since the dimples were drilled out, I substituted 3/8 inch long #6 self-tapping screws. The alternative would be to cover the dimple holes with tape, but using screws seemed to add to the structure of the trap.

CT1 trap, reassembled on the antenna with a protective
layer of electrical tape.
Weatherproofing is held in place with a layer of tape, wound from the bottom to the top. I also added a layer of tape over the bottom screws, in order to keep the weather off the aluminum wire connection.

With all the components cleaned and re-assembled, it is time to assemble the antenna. Which we will do in our next installment.


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