They're pretty standard, though quite likely rat shack doesn't carry them (and I know better than to point you to a local  - dutch - source).

Briefly,  this is one of the "new standard" DC plugs and any reasonably electronics shop should have them.

Be careful though - the input voltage is 4-7 volts (charging happens above 6 volts), so don't make a just a cable to connect to your 12V power supply as you *will* blow things up with overvoltage.

The 12V car adapter is actually a pretty good deal because it contains a switched mode power supply so it doesn't run hot and it's input current is actually *lower* than the 0.5A the rig uses while charging.

If you'd create your own cable, then you either need to create your own SMPS (I usually don't have luck with these), use a 7V power supply (and make sure you never forget!) or use a series stabilizer that will run hot as hell.

Geert Jan

I've rebuilt several dozens of battary packs over the years, on the average with positive results.

One skill you'll need to learn yourself is how to open up spot-glued boxes w/o destroying them. I recommend practicing with a few wall warts first. There's a couple of tricks:
- you need to know how the unit was put together, that that is, what fits on what. Once you know that, it's usually a matter of excerting force the right way and the box just springs open. Bonus points of the unit isn't completely full with marks from the scredriver you used to pry it open.
- Sometimes (but this mostly applies to wall warts), I use a vice to put the right pressure  and open up the box
- Some welds get very brittle under low temperature conditions. Dumping the thing in the freezer one night might help
- Some packs are hopeless. I'm defeated by the packs used in IC-W2E/A and FT-23 radios.

The aim of the game is to break the glue seal and not the plastic itself. This way, you can re-close by just adding some glue.
Again, this is an aquired skill which will help with all kinds of other repairs.

Then the replacement.
NiCD/NiMH don't like heat. use spot-welds if you can, If not, use a hot iron with a thick tip so you can work quickly - a lot better than struggle for 20 seconds with a small iron for the whole cell to heat up enough to solder - you've ruined the cells if you do that.

If cells contain small holes, these are overpressure vents and MAY NOT be closed, so watch you soldering.

Simple AA cells can easily do 200A short circuit current, and this *will* cause nasty burns and fires.
It is for this reason that the original cell will have a short circuit protection in the pack. You'll need to transplant the protection.

Alkaline packs don't have protection. When used with rechargables, a short will have nasty consequences. This is the reason manufacturers try to block this scenario.

Geert Jan

The 790 (and friends like the 290 and 690) have a processor that only supports a limited number of pre-configured shifts.
See, for the 290, http://www.qrz.com/download/mods-e-h/ft290.txt

If you look up the mods pages, you'll find different jumper settings for different parts of the world. This affects repeater shift, step size (big and small), and frequency range.
Not sure if the 790 has the same CPU but I think it does - clearly some config settings were intended for the 690 (the 140-144 MHz setting makes a lot more sense as 50-54 MHz and a shift of 1 MHz is used on 6 meters)

Every time you change the jumpers, you'll need to reset the processor. Note that some jumper settings may cause the processor to think it's a 290 and not cause lock at all - I don't remember oif the 790 had a different frequency plan but I don't think so.

I'm not sure if your required setting is among those selectable. ZA uses both 1.6 and 7.6 shift; you can't have both, for sure.

The trick to use spilt frequency, as described by others in this thread, may work for you.

If you work on the rig, be aware that the CPU is battary-backupped, ESD sensitive, and spares no longer available so be careful; ruining the CPU ruines the rig.

The 790 makes a fine backend rig for SHF work with transverters because of it's high first "IF". I'm sure SHF folk would be interested if you would want to sell it. It's less desirable for band plans the rig wasn't built for when the CPU firmware was written. If your main thing is doing repeaters, you may find that a newer, FM-only rig, with per-channel selectable repeater shift, may be more to your liking.

Good luck!

Geert Jan PE1HZG

Like so many, I live in a place where a full-size 80m dipole unfortunately is infeasible. My eye fell on the Diamond BB-7V vertical: vertical, 6 meters long, does 80-10 meters.

I'm quite confused about this thing. When working on 80 meters, the radiator is only 6 meters instead of the 20 needed to make a quarter wave. There is no counterpoise, so I wonder to what we're radiating against.
They claim swr <1:2.0 everywhere and the only way I can see this is through an attenuator, with the 6m radiator connected to the other end to provide some accidental radiation.

Wish is the foundation of many misunderstandings and while I like this antenna to work, I'd feel better if I would understand *how*.

Eham reviews http://www.eham.net/reviews/detail/5936 are surprizingly good, ignoring the machining bit.
And, I'm a fan of Diamond products which in general are well built and survive many years of weather, so I'm not after bashing this thing.
However, if I want it to work, I'd like to understand how it can work and I would appreciate someone enlightening me on this.

Thanks,

Geert Jan

Nobody seems to be answering so I'll give it a try. I have repaired the lamp on an FT-290 (not the RII model).

I remember that it was straightforward but the lamp was very small. Just take the thing apart and see how it goes. As a matter of course, I usually mod the circuit to make the lamp run on -10% of the nominal voltage. You'll have less light but the lamp will last forever.

Just go in and do it. You may want to check the documentation for shape, size, and voltage first, and make sure you can get a replacement. I just stock these small lamps.

Good luck!

Apart from the beeps, does the unit function at all, that is, does it receive and transmit?

You should check the service manual, but a guess would be that this is the way the box indicates that the PLL is unlocked.

Geert Jan

I'm rebuilding a MFJ941B antenna tuner that in a former life had suffered sever damage because of a coil switch burn. The switch was replaced, but the repair was a mess and when I got the unit, I had no recourse but to rebuild.

Anyway, the tuner uses a T-configuration: two variable capacitors in series, coil with switched taps to ground.
The question is where to put the taps.

Just connecting every other winding of the coil or so gives a very uneven selection of inductance. Doing by experimentation doesn't work either (can we simulate all kinds of impedances, including inductive and capacitive ones, over a wide frequency range?)

Many years ago, I heard a casual comment that for tap selection of a fixed coil in a tuner, the taps should be so that the inductance should be (0.5*sqrt(2)) apart, so:
- 1st tap: full inductance L
- 2nd tap: 0.7 * L
- 3rd tap: 0.7 * 0.7 * L
- 4th tap: 0.7 * 0.7 * 0.7 * L
et cetera

But, I'm not sure I heard the hyphthesis correct and besides, I don't understand the rationale behind it.

I plan to select the taps using resonace with a dip meter, as soon as I figure out where the taps should be.

Does anyone understand the rationale, or give another suggestion I should follow?

Yes, I asked MFJ, and they said me I should "see what they did" as they didn't have documentation, and besides, the unit is old. Doesn't matter, as knowing this (which I haven't seen in construction articles about tuners) might help other tuner constructors (or re-constructors).

Your wisdom appreciated,

Geert Jan PE1HZG

You may also want to check the impedance of the cable.
This is how to do it:

On one end, connect a scope and a pulse generator with sharp raise time (a 555 will do). Leave the other end open for now.

Check on the scope that you see the reflection of the pulse through the cable. You may have to tweak the frequency of the pulse generator a bit.

Next, on the uptil-now open other end of the coax, attach a potmeter, 100 ohms, or a couple of fixed resistors. Adjust till the reflections are gone. The resistance of the resistor is now the impedance of the coax.

There. You know the impedance, you know the dampening, you may not know the type of the cable but you have it's parameters, yes?

73,

Geert Jan

I received an older piece of test equipment that is in a box with a top lid that has black "foam" in it to protect the contents. Unfortunately, time has done it's work and the foam has disintegrated to a sticky black goo. I'm concerned that, if I start to try to wipe the goo away, I'll just rub it in.

Users of old test equipment probably have come to this before: what's the best way to remove the goo?

Thanks, 73

Geert Jan PE1HZG