Edited By Dave Karr, KA9FUR
You *WILL* want to have a cordless drill or cordless screwdriver with
an assortment of Torx bits. If you don't have one, go get one.
If you decide to work on this radio with a regular manual Torx screwdriver,
you will end up wasting a lot of time and probably swearing a lot.
Remove 1/4 turn from the last helical resonator in the front end (L405/C5) to get radio to tune up better at high end of ham band. Basic instructions:
I've also found that the final LO coil (C6) may need to have somewhere between 1/8 and 1/4 of a turn removed from it as well. If when you tune up the radio and find that any of the slugs is about ready to fall out you need to remove a portion of that coil.
If you are careful, you can avoid completely disassembling the front end as described above and simply unscrew the tuning slug and remove a portion of a turn through the hole in the top of the casting. You will need wire cutters that have a long skinny head to them in order to get them down into the hole to make the cut. Don't forget to hold the end of the coil with a set of needle nose pliers to keep it from falling down inside the casting.
Without this mod, the VCO won't lock above about 441 MHz. I'd be hesitant to trust the VCO to be able to lock at all temperatures at anything above maybe 438 MHz without doing this mod. Of course, every radio is a little different, so you might find some Deltas that work better higher in the band than others.
After doing the mod, the receive VCO will lock from about 410 to 475 MHz at the widest extremes, with the "safety region" being about 420 to 460. If you're going to use the radio to Rx low in the 70 cm ham band (below 435 MHz), don't bother doing the mod.
The mod involves removing C219, a 12 pF SMD cap on the bottom side of
the board. A perfectionist (Dave Karr, for example) would replace
this cap with something around 3.3 or 4.7 pF which would better center
the tuning range in the middle of the ham band, but it's probably not necessary.
Just keep an eye on the VCO steering voltage - 5V is pretty much the center
of the range. It will lock anywhere from about 2.5 to 8.5 volts.
The GE manual says to set it for 7.5 volts at the highest operating frequency,
but if you're using it as a single-freq radio, it makes more sense to center
Finding C219 is the hardest part of the operation, so I took two pictures to show exactly where the cap is.
The sacrificial component in this mod is C260, a 4.7 pF ceramic cap
(standard leaded part, not SMD). The easy version of the mod is to
just remove it. With it removed, the VCO will lock from *about* 420
up to around 475 MHz. I say *about* because with it completely removed,
the ferrite slug has to be just about all the way into the L209 core in
order to get a good middle-range lock down at 420 MHz. The "correct"
version of the mod is to replace the 4.7 pF cap with a 2 pF cap (NP0!!!).
With a 2 pF cap, it tunes up nicely from 410 to 460 MHz no problem.
So the choice is
yours. If the radio definately won't be used low in the band (below 430 let's say), then you can just remove it. If you want to make the radio a general-purpose go-anywhere-in-the-ham-band transmitter, then put 2 puffs in there.
L103 is the variable inductor that tunes the buffer amplifier that follows the VCO output, which in turn drives the tripler stage. Up above about 440, the slug ends up being just about all the way out at the peak. I haven't bothered to mod this stage since it works just fine with the slug all the way out. Intentionally detuning this stage by running the slug further into the form has shown that the meter reading can drop way way way down before it quits working, so there seems to be plenty of headroom, hence I don't think it's worth messing with this stage to optimize it. Just tune for the peak per the instructions, and if it so happens that the slug ends up poking out of the form, no big deal as long as you have less than -0.5 volt ("less" meaning "more negative") it will be fine.
Since this (C260) is a leaded component, it's on the top side of the board.
There are two vintages of these Delta-S radios. For the most part they are pretty similar. Most of the areas of the TRS board are laid out the same, though there are a few variations, especially in the area of the reference oscillator. All of the stuff I've sent so far was done using one of the radios that has TRS board p/n 19D901350Gxx. The other board is 19D900775Gxx. If you can't correlate what I've sent to the other board layout, let me know and I'll figure it out. The manual I have doesn't have either of those two part numbers in it, but it seems to be more similiar to the 19D900775Gxx boards. Go figure.
The only tools you need to tune up these radios are a DVM, a diddle stick with a square end, a greenie, and a 3/32" allen wrench (or hex screwdriver which makes it a lot easier). If you don't have a square diddle stick in your collection, the three-pack of diddle sticks that Radio Shack sells has one in it. I haven't been able to find a source for the square diddle sticks anywhere else (GC Electronics doesn't seem to sell them separately), so Radio Shack is the easiest source.