The following information specifially applies to the 403-433MHz Delta-S

By Jeff Depolo, WN3A
26-Feb-2001

Edited By Dave Karr, KA9FUR
17-Dec-2001

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.
 

Receiver Modifications



FRONT END MOD


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:

  1. Remove three Torx screws that hold the audio amps to the chassis along the right side of the radio (T-15 I think).  Remember where all of these Torx screws go, both in this step as well as in the following steps, because they are various lengths.
  2. Turn radio over and take off bottom cover of radio by removing the four Torx knobby things that attach the radio to the vehicle mounting plate (T-25?).
  3. Unsolder the PA connections (two large solder pads with reinforcing washers).
  4. Remove all of the Torx screws that hold the main board to the chassis (T-15 I think).  There are 9 screws.
  5. Remove all of the Torx screws that hold the front end and IF filter casting.  These are the screws that go through the long narrow shielding strip on the bottom side of the board.  There are 22 screws.
  6. Remove the two smaller Torx screws that attach the system connector at the front of the radio (T-10 I think).
  7. Remove the entire board from the radio chassis.  There are several slide-in pins that connect the main board to the PA; be careful when removing the board so you don't bend the pins.  You'll have to wiggle the front connector and board to get it to slide out of the chassis.  When you remove the board, the front end/IF filter casting should just fall off in the process since all of the screws have already been removed.
  8. Cut off 1/4 turn (NO MORE!) of C5, the fifth coil from the front of the radio (the last coil in the RF front end filters).  Be careful not to squash or otherwise deform the coil when you go to snip it -- it is soft copper wire and will bend easily).
  9. Undo everything above (except for #8 of course).
After this mod, receive performance at 440 MHz improved slightly (went from 0.25 uV for 12 dB SINAD to 0.22 uV), and slug is now about 6 turns into casting.  I don't think we need to optimize this any further.

Editor's notes:

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.


RECEIVE VCO MOD


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 it
around 5V.

Finding C219 is the hardest part of the operation, so I took two pictures to show exactly where the cap is.

  1. Turn radio upside-down and remove bottom plate.
  2. Remove all of the screws that hold the shield that covers the bottom side of the VCO stuff toward the rear of the radio (about 17 T-15 screws). Remove the shield.
  3. Look at my pics to figure out where C219 is.  It's in about 3 1/2" from the rear of the board, and 2 7/8" from the left side.
  4. Once you've found C219, make it go away.
  5. Undo steps 2 and 1.


Transmitter Modifications



VCO UP-RANGE MOD

Without this mod, the VCO won't lock reliably above about 442 MHz.  You can use a #10 brass screw instead of the ferrite slug in L209 (the variable inductor that you tune to center the VCO range) to up-band it also, but this is a little cleaner.  Again, depending on the specific radio, you might not have to do this mod to make it work at 441.  If you do this mod though, it would be better centered on the ham band to eliminate any possibility of problems.  It only takes a couple of minutes, so you may as well do it.

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.

Instructions:

  1. Remove all of the screws that hold the top shield that covers the whole VCO/synthesizer area of the board.
  2. Find L209 (the variable inductor that you normally tune to center the VCO).  Immediately to the right of the casting surrounding L209 is C260, the target component.
  3. I've included several pics showing where C260 is looking at the top of the board.  I also included a pic showing where the pads are on the bottom side of the board so you can use some solder wick to get the part out cleanly if you plan to replace it with 2 pF.  To get at those pads you need to remove the shield on the bottom side of the radio as well.
  4. Replace the shield and screws.

 
 

Additional notes


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.