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Jul 25, 2010

HTX-100 Modifications

Modifications for the Tandy HF HTX-100 USB/CW Tranceiver


  • Extended Frequency Coverage from 26.000 Mhz to 30.000 Mhz
  • Simple RF Wattage Enhancement to 35 Watts
  • Enhancement of Microphone Frequency Response

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Fair Warning and Friendly Advice To Very Anxious and Excited People

We are not responsible for any damage on your part. If you don't know
don't do it. Let someone with more experience do it.
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Thank you for your interest. The Tandy HTX-100 Amateur Radio Transceiver is
a close cousin of the Uniden HR-2510 (Listed as the Amateur Radio Mode
HR- 2510 (UT-550B). HTX for Ham Tranceiver, HR for Ham Radio, UT for Uniden
Transceiver. We can assume the A suffix for the HTX-100.)

My first HF radio as a Novice was a HTX-100, so there was always a lingering
question as if the Tandy model was capable of similar capabilities. I had
spent many hours examining the mini-schematic diagram at the end of the manual.

This information was derived from the buying of a HR-2510 service manual
at the local hamfest. When I examined the PLL schematic, I realized that 
I was looking at a prior generation of the same PLL circuit. It helped
to complete the purchase.

The Uniden HR-2510 microcontroller u1201 is a 46 pin Dual Inline Package.
On the diagnostic voltage table for the microcontroller, there are two pins
labeled (34)AMATEUR and (35)GND aside of each other. They are pulled to GROUND.

#34 will open TX/RX from 26.000 Mhz to 28.000 Mhz, #35 will open 29.7 to 30 Mhz.

On the schematic for the HTX-100, there are similar pins 28 and 29 on U1208
pulled up by a resistor pack to Vcc.

The Tandy model microcontroller is a surface-mount device and the pins on the
chips don't bend: they SNAP. The steel used on the pins are quite different.
Do not attempt to physically pull on the pins of the package.

Percy KF2AT wrote the mods for the HTX-100.
Use common sense, a grounded soldering iron, and enjoy the mods.

Adam Kb2Jpd

*Extended Frequency Coverage from 26.000 Mhz to 30.000 Mhz
________ _________ ________ ____ ______ ___ __ ______ ___

Thanks Percy for being the first one.

Remove bottom cover. With the radio upside down and display facing you remove
the synthesizer board with the four screws, this is near the front of the radio.

This gives access to the small board underneath with the surface mount
components on it. Locate the processor chip IC401 (UC-1208).Locate pins 28 and
29 which are on the right hand side of the chip and are 4th and 5th from the
top. Note they are bridged with solder.

CAREFULLY cut the copper track NEXT to the pins. DO NOT attempt to unsolder the
two pins form the board. Use a GROUNDED soldering iron. Locate the two rows
of connector pins above the chip.

On the bottom row locate the last pin on the right. Verify with a VOM that this
pin has + five volts. Solder a 10k (1/4 w) resistor between this pin and pins
28 and 29 on IC401.The synthesizer will now tune from 26.00 Mhz to 30 Mhz.

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*Simple RF Wattage Enhancement to 35 Watts

With the radio in the same position as before, locate VR5 in the top
left hand corner. This is the RF high power preset.

Lift and bend the wiper so that is is permanently disconnected form the track.
Solder a 47k resistor (1/4 watt) between the wiper and the left hand side of
the track.

This should increase the RF power output to around 35 watts but you will now
not be able to trim the power output. The Low power trim will function as
before.
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Enhancement of Microphone Frequency Response

1)  To increase average depth of modulation
--  -- -------------- ----- -- ----------
Remove top cover of the radio and with the display facing you and the radio
upright locate IC3 (MJM45585) near the bottom right hand corner of the main
large board.

Just to the right of this IC locate c75 (56 pf) and remove it. This will
increase the gain of the microphone Preamp considerably but without
overmodulation. If you find the compression excessive(i.e. background noise)
open the microphone and insert a 470 ohm resistor between the hot side of the
mic insert and the input (white lead).

This will reduce the gain somewhat but will still give you much increased
"talk power" (percy's words, not mine. Adam).

2) To increase audio response in the high mids
-- -- -------- ----- -------- -- --- ---- ----

This simple mod will recrease the audio response in TX in the 2 to 2.7 khz
region giving higher articulation and move intelligibility under weak signal
conditions.

Locate IC3 as before on the large main board and the radio positioned as before.

Locate a 5.6k and a 4.7k resistor just below IC3.
Solder a 0.015 mf (16v) capacitor between the left side of the 4.7k to the
right side of the 5.6k resistor.

These two mods work together to give improved readability on SSB under DX
conditions.


Note: I haven't tried or verified this, proceed at your own risk.  And do not transmit outside of legal bands!
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