|Arguably one of the most popular ham radios ever made was the Uniden President HR2510 mobile 10-meter transceiver, and it's cousin, the Radio Shack/Realistic HTX100. They are probably among the most modified, as well. On this page I will detail the results of some of my tinkering with these rigs. It is not my intention to cover ground that has already been covered by others; there are many fine resources on the web describing various mods and enhancements to these rigs. What will be covered here is information that I have found through my own experiments that I have not seen covered elsewhere, as well as my own opinions on some of the other published mods.|
|RX IF Bandwidth|
|RX Audio Response|
|PLL Alignment Errata|
|More obscure 2510 notes
Bandwidth (or, The Best of Both Worlds...)
|Over the years, I have talked to several people who have owned both
the HR2510 and the HTX100. The comment inevitably comes up that the
HTX100 has a 'better receiver' that the HR2510. But how can this
be? Everyone knows that the 2510 and the HTX have virtually identical
When I finally managed to acquire an HTX100 last year, I decided to
investigate to try and prove or disprove this notion once and for all.
Examination of the schematics for both radios showed that the RF sections
were indeed almost identical. I reasoned that the 2510, being an
'allmode' radio with AM included, may use the common CB radio design practice
of having a wider 'compromise' IF crystal filter to improve the RX audio
response on AM. The center frequency of both filters is 10.695 MHz,
so assuming that the carrier oscillator signal is placed on the slope of
the filter, a check of the carrier oscillator alignment frequency for USB
should tell us the approximate bandwidth of the filter. Sure enough,
the USB carrier oscillator frequency is different in the two radios!
Shown below is a chart of the carrier oscillator frequencies for both rigs,
with a LSB value extrapolated for the HTX100.
What does this tell us? If we assume that the USB and LSB carrier frequencies represent the edges of filter, we can see that the 2510 has a 5 KHz wide filter, while the HTX has a 3 KHz filter! This is enough to make a noticeable difference in the performance of the receiver.
The next step was to try to verify this theory on the test bench. I connected each radio to a signal generator, and attached a digital voltmeter to the output of the AGC rectifier as a quick and dirty measurement technique. I then stepped across signal generator frequency in 100 Hz increments using the main tuning dial and recorded the AGC voltage at each step. The results are shown in the graph below.
Click HERE for a 2D version of this graph
|As can be seen from the graph, the HTX filter is indeed significantly
narrower than the 2510 filter. The next question becomes 'Can an
HTX100 filter be transplanted into a 2510 for better performance?'
The answer is, yes, but....
The filters are physically identical, and the HTX100 filter is available through Radio Shack Parts for around $55. The part number from the service manual is BFLY0285001, and the Radio Shack SKU is RSU10569036. Be sure to give them the HTX100 catalog number, 19-1101, as well. You can also swap filters with an HTX100, and realign the HTX's carrier oscillator accordingly. This filter is not available directly from Uniden, I tried. The only downside to this mod will be that, since the frequency offsets for the display readout are controlled by fixed values in the CPU microcode, your frequency display will be off by 1 KHz in both USB and LSB modes. AM, FM, and CW will display normally. The Chipswitch replacement CPU will not compensate for this either, and the people at Chipswitch haven't shown any interest in releasing a new version to allow for this mod. I have come up with a fix for this for USB by shifting the RIT voltage. I did not concern myself with fixing LSB, since 99.999% of all 10-meter sideband activity is USB anyway (and we all use our 2510's strictly on the ham bands, right?).
If you operate quite a bit of AM, you will notice that AM signals do sound more muffled with the narrower filter. This is normal, and can be compensated for somewhat by off-tuning the signal a little with the RIT or the main dial. If you operate a lot of AM, however, this mod may not be for you.
HR2510 Carrier Oscillator alignment for 3 KHz filter
RX Audio Response
|I always thought the RX audio on the 2510 sounded a little wimpy, even
with an external speaker. Rogerbird
has a good
mod on his web site to increase the overall gain of the audio stages,
but I didn't think this was my problem. A sweep test of the audio
chain from the collector of Q109 to the speaker showed an overall response
of about 390 Hz to 2600 Hz. OK on the high end, but a little restricted
on the low end. The main limiting factor on the low end seems to
be the R/C filter network made up of C105, C107, and R148, located between
Q135 and IC103. I paralleled C105 and C107 each with a high-quality
computer grade .1 uF capacitor, and this seemed to extend my low end cutoff
point to about 280 Hz.
|There are lots of good descriptions of mods floating around to enable
the RIT to function on transmit, so I won't re-invent the wheel here. Rogerbird
has several listed on his page. The only thing that I can add is
that I placed a short across diode D160 when I did my mod. It isn't
needed any more once the RIT is enabled for transmit, and may introduce
some non linearity on the low end.
Note: DON'T follow the alignment instructions for Mod #1 on Rogerbird's RIT page. These adjustments control the frequency of your carrier oscillators, and will knock your audio out of whack if not adjusted to spec. Use the instructions in Mod #3 or Mod #4, they are correct.
I always wondered why, when we have a radio that tunes in 100 Hz increments on the main dial, everyone seems fascinated with having 3-5 KHz of 'slide' on the RIT control. Seems kind of silly. My preference was to reduce the range of the RIT circuit by placing a 22 K resistor in series with the high end of the RIT pot and a 10 K resistor in series with the low end. This reduces the overall tuning range of the RIT quite a bit, and makes fine tuning a lot less touchy. You can see this mod in my schematic of the RIT shifter for the crystal filter mod. It's just personal preference on my part, but that tiny knob doesn't make a real great VFO.
|Did you ever wonder what those 2 extra adjustment pots on the synthesizer
board are? The HR2510 service manual makes no mention of them, but
the HTX100 service manual tells the secret: they are balance adjustments
for IC304 and IC305. To adjust them, you need a good scope with at
least 60 MHz bandwidth (100 MHz would be better). Preset the rig
to 29 MHz. Connect the scope to TP305, and adjust VR302 for the cleanest
sine wave. Repeat using TP306 and VR301. That's all there is
|Anyone who has opened a 2510 has noticed the melted plastic of the
LCD holder around the display lamp. This seems to have been a major
design screwup on Uniden's part. My solution to the problem was to
drill several small holes in the top and sides of the LCD holder for cooling.
I then removed the stock lamp and suspended a pair of 12v micro-lamps (272-1092)
from Radio Shack through two of the holes in the top. Since I never
liked the ugly amber color that Uniden chose for the backlight, I topped
it off by making a red filter to fit behind the LCD unit from a red Rolodex
plastic card protector (item #67649), available in a pack of 5 colors from
your friendly office supply store. (Note: the blue ones work nice
in vintage Drake gear). Inserting the red filter requires careful
disassembly of the LCD holder, so don't try this if you are all thumbs!
The result is a pleasing, evenly lit red display that is easily visible
under most lighting conditions. Even if you don't go the full route
and install new lamps with a different color filter, the cooling holes
in the LCD holder are probably a good idea.
|This is where I get to express my opinion of some of the other mods
floating around out there. Constructive comments are welcome; flames
will be ignored.
The '73 Magazine Repeater Split Mod'
ALC/AMC mods (generally involving disconnecting
the ALC/AMC entirely)
Power Increase mods
Repair and Mods for lots of interesting rigs
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More to follow....
All Articles and Pages are ©
Copyright 2001, all rights reserved, by Jim
Shorney (except where noted).
HR2510 and President are registered Trademarks of Uniden Corporation.
Realistic and HTX100 are registered Trademarks of Tandy/Radio Shack.
Chipswitch is a Registered Trademark of the Chipswitch Company
All other Trademarks and Copyrights are those of their respective owners.
The contents of this page are provided for educational purposes only. All information contained herein is provided as-is, without any warranties express or implied, and is accurate to the best of my knowledge; however, the author assumes no liability for lost/damaged sanity, time, productivity, or property resulting from the use of information contained in this page. The modifications provided on this page may void any existing manufacturer and/or vendor warranties and invalidate the F.C.C. type acceptance of the HR2510 and HTX100. The author is not responsible, and assumes no liability for any fines or penalties imposed for illegal operation resulting from the modifications on this page.
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