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Re: RE: S band for 6 bucks



A couple of people have written to ask me if I plan to create a webpage 
showing how to do the mods on the $6 downconverter or if I have any 
photos.  Sorry, but I have no plans for a webpage and I did not take any 
photos.

This is NOT a beginner's project.  The mod is very straight forward, * 
IF * you have very good soldering skills and are comfortable working on 
surface-mount PCBs.

The PCB must be removed from the metal housing in order to get to the 
bottom side of the board so that you can remove and replace the crystal. 
The three coax connectors are easily unsoldered from the PCB by using 
"solder wick".

The combline filter element has to be unsoldered from the PCB before the 
PCB can be safely removed from the metal housing.  Trying to pick up the 
PCB with the filter element still attached is too risky since the solder 
pads would be easily damaged.

I believe that the filter element is copper plated steel.  I did not 
think to check it with a magnet to see if it was steel before I 
reassembled the unit, and I do not plan to disassemble it again just to 
check it out.  What ever it is, it is a good heat sink and not easily 
unsoldered with a small soldering iron.

 It will require a good soldering iron with a medium sized tip or a 
soldering gun to unsolder the combline filter element from the PCB. 
Working very close to a small PCB loaded with very small surface-mount 
components SHOULD make you nervous about using a soldering gun.  If you 
decide to take this approach, be very careful.

I removed all of the mounting screws securing the filter element and 
applied heat to the top side of the filter element, on one end of the 
filter element, above the point where it is soldered to the PCB.  By 
applying a slight upward pressure on the same end of the filter element, 
it was easy to separate the filter element from the PCB as soon as the 
solder melted. This will prevent damage to the solder pad on the PCB 
from the soldering gun.

Once the filter element is separated from the PCB, it is easy to tilt up 
the PCB so that it can be slid out from under the three coax connector 
pins.

Once the PCB has been removed from the metal housing, removing the old 
crystal and replacing it with the new one is very straight forward, 
assuming that you are comfortable working on this type of equipment.

The LO was originally 2380 MHz., divided by 256 and locked to a 9.29687 
MHz. crystal.

With a new downconverter input freq. of 2.4 GHz. and an IF output of 144 
MHz., the LO needs to be changed to 2.256 MHz.  I have not found any 
published specs for this downconverter, so I do not know what the 
crystal specs are.

I happened to have an 8.8125 MHz. crystal that I had bought for use in 
converting an AIDC downconverter.  The crystal that I have was 
manufactured by JAN.  I bought it from someone who was selling 
modification parts for the AIDC downconverters.  If I had bought it from 
a crystal manufacturer, I would have ordered it from  "International 
Crystal".  It may, or may not be the correct crystal for this 
downconverter, but it is what I had available, and although it is not 
exactly "on freq" it works OK.

The actual LO freq. is 2.25588 MHz. instead of 2.256 MHz.  I am using an 
HP 5340A, locked to a Rubidium standard, which is in turn disciplined by 
a Cesium standard, compliments of the GPS satellites..  So, yes, I do 
know what the LO freq. really is. :-)

If you order a replacement crystal and you do not know what the crystal 
specs are, you can send them your old crystal and they will test it to 
determine what is needed.  I have done this many times and I recommend 
ordering from International Crystal.  If you do this, please post the 
information on the AMSAT BBS so that it can help others who want to 
obtain the correct crystals.

Now, all that is left to do is to retune the combline input filter. 
Looking at Roger's photos at:
http://www.themidwives.org/CalAmp/ (thanks kc8zfn).

The first photo shows the downconverter with the "N" connector at the 
top of the photo.  On the right side of the photo is the combline input 
filter.  There are a total of nine Philips head screws on top of the 
filter that are locked in place with black thread lock.  The seven 
screws on the right hand side of the filter, closest to the outside edge 
of the downconverter are the tuning screws.  The other two screws are 
longer and they are used for grounding the top cover of the filter. 
Leave these two screws alone for now.

I used finger nail polish remover to soften the black thread lock on the 
tuning screws.  This stuff is very tough. I had to let the finger nail 
polish soak for several minutes before the screws could be tuned.  They 
are very soft and are easily damaged.

Once you remove the filter's cover, you should be able to see where 
these two screws scratched the inside bottom surface of the metal 
housing of the filter.  These two screws should be vertical.

As Roger stated:
"There are two long "grounding screws" which run between the outside
two fingers. Not much care was taken by the assembler to keep these
vertical, so if you have a "dead" converter you might want to check to 
see if one
 of these screws has come in contact with a finger".

The screws in my downconverter were straight, but if you find that your 
screws are not, I suggest that you follow Roger's advice.

The seven tuning screws should be "dressed" flat on the ends of the 
screw.  If they are not, it will cause problems when you retune the 
filter.  Making the end of the screw smooth and flat will permit maximum 
capacitance between the tuning screw and the combline filter, before the 
screw grounds out on the filter element.

I retuned the filter by observing the IF output of the downconverter on 
a spectrum analyzer.  However, if you have a good clean and stable, but 
low level 2.4 GHz. test signal AND you keep the downconverter's input to 
your receiver from overloading it, you can retune the combline filter 
without a spectrum analyzer.

Adjusting the crystal's trimmer capacitor is also simple.  If you have a 
freq. counter that can display the LO freq., it is very simple to adjust 
the LO's freq.  Mine tunes smoothly through out the range of the 
trimmer.   A small single turn pick up loop, placed near the top of the 
open downconverter provided enough signal for my counter.  My loop is 
about 3/8" in diameter and this not critical at all.  Use what ever you 
happen to have.

If you do not have a frequency counter, simply adjust the LO so that 
your 2.4 GHz. test signal produces an IF output as close to 144 MHz. as 
possible.

The only mods that I have made to this downconverter were to retune the 
combline input filter, replace the LO reference crystal, and touched up 
the LO reference crystal's trimmer capacitor to get the LO as close as 
possible to 2.256 GHz.  I have not made, nor do I plan to make any other 
adjustments or modifications to the downconverter.

With an input level of -70 dB at 2.4 GHz. the IF output level is -38 dB 
at 144 MHz.  The modified downconverter measures to within 1 dB of what 
the unmodified downconverter measures at 2.5 GHz.

I have modified several Drake, AIDC, and other models of Cal Amp 
downconverters.  This Cal Amp p/n 130215  is the simplest one that I 
have modified so far.  The seven-pole combline filter on the input is 
very impressive.  Properly tuned, it should really help keep unwanted 
signals out of the downconverter's input.

If you do not have the necessary test equipment, hopefully you can find 
a friendly Ham near by who is willing to help.

Good luck & 73,

Woody
KJ4SO
AMSAT # 5562

.

> ----- Original Message ----- 
> From: "Woody" <kj4so@nc.rr.com>
> To: <amsat-bb@amsat.org>
> Sent: Monday, June 13, 2005 5:07 PM
> Subject: [amsat-bb] RE: S band for 6 bucks
>
>
>> Hi All:
>>
>> I replaced the crystal in the Cal Amp 13215 downconverter today and 
>> all went very well.  Observing the LO output on a spectrum analyzer, 
>> the signal is strong, clean, & stable.  The crystal was manufactured 
>> by JAN and is slightly low in freq.  The LO is 2.25588 MHz. instead 
>> of 2.256 MHz. I prefer "International Crystal" crystals, but I 
>> already had the JAN crystal.
>>
>> With an input level of -70 dB, the 144 MHz. IF output level is -38 
>> dB. This is the same level that the unmodified Cal Amp 130215 
>> downconverter shows with an input of 2.5 GHz.
>>
>> I retuned the combline input filter and touched up the LO reference 
>> crystal's trimmer capacitor to get the LO as close as possible to 
>> 2.256 GHz.  I have not made, nor do I plan to make any other 
>> adjustments or modifications to the downconverter.
>>
>> I have modified several Drake, AIDC, and other models of Cal Amp 
>> downconverters.  This downconverter is the simplest one that I have 
>> modified so far.
>>
>> The seven-pole combline filter on the input is very impressive. 
>> Properly
>> tuned, it should really help to keep unwanted signals out of the 
>> downconverter's input.
>>
>> Thanks again, Drew for the "heads up" on the $6 S-band 
>> downconverters.
>>
>> 73,
>>
>> Woody
>> KJ4SO
----
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