Astro Spectra VHF Preamp Play

[Astro Spectra]

In these past posts:

http://batboard.batlabs.com/viewtopic.php?f=1&t=92890
http://batboard.batlabs.com/viewtopic.php?f=1&t=88911
http://batboard.batlabs.com/viewtopic.php?f=1&t=67938

there has been discussion about the preamp option in the Spectra and Astro Spectra radios.

Recently I’ve had occasion to work on a few Astro Spectra radios and since these were destined for amateur radio use I thought the receive performance could do with some attention in the sensitivity department.

Note that receiver sensitivity is not the only receiver parameter that matters. Even today in the XTL series Motorola offers two receiver options that trade off sensitivity for blocking performance.

To fully modify the front end board from standard to the pre-amp version requires quite a bit of work with adding, removing, or replacing various resistors. Not only on both sides of the front end board but also under some of the soldered down shielding cans.

However, of the various changes one resistor, R3206 in the emitter of the 1st RF stage transistor, makes the most difference to sensitivity. The good news is that by simply removing the cover of the front end board cavity it is easy to change this resistor without removing the front end board and associated coax cables.

R3206 normally has a value of 68 ohms; it is in parallel with a 51 ohm resistor on the component side of the board giving 29 ohms. In the pre-amp version these two resistors are replaced with a single 10 ohm resistor. We can achieve the same thing by soldering a 15 ohm resistor across R3206. You could replace R3206 with a 12 ohm resistor but it is quicker and easier to just solder a new 1206 sized resistor across the back of the existing resistor.

This is a picture of a Range 1 board with the red arrow pointing to R3206:



The green arrow points to the range identifier (note the digit 1 revealed by the resistor placement) and the standard (S) vs preamp identifier (P).

This is a picture of a Range 2 board:



Note that this particular board has a 27 ohm resistor in place of R3206 and this appears to be how it came out from the factory. In this case the resistor was removed and replaced with a 12 ohm resistor. Placing another 27 ohm resistor across it would have achieved the same effect.

Here is the performance unmodified vs modified with all measurements were made at 146 MHz:

Radio #1: -119 dBm (0.25uV) before -122 dBm (0.18uV) after
Radio #2: -115 dBm (0.4uV) before -120 dBm (0.22uV) after
Radio #3: -114 dBm (0.44uV) before -122 dBm (0.18uV) after

Generally a worthwhile improvement but as you can see your mileage may vary.


Postscript: One of the radios was DOA needing a new front end board. An old Range 1 Spectra provided the donor board. This opportunity allowed the effect of Range 1 vs Range 2 non-preamp front ends to be compared using a range 2 front end borrowed from one of the other radios being worked on (different front ends on the same back end radio):

Range 1 radio

144 MHz -119 dBm
146 MHz -119 dBm
150 MHz -119 dBm

Essentially no difference.

Range 2 radio

144 MHz -117 dBm
146 MHz -119 dBm
150 MHz -120 dBm

Reduction of 3dB at the low end of 2M.

 

[Walking_Catfish]

Hey... Thanks for the info. Perhaps I will try it soon!

[RADIO43]

Pretty cool.... Any chance of a similar mod on a UHF Astro Spectra?

[Astro Spectra]

Unfortunately the UHF front end is not as straight forward as with VHF design.

The 1st RF stage transistor is not fitted in the standard UHF front end. See the unused solder pads in the red circle in this image:



You can do the mod by moving one chip cap and installing 5 new resistors, 2 new caps, the transistor and a zenner. You can buy all the parts from Mouser or Digi-Key, except for the exact Moto 82D49 transistor. This is the same device as used in the mixer and also as the 1st stage of the VHF preamp. There’s bound to be a suitable standard replacement you’d need to find the right one. Another option might be something like a SPF5043Z high IP3 pHEMT MMIC LNA from RFMD.

Probably the hardest part is that, unlike the soft substrate VHF boards, the UHF ones are on a ceramic substrate that is hard to solder well on.

[Bill_G]

Preheat the ceramic slab with a heat gun. I have to reattach UHF presel shields often. The heat gun does a good job of getting everything up to temp so your iron can work quickly.

[N4KVE]

Will upgraded my UHF Spectra with the "preamp in the red circle". Definitely made an improvement. There's a guy on E-bay selling the components as a kit for about $14 in the "Spectra" section. GARY N4KVE

[kk6rq]

Regarding adding the 12 ohm resistor in parallel-- assuming that's all you do, confirm there's no need
to move the S/P jumper? In fact is there any functionality by doing so or is it simply a visual indicator?

Thanks,

Bob
KK6RQ

[Astro Spectra]

The jumper is just ID. I'd leave it so you can always return the board to standard with the least work.

But I do write on the board with a felt pen to note the mod for any future tech who might be otherwise left scratching their head...

[kk6rq]

Thanks; all they had locally were 11 ohm chips, which I assume are close enough! Off to the races.....

[kk6rq]

Awesome! Average .4 is now .2 at 146MHz on a J-split! I already have a preamp on my radio but I can now
upgrade all my friends for about .08 a pop!

-Bob
KK6RQ

[Astro Spectra]

Thanks for the feedback. Nice 6dB improvement there. Be aware that the receiver will have slighly worse IMD performance but I've not expeieced any problems even at high sites where amateur gear folds.

[4n6inv]

I did the mod this morning, and drove 55 miles to Abilene. The result: Practically blew me out of the car! Awesome! Thanks for the information! Incredible difference. I'm out in the middle of nowhere, so IM isn't a real issue out here. Being able to hear? Priceless! Thanks again!

What I did was; I removed both resistors and replaced them with two 20 ohm resistors in parallel. Got to thinking that maybe; just maybe, there was a method to their madness. My rationale? Perhaps they piggybacked the resistors to share some heat dissipation. Probably not; but, the end result: 10 ohms. And one hot receiver!