Deaf VHF Astro Spectra (100 W unit)

[Tim]

Hi Folks,

I've got a VHF Astro Spectra in my car, and it has recently
become relatively deaf. Now appears to be about 1.5 to
2.0uV to break squelch.

I opened up the PA compartment and noticed that there are a couple
of glass type components that I believe may be used in the RF switching
circuit. (I don't have any schematics).

One side of these devices is dark, leading me to believe they are no
longer working. (looks like the smoke couldn't get out).

Might this be the problem?

The antenna is mounted on the back deck of the vehicle, and a low band
antenna (connected to an X9000) is located on the other side of the
deck. Probably about 2.5 feet away.

If indeed they are no longer functioning properly, could the proximity of
the two antennas be the issue?

Thanks,

Tim

[Will]

Common problem with high power transmitters and antenna VSWR.

Reading your post I would believe the antenna switch diodes are OK. Your RF amp on the RX frontend is bad. Very Common.
The poor little transistor gets all the RF energy down its base.

[Tim]

Hi Will,

Thanks.

Anybody have a schematic of that portion of the circuit that I could
get? Would help in seeing which part has been zapped.

Guess I need to move one of the antennas!

Thanks,

Tim

[Mike B]

Been there done that. A PIN diode CR3903 on the bottom of the HLD6063 VHF high power amplifier board shorts out from the high power RF energy the low band X9k Tx provides to the Spectra antenna. CR3903 is Motorola part number 4880142L01, with comments: SOT 23 MMBV 3401. It is a 3 pin surface mount (SOT 23) that looks like a transistor, but it is a PIN diode that only uses two terminals. All of the other Spectra T/R PIN diodes are really huge and handle the X9k high power just fine, except for this tiny fragile CR3903 that is intended to kill the very last bit of Spectra Tx RF energy before entering the Spectra receiver connection. When this PIN diode dies your Rx sensitivity looses about 10x of what it should be.

After I replaced this dead PIN diode, the Spectra Rx sensitivity was completely back to normal.

On my vehicle I observed my side-by side mounted at opposite sides of the roof VHF and low band antenna whips had different timing when swaying back and forth. When driving off road they would swing sideways out of phase physically and actually touch sometimes. Buy at least two of these PIN diodes just in case you do not get the Spectra and X9k antennas far enough apart the first time. Another problem is do not sneeze during the repair because this SOT 23 part is so tiny if you drop it, you may never find it again.

[Tim]

Hi Mike,

Long time no talk!

Thanks for the info - I've got the spectra apart & will check that this afternoon.
Digikey has the 3401s for .44 each, so at least it's an easy to get part.

I understand about the sneeze.... I do a lot of hand assembly of SMT stuff, and I
always get several extra parts.... my lab floor is a black hole for SMT parts... when
they fall, I NEVER can find them. Someday when I'm gone and they tear the place
down, somebody's gonna find lots of stuff!

Thanks again,

Tim

[Will]

You can check that PIN diode from the top of the PA board with an ohm meter.

I use a better PIN diode that seems to hold up better. Infineon BAR6403WE6327XT

[Tim]

Hi Will, alll.


Thanks for the info. That BAR6403W looks pretty good... only was wondering
about the on-resistance. @10ma, the BAR is 2.1 ohms vs 0.7 ohms for the other
one.

Actually, it's a non-issue now, as I KNEW I would sometime be able to use some
parts out of all of those old Analog Spectras that I have squirreled away.... gee,
they even have the same Moto part number for the diode in the PA section.

Sometimes ya get lucky!

Should I run out of 'spares', I'll get a couple of the BARs also.

gee, don't guess this front end would withstand even a 'small' EMP.

It's back up and running <albeit with a different issue>.

Thanks,

Tim