Cobra 148GTL SSB problem...
TRAMD300
I hope someone can help me out with this strange problem.
here's the scoop...
AM side of this unit works perfect and the recieve on side band is great,
the problem is on the transmit side of both LSB & USB.
While monitoring the transmit signal with another unit using headphones,
I am un-able to clarify anything in on SSB (monitor radio is working perfect)
The signal that is transmitted is garbled and scratchy.
It appears to have too much modulation, so I turn the dynamic down all
the way and the problem goes away, but now there is no output power
until I increase the dynamic level which gives me the first result.
All the mod. limiters are in place, and the resister 131 has not been
removed. basic stock radio.
Now, I found out if I take the mic. 2 or 3 feet away from my mouth,
it sounds fine until I get closer and a power signal is indicated on
the meter. I am using a stock mic. and the mic. is working fine.
Has anyone encountered this problem?
Is there any suggestions to help me fix this problem?
I wish I could explain it a little better.
Any help with this will be greatly appreciated...
Thank you....
Steve Hanis
having regulation or rfi problems.
Steve.
TRAMD300 wrote in message <19990502153514...@ng-fv1.aol.com>...
BCNY
to antenna, any matchers in line , SWR meters too - double ground.
Examine coax, etc.
Billy-Bob-Jo
GT40 425HP
it could be the power cord or chokes at the back of the radio
It's just me!
Check the capicator on the power supply which is only working on the SSB
mode. Many of the 148's had a bad cap installed at the MFG.
Sorry, I don't have the schematics available right now, but it is easy
enough to find.
M:)
Walt Bryant
Sounds like someone changed r126 or r124
to a smaller value they should be (10K ohms)
if this was done you will need a drawing to check out that circuit so
you can find the cap
that is bad. Hope this helps 73s Oldtimer
Michael A. Willey
Start by checking CAP "C-172" it is supposed to be 1000uf, however the
ones from the factory don't own up to the specs. This has been
confirmed by a Tech at Cobra. Also check the following CAPS C-402,
C-151, C-152 and C-191. These have been known to have failed also when
mod's were done improperly.
You might also want to check the VCO, as it is also suceptable to
failure with improper mod's.
I have found that on some of the 148's (usually from Tiawan), that the
best thing to do is if you find a bad cap at C-172 or C-402, just go
ahead and change the VCO. Don't even play with it, as you will probably
just chase your tail looking for the problem.
M:)
Roygbiv779
THROUGH THE MIC.
TRY BYPASSING MIC + AND - TO MIC SHIELD WITH A COUPLE OF .001 uF CAPS. ALSO
CHECK ALL SOLDER CONNECTIONS IN THE ANTENNA SYSTEM. A COLD SOLDER JOINT ACTS
LIKE THE GALINA XTAL OF XTAL RADIO DAYS. THEY RADIATE TOO.
ALSO CK S W R , BAD SHIELD GROUND WILL SEND R F RIGHT BACK INTO THE SHACK. TRY
DOING WHAT EVER YOU ARE DOING ON A 50 OHM DUMMY LOAD INSTEAD OF AN ANTENNA IF
YOU HAVENT YET. IF THAT WORKS, THE ANT. IS THE PROBLEM.
GOOD LUCK
73'S 675
Handy Andy
A couple of more things to add...
If this radio claims to be made in Malaysia, the following applies...
Check for L9, by MB8719 PLL. By pin 9. Opposite end of notch.
If it's a 10 ~ 100 ohm resistor. Theres part of the problem.
Remove this component, use a jumper wire with a ferrite bead on one
lead to replace the component. This supplies the MB8719 with the full
8 volts. The original problem was with current drain and the delay in
capacitor charging, causing the voltage to drop below regulation point
on the MB8719, and in turn, the poor modulation performance you're
experiencing.
The MB8719 IC1, is identified as RCI 8719. If this is the case, this
IC has an internal 5 volt regulator, and has marginal performance.
If you can, replace the RCI version, with a true MB8719.
The clarifier mod can be done with little problems, and should not
warble after correcting L9.
Locate MB3756, IC4 - voltage regulator/switch. Check to see if jumper
J2 is on the board. It's located just in front of IC4, it's used to
supply voltage from the power switch, and bypassed RF power supply
choke and filter, T1, on older [pre-1994 Cobra models].
A voltage drop occurs when T1 powers IC4 and Audio amp. This drop can
be severe enough to cause IC4 to become unstable at regulation.
If J2 jumper is not there, make one using some simple hookup wire, and
cut the trace arriving to Pin 2. A good break point can be found where
the trace "ducks under" the MB3756 and is close to the chassis panel.
Once disconnected, the power to the MB3756 is through the power switch
and the voltage drop across T1 is removed, making the constant 8 volt
supply steady and you get better performance.
Hope this helps...
On Sat, 8 May 1999 03:52:24 -0400, While the Monitor displayed an idea
about "Re: Cobra 148GTL SSB problem..." from '"It's just me!"
<i...@nospam.com>'. The Keyboard responded with this:
:+> TRAM:
:+>
:+> Check the capicator on the power supply which is only working on the SSB
:+> mode. Many of the 148's had a bad cap installed at the MFG.
:+> Sorry, I don't have the schematics available right now, but it is easy
:+> enough to find.
:+>
:+> M:)
:+>
:+>
:+> TRAMD300 wrote in message <19990502153514...@ng-fv1.aol.com>...
:+> >Cobra 148GTL SSB Transmit problem
:+> >
:+> >I hope someone can help me out with this strange problem.
:+> >here's the scoop...
:+> >AM side of this unit works perfect and the recieve on side band is great,
:+> >the problem is on the transmit side of both LSB & USB.
:+> >While monitoring the transmit signal with another unit using headphones,
:+> >I am un-able to clarify anything in on SSB (monitor radio is working
:+> perfect)
:+> >The signal that is transmitted is garbled and scratchy.
:+> >It appears to have too much modulation, so I turn the dynamic down all
:+> >the way and the problem goes away, but now there is no output power
:+> >until I increase the dynamic level which gives me the first result.
:+> >All the mod. limiters are in place, and the resister 131 has not been
:+> >removed. basic stock radio.
:+> >Now, I found out if I take the mic. 2 or 3 feet away from my mouth,
:+> >it sounds fine until I get closer and a power signal is indicated on
:+> >the meter. I am using a stock mic. and the mic. is working fine.
:+> >
:+> >Has anyone encountered this problem?
:+> >Is there any suggestions to help me fix this problem?
:+> >I wish I could explain it a little better.
:+> >Any help with this will be greatly appreciated...
:+> >Thank you....
:+> >
:+> >TRAM...@aol.com
:+>
:+>
Regards!
:+> Andy <+:
. |
. |
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