SSB Warble ?
mike
Here is One for You to kick around.
I have a Cobra 148GTLDX, the old Model. After having a Freq. Counter
hooked up and in the shop twice
to have this Warble Fixed, I am told that it is still there.
My Question is what causes it and how to get rid of it ? The Radio seems
to be Driving My TS DX400 well.
On SSB it swings to the end of the 500w Scale and needs more Meter.
Anyway.................
This Warble is Driving Me Crazy, Please Help !!
TIA
Mike
tn...@mucks.net
wrote:
Pick and choose what fixes are appropriate.
In my mind the single biggest problem is that RF
is getting into the input of the voltage regulator.
This causes output voltage fluctuations that in turn
shifts the VCO which causes the audible warble.
This can be remedied by replacing and increasing
the size of the input cap.It may also help to add
other filtering components at the input and output
of the voltage regulator. Here are the files. I will
paste them to the E-mail below the asterisk line.These files
were compiled. They are not of my authoring but they should
help.
*********************************************
Are there any techs out there that could/will post the modification
that will correct the problem that the 148F GTL has when you transmit
without the engine running- anything under 13.4V and the transmit on
SSB goes all to hell. I don't have a schematic on hand for this
Well, I thought I was the only one out here that had problems too.
Sorry to see you're in the same spot I was in earlier - I found my
problem to be with their [Cobras] upgrade to the 148F. The RF Choke
or T1, is the culprit - the voltage drop across the coil is too great
- about 1.5 to 2 volts under load. The MB3756 is a voltage
regulator/switch and requires at least 11.5 volts to regulate, and
maximun of 16 - else it crowbars and can self destruct, [Believe me,
I know] and the TA7222P audio chip - and everything else after it, is
all handled through the RF Choke. [Up to and including Driver and
Final - quite a bit of current crunchers on their own.] The original
design bypassed the RF choke section and used a zener diode rated at
16-18Volts along with a electrolytic and disc cap to help filter
ripple before going to the MB3756. In the newer models, the RF
choke [T1] handles all the noise filtering and when put under a load,
like the TX current draw, starts dropping voltage and the regulator
can't regulate enough current and tops out on voice peaks during
transmit, either on AM or SSB.
Also-----
The radio I have, with it's inherent faults, can be considered a lost
cause. The newest problem I have found, and the purpose of the posted
replies, is with the Voltage regulator itself. D44 was originally a
16-18Volt zener, now has a lower voltage rating - I believe around
15.2V in the newer ones. Many of todays cars produce around 14.4 volts
to charge and run the ignition system. The MB3756 voltage
reguator/switch will cut out due to a defective design, in both the
component itself and the zener used on the newer models, while the
engine is running [due to clamping at the diode] or distort when the
engine is off [lack of voltage to regulate]. The MB3756 in these
newer ones may be a defective chip, and if you have a radio that
"Warbles" or "Pops" during modulation - observe the freq counter and
TX/RX light - if strong, sudden vowel sounds and loud consonants type
of modulation causes the TX/RX light to go green or yellow or drop
frequency - suspect the regulator - it's close to crowbar'ng itself to
keep regulation stable when the engine is running or dropping out when
power supply voltage goes below threshold level. It's very sensitive
to voltage swings.
In my situation, the load imposed was from a faulty capacitor [quite a
few really] in both the input filtering, [1000uF 25V electrolytic was
leaky] audio out to AM regulator [another electrolytic shorted] and
due to transients, affected the MB8719 constant 8.2 volts, [three
tantalums - one rated 10 volts quit during first power up on the
bench.] All total sum left me with little left componentwise in the
stockpile, and a piece of junk...
Also------
It is a low voltage drop out problem due to some component changes
made when Cobra started using Malasian manufacturing plants.Don't even
listen to all these hoky opinions of what is causing the
problem-especially the dork that says it is a VCO problem. All it
takes is a jumper a cut trace 1 diode and 1 capacitor and the problem
is gone.
Hope this helps.
Well, lately, I've been getting a lot of mail from others regarding
the 148 GTL-F model from Cobra - and this is the one that is made in
Malayasia.
One of the Emails I've sent - even though this may cover more than
what you want to deal with at this time - is added to this message.
Please, keep this as an archive for your reference and general idea -
let alone your
own good. This is a lengthy message - and is a generic response - but
it covers a lot of bases you'd need to cover ground with while
troubleshooting this chassis...there is a brief explanation - along
with some other advice that pertains to audio problems arising from
this poorly thought out chassis design.
There may be more than one type of problem - or worse, a problem that
can arise later from a failure on another part of the circuit board
that you may need
to fix later...the text attached - has some other key components you
can offer as a list to your tech - so they can verify these parts are
working like they should. PLEASE CHECK TO SEE IF THE PLL MB8719 HAS
THE RCI LOGO, STAMPED ON THE CHIP - IF THIS IS THE CASE - THIS
MESSAGE APPLIES TO YOU...
Reply enclosed...here is the lastest revision of my notes on this...
Date: Thursday, January 29, 1998 2:31 AM
Subject: Re: Cobra 148f warble
[...my stuff snipped...]
Yeah, it's the malaysian chassis... it has the RCI-8719-A chip which I
heard is a better one than the original RCI 8719. I do, however, have
an MB8719 chip (from a 1981-built 2000) sitting in the parts bin.
[Well, we might need that, after this is all over...it is the LAST
resort...]
Ok, to bring you up to speed on this...
When Cobra changed from Uniden to Malaysia - they also decided to make
some changes to the chassis - notably - condensing the noise from
electrical power supplied - through the RF choke - for ALL of the
radios power supply needs.
This poses a problem - due to excessive current draw, and the
likelyhood of RF contamination in areas you don't want RF to be in -
and this includes the VFO/BFO sections.
The Warble can be from excessive current draw from RF choke T1.This
choke of the earlier Grant and Cobras served to filter ignition and
other noises from the Audio Amplifier and Modulator circuits [The
TA7222P & TR41-42 modulators.] With the rest of the RF prone areas
going through the MB3756 voltage regulator/switch combination for
RX/TX and PLL constant 8V regulation.
The warble is due to poor regulation from the MB3756 - it is losing
power from the current draw at the filter choke - this can be fixed by
splitting the power feed to the original form - and requires you to
remove one trace, and install a jumper...to reduce the current demand
at T1 to power everything. The MB3756 has good ripple rejection - but
is flakey when voltage surges and current drops are
beyond the "operating window" of 11 Volt minimum - there are 3
progammable outputs - one is left on to supply 8 volts constant that
the PLL uses - with two other outputs being the TX/RX voltages.
[I would go with this first - to see if it clears up the warble - it
might..just don't run the radio "flat out" if you can avoid it - for
the warble is also current dependent and that means leaving in the
limiter to provide some "cushion" to protect the PLL regulator
portion from drawing more power than the regulator can provide.]
Install Jumper [ferrite bead of #16AWG wire would be nice - but not
necessary] at J44 - by the MB3756 chip - inside chassis wall on
right-side, middle - front facing you with component side up...You can
also install a 0.047uF disc ceramic by this area, some boards did not
come with this - mainly mine [C136]. A spot for an Electrolytic
or Polarized capacitor is also located here - you may want to use a
100uF Electrolytic or 47 uF tantalum here to provide some filtering
for the Regulator.
[Due to labeling - Verify that J44 does connect to the Pin 2 of the
MB3756 chip.]
Also, check and verify that there is a diode in location D44 - you
should have a diode similar to a 1N4148 - because of board revisions,
and possible "dropped" components during soldering by machine - this
component should be checked to see if it's working, and not blown
open - if it has - RF contamination past the RF choke T1 can occur and
problems with stability - or blown fuses - are going to happen.
D44 can be replaced with a 1N4148 with a 100PIV and a 200-300mA
reverse surge rating - the diode from Cobra, in this area, is marginal
in performance. Considering the cost of this component - it's a good
idea to replace it now - before you find problems with it later.
The power supply feed to the RF choke that also "splits" to the MB3756
chip - is by Pin 2 on the IC4 [MB3756] - if you follow the trace side,
the trace "dips" - towards the chassis wall as you follow it towards
the rear of the chassis - cut clean here - to break the secondary feed
from the RF choke.
[The earlier mods that I've done with this radio - have now been
refined][You should check D44 - and use ferrite beaded lead on J44 for
best ][results...this helps keep RF from affecting the MB3756 ...
]
This completes the conversion of the feed to the original form -split
only by the power switch feed and the regulator is no longer forced to
accept current swings from T1 [RF choke] as it attempts to draw
current through itself and feed the Audio section and PLL regulation -
they are now separate...
You may also want to check on the condition of the Electrolytics in
the TA7222P area - mainly C175 and C176 - both are rated at 10 volts -
you can increase to 16 volts to provide more headroom with less chance
of failure. C175 is a 470uF while C176 is simply a 47uF electrolytic.
And C172 should be rated 1000uF
at 28 volts. It is a good idea to check these components - mine went
south after the first week of use. Another one to check in the VFO
area, is C81 and C80 - both are tantalums 2.2uF/25V, and C84 - on
bottom side 1000uF/10volts - all are suspect due to the warble
affecting leakage around these components in a frequency sensitive
area [the Balanced modulator AN612, has C95 and is rated at 10 volts
47uF - it may need replacement because of voltage swings from the
"warble" being a voltage sensitive area and feed from the MB3756].
Well, did I scare you away yet - this stuff is a lot of work for one
night - care for more? OK - you asked...;-) [Remember - Malaysian
components were cheap and lacked quality...]
By the MB8719 - the ORIGNAL used a ferrite bead jumper known as L9 by
Pin 9 of the MB8719 [we're at the PLL now...] on the 148F chassis -
it's now a resistor of approx 10 ohms at 1/4 watt - I know it's silly
but they did it, not me. This resistor acted as a current limiter for
the [now] 5 volt tolerant PLL [RCI-8719].
Since you have the "A" PLL version, try using a lower value resistor
here [L9] to 5 ohms - but use a 1/2 watt - sure it's bigger - but you
do have room. This will provide current headroom without droppng
voltage heavily [ this is why the C89 cap was such a large value to
compensate the current draw and keep voltages stable ].
What I've done - and works, is to use a 1N4148 diode here - with
banded end towards Pin 9 - this acts as a wattless dropper and
provides the PLL with plenty of current [100PIV 200mA surge minimum]
and keeps the board from "pulling" current away from the PLL during
"peak" modulation times. HOWEVER, the MB8719 in mine only worked with
a 5 ohm resistor and diode placed piggyback [up/down] on the board
[banded side to the board on the pin 9 hole feed - with the anode tied
to the resistor - with it's freed end towards the other
hole].
The clarifier mod is like the Grant/148 of older chassis - WITH ONE
EXCEPTION - you use R44 in line with the voltage [8V+ to clarifier]
feed to the constant 8Volt supply - this reduces current/voltage to
safe levels and provides the proper drop to operate the radio within
the RCI VFO recommended ranges. So, that means - remove R44 from the
board, and use it in-line [soldered in parallel] with the power feed
to the 8 volt constant - this may reduce slide - but keeps the MB8719
within operating range.
***Ammendum***
UHIC 007 [IC2] can also cause problems - since you've already replaced
this component - and still have "warble" problems - the most suspected
parts are the L9 region - explained above - or the T1 feed needs to be
broken at IC4 [MB3756]
WARNING!
Due to numerous problems with this piece of SH*T [BCD failure -switch
spiked PLL/BCD channel switch was poor quality - IC2, UHIC007 - bad,
numerous bad capacitors and poor soldering joints [please check your
circuit leads for solder beads and use a Geiger counter - mine was
radioactive - moreso than typical background
radiation - which I found quite suprizing - the Grant that I still
have is/was silent] - I've simply given up past this point - and I'll
also let you know - that I no longer have this radio in my possession
- it was simply a "refurb" that got past quality control - whatever
that means nowadays - and is either floating down a
sewer - or is being used as a boatanchor - in it's true meaning in
life...
The above was from my notes of an era long past - I hope these notes
can still apply to your situation - good luck!
**************************************************
Most of these files were compiled from posts
autrhored by:
mike
Thank You for Posting this Information for All to See and Learn from !!
I will Present it to the Tech and hope He can get the Radio working properly.
Thanks again
Mike
To All others in the know, Please post Your thoughts and experiences !!
bgri...@my-deja.com
mike <mikea...@earthlink.net> wrote:
> Hello All
>
> Here is One for You to kick around.
> I have a Cobra 148GTLDX, the old Model. After having a Freq. Counter
> hooked up and in the shop twice
> to have this Warble Fixed, I am told that it is still there.
> My Question is what causes it and how to get rid of it ? The Radio
seems
> to be Driving My TS DX400 well.
> On SSB it swings to the end of the 500w Scale and needs more Meter.
> Anyway.................
>
> This Warble is Driving Me Crazy, Please Help !!
> TIA
>
> Mike
>
>
The warble is coming from the fact that the radio was modified
incorrectly. The DC source for the varactor diode is unstable...
Professor
Sent via Deja.com http://www.deja.com/
Before you buy.
AB7RS
of low current,check your power cord.
tn...@mucks.net
wrote:
>tnom
>Thank You for Posting this Information for All to See and Learn from !!
>I will Present it to the Tech and hope He can get the Radio working properly.
The real thanks should go to Andy at cod...@iserv.net
All i did was research Deja-News for answers to the 148
problem. I then just copied the info. His posts kept on popping
up in the search.
The 148's of recent years have had this problem on SSB.
Even the stock 148's have warbled on SSB straight out
of the box.
What i don't know is if the present day 148's have had this
problem corrected. Cobra was made aware of it long ago.
If the chain of command was not broken, and if they are
the least bit responsible, the new 148's should be fine.
Maybe someone could affirm this?
Bill Nelson
: tnom
: Thank You for Posting this Information for All to See and Learn from !!
: I will Present it to the Tech and hope He can get the Radio working properly.
The tech should have know all those possible failure modes - and already
eliminated them as possibilities. There are very few things that can cause
such problems. I would seriously question the qualifications of the person
to call himself a Tech.
--
Bill Nelson (bi...@peak.org)
Handy Andy
Thanks tnom...
That radio was a bear!
Grizzly one at that!
The easiest fix was to Take T1 off on the MB3756, cutting the trace,
and run the Regulator straight to the power switch thru the jumper
that was removed on these radios, that still had the foil traces from
their predecessor the Uniden made 148 GTL and the Grant XL. Go figure?
The rest of the mess dealt with a 10 ohm resistor labled L9 by Pin 9
of the MB3756. Voltage starvation, causing the RCI-MB8719 to never get
a stable regulated voltage to the VCO-loop.
Change that to a ferrite bead, and pretty much brought the radio back
to stability except in worse cases of audio overdrive.
Hope this helps...
While Dancing with the Mouse on 30 Sep 1999 15:50:15 -0500, the Modem
interrupted with an MNI from "tn...@mucks.net", and the MotherBoard
buffered this to Video RAM:
:+> On Thu, 30 Sep 1999 09:07:48 -0700, mike <mikea...@earthlink.net>
:+> wrote:
:+>
:+> >tnom
:+> >Thank You for Posting this Information for All to See and Learn from !!
:+> >I will Present it to the Tech and hope He can get the Radio working properly.
:+>
:+> The real thanks should go to Andy at cod...@iserv.net
:+> All i did was research Deja-News for answers to the 148
:+> problem. I then just copied the info. His posts kept on popping
:+> up in the search.
Regards!
:+> Andy <+:
.
... You're a resourceful CB'er when... .-.
... You use your Antenna to double as a / |
... Fishing pole... o / (^)
... and have the trophies to prove it! |/\ /_\
. '------
. http://www.iserv.net/~codyspc