Need Information For Cobra 25LTD Classic
Power House / FeMaster
This part is located right next to the final. The one in the rado I am
working on is burnt black, and the color bands can not be read. This
resistor connects one leg of the final to the ground section of te
circuit board.
The final is blown (literaly into pieces), so I am considering upgrading
to the 2SC1969 final. If I do this, will I need to change the value of
this resistor, or another one, or leave them the same?
One last thing what would have caused this to happen?
Thanks,
Power House
===============================================
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===============================================
Handy Andy
On Tue, 1 Dec 1998 06:12:54 -0600 (CST), The Monitor was merrily
roasting the Modem over a campfire, when the Modems buffers coughed up
something about "Need Information For Cobra 25LTD Classic" from
'Power_...@webtv.net (Power House / FeMaster)' and dumped this:
:+> I need to know what value (or colors) R40 is in the above listed radio.
:+> This part is located right next to the final. The one in the rado I am
:+> working on is burnt black, and the color bands can not be read. This
:+> resistor connects one leg of the final to the ground section of te
:+> circuit board.
Usually, it's a 100ohm, if the Final was a 2078. About 1 ohm if it was
a 2166. Theres a ferrite bead on the one I'm thinking of, or a
resistor [brown body]/ferrite coil [green body] combo soldered
together in this area. This controlled bias in the base of the final.
Offering 0 DC bias [ground] with the coil being more reactive at RF
frequencies. The 100 or 1 ohm resistor is a broadbanded impedance
matching component to provide some impedance stability for the base
and acted as part of the mis-match protection of the final for the
impedance matching network.
:+> The final is blown (literaly into pieces), so I am considering upgrading
:+> to the 2SC1969 final. If I do this, will I need to change the value of
:+> this resistor, or another one, or leave them the same?
Well, if you decide to change, look at the driver, is it a 941 in a
small TO-92 with a heat sink in a package? Then the final was a 2166.
This drive transistor will not work well enough to drive the 1969.
You'll need to make some changes.
Write back, the reason why I'm stopping here, is because Cobra changed
Driver and Final Combos in some chassis designs. I can tell you what
might help, but I'm not going to waste bandwidth over telling two
different ways.
A lot of us out here that work on them, know what I'm talking about.
Each person is different, and I don't want to create a flame war over
how to make a radio work.
:+> One last thing what would have caused this to happen?
Many things...
Component failure, RF base lead coil could have opened, sending idle
current to the resistor heating it up and in the process blowing the
final. [Especially if the resistor was smaller than a 1/4 watt]
Improper tuning high-SWR [rare occasions] made final fail; upon
failure smoked the resistor until it opened.
It's an NPN, if the collector heat sink insulator failed, the NPN
junction, being more like a diode, would send a lot of battery juice
to chassis ground instead of board ground. They could have tried
running it like this when it quit working.
Polarity reversal? Highly doubt it. But, if the protection diode quit
working, it could cause reverse current from a short to go through the
resistor and coil combo, opening both. But I can't see how a final
could go.
If you say the final is blown to pieces - did lightning strike the
radio?
Too many fingers and one too many screwdriver tips? A small spark and
poof! Oops! Could have tried a direct jumper straddling the driver and
final together, but you would see solder blemishes that are different
from the rest of the radio that was wave soldered.
Overvoltage? A possibility. But Driver and audio may also be affected.
Is this the same radio you posted about earlier?
If so, Then this goes back to the capacitor hunch said earlier.
The way it sounds, it sounds like you have a lot of work ahead of you!
Good luck!
. //^^^^^^^^^^\\
. << OH DRAT! >>
Regards! \\ !@#%#@! //
.:+> Andy <+: ''-, ,-''
. ||||
... You've gotta' be a CB'er when... ,---------,
... You spend more time - /________./|
... fixing your radio, |=== oo o|/
... Than talking on it! `---------'
http://www.iserv.net/~codyspc/CBRADIO.HTM
StAnGWaReZ
>. //^^^^^^^^^^\\
>. << OH DRAT! >>
>Regards! \\ !@#%#@! //
>.:+> Andy <+: ''-, ,-''
>. ||||
>... You've gotta' be a CB'er when... ,---------,
>... You spend more time - /________./|
>... fixing your radio, |=== oo o|/
>... Than talking on it! `---------'
>http://www.iserv.net/~codyspc/CBRADIO.HTM
>
do you kake these on your own...There pretty funny....only thing is is that the
pics don't line up for me....
Ryan
formerly known as RPARZ
replys can be sent to rp...@hotmail.com
Power House / FeMaster
>About 1 ohm if it was a 2166.
Well, the final is a 2075...Is this what you meant instead of 2078?
>Theres a ferrite bead on the one I'm thinking
>of, or a resistor [brown body]/ferrite coil [green
>body] combo soldered together in this area.
Yes, a resistor with a ferite bead attached to one leg of it.
>Well, if you decide to change, look at the
>driver, is it a 941 in a small TO-92 with a heat
>sink in a package? Then the final was a 2166.
>This drive transistor will not work well enough
>to drive the 1969.
The fianl is a 2075...Can't tell what the driver is...The nut is in the
way, and I don't have time fuss with it at this second. If you need to
know the number, let me know..
>Is this the same radio you posted about
>earlier? If so, Then this goes back to the
>capacitor hunch said earlier.
Nope, this is a different one...
>The way it sounds, it sounds like you have a
>lot of work ahead of you!
>Good luck!
Sound lke I will need it...Now that you know what final was in there, do
you think it would be possiblr to go with the 1969?
Later...Power House
Handy Andy
When I post, I use a Mono-spaced font like Fixedsys or Courier. A
proportional font like Arial or Comic Sans causes problems. The stuff
at the bottom of the post is called a signature. It's a text file that
if you have a Newsreader, like Outlook Express, or Free Agent, they
can place it on the bottom of the message before it's sent. They can
offer to select a mono or proportional font as you view, on the fly.
Although reading some of the posted comentary makes Comic Sans Font a
highlight to the day...;-)
Since I'm more into Newsreaders. I find that mono-spaced fonts are
easier to use for people wanting to diagram an idea or design. They
post something in a non-proportional font which makes it easier for
others to look it over and make suggestions. This happens
[mono-spaced renderings] on the rec.radio.amateur.* groups and eng.*
and sci.* groups. It's quite commonplace.
Since it would look like "garbled" text while using Arial or some
other "compressed/proportioned" font, you should be able to switch
over your readers' font to a non-proportional. And soing so, would
allow you to see some of the stuff on other NG, even rec.animal groups
and other very well done signatures.
On 2 Dec 1998 08:23:41 GMT, The Monitor was merrily roasting the Modem
over a campfire, when the Modems buffers coughed up something about
"Re: Need Information For Cobra 25LTD Classic" from
'stang...@aol.com (StAnGWaReZ)' and dumped this:
:+> >
:+> >. //^^^^^^^^^^\\
:+> >. << OH DRAT! >>
:+> >Regards! \\ !@#%#@! //
:+> >.:+> Andy <+: ''-, ,-''
:+> >. ||||
:+> >... You've gotta' be a CB'er when... ,---------,
:+> >... You spend more time - /________./|
:+> >... fixing your radio, |=== oo o|/
:+> >... Than talking on it! `---------'
:+> >http://www.iserv.net/~codyspc/CBRADIO.HTM
:+> >
:+>
:+> do you kake these on your own...There pretty funny....only thing is is that the
:+> pics don't line up for me....
:+> Ryan
:+> formerly known as RPARZ
:+> replys can be sent to rp...@hotmail.com
Regards!
:+> Andy <+:
. |
. |
. |
Regards! =
.:+> Andy <+: __]|[__
. _/_______\_
. |OO|=====|OO|
... You could be a CB'er when... [===|===|===]
... The radio and antenna... |_| |_|
... Cost more than the car is worth!
Power House / FeMaster
><>2078. About 1 ohm if it was a 2166.
>>Well, the final is a 2075...Is this what you
>>meant instead of 2078?
>Well, 2075 is new to me. 2078 is the usual I
>see in a simple Cobra.
Well, I have some new information that may help you...Or may confuse you
further...I was told by MANY people (including Cobra themselves) that
the value of that resistor was a 22-ohm...
>The 2075, check the pinouts. If it's got a BCE -
>the same leg should solder to a ground
>lug/pad area on the bottom as you would find
>on the typical AM radio using a 2078. This
>means that, most likely, it's pin compatible with
>the 1969. Some that use a different pair of
>combos have EBC types of pinouts with tab to
>collector, or BEC with Emitter to tab - like the
>HR2510.
Well, I'm also working a a REALLY cheapo Midland, and it has a 2078 in
it. The 2078 has the RIGHT (when looking at it from the side with the
writing) pin connected to ground...The 2075 has the LEFT pin connected
to ground. I don't know how to figure out which pin is which (Base,
Collector, Emiter), if there is an easy way to do this, I could be more
help in this department...
>>Yes, a resistor with a ferite bead attached to
>>one leg of it.
>Well, this makes it a tough call, I would think
>that the 2075 might be a sister to the 2166 -
>it's one of those transistion ones. Can be a
>driver, and a final.
Well, don't know...ihope the above information helps out with ths...
>Well, take a look at the final cap [in tank
>circuit] body, is it a 271 or 331 printed on it? If
>any other value, my guess would be that there
>may be another problem that I haven't caught
>yet. I have a suspicion or two.
Where would I find this? As you you can tell, I'm an amateur repair
person...I can do a lot, but I still have a lot to learn...
>Check the driver, and the pinouts of both
>Driver and Final. I'm assuming BCE for the
>Final [like the 1969/2078 and perhaps ECB
>[like 2314] for the Driver. This can give you a
>headache!
Well, like I stated above, I don't know how to figure out the
pin-outs...
>Well, a lot of it has to deal with the case style,
>the ones that have a hole in the body [not like
>the ones with a hole in the tab], and are
>slightly smaller than the TO-220 package, are
>more than likely, a TO-126 style thats more
>durable than the TO-92.
It would be the TO-126 style...Hole though the body, and slightly
smaller than than the TO-220.
>You may have an easier time with this
>conversion of it. When you check numbers, it
>just might be a 2314. If it's not - stop! Email
>me... We might be in over our heads.
OK, got the number off of the driver. It's a 1957...
>I don't recollect ever coming across a 2075.
>However, to use a 1969, you do have to
>substitute some different values. For R40? I'd
>be conservative and try a 10 ohm and the
>ferrite bead. This should be enough. At worse,
>try different values up to 100 ohms.
Well, now that you know about the 22-Ohm value, are there any other
suggestions?
>What this radio sounds like, is similar to a
>TRC-443.
Don't know if it helps, but according to my PLL Data Book here are some
radios with simmilar boards are:
Cobra 21 GTL/LTD
Midland 200M
President J, AR/AX44, AR/AX711
Uniden PC66
What a chore! I was hoping this would be easier...
Later...
Handy Andy
On Wed, 2 Dec 1998 20:11:15 -0600 (CST), The Monitor was merrily
roasting the Modem over a campfire, when the Modems buffers coughed up
something about "Re: Need Information For Cobra 25LTD Classic" from
'Power_...@webtv.net (Power House / FeMaster)' and dumped this:
:+> >Usually, it's a 100ohm, if the Final was a 2078.
:+> >About 1 ohm if it was a 2166.
:+>
:+> Well, the final is a 2075...Is this what you meant instead of 2078?
Well, 2075 is new to me. 2078 is the usual I see in a simple Cobra.
What I would do, is getting it to key up first, so replace the 2075
with a 2078 if it's pin compatible. Key up the radio, and you may also
have to replce the driver, so do that, and keep trying until you get a
radio back. Then try the below.
The 2075, check the pinouts. If it's got a BCE - the same leg should
solder to a ground lug/pad area on the bottom as you would find on the
typical AM radio using a 2078. This means that, most likely, it's pin
compatible with the 1969. Some that use a different pair of combos
have EBC types of pinouts with tab to collector, or BEC with Emitter
to tab - like the HR2510.
:+> >Theres a ferrite bead on the one I'm thinking
:+> >of, or a resistor [brown body]/ferrite coil [green
:+> >body] combo soldered together in this area.
:+>
:+> Yes, a resistor with a ferite bead attached to one leg of it.
Well, this makes it a tough call, I would think that the 2075 might be
a sister to the 2166 - it's one of those transistion ones. Can be a
driver, and a final.
Well, take a look at the final cap [in tank circuit] body, is it a 271
or 331 printed on it? If any other value, my guess would be that there
may be another problem that I haven't caught yet. I have a suspicion
or two. Check the driver, and the pinouts of both Driver and Final.
I'm assuming BCE for the Final [like the 1969/2078 and perhaps ECB
[like 2314] for the Driver. This can give you a headache!
:+> The fianl is a 2075...Can't tell what the driver is...The nut is in the
:+> way, and I don't have time fuss with it at this second. If you need to
:+> know the number, let me know..
Well, a lot of it has to deal with the case style, the ones that have
a hole in the body [not like the ones with a hole in the tab], and are
slightly smaller than the TO-220 package, are more than likely, a
TO-126 style thats more durable than the TO-92. You may have an easier
time with this conversion of it. When you check numbers, it just might
be a 2314. If it's not - stop! Email me... We might be in over our
heads. The 2314 has a heat sink surface on the backside, that needs
to use an insulator/washer before mounting to the heat sink. If the
pinouts are different, the heat sink for the body of the transistor
may be on the wrong side.
:+> >The way it sounds, it sounds like you have a
:+> >lot of work ahead of you!
:+> >Good luck!
:+>
:+> Sound lke I will need it...Now that you know what final was in there, do
:+> you think it would be possiblr to go with the 1969?
Yes, but I can't guarantee results unless I'm more familiar with the
chassis. I caution you, I don't recollect ever coming across a 2075.
However, to use a 1969, you do have to substitute some different
values. For R40? I'd be conservative and try a 10 ohm and the ferrite
bead. This should be enough. At worse, try different values up to 100
ohms. Be careful in selecting. You can get thermal runaway, a hot
transistor, or reverse swing audio.
What you are doing is setting base bias. The 2078 needs more RF [RF
choke/bead to ground] and less ground [DC <.1 volt and 100 ohms +
impedance] to keep Class C. With a 1969, the ground required is
different to keep Class C, which is not what it's can run the best at
- but workable.
What will need to be worked on, is mostly in the driver to final
section.
Extra turns or two of the coil that supplies the collector and audio
drive to the driver will have to be changed. There may be a 1 ohm
resistor that is from the audio tap of the transformer - get rid of
it. I substituted a hand wound coil in this section of about 4 turns
using a 1/4" drill bit for form, with #16AWG wire. Spread apart if
needed, and check the heat from the driver. Too hot? Then too much
inductance, back it off. The cap between the final and driver may need
to be increased. To a 391 [390 uF].
And the Final output tank cap may need to be changed to a slightly
higher value, from 270pF [271] to perhaps 330pF to 470pF [331 - 471
respectively]. This part is tough, because of the coil used in the
output tank section. Some windings use no slug, while others do. So,
experiment and be ready to find some suprizes. It's sounds like you
already have! ;-)
There might me some tweaking that is needed to supply whats needed in
this area. Since I don't have the radio in front of me this is about
as much as I can give you to work with at this time. I'm working blind
over here without the radio in front of me.
What this radio sounds like, is similar to a TRC-443. I've done a
power mod to it like listed above. Each chassis and support are pretty
much the same between brands, but small changes in parts selection
makes a big difference between radios.
Good luck. Like I said before, you've got some work ahead of you.
Regards!
:+> Andy <+:
... You could be a CB'er when...
... You spend more time at a local radio shop...
... Than your Wife does shopping!
TRA1N
>Subject: Re: Need Information For Cobra 25LTD Classic
>From: Cod...@Iserv.Net (Handy Andy)
>Date: 12/3/98 6:50 PM Central Standard Time
>Message-id: <3667232a...@news.iserv.net>
tr...@aol.com
tr...@pixelation.com
http://members.theglobe.com/CyberTrain/griffy.html
THOSE WHO CAN, DO
THOSE WHO CAN'T.....SEEK THOSE WHO CAN ! ! !
JUST CALL ME BUBBA ! ! ! !
Power House / FeMaster
><>style, the ones that have a hole in the body
><>[not like the ones with a hole in the tab], and
><>are slightly smaller than the TO-220
><>package, are more than likely, a TO-126
><>style thats more durable than the TO-92.
>>the TO-220.
>Shoot! Then you'll have to scrounge or locate
>a replacement driver of the exact type. The
>1306/1307's are TO-220 [like the 1969].
Great...OK, Find the same Driver type. Is there a way to test these
without a special transistor checker?
>> OK, got the number off of the driver. It's a
>>1957...
>Yikes! How old is this radio? We've got to get
>Malaysia up to date!
Manufacture date: October '92
>Well, now that you know about the 22-Ohm
>value, are there any other suggestions?
>Well, I have a better idea of what you're up
>against. If you want my honest opinion, I have
>to lay out some facts that you've posted...
>The Final is melted, fused. A burnt resistor.
>No way of telling if the darn thing is going to
>key up when Final or Driver and/or both are
>replaced. All of the above cites some type of
>abuse, either lightning, power leads reversal,
>or something was plugged into the antenna
>jack that did some serious damage. None of
>the above would occur if the radio was used
>properly, except for lightning.
Actually the final is still there...A piece of the plastic casing blew
off if it, exposing a small section of the metal workings inside.
Also, the radio DOES key up now. Using a modulation light dummy load, I
can get a weak glow out of the bulb. I removed the resistor that burnt
, and checked it. It still APPEARS to still function...I get a 30-Ohm
reading out of it. It seems like the radio would repairable (from my
stand point anyway), but I may be missing something here, that you may
catch.
Since the 1969 is not possible, should I just try replacing it with the
same final. It seems like all that would need replacing would be the
final and te resistor...Maybe something in line between the driver and
the final also. Any thoughts would be cool....
Handy Andy
On Thu, 3 Dec 1998 20:21:36 -0600 (CST), The Monitor, aptly displayed
"Re: Need Information For Cobra 25LTD Classic" from
'Power_...@webtv.net (Power House / FeMaster)' and in it's
confuzion, sent this:
:+> >Well, 2075 is new to me. 2078 is the usual I
:+> >see in a simple Cobra.
:+>
:+> Well, I have some new information that may help you...Or may confuse you
:+> further...I was told by MANY people (including Cobra themselves) that
:+> the value of that resistor was a 22-ohm...
Hmmm.. I went over some stuff over here. Now, if anyone following this
finds it wrong - post a correction quick!
The 2075, and the 1307, have the same pinouts.
I believe they are CBE, which is backwards to the 1969 - and you will
have some trouble ahead. It might not work. However, if you can get a
1306 or 1307 - these will do just fine in a pinch. These two are the
driver/final pairs found in an older 858 and other SSB-based chassis
designs. [THX Carl!]
:+> Well, I'm also working a a REALLY cheapo Midland, and it has a 2078 in
:+> it. The 2078 has the RIGHT (when looking at it from the side with the
:+> writing) pin connected to ground...The 2075 has the LEFT pin connected
:+> to ground. I don't know how to figure out which pin is which (Base,
:+> Collector, Emiter), if there is an easy way to do this, I could be more
:+> help in this department...
Well, I'm possibly going to get myself in hot, hot water over this by
divulging tech secrets. ;-) [Actually it's not hard to figure out Base
Emitter and Collector.]
Most RF designs are common Collector, it makes it easier [cheaper] to
get good gain with some linearity problems that "supposedly" can be
ironed out. Find board ground, and the Collector is connected to that,
you've found one.
Well, to find out, use a strong light to backlight the board. Locate
driver, the output lead goes [usually] to a section between the Driver
and Final [Duh...ok...quit guys ;-] The only thing stopping the output
of the driver from going direct to the final, is a capacitor. The
value ranges from about 33pF to 68pF. This, along with some resistors
and a coil or two, help shape the waveform going to the Final.
The lead that comes off the cap, may also follow through a resistor.
In' checking my notes, the Cobra 25 uses a 24 ohm, while the Cobra 21,
uses a 1 ohm. Theres' a little story behind that I'll save for later.
This is why things get confusing. Because of these low values, and
board "refinements", it get's harder and harder to remember...
[The Mind is the 2nd thing to go, I can't remember what the 1st one
was...]
Where was I? Oh yes...;-)
Concentrate on what the driver does send to the final.
In some cases, not many, there will be a feedback circuit at the
Final, SSB radios use a Feedback network at the Driver, some use it at
both stages. I don't think you'll have that problem, but keep it in
the back of your mind when looking for the Base lead. The input, or
base, is the lead that goes directly to the transistor. There is
nothing, or little, impedeing it's path. Usually there is the coil and
resistor [sometimes piggybacked to each other] that "straddles"
[trace, component path, to ground] the lead, but otherwise driver to
final has very few components in the path routing and remains a direct
connection.
Now, you may find a tank coil in this section, instead of a more
direct connection with the cap. Cobra 29's PA-288AA board uses a coil
to isolate the driver output from the final. The coil uses the Base
lead to Final as a means to apply DC bias to ground, the base lead
flows to ground, DC level, and uses a resistor [possibly a 10-24 ohm]
to keep DC bias close to, but not at, ground level. The RF flows to
the final from the coil, through another low value cap, and is impeded
from flowing into ground because it's easier to go into a 2.5 ohm
impedance at the final, than a broadbanded resistive impedance to
ground. This is a parallel circuit, when dealing with a transistor
turning on and off or operating linearly, the impedance can change
from a high-impedance 1000 ohm state, to very low <1 ohm impedance
very quickly. The resistor is used to maintain some parallel impedance
that's constant - instead of trying to drive a complex impedance
directly.
I told you the above because I wanted to warn you about my suspicion.
If the Driver/Final section uses a Tank coil to isolate each other, It
may have opened or blown apart internally. You may have a bigger mess
on your hands than it's worth, RF can't flow well enough to make the
circuit work. The number on the side of the can can help, but unless
you have a few scraps of other radios around, the pinouts do change
with numbering, and also the tuning!
EXTRA NOTES!: Check the Driver to Final section for shorts, especially
the caps. One may have shorted to ground from age or static. It could
have been zapped. Sudden Disc cap failures are not uncommon. Same cap
rule applies to direct matching networks that don't use a coil to
isolate voltages. Lift a lead or two to verify. This area is hard to
troubleshoot with failed caps, good/bad coils and open resistors in
the same parallel circuit design.
Use the strong light, the lead that is verifiably going to ground is
the collector, the lead that takes in the output from the driver is
the base, and to verify the emitter, the power supply feed from the
mod transformer goes into the tank coil output section and into the
final through a direct path through the coil.
Since you've said the ground path is on the left, [orient the radio
with the transistor facing you, leads down, or component side up] if
that is correct, then the 1306/1307 combo is my most likely candidate
to try getting the radio to work again, if at all.
:+> >Well, take a look at the final cap [in tank
:+> >circuit] body, is it a 271 or 331 printed on it? If
:+> >any other value, my guess would be that there
:+> >may be another problem that I haven't caught
:+> >yet. I have a suspicion or two.
:+>
:+> Where would I find this? As you you can tell, I'm an amateur repair
:+> person...I can do a lot, but I still have a lot to learn...
These values are printed right on the Disc caps' body. I still have a
page helping to explain it better than this message. I think it will
help...
Try...
http://www.iserv.net/~codyspc/colorcds.htm
This page might help you figure out the numbering scheme used on Disc
caps today. I've typed it up, but I don't know if it can provide an
answer as to what you need to look for. I just use it as an archive in
case I leave my notes behind at home.
:+> >Well, a lot of it has to deal with the case style,
:+> >the ones that have a hole in the body [not like
:+> >the ones with a hole in the tab], and are
:+> >slightly smaller than the TO-220 package, are
:+> >more than likely, a TO-126 style thats more
:+> >durable than the TO-92.
:+>
:+> It would be the TO-126 style...Hole though the body, and slightly
:+> smaller than than the TO-220.
Shoot! Then you'll have to scrounge or locate a replacement driver of
the exact type. The 1306/1307's are TO-220 [like the 1969].
:+> OK, got the number off of the driver. It's a 1957...
Yikes! How old is this radio? We've got to get Malaysia up to date!
:+> Well, now that you know about the 22-Ohm value, are there any other
:+> suggestions?
Well, I have a better idea of what you're up against. If you want my
honest opinion, I have to lay out some facts that you've posted...
The Final is melted, fused.
A burnt resistor.
No way of telling if the darn thing is going to key up when Final or
Driver and/or both are replaced.
All of the above cites some type of abuse, either lightning, power
leads reversal, or something was plugged into the antenna jack that
did some serious damage. None of the above would occur if the radio
was used properly, except for lightning.
You're in for a lot of work, and a 1969 won't fit because of the
backwards pinouts.
It can be repaired, but take a moment to reflect on what you'll have
to put into it if a simple replacement doesn't work. I've got a
suspicion or two that the radio is better off supplying you parts to
keep other radios going.
Either way, you've bought yourself a lot of time and money to put into
that radio that it may be worth. It's up to the owner.
This is how I see it.
Sorry to have wasted your time.
Regards!
:+> Andy <+:
. -----========----==========-----==========_---_--
. ^-^-^---.____\\ `~~-' //____.---^~~~~-~-~
. -- - \\ . ' // --
... You just might be a CB'er... | |
... If you can name all the . / /
... CB's used in... __./ ,/ ...The Movie
... /' .__./ TWISTER.
... / / ,. .'
... /\ ..'/\^\../ /.../~~'', .
...___| H|________---//<<<(((|||)))>>>\\-_______________
http://www.iserv.net/~codyspc/CBRADIO.HTM
StAnGWaReZ
>can get a weak glow out of the bulb.
where can i get these? I saw them years ago, but havent seen any recently...
Ryan
formerly known as RPARZ
CoaxKid
We have them, $4.95
Coaxkid.com
StAnGWaReZ
>Coaxkid.com
>
>
thanks...I ordered from you before...I might be putting in another order for
the xmas season....
Handy Andy
On Fri, 4 Dec 1998 21:59:06 -0600 (CST), The Monitor, aptly displayed
"Re: Need Information For Cobra 25LTD Classic" from
'Power_...@webtv.net (Power House / FeMaster)' and in it's
confuzion, sent this:
:+> >Shoot! Then you'll have to scrounge or locate
:+> >a replacement driver of the exact type. The
:+> >1306/1307's are TO-220 [like the 1969].
:+>
:+> Great...OK, Find the same Driver type. Is there a way to test these
:+> without a special transistor checker?
It's really not difficult.
You need an ohmmeter. One that presents at least enough voltage to
check diodes. A transistor is really two diodes with the "shared"
diode layer bonded to the base lead. Looking at it this way, can help
you determine the pinouts too.
Many VOM's are correctly polarized for checking semiconductors.
Since you're looking at a typical NPN device, finding the base lead is
pretty easy. If you know which leg was ground, place the negative lead
there. The positive lead can help you find the base.
For starters though, try this on a new, unused transistor. If you're
checking parts that are inside a radio, get to know the readings and
hookup for the part, so use a new prime part to help get you ready.
The 1969, is a good start.
Since the part is BCE, and we know it's an NPN, the Base lead will be
positive. And it's the one on the left, when the body is facing you,
leads down.
Holding the negative lead of the VOM, a hand helper can hold the part
better when you check for good or bad. The negative lead being the one
that would be used for the ground pad if the transistor was in the
circuit. For giggles, hold the positive probe side to the leftmost
lead, and the rightmost lead to the negative probe tip. You should see
something ont the meter.
Touch the posistive tip to the other lead and note the reading you get
while holding the negative lead on the right leg.
One lead test to these legs, one should show conduction, or close to a
dead short. The other lead should not conduct. If you don't get this,
reverse the leads of the VOM and test again. The VOM may not have it's
internal wiring set properly. You should get a reading now. And if you
do, it would be in your bests interests to get a VOM that does show
polarity properly when checking diodes. Some meters just check
continuity and resistance, get a meter that allows you to check
diodes. It's cheap insurance and a good tool for any test bench.
With the part out of the circuit, this is the easiest way to find the
base lead. In the circuit, and just going over parts, the previous
message should help you find out what you're up against.
Most, if not all amplifiers use NPN for higher currents. While
switching transistors, use PNP.
Try this with other transistors in your bucket bins on the workbench.
These should be oriented the same way in the helpers jaws. Check one
lead to the others. Then reverse polarity and check again. You'll find
that only one lead will conduct to the other two leads and only one
way. Theres your base lead.
This principle is the method used to check transistors out of circuit,
and whether they are good or bad.
Some bad transistors, won't conduct, or show open, no matter which way
you orient the test probes to the leads. [Blown open]
Some bad ones, show short between two leads, either polarity
direction. [Blown shorted]
Others can throw you for a loop, they can show conductivity to two of
the three leads with correct polarity. While the third doesn't. These
might be IGFET types, verify by looking up the number, you may find
one in a radio that uses these parts. Just so you know, because a
regular bi-polar like the 1969 can also blow in this fashion, open
base, collector, or emitter - either way, it's shot.
Eventually you'll find out which one is the base lead and it's
polarity. Using this technique can get tiring, but when working blind
without a schematic, the more transistor types you test and remember
their pinouts, the easier it is for you to find the right replacement
or substitution.
Most radios that are engineered, usually use transistors that follow
pinouts close to the rest of them. I call them right handed, or left
handed engineering. Because of the orientation of the leads and the
pinouts, BCE or ECB. And then there are some that are ambidextrious,
and use EBC, or CBE and all the others listed above...;-)
:+> >> OK, got the number off of the driver. It's a
:+> >>1957...
:+>
:+> >Yikes! How old is this radio? We've got to get
:+> >Malaysia up to date!
:+>
:+> Manufacture date: October '92
Well, it's not too old, but something tells me from below, that the
parts are completely unacceptable for even casual use.
...[...]...
:+> >replaced. All of the above cites some type of
:+> >abuse, either lightning, power leads reversal,
:+> >or something was plugged into the antenna
:+> >jack that did some serious damage. None of
:+> >the above would occur if the radio was used
:+> >properly, except for lightning.
:+>
:+> Actually the final is still there...A piece of the plastic casing blew
:+> off if it, exposing a small section of the metal workings inside.
This is what I find too, only the "melted" is the junction, when it's
fused together. One person has described that the "epoxy" that is used
to house the part can fail, or it's no longer sealed from the outside.
If you have an older transistor book handy, you'll see how the
junction is bonded to the heat sink, and an epoxy shell is glued over
it.
Sometimes, lead bending or poor epoxy glue can fail. This allows air
and moisture to leak into the junction. The part that broke off was
explosively ejected when the part failed. Bad sealing of the epoxy to
the transistor heat sink casing, ruins the part before it's time,
because it was contaminated or placed under a lot of stress during
assembly. Air and moisture can accelerate the parts' failure and in
some cases, blow off part of the case when heat was too great and the
pressure of air expanding, forced it's way out.
It's not a normal condition. And a lot if it stems back to the quality
of the assembly done. Considering the source of where these parts are
coming from, you're not the only one finding this.
:+>
:+> Also, the radio DOES key up now. Using a modulation light dummy load, I
:+> can get a weak glow out of the bulb. I removed the resistor that burnt
:+> , and checked it. It still APPEARS to still function...I get a 30-Ohm
:+> reading out of it. It seems like the radio would repairable (from my
:+> stand point anyway), but I may be missing something here, that you may
:+> catch.
Re-check the meter and re-zero it. Then check again, if it's 30 ohms,
then try what everyone else is saying, the 22 or 27 ohm. And replace
it anyways, with at least 1/4 watt rating. The bead can be used again.
Since you can't find a "true" 30 ohm resistor, theres' 33 ohm or 27
ohm. This is why I like digital meters, they're more exact.
For better results, use a 5% precision tolerance value. Or, if you
don't care, use whats available. Just make sure it's carbon
composition, not a wirewound type.
To get a weak glow, means several things.
You can be using the pre-driver, and the energy produced is leaking
past the driver and final.
Or the final is bad and the driver is forcing RF past the transistor.
There may be hope, but I appologize heavily for not knowing more about
the age of the radio. At one point, when Cobra was no longer being
made by Uniden, other companies less experienced took over production.
It sounds like you've got one of these models, made by an
inexperienced company. You're not the only one suffering the fate of
having to "re-build" one of these radios.
:+> Since the 1969 is not possible, should I just try replacing it with the
:+> same final. It seems like all that would need replacing would be the
:+> final and te resistor...Maybe something in line between the driver and
:+> the final also. Any thoughts would be cool....
Since I don't have a schematic of a 92' model, I'd also replace the
driver too. It may have been weakened by use after the final failed.
If you find that the only thing standing in the way of the driver to
the final is a capacitor versus a tank coil. If so, you shouldn't have
to do any extra work, but check the cap to make sure it's not showing
a short across the leads. Use the meters X100,000 or 100K setting, not
the 1M setting, to see if the cap is leaky. If you use the 1M
resistance scale, stray resistance may give you a false reading.
If it's a tank coil, well, be ready to replace that too. You can
unsolder the coils metal shield and unbend the tangs holding it onto
the board, and gently removing the case will expose the coil. Examine
the coil, the winding should have a uniform color, discoloration not
caused by an enamel sealant to protect the coil form and keep it's
place. If you see discoloration or a frayed lead, expect the coil has
opened and needs to be replaced. You can't always re-solder the
connections because of the length of winidng of the coil needed is
included in the overall length to even connect it to a post on the
coil form body.
If you suspect a tank coil is bad, remove the entire case and coil
from the board, look underneath, there might be an SMT cap too, this
can fail and de-tune the coil.
.
.Regards!
.:+> Andy <+:
.
... You might be a CB'er when...
... You use the Radios' Owners Manual
... as a Placemat, to prevent Skid Marks!
Ramrod
answering a question. Once again Andy proves to be a true professional. Keep up
the good work . This is what rec.radio.cb is all about.
Power House / FeMaster
>>weak glow out of the bulb.
>where can i get these? I saw them years ago,
>but havent seen any recently... Ryan
I created my own. a standard PL-259, two short pieces of wire (as short
as possible), and a #47 pilot light (bulb available from Radio Shack [2
for $1.19]). Gives me about a 1.22-SWR, not the best SWR, but it works
for certain testing procedures. Just don't run more than 4-watts into
it...