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Meter swinging backwards on Cobra 29 ltd

A

animal

Member
Jan 29, 2010
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0
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I recently got a Cobra 29 ltd Classic that when you key the mic, the meter swings BACKWARDS... :censored:
So far I've tried it in the mobile, that has a 5' antenna on a spring. My SWR is 1.3 with that setup.
It's had a "swing kit" put into it, before I got it.
Deadkey is 4, swing to 18.
32's I've gotten have all been good, but still the meter swings the wrong way...
Any suggestions would be greatly appreciated, for this relative noob.
 

when ya say meter swings backwards im assuming your
talking about the meter on radio.if yes then id check it with
an external meter.this will be accurate .also make sure that
the toggle swith for swr/cal/s-rf is in the rf position.if
ya done all this already im just gonna guess its maybe
a bad meter.cause i havent seen a meter on radio
swing backwards when at the same time an external
meter is reading ya got forward swing.hopefully
someone else will chime in and can help more than i did ..lol
 
do you mean that when you talk into the mic the meter moves toward the left instead of the right?

if you are sure that the radio is getting enough current, and the radio has had work done to it already, there is a chance that you are experiencing downward modulation.

downward modulation occurs when someone tunes the coils in the transmit section for maximum deadkey instead of maximum modulated wattage.

if this is what you are experiencing, the transmit coils need to be tuned for max PEP watts.

this is done by either using a 1000hz tone generator held up to the mic while its keyed, or by saying "ahhh" into the mic while peaking each coil.

dont try to use a whistle to tune by, use your voice.

i agree that you should confirm what is happening on an external meter before you proceed.
if the wattmeter has an AVERAGE setting, use it and make sure the needle kicks forward when you talk.
if it kicks backwards, you have downward modulation.
a PEP meter will still swing forward most of the time even if you have downward modulation.
LC
 
Sorry for not making it clearer, but yes the meter on the radio moves to the left when I even key up.
The power is coming directly from the battery, via 10g wire.
I'll check the radio with a RS 1-534 meter which has both pep and avg, in the morning.
It's a good thing I don't have to whistle to tune it, because my whistling is pretty pitiful...

I'm hoping to learn about doing basic tuning and repairs, and appreciate all of the advice.
animal
 
Last edited:
'Downward modulation', 'reverse modulation', call it what you want, but it still amounts to 'over modulation' or greater than 100%. That means something wasn't done correctly when the radio was modified. It is a very common thing to see, and any radio can be over modulated.
- 'Doc
 
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Just another observation, does the meter appear to have ever been replaced at all? If so could it have possibly been wired backwards?

If you're showing a good modulation on an external meter, then odds are it is something with either the metering circuit or the meter itself in the radio.
 
Well the latest test show the following:
On an external RS 21-534 meter in average setting it has a 7w DK, swings to 10.
In PEP it has 7w DK swinging to 18.

When the s/rf switch in the transmit mode, the internal meter swings to the left as soon as you DK the mic, but in swr gives a fairly accurate swr reading. In cal mode, the swr cal knob has NO effect whatsoever.

AFAIK,
the only mods done to the radio have been the Cobra 29 LTD super modulation modification
mod. Whether or not the meter has been replaced, I've no idea.

When I get the rest of it sorted out I'd like to do the Schotty diode/transistor receive upgrade, and possibly a variable DK hooked to the RF gain knob.

All radio checks have been positive, with good clarity reported.
 
first off, you do not have downward modulation.

most likely someone turned down the pot that controls the RF meter in your radio.

this variable resistor (labeled VR on the PC board) does not have any effect on your transmit power, it just controls what the meter reads.

this is sometimes done when the power of a radio has been turned up such as yours, because the needle will smack against the right side of the meter when you talk into the mic.

sounds like someone turned yours down just a bit too far.
the VR to turn to change this in your radio is VR5.

just key the mic with the mic gain all the way up, and say "ahhhh" into the mic while adjusting the VR so that the needle goes almost all the way to the right but does not slam against the side on voice peaks.

wherever the needle moves when you deadkey is just a relative indication that your radio is working as it should and does not correspond to any certain wattage.

i will say that the way you describe it sounds like you are used to the meter being over to the left on receive and when you key the mic it moves over to the right.
well, the skip cycle is returning and most likely you radio has anywhere from S-3 to S-5 of skip/noise coming in.
if your meter is set so that the needle only moves to S-2 on TX, then of course its going to move over to the left when you key the mic.


one other thing, if all you did was the "super modulation modification", then how did you get a deadkey of 7 watts?
this mod is intended to lower the deadkey from 4 watts down to 1 or 2 watts so you can properly drive a linear amp.

i dont think you had the "swing mod" done, it sounds like someone peaked the transmit section and turned up the modulation pot (VR4).
they might have also changed the final transistor to a 2SC1969, IRF520, or an ERF2030.
im not saying anything is wrong with your radio, it actually sounds like its working just fine. just saying that the mod you mentioned may not have been what was done to your radio.

have fun,
LC
 
LC,
Thanks for the reply, and the information!
I'll go thru the VR 5 tuning, and will post the results.
Well, the parts for the modulation mod are on the board, soldered and everything...
I thought the DK was a bit high for that having been done also. I'll check and see if D11 has been cut, because that'd raise the DK too, wouldn't it?
I'm used to seeing about S5 or so of noise, due to where I live in Ft. Worth, not too far from I-30...
Would the VR5 also influence the swr calibration knob not having any affect too?
Also, I'll check the VR4.

Thank you so much for the help... hopefully one of these days I'll know enough to be an asset too.
animal out
 
D11 would be jumpered if anything, and you should DEFINITELY put it back to normal if you find it has been replaced with a jumper or jumped on the solder side of the board.

doing the jumpering only gains you a watt or so, and IMO is nowhere near worth the risk involved.

what final do you have in your radio?

the VR5 tuning will not affect anything but what the meter shows when you key the mic with the meter in "S/RF" position.
it will have no effect on what it reads when you are in the CAL or SWR positions.

later,
LC
 
Has the meter worked properly since you have owned the radio? With the meter deflecting to the left on key up I wonder if the wires to it have been reversed at some time like someone else suggested. Of course the lower the noise level on receive, the higher the meter would be showing to the right. Everything working backwards. This would also explain the SWR cal not working.
 
loosecannon said:
do you mean that when you talk into the mic the meter moves toward the left instead of the right?

if you are sure that the radio is getting enough current, and the radio has had work done to it already, there is a chance that you are experiencing downward modulation.

downward modulation occurs when someone tunes the coils in the transmit section for maximum deadkey instead of maximum modulated wattage.

if this is what you are experiencing, the transmit coils need to be tuned for max PEP watts.

this is done by either using a 1000hz tone generator held up to the mic while its keyed, or by saying "ahhh" into the mic while peaking each coil.

dont try to use a whistle to tune by, use your voice.

i agree that you should confirm what is happening on an external meter before you proceed.
if the wattmeter has an AVERAGE setting, use it and make sure the needle kicks forward when you talk.
if it kicks backwards, you have downward modulation.
a PEP meter will still swing forward most of the time even if you have downward modulation.
LC
Click to expand...
Your correct destining w in final plate load will restore forward modulations
 

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