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Cobra 29LTD Chrome VFO windup...

sunbulls

Sr. Member
Apr 25, 2017
891
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Back in early days of CB when manufactures incorporated crude tube oscillator circuits, key up frequency drift was common. I’m grateful that issue was addressed in later years with better designs, especially with the advent of PLL’s circuits. I’m fond of those antiques for many reasons, but bad frequency drift is abhorrent IMO. I for one would never try to duplicate the same fault that I’ve been passionate about trying to correct for years. Instead, I’m far more prone to spend countless hours trying to stabilize some of those old sets. I place this mod in the same category as diode clipping, using metal screwdrivers for tuning cans, or stripping out the cover threads by using the wrong sized screws.
 

Shockwave

Sr. Member
Sep 19, 2009
3,779
3,270
273
Years ago people would pay extra to have the VFO supply regulated to avoid this "problem". Now they pay extra to have the problem recreated. The sad part is only about 50% of these mods that I've heard actually "land" on frequency after the slide. Some people inject a voltage through an electrolytic to the varactor but never realize the polarity of the voltage across the cap changes between RX and TX. If you see that problem in a particular mod, use a non polarized electrolytic so the normal voltage can settle across it without loading. Mods that directly ground one side of the cap do not have this problem.
 
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Mar 7, 2019
24
12
13
42
Va
Mod don't work. What am i doing wrong. Tried several caps. Could someone be more explanatory. Thanks. I know some people think its stupid. Its just something to piddle with.
 

nomadradio

Analog Retentive
Apr 3, 2005
5,215
7,412
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Louisville, KY
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Where are you connecting the capacitor?

What size cap are you using?

In what kind of radio? The schematic callout-numbers for parts in a radio will change from one model to the next, and from one "version" of the same radio model to the next.

73
 
Mar 7, 2019
24
12
13
42
Va
An 2004 Cobra 29. Old style audio chip. Connected positive of cap to junction of d18 and r99 like it said and there is a ground right next to it. Tried a 470uf, 220uf and 1000uf. Nothing.
 
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nomadradio

Analog Retentive
Apr 3, 2005
5,215
7,412
573
Louisville, KY
www.nomadradio.com
Try setting the receiver you're using to hear this radio to upper sideband.

You should hear the pitch of the 'squeal' from the Cobra 29's carrier more clearly that way. If the pitch comes up steady and doesn't change, that's a puzzle.

What you should hear is a high pitch that falls off. Moving the receiver's clarifier may make this easier to hear.

This trick is easiest to hear when it's keying on a weaker station, or when it's below S-9 on the other end. A receiver in the same room gets too high a signal level to hear this trick clearly, even if it's working perfectly.

73
 

Highway Man

Well-Known Member
May 18, 2019
359
416
73
TN
pardon my ignorance but...
what exactly is this supposed do. Keys off frequency and slides on frequency right?
my question is "WHY"?
 

Shadetree Mechanic

808 On The North Side of Dover
Oct 23, 2017
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If I had it on my radio, I would use it about twice. My interest in this mod is about how it works; What exactly does the circuit do that is modified? Maybe experiment with the cap values to see how it reacts to the change in capacitance.
 

nomadradio

Analog Retentive
Apr 3, 2005
5,215
7,412
573
Louisville, KY
www.nomadradio.com
How it works?

Okay, for starters, when you key the mike the voltage that feeds the front-panel clarifier knob gets shut off. That knob is not allowed to function in transmit mode, since they changed this rule around 1974. SSB radios older than that will have an "open" clarifier that functions on transmit.

Next, the transmit-only voltage becomes active. It feeds into a pair of resistors that place a fixed voltage onto the clarifier circuit. Generally feeds a voltage roughly equal to what comes out of the front-panel control set to 12 o'clock.

The capacitor is placed from that resistive voltage divider to ground. This cap is empty in receive mode, and when you key the mike, that transmit-only voltage has to charge up this cap. There will be drop in the voltage across the top one of those two resistors as it charges up. This causes the "slide" up from below the channel frequency. Once this cap charges up the transmit frequency will be the same as before it was modded.

When you unkey, the capacitor drains off, and the radio's frequency is once again controlled by the receive-only clarifier voltage.

As to "why?".

Not my department.

Except maybe "Because the customer asked for it".

73
 
Last edited:

Low_Boy

Sr. Member
Jan 21, 2010
1,836
1,106
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Rochester N.Y.
I would like to play one day and see if I could figure it out. I think it is really neat. Much more easy on the ears than a roger beep. Also it would probably make me make more sense of just what goes on when a radio is keyed
 
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