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cobra 25ltd c91 vs. cobra 29ltd C71

Uncle Ronnie 336

Well-Known Member
Apr 28, 2020
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ok so off of andy's advice that limiter release cap in these rigs get changed on the 29s and it makes the wattage ABSOLUTELY SLAM
but when i try it on the 25, it supresses the modulation bigtime. looking at the schematic im not really seeing a difference in the layout of the 2 limiter sections so why does it act opposite? a little bit stumped on that one because it did that on both a 66xl and early 95 china 25
 

There are slight differences between the 29 and 25 - or 78 versus 68 series.

The parts and layout may look similar, but it's with the Parts PLACEMENT that makes one mod that works well on one type, fail in being tried on the other...

Here. Read this Thread about the PC-66

Note the layout and PARTS placement - how it's wired in.

Then go here...another spot for the 29

Both look similar and you'd be right, but the values need fixing.
1657508308069.png

So, @NZ8N has the better way - at least it's more universal.

When the MOD limiter clamps too hard, it means the section is TOO sensitive to changes - so you'll have to tame it - use a 1KΩ or even 560Ω ohm resistor ACROSS C71 or C91 to lessen the filtering - those caps "Sample and Hold" until the limiter transistor bleeds it down - and there is nothing else there to lessen those spikes. When the cap stores that charge - there is only the BASE of the limiter to drain it off - so do what NZ8N and many others do, just strap a resistor of some low values across the two legs of that sample cap and run with it.

In the above, the 29 graphic, you can keep or remove C70 - but just solder a low-value resistor in across C71 or C70 (whichever is convenient) - to reduce the effectiveness of the caps charges that hold the limiter engaged longer - by draining it faster.

C91 of the '29 76/78 series and C71 of the 66/68 / 25 series of the older units, don't know of the newer SMD ones if those are even still using a similar design.

Many simply "pull the limiter". But in doing so - remove any chance to keep the signal from entering into stereos and amps and speakers - of your neighbors.
 
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thank you for the info. so it appears on the 25 it's c98 that i should put a 1k in place of. should i leave the 33uf in c91 or change back closer to stock?
 
This switch is to suit your tastes, if the others like what they hear, then mission accomplished. You have got your radio to work so you can be heard like you want it to do.

To go back might make it sound flatter and boomy - with a bass tone thru may radios that don't go over as well.
 
ah ok. so yea, i'll leave c70 (98 on the 25) and tack in a resistor to see what happens. never seen power drop like that from lowering the release cap on any other radio. only the 25/66
 
just tacked 820ohm across c91 (changed to 33uf). that did the trick. clean on adjacents, fast and robust. thanks again andy, and if we can get this thread pinned so everyone knows how to make the meter do the bit without making the neighbors hear the ghost watts on the (very common) cobra 25, this reference would be a good thing to eyeball
 
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