Do we really need to change bypass caps and tantulam capacitors on older cobra cb models or old radios?
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Myth or Bust?
- Thread starter kaos513
- Start date
Some will call it a mixed bag.
Tantalums and ceramic caps are very reliable parts.
Replacing them "just to do it" because "something might happen"...
Well, the motor in my car might blow up tomorrow. I won't run out and replace it because of what might happen.
A lot of those Tantalums are really not in a position to be subjected to overvoltage, like in the PLLs loop filter.
Tantalums and ceramic caps are very reliable parts.
Replacing them "just to do it" because "something might happen"...
Well, the motor in my car might blow up tomorrow. I won't run out and replace it because of what might happen.
A lot of those Tantalums are really not in a position to be subjected to overvoltage, like in the PLLs loop filter.
If you've got an 858 chassis radio (Cobra138XLR,139XLR), I highly recommend replacing the original tantalum cap at C179. Use another tantalum or a regular electrolytic of the same capacity and voltage ratings.
The originals have a tendency to do a very good impression of what happens to a hand grenade five seconds after the charging handle is released.
The originals have a tendency to do a very good impression of what happens to a hand grenade five seconds after the charging handle is released.
My motto is never trust a fart or an old tantalum. Trust 100% in GOD and 99.9% with ceramic caps applied in a low voltage circuit.
The larger Electrolytic Capacitors are the one needing changing most often. If you hear an alternator whine that you have not heard in the past or 60 cycle hum that's an indication.
Tantalum? No - unless it blew up from reverse voltage or the case got smashed - they're usually pretty durable. Their innards, like Beryl - based RF Pills - are toxic though so be careful around the debris if one does blow up - in their reduced form, they are safe. But with things used for processing in refining them - along with the parsing techniques to isolate it from the ore and niobium - a sister element usually found with it - that can pose a considerable health risk.
Disc? That may be a yes, but you didn't specify any particular type so in general - only as needed. I've had a gauntlet of various Disc caps Mylar's, Polystyrenes Silver Micas - all have failed in circuit but usually due to puncture or excessive power being put thru them - although as for length of time for a factor of age, no, but time as a factor of any given moment of stupidity from the operator - definitely yes...
Disc? That may be a yes, but you didn't specify any particular type so in general - only as needed. I've had a gauntlet of various Disc caps Mylar's, Polystyrenes Silver Micas - all have failed in circuit but usually due to puncture or excessive power being put thru them - although as for length of time for a factor of age, no, but time as a factor of any given moment of stupidity from the operator - definitely yes...
The tantalum in question, C179 in those Uniden-made 858 SSB radios is just a victim of its voltage rating. A 25-Volt capacitor is safe enough when the modulation waveform reaches a peak value of under 20 Volts with the AM modulation limiter working like it's meant to.
But cranking the audio on that radio puts peak voltages up to 30 Volts on that part. We routinely change that cap to a 35-Volt part before I'll key it more than once or twice.
It's been a long while since I last saw one of those radios with the AM modulation set for a 100% limit. I simply assume that any original part in that spot has already been compromised, whether it has shorted yet or not.
But that is the only tantalum cap I know to be a chronic breakdown failure. The rest of them are a lot more reliable than the aluminum electrolytics after 30 or 40 years.
Just my experience. Your mileage may vary.
73
But cranking the audio on that radio puts peak voltages up to 30 Volts on that part. We routinely change that cap to a 35-Volt part before I'll key it more than once or twice.
It's been a long while since I last saw one of those radios with the AM modulation set for a 100% limit. I simply assume that any original part in that spot has already been compromised, whether it has shorted yet or not.
But that is the only tantalum cap I know to be a chronic breakdown failure. The rest of them are a lot more reliable than the aluminum electrolytics after 30 or 40 years.
Just my experience. Your mileage may vary.
73
I only called out C179 because I've had one detonate inside a 139XLR. Inside of the case had a nice black spot and the metal looked like something had scratched it. Just glad the case was on when it let go.
On the plus side, the remaining shrapnel shook out really easy. Which is nice.
Replaced it with an electrolytic and went skip shooting that afternoon.
On the plus side, the remaining shrapnel shook out really easy. Which is nice.
Replaced it with an electrolytic and went skip shooting that afternoon.
do the ceramics in the powersupply for sure and I've had bypass caps screw me over too.
https://www.worldwidedx.com/threads/cobra-148gtl-shorted-final-on-rx.221534/
https://www.worldwidedx.com/threads/cobra-148gtl-shorted-final-on-rx.221534/
