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Cobra 148 - modulates normally then goes nuts

Big Kahuna

Big Kahuna

Sr. Member
Jul 31, 2008
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Interesting one this evening.

Philippines Cobra 148 GTL - hasn't been run in many years I suspect. Just began testing tonight.

Powers on, RX and TX in all modes, PA works with good audio no distortion.

However, on AM if I talk normally when the audio reaches a certain level it suddenly turns to loud static and spikes (no voice anymore) and is stuck that way until I unkey. If I whisper it doesn't happen just when the audio hits a certain level.

R131 intact, TR26 intact, tried adjusting dynamike and VR7 and didn't seem to effect the problem other than adjusting the audio as expected. VR10 and VR11 seem set respectably.

It was late so I had just started with giving it a quick test. Cleaned the control pots but no change.

Going to start the alignment process tomorrow but if anyone has any ideas let me know :)
 
Last edited:

Those capacitors are over40 years old, if the radio has the mike jack on the left side of the case.
Capacitors tend to dry out, the oil can dissipate.
If it is one of the older ones, recapping the radio would only improve it.
That's just my take on the problem you describe, it sounds like a bad cap.
Good luck!
 
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Pete Fender said:
Those capacitors are over40 years old, if the radio has the mike jack on the left side of the case.
Capacitors tend to dry out, the oil can dissipate.
If it is one of the older ones, recapping the radio would only improve it.
That's just my take on the problem you describe, it sounds like a bad cap.
Good luck!
Click to expand...

Yeah it’s one of the old side mic ones and probably could use a recap as it was sitting for a long time apparently. In all my years dealing with older radios I can't remember this particular issue though.
 
Turned on radio this morning let it warm up then started the alignment process. VCO voltage was a tad low but only off by .2

Aside from needing some tiny adjustments on L23,22,59 everything else seemed pretty good.

Went to test it out - problem will not present itself again. Now I'm really baffled. Maybe it just needed to warm up all those old caps :)

I'm sure as soon as I think I'm all good and put the cover screws back in it will do it again.
 
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Some Cobras (Uniden) were made in Taiwan or Malaysia, they changed to 1/8 Watt components.
The Filipino Cobras kept using 1/4 Watt resistors.
Same with caps.
Most capacitors were 16 Volt, but Filipino ones were rated 25 volt.
All of them, after a long time, eventually "dry out,"
and fail.
The fact it started working after using it, is most likely from heating it up and re-dispersing the oil inside the capacitors themselves.
I personally would recap those older Cobras, as they will work better and upgrade to 25 Watt caps of the same value.
Although I only mentioned Cobras, this includes all older radios, I worked with a neighbor in the 1980's that had a fully equipped shop.
He used to do work in Avionics and was an FCC trained tech.
 
Pete Fender said:
Some Cobras (Uniden) were made in Taiwan or Malaysia, they changed to 1/8 Watt components.
The Filipino Cobras kept using 1/4 Watt resistors.
Same with caps.
Most capacitors were 16 Volt, but Filipino ones were rated 25 volt.
All of them, after a long time, eventually "dry out,"
and fail.
The fact it started working after using it, is most likely from heating it up and re-dispersing the oil inside the capacitors themselves.
I personally would recap those older Cobras, as they will work better and upgrade to 25 Watt caps of the same value.
Although I only mentioned Cobras, this includes all older radios, I worked with a neighbor in the 1980's that had a fully equipped shop.
He used to do work in Avionics and was an FCC trained tech.
Click to expand...
The Taiwan radios came before the Philippine radios. They were not downgraded.
 
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Electrolytic capacitors contain chemistry that can "repair" a breakdown defect after power is removed. The dielectric (insulating layer) that makes it a capacitor in the first place is an incredibly-thin layer of oxide on the surface of thin aluminum foil. The liquid inside is formulated to react with the aluminum to create this layer in the first place. After a breakdown, that same chemistry can "paint over" a rupture in the surface of the foil. Makes it appear to 'fix' itself.

Always temporary. Just no good way to predict how temporary.

73
 
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