Cobra 29 GTL Leaked capacitor after 2 years?

[doffo]

Hello,

Decided to have another look inside a Cobra 29 GTL I picked up a couple years ago. Seen that one basically blew its entire casing off (I have the picture somewhere on my computer). Replaced that capacitor and the one near it that looked a little rough. Radio did work ok.

Last night, I noticed the 47uF 50v Nichicon Capacitor I put in has already leaked from up top and in a dome shape... Surely something else is causing this?? There is a thread here that someone had little modulation on his 21 GTL, but turns out it was that same capacitor that gave up and leaked all over. Is there something neighboring in the area that is causing that one particular capacitor to just go bad? I had the radio switched off before I put it on the shelf. I did replace a couple more capacitors near the audio chip, and the one that is behind the small transformer right in front of the power connector. (Think that was a 100uF capacitor) In this picture from the thread, it is that capacitor with all that white residue on top of it.



Overall, the radio isn't too pretty and I would rather recap the entire board if the 29 GTL was in much better cosmetic shape. I was just more curious of the fact of what may be causing that particular capacitor to go bad. Here is the link to the thread that explained it.

Cobra 21 GTL Little Modulation

 

[Amperex]

Excessive ripple current? This cap brand you used can have as low as 105ma Ripple current. I would use Panasonic part number EEU-FS1K470uF 47uF 80 VDC rated at 490ma ripple current.

 

[doffo]

Excessive ripple current? This cap brand you used can have as low as 105ma Ripple current. I would use Panasonic part number EEU-FS1K470uF 47uF 80 VDC rated at 490ma ripple current.

Very well may be it. I just found it surprising that after short use for a couple days, then shelfing it for a few years, it leaked the way it did. First time any of the radios I have do that.

 

[Lkaskel]

Very well may be it. I just found it surprising that after short use for a couple days, then shelfing it for a few years, it leaked the way it did. First time any of the radios I have do that.

Maybe try measuring the voltage at the cap while in RCV and XMT. That would be interesting to see. Also, if you have a scope try the same thing to see if you get a sawtooth (ripple current) reading.

How to Measure Power Supply Ripple on an Oscilloscope

See how you can measure power supply ripple and noise with an oscilloscope in this article.

resources.pcb.cadence.com

 

[doffo]

Maybe try measuring the voltage at the cap while in RCV and XMT. That would be interesting to see. Also, if you have a scope try the same thing to see if you get a sawtooth (ripple current) reading.

How to Measure Power Supply Ripple on an Oscilloscope

See how you can measure power supply ripple and noise with an oscilloscope in this article.

resources.pcb.cadence.com

I just hope it isn't a common problem with every GTL radio out there. Least maybe certain revisions? But good idea to see the voltage on it during transmit or receive. Thanks for chiming in.

 

[nomadradio]

Doesn't make much sense. The empty spot appears to be C117. It's connected to the base of TR21. Should be a low-energy point in the circuit. Not sure how enough current can reach this cap to make it bulge.

Or is this a different cap than C117?

73

 

[doffo]

Doesn't make much sense. The empty spot appears to be C117. It's connected to the base of TR21. Should be a low-energy point in the circuit. Not sure how enough current can reach this cap to make it bulge.

Or is this a different cap than C117?

73

The cap that did bulge was C48, a 470uf on the Cobra 29 GTL. Same area by the looks of it. I did find a schematic to be sure I didnt put in the wrong value back in it, but it does say 470uF.

 

[nomadradio]

C48 is a horse of a different color. It handles all the AC current feeding audio power from the audio chip to the audio transformer. That part works hard, and if it has too much resistance inside, it will get hot. And heat is bad juju for electrolytic caps.

Makes me wonder if it wasn't a one-lung-sam chinesium capacitor that had a high internal resistance from the start. The same part used elsewhere in the radio as a filtering cap with the negative side grounded might have stood up okay. Much less AC current through it that way. The common acronym for this property is ESR for equivalent series resistance. Cheap caps are most likely to have this defect.

73

 

[Amperex]

C48 is a horse of a different color. It handles all the AC current feeding audio power from the audio chip to the audio transformer. That part works hard, and if it has too much resistance inside, it will get hot. And heat is bad juju for electrolytic caps.

Makes me wonder if it wasn't a one-lung-sam chinesium capacitor that had a high internal resistance from the start. The same part used elsewhere in the radio as a filtering cap with the negative side grounded might have stood up okay. Much less AC current through it that way. The common acronym for this property is ESR for equivalent series resistance. Cheap caps are most likely to have this defect.

73

Radio is feed pure DC, there is no ripple current. Even if an audio buffer 1/2 watt audio is loud and cap should survive any audio current. I think you bought a bad capacitor.