[R-390] Lightning crashes in audio

[Stan Gammons]

Decided to pull the IF deck, flip it on it's side and reconnect the 
cable to J512.  Cables from the RF deck were not re-connected. I've been 
guessing C511 might be the problem, so I took some freeze spray and 
sprayed C511. All quiet on the home front after I did that. Will have to 
look through my caps to see if I have one of those. Seems like I do.  If 
not, I'll have to order some from Mouser. Hopefully that is the problem.


73

Stan
KM4HQE


On 8/18/20 6:09 PM, Cecil wrote:
> Check out the Sencore analyzers. They test at rated voltage up to 999v I think. Love mine!
>
> Cecil
> K5DL
>
> Sent from my iPad
>
>> On Aug 18, 2020, at 12:27 PM, David Olean <k1whs at metrocast.net> wrote:
>>
>> I had answered an e mail from Jacques yesterday, about silver mica caps, but a WIN 10 hiccup removed all evidence of it along with his e-mail.   I have found at least two defective sm capacitors that were both testing fine in two different capacitance meters that both measured dissipation.  I had a 3rd meter that measured ESR and it checked OK on that one too. In each case the problem was thunderstorm static with no antenna attached.
>>
>> Cap #1 was in the IF circuit of an R-392 receiver. The cap was located across the  2nd IF transformer winding, so it only had RF voltage across it. I definitely isolated the static noise to that area with an oscilloscope. When I removed the cap and replaced it, the noise was gone, but the only way I could find the trouble was to use my 0 to 450 Volt DC variable power supply and put some DC voltage on one side of the capacitor, and a high impedance voltmeter (Fluke DVM) on the other side of the cap. If I kept DC voltage below 10 volts, there was no real dc leakage visible. The DC levels would drop down to a few millivolts.  But as soon as I increased the voltage above 12, I started to see significant leakage. The DC level started to soar across that capacitor.    A second silver mica cap was causing similar noise in an R-390A receiver. It happened on only one of the four rf bands in the R-390A RF section.  I think it was the 1-2 MHz band.  Again, the capacitor was across an RF coil, and it was causing static noise. It was also failing at a low voltage.
>>
>> After seeing this, I am more appreciative of my brute force DC voltage testing with a high impedance volt meter. I trust it more than any low voltage testers. To be fair, my GENRAD RLC meter has a provision to apply up to 30 volts across any capacitors I may want to test, but I need a four lead test fixture to access that function.  My DC power supply is also great for testing electrolytic capacitors. I can see when the leakage current starts to rise.  I can also charge the cap up and do a relative test on how long it takes to discharge. The new electrolytics are so much better than old ones from even ten years ago.
>>
>> 73
>>
>> Dave K1WHS
>>
>>
>>
>> ______________________________________________________________
>> R-390 mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:R-390 at mailman.qth.net
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html