[R-390] Carrier Level Meter Issues

Wed Mar 9 02:23:49 EST 2022

When doing any kind of leakage testing, you have to think about unintended consequences.  
If you plan to measure current by putting the VOM or VTVM in series with the DUT (Device Under Test) plus the power supply then if the DUT fails in a short, the power supply will force the maximum current that it is capable of supplying through the meter.  That will do wonders to the pointer of the analog meter movement or the circuitry of a DMM.  Not to mention heating up the DUT.
You should limit the maximum current so as not to overstress either the DUT, the power supply or the test equipment.  If you have a power supply that you can set current independently of voltage then with the voltage set to near zero, short the power supply leads together and adjust the current setting pot to set a maximum current of perhaps 25 mA.  Then if the DUI fails, the power supply voltage will fold back to whatever voltage is required to supply that set current.
Don't have a fancy power supply?  You can select a resistor to provide a reasonable current at the test voltage you plan to use.    Measure the voltage drop across the resistor.  Each volt dropped across the resistor divided by the resistor's value is the total leakage current.  
By selecting a current limiting resistor of 1, 10, 100, 1k, 10k and so on ohms, one volt measured across that resistor is equal to one amp, 100 mA, 10mA, 1mA, 100 uA and so on.  You get the idea.
Now you can run the leakage test without having to closely monitor what is happening.  If the DUT shorts, then the power supply, meter and DUT are protected form overstressing and this will help keep that magic smoke inside the power supply, meter and DUT.  
This same sort of setup can also be used to reform electrolytic capacitors that have been setting around for ages before you install it into the equipment you are repairing.
JimLogic: Method used to arrive at the wrong conclusion, with confidence.  Murphy

-----Original Message-----
From: Larry H <larry41gm at gmail.com>
To: Gary I. Biasini <gary.biasini at shaw.ca>
Cc: R-390 Forum <r-390 at mailman.qth.net>
Sent: Tue, Mar 8, 2022 11:42 pm
Subject: Re: [R-390] Carrier Level Meter Issues

Hi Gary,  I'd start with a vom or vtvm.  If nothing there, apply about 200
vdc (because of the circuit it is in) from can to one cap end and then
the other one.

Regards, Larry

On Tue, Mar 8, 2022 at 9:13 PM <gary.biasini at shaw.ca> wrote:

> Larry,
>
> How would you check cap to can leakage?  With an ohmmeter or at a higher
> voltage?
>
> Thanks.
>
> Gary
>
> -----Original Message-----
> From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On
> Behalf Of Larry H
> Sent: March 6, 2022 8:30 PM
> To: R-390 Forum <r-390 at mailman.qth.net>
> Subject: Re: [R-390] Carrier Level Meter Issues
>
> Hi all,  There is a second type of leakage in this style of cap (cap in a
> can), from cap to can (which is grounded).  This is just a damaging to the
> agc circuit as the other normal type of leakage.  Both types of leakage
> need to be checked.
>
> Regards, Larry
>
> On Sun, Mar 6, 2022 at 9:27 AM <gary.biasini at shaw.ca> wrote:
>
> > Renee,
> >
> >
> >
> > Thanks for your thoughts.  What do you use to heat the C551 can to
> > disassemble it?
> >
> >
> >
> > Thanks again.
> >
> > Gary
> >
> >
> >
> > From: Renee K6FSB <k6fsb.1 at gmail.com>
> > Sent: March 6, 2022 10:16 AM
> > To: Jordan Arndt <Outposter30 at shaw.ca>; gary.biasini at shaw.ca; R390A
> > List < R-390 at mailman.qth.net>
> > Cc: 'R-390 Forum' <R-390 at mailman.qth.net>
> > Subject: Re: [R-390] Carrier Level Meter Issues
> >
> >
> >
> > Hi All-
> > I replace C551 on sight....on 16 (IIRC) IF decks I have never found
> > one that does not have leakage current ( maybe just my bad luck,
> > tested on Sencor LC102)...oil/paper....i went with mylar anywhere
> > between 1.5uf or
> > 1.8 uf ( I liked these best) to 2.2uf.
> > I machined small rectangular aluminum cans (with teflon feed thru for
> > the
> > leads) for them to fit in the original space. and as Jordan suggests
> > the 1.5uf will fit in the original can......I drill a small hole
> > before heating to disassemble the can. I do not like hot oil
> everywhere......
> > some of the postage stamp Micas are starting to have shorting issues
> > due to impurities, moisture intrusion and what is called silver
> > migration...so do not rule them out because they are mica.
> > Renée, K6FSB
> >
> >
> > On 3/6/22 4:25 AM, Jordan Arndt wrote:
> >
> > Hi again group...
> >
> > Any leakage of C551 will dramatically alter the AGC characteristics,
> > with the results of leakage being seen as a longer to much longer AGC
> > recovery time and a lower and "softer" AGC voltage.
> >
> > I rebuilt C551 in my three 390As using non-polarized 1.5uF @ 450Vdc
> > film caps which just fit inside the original square can. The result is
> > a "stiffer" AGC voltage, and a noticeably quicker return to receive
> > than with a 2uF cap.
> >
> > I use a restored Sprague TO-6 to detect leakage, and these film caps
> > show no measurable leakage at all, where all of the original C551 caps
> > clearly showed varying degrees of leakage. I'd expect that most, if
> > not all, of the original C551 caps are leaky by now and should be
> replaced/rebuilt...
> >
> > 73...Jordan VE6ZT
> >
> > ----- Original Message ----- From:  <mailto:gary.biasini at shaw.ca> <
> > gary.biasini at shaw.ca>
> > To: "'Larry H'"  <mailto:larry41gm at gmail.com> <larry41gm at gmail.com>
> > Cc: "'R-390 Forum'"  <mailto:R-390 at mailman.qth.net>
> > <R-390 at mailman.qth.net>
> >
> > Sent: Saturday, March 05, 2022 10:13 PM
> > Subject: Re: [R-390] Carrier Level Meter Issues
> >
> >
> >
> >
> >
> > Larry,
> >
> >
> >
> > Thanks. In fact, I started on one tonight which will be the subject of
> > a separate post once I do a bit more work.
> >
> >
> >
> > I checked the manual for the IT-11 and in fact, when properly
> > calibrated, on the setting for “Paper, Mica, etc.” capacitors – i.e.
> > anything other than Electrolytic or Mini-Electrolytic capacitors, the
> > eye will not open if the leakage is more than 2 ua so, the C551 that I
> > tested leaks less than 2ua.
> >
> >
> >
> > By the way, the threshold for Mini-‘lytics is 15ua and the threshold
> > for Electrolytics is the 2ma.
> >
> >
> >
> > Thanks again for your response.
> >
> >
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