[R-390] AGC voltage issue -- SUCCESS!

[Ryan Scott]

Greetings all:

Having spent another few hours circuit tracing this AMELCO module, I
discovered the gain from V501 (measured on the switch side of C553) was in
the dirt.  All the other stages had decent gain when compared to my good
EAC module.

Further tracing and comparisons to the good EAC module revealed ***L505
shorted***!  on the good EAC module, the DC resistance of this coil was
around 74 ohms, on the AMELCO it was a short (about .5 ohms)...  I tacked
in a 10mH inductor from the junk box and I now have loud audio and about
-8V AGC with a strong AM BCST station.

It all makes sense now as to why the signal was barely there on the output
of V501 on the other side of C553.  But what caused the inductor to short?
 Previously, I mentioned the original C553 was sensitive to tapping.  It's
connected to the inductor.  Once I replaced the cap, the intermittent audio
issue went away and I was left with low audio.  I'm suspecting the
unsoldering and re-soldering multiple times caused it to short, and my
final soldering caused it to fail?

At any rate, I will clean this module up and replace C551 with a voltage
equivalent cap and move on to the next module or radio.  (I have 11
R-390a's, one is fully restored and working great [ebay purchase], the
others are well, work in progress as they say).

Thanks again to all on the list for the help.  This, along with all the
resources on the net is what drew me to these radios -- and to be able to
actually work on something without surface mount devices. (I work as a RF
analog semiconductor test engineer at MAXIM Integrated).

All these resources and interest from the group will keep these radios
playing for another 50 years.

Thanks again!

Ryan
N7QJ

On Tue, Sep 16, 2014 at 7:12 PM, Larry H <dinlarh at att.net> wrote:

> Hi Ryan,  The reason I though a 400v rating would be ok for c551 is that
> the caps in the power supply and most in the audio deck are rated at 300v.
> I think the reason they are not rated higher is that the power supply is a
> 'choke input' and the voltage regulator is active immediately (putting
> enough load on the rectifier to minimize the initial 'power on' high
> voltage condition you see in many other power supplies).  Granted my 2
> r390a's here have 26z5's, so I don't see instantaneous B+, but my B+ comes
> on quite fast.  It goes to 260v very quickly and then holds at 250.  I used
> my 11 meg ohm vtvm to measure the voltage at F103 so as not to load the
> voltage down.  Even if you have the SS recs, I don't see this changing much
> because of the design mentioned above.
>
> Regards, Larry
>
>
> On Tuesday, September 16, 2014 5:11 PM, Charles Steinmetz <
> csteinmetz at yandex.com> wrote:
>
>
>
> David wrote:
>
> >Self-heal events (characteristic of metallized film) are harmless in
> >C551, the AGC Time Constant cap.
>
> While the radio is operating normally, that may be so (although C551
> can end up with >200v across it with strong signals, as you noted in
> your AGC article, and my tests indicate that a metallized film cap of
> any voltage rating has self-healing events above 200v).  But during
> the startup condition I described in my message, C551 may have >400v
> across it for some seconds every time you turn on the radio,
> positively ensuring that there will be self-heal events.
>
> If your point is that self-heal events at this location would not
> matter even if they occurred, I disagree.  Any self-healing current
> would flow through the grid of V506A.  Cumulative occurrences would
> damage V506's cathode.
>
> Best regards,
>
> Charles
>
>
>
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-- 
Ryan P. Scott
n7qj at arrl.net
503-880-1214