[R-390] Saturday's Line & Local Report

[Craig Heaton]

Hello Charles,

This has been fun, been chasing the loud audio for a while, since it was
brought home in Feb. 2007. That when the caps were ordered. As stated
before; moving either the IF or the AF module from the Motorola to the
Amelco has had no effect on the audio. 

The caps in C603 have the leads crimped and soldered. Good connections to
the pins, no resistance. Socket of C603 is good to go. Last night I even
soldered the eyelets to the screws. I don't have any caps here that fit the
bill to tack under the chassis.  

This evening I'll trace all the AF wiring again for the umpteenth time. 

Tomorrow I want to compare IF output of the Motorola vs the Amelco. Yes, the
IF gain of both receivers are set @ -7VDC 150 micro-volts 455KC.  

Craig,



-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Charles Steinmetz
Sent: Saturday, January 04, 2014 5:10 PM
To: 390 list
Subject: Re: [R-390] Saturday's Line & Local Report

Craig wrote:

>with an antenna connected there was about 1.0VAC with the Local gain 
>set at 4.

The setting of the local gain pot will not affect the amount of audio at the
junction of C603B and R606.

>Then  *   *   * a local AM BC station was tuned in  *   *   *  and I 
>could watch the wave form on the O'scope bounce.

So, it appears that C603B is not doing its job.  Since you have the AF deck
out and can access the bottom of the chassis, you should try tacking in
another cap below the chassis, directly at the junction of
R606 and the AF B+ choke (L603) (positive end at that junction, negative end
to a convenient ground).  This bypasses the octal socket connection to the
existing cap (C603B), just in case there is something amiss with the socket
or inside the C603 can.

>since the AF module from the Motorola has been placed in the boombox 
>with no change

"No change"??  Am I understanding you correctly -- neither AF module works
correctly in the broken radio?  I thought you said the Motorola AF deck
worked correctly in the broken radio.

>I'd say the Amelco is picky about its diet of capacitors. At present 
>both R-390/A's have Xicon 33uF 350V caps in them. The Motorola is happy 
>with its diet.

Somehow, C603B is either not a properly working capacitor, or if it is,
there is something wrong with the wiring at the octal socket or inside the
C603 can and C603B is not really connected to the circuit as it is supposed
to be.

The audio is coming from the plate of V601A, through R605 and 606 (a total
of 58.2k ohms), and thence on to the grids (through R611 and
622) and screens of V603 and V604.  C603B should be putting a reactance
(think of reactance as AC resistance) of around 5 ohms to ground at audio
frequencies.  The voltage divider formed by R605+606 and C603B should reduce
the audio voltage at the junction of R606 and C603B to about 1/10,000 of the
audio voltage on the plate of V601A -- i.e., only millivolts or less.  But
according to your observations, it is not doing this.

It has nothing to do with the Amelco AF deck being "picky" or "needing" more
or better capacitance than the Motorola AF deck.  It just needs a properly
working 30uF capacitor hooked up to the junction of L603 and R606, and for
whatever reason, there isn't one.  Maybe you got a bad cap (it happens).
Maybe the crimp connection between the capacitor lead and the crimp terminal
is flaky.  Maybe the pin(s) of C603 aren't making good contact with the
octal socket.  Whatever the reason, C603B is either faulty or it isn't
actually electrically connected where you think it is.  (That is why I
recommend tacking in a cap under the chassis as a test procedure, to
eliminate the socket and internal can wiring from the equation.)

>Mouser, part # 140-XRL350V33.

When did you buy those?  Mouser shows that part as obsolete, no longer
stocked.

Best regards,

Charles



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