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

[Charles Steinmetz]

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