[R-390] Local & Line potentiometers

[Craig Heaton]

Charles,

A little time was spent on the beast this evening. 
1: No ground problems at the end of the pots, even added a jumper once
again. No change
2: Checking the audio at the CCW ends, less than 5 millivolts with the
O'scope. 

Going to check a few more items tomorrow; swap IF's between the Motorola and
Amelco again. This should rule out AGC issues there. Then on to the RF
section.

Thanks,
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: Monday, December 30, 2013 4:37 PM
To: 390 list
Subject: Re: [R-390] Local & Line potentiometers

Craig wrote:

>Resistance wasn't quite up to snuff. Wires go to correct terminations, 
>no shorts, no grounds, etc.

If the end-to-end resistance of each pot is between 1k and 10k, they should
work fine.  They do not appear to be your trouble.

>With a speaker connected to either audio circuit, knob at zero, at 
>times the RF gain has to be decreased to save the ears. IF gain is set 
>correct, -7volts @ 150 micro-volts.  Transformer used to match 
>impedance, speaker to receiver.

Either you have WAY too much audio at the CW ends of R104 and 105, or you
have a ground problem at the pots (CCW ends not firmly grounded), or BOTH
audio amps (V602A/V603 and V602B/V604) are broken.  The chance of both audio
amps being broken is slim, so that's the least likely.  Using a scope, check
for audio at the CCW ends of the pots (terminal 3 of R104 and R105) to rule
that out.  There should be no more than a millivolt or so of audio there.

If these check OK (and I expect they will), then you have WAY too much audio
coming from the detector/limiter/first audio amp and follower.  That would
suggest an AGC problem.

>Could a person just add a resistor where the wire from S104 (connection 
>4) and the wire from P120 meet? If my memory is correct, this point is 
>on the line gain pot and easy to access.

There is something broken.  Find out what it is, and fix it.  Don't hide the
problem by modifying the radio.

Best regards,

Charles



______________________________________________________________
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