[R-390] FW: Thursday's Line & Local report

[Craig Heaton]

Good Evening Charles,

Glad you chimed in! I've printed out your suggestions and will follow thru
in the next couple of days, if not sooner. Lets rule out a few items.

1: I've disconnected every wire to the Local & Line pots, measured the
resistance end to end. Neither has a broken track. Local measures about
2.55K, wiper is good. The Line pot measures about 2.18K, wiper is good. Both
are log tapers. As already stated, I had a 2.5K log taper in the junk box to
replace the as found 5K. 

2: C603: As stated in an earlier post, all three sections were checked with
a T0-6A. Values are about 30uF, leakage under 0.01mA @300VDC. Should be sort
of easy to check in-circuit. Just have to remove the clamp screw, raise the
can, & connect a clip lead. Last time I battled the Amelco, the caps were
removed from the can, checked to make sure all wires were covered with tape,
no shorts, polarity is correct. But could look again. At this point I'll
paint my face blue and dance around a Christmas tree if it helps.  

Charles & all; I believe in past topics the fact, radio is not my ball of
wax, has been mentioned. Your help is greatly appreciated. At this point, my
fingers are still crossed the issue isn't AGC.

As a reference point, I thought about checking the last tube output of the
RF, last tube output of the IF, of both the Motorola vs the Amelco. Connect
a sig-gen to the antenna input and record a no signal vs a couple micro volt
signal for each section. Maybe toss in a AGC vs MGC to the mix also. Tube
extenders and an O'scope should get this done.

Regards,
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: Friday, January 03, 2014 6:16 PM
To: 390 list
Subject: Re: [R-390] FW: Thursday's Line & Local report

Craig wrote:

>Now the voltages on terminal #1 of the Line & Local pots agree with the 
>normal Motorola. The audio hasn't changed.

OK, so everything before the pots is OK.  The problem must be from the
wipers (terminal 2) of the pots forward.

Just as a sanity check, measure the audio on terminal 2 of the pots and
verify that it goes from 0 (just a few mV) to the same as terminal 1 as the
pots are rotated from full CCW to full CW.  (For example, if the carbon
tracks were broken internally at terminal 3, the radio would be near full
volume all the time.  It's very unlikely this is the problem, but it needs
to be ruled out.)

If that is OK (essentially no audio on terminal 2 of the pots when they are
fully CCW), the question is how audio is getting to the amplifiers (V602A,
602B, 603, and 604) to be amplified when the pots are all the way CCW.  A
second question is whether the amplifiers are working properly.  For
example, if the NFB resistors R612 and R626 were missing or open circuit,
the amplifiers would have too much gain.  Still, if there were essentially
no audio on terminal 2 of the pots (and, therefore, no audio on the grids of
V602A and 602B (pins 2 and 7) with the pots fully CCW), even if the amps had
too much gain there still should not be very much audio at the plates of
V602A and B.

So, the task is to find out (i) how audio is getting into V602A, 602B, 603,
and 604, and (ii) whether V602A, 602B, 603, and 604 are operating properly.

If the wipers of the pots have essentially no audio on them, the grids of
V602A and 602B should have essentially no audio on them.  The only other
obvious way for audio to get from V601A or B into V602A, 602B, 603, and 604
is through R606, to the grids and screens of the 6AK6s (V603 and 604) -- but
this would only occur if C603B were not bypassing the audio to ground. You
say you have checked C603B, and that it is a new cap.  How did you check it?
Did you measure it in-circuit (at the junction of C603B and R606)?  Again,
as a sanity check it is probably worth looking at that junction with a scope
to see if there is any audio on it.  (There will be some 120 Hz hum, so a
voltmeter reading could mislead
you.)  Is it possible when the cap was rebuilt the + or - end was connected
to the wrong terminal of the octal plug?

If all that checks out, is it possible someone rewired the audio deck for
some unknown reason, and added another path for audio to get into V602A,
602B, 603, and 604?

Best regards,

Charles



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