[R-390] Today's Line & Local experiment

[Craig Heaton]

Good Almost Afternoon all,

Charles,

Yes V601!! I'll blame it on I can read my typo's, or morning coffee hadn't
done it job, didn't put on my glasses yet. 10dB, I'll take that if it's the
problem. Swapping C603 from the Motorola isn't going to happen anytime soon.
It has been swapped by itself before and the entire AF module from the
Motorola. Purchasing another set of new caps for grins & giggles might
happen first. 

Next step is to ring out all the wiring in and out of the AF module of the
Amelco. The search will continue for shorts, grounds, opens, and who the
heck wired this thing? 

Something is amiss? Might run through the Motorola and do that AGC thing
with the HP 8640B and TB102. For others on this e-mail reflector it would be
nice to have good numbers of the AGC line when all is right.

It's just about noon here in the PNW. Any other typo's will be blamed on my
favorite adult beverage.

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: Thursday, January 02, 2014 10:39 AM
To: 390 list
Subject: Re: [R-390] Today's Line & Local experiment

Craig wrote:

>Checking the electrolytic caps in the Amelco AF module isn't too much 
>of an issue. No extras on hand.

Just swap in C603 from the other unit to see if that is the problem.

>P120 pin #15. Goes from the grid of V602 thru S104 to terminal #1 of 
>both Line & Local pots

???  According to my schematics, the wire at P120/J620 pin 15 does not
connect to either V602 or to the audio pots.  It connects the grid of V601A,
through R602, C601, and S104, to the cathode circuit of V601B when the AUDIO
RESPONSE switch (S104) is in the WIDE position (but not when S104 is in the
NARROW position).

This provides negative feedback around V601A and B and reduces the gain of
V601A by about 10dB, to match the audio level in the WIDE position to the
audio level in the NARROW position (i.e., to match the insertion loss
through the NARROW filter, FL601).  Even if it were not connecting, it would
only raise the audio level by 10dB in the WIDE position (and not at all in
the NARROW position) -- not enough to cause unbearably loud sound.  You can
check it by switching the AUDIO RESPONSE switch to the NARROW position.  If
the level sounds approximately the same when you do this, the NFB is
connecting properly through S104 and P120/J620.

Or did you mean P120 pin 2?  That connects the output of the cathode
follower, V601B, to terminal 1 of both LINE and LOCAL pots (it also connects
through R608 to the grid of V601B for biasing).  That is definitely not your
problem -- If it were not connecting, you wouldn't have any audio or DC on
the LINE and LOCAL pots.

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