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

[Craig Heaton]

Good Morning Charles & all,

The Line pot is still a 2.5K log taper, 5K pot (Local) that was there when I
brought it home was a log taper. The 5K has been replace since, with a 2.5K
log taper from the junk pile. As of yesterday, both pots are good.

Getting the Motorola & Amelco upside down at the same time is a pain in the
backside! But been there before. I hate trying the same thing over and
expecting different results, if you know what I mean. 

Next trip to the shack I'll lean towards a broken wire in either of the two
plugs which connect to the AF module. Don't have a TM in front of me, but a
shot in the dark..............P120 pin #15. Goes from the grid of V602 thru
S104 to terminal #1 of both Line & Local pots? Checking the electrolytic
caps in the Amelco AF module isn't too much of an issue. No extras on hand.

Off to do house work,

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: Wednesday, January 01, 2014 9:23 PM
To: 390 list
Subject: Re: [R-390] Today's Line & Local experiment

Craig wrote:

>Might add at this point, the beast arrived in this condition. R105 was 
>a 5K pot when I brought it home and started restoration. So, it has 
>been too loud for some time for other owners.

Changing R105 from 2.5k to 5k would not lower the volume, so it is unlikely
that's why it was done.  But since there is evidence that the pots have been
monkeyed with, it is worth checking to make sure both of them are still log
(audio) taper pots, not linear.

According to your data, with an antenna connected, the Amelco is less than 5
dB louder than the Motorola (4.5v vs. 2.6v).  We would not expect a 5 dB
difference to be characterized as unusually loud, or "blasting," or "have to
turn the RF gain down even with the audio pot at zero."  This suggests that
the problem is after the volume control (i.e., V602/603/604 and associated
circuitry).

At this point, I'm inclined to suspect a bad decoupling capacitor, as
suggested previously by Steve and Drew.  In particular, C603B.  C603 is a
plug-in cap, so you can easily swap in C603 from the other AF deck.  That's
the first thing I'd check at this point.

Best regards,

Charles



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