[R-390] A bit more on my IF deck
JMILLER1706 at cfl.rr.com
JMILLER1706 at cfl.rr.com
Tue Mar 7 13:40:22 EST 2006
It could be a difference in tube performance, but also keep in mind
that there is AGC on some grids and you could be seeing some residual
AGC bias voltage. Also, with the higher line voltages today (120+
VAC), the B+ lines will run somewhat higher than spec. which could
change the operating points of the tubes. To compare against the
voltages in the maintenance manual, I think you should also be using
the spec-ed AC line voltage input. A variac cranked down to 115
probably. Finally, these are very high impedance circuits. If you
measure grid voltage with a standard VOM, you could be dragging the
voltage down through the meter. Need to use a very high impedance VTVM
like they had in the old days for working with tube circuits.
----- Original Message -----
From: "n4buq at knology.net" <n4buq at knology.net>
Date: Tuesday, March 7, 2006 1:01 pm
Subject: Re: [R-390] A bit more on my IF deck
To: JMILLER1706 at cfl.rr.com, R-390 at mailman.qth.net
>
> Pin 2 has a path to ground through R545, R546, and R547 which add
> up to
> 500K. The path through R544 meanders through the system and there
> are a
> few paths to ground I see; however at 2.7M, it is a bit negligible
> on the
> overall resistance. I'm thinking the 500K listed in the manual is
> theoretically incorrect, but I'm wondering how close that needs to
> be.
> Since I'm not getting the -0.4V on that pin, I'm trying to figure
> out why.
> It may just be a weak tube. I need to try some subs and also
> check that
> pin on my other R390A just to see if I see any differences.
>
> Thanks!
>
> Barry - N4BUQ
>
>
> > Let's see.... from pin 2 of V-506 it looks like paths to ground are:
> > R547 (220K) + R544 (2.7M) to ground thru whatever resistance to
> ground
> > is in the B+ line in the power supply.
> > In parallel with:
> > R234 1.5M which is on the AGC line in the RF chassis.
> > What's the math for those parallel resistors? I get about 1M,
> which
> > doesn't agree with the 500K you're reading...is there another
> > resistance to ground I am missing? Look all along the AGC line
> to see
> > if there are others in the RF deck...I can't find any others.
> > Or am I reading this all wrong?
> >
> >
> > ----- Original Message -----
> > From: "n4buq at knology.net" <n4buq at knology.net>
> > Date: Tuesday, March 7, 2006 11:46 am
> > Subject: Re: [R-390] A bit more on my IF deck
> > To: JMILLER1706 at cfl.rr.com, R-390 at mailman.qth.net
> >
> > > Oops!!! In my post below, I wrote Pin 6 of V506. I meant Pin
> 2
> > > of V506A
> > > (the grid of the AGC Time Constant Tube). Sorry.
> > >
> > > Barry - N4BUQ
> > >
> > > > Isn't V-506B the AM detector? If I am reading the schematic
> > > right, V-
> > > > 506B doesn't connect to the RF deck,... I htink it connects
> to
> > > the AF
> > > > deck (it provides audio to the AF deck).
> > > >
> > > > ----- Original Message -----
> > > > From: "n4buq at knology.net" <n4buq at knology.net>
> > > > Date: Tuesday, March 7, 2006 9:12 am
> > > > Subject: Re: [R-390] A bit more on my IF deck
> > > > To: R-390 at mailman.qth.net
> > > >
> > > > > Okay, I think I'm carrying on a conversation with myself
> here,
> > > but
> > > > > I'll
> > > > > continue...
> > > > >
> > > > > When I pull the main connector from the IF deck, the
> > > resistance at
> > > > > Pin 6 of
> > > > > V506 goes very close to 500K (what the book says it should
> > > be).
> > > > > If I
> > > > > reconnect the IF connector and unplug the RF deck
> connector,
> > > the
> > > > > same
> > > > > thing: 500K. So, I checked the resistance at pin "E" of
> the
> > > RF
> > > > > deck (the
> > > > > point where Pin 6 of V506 connects to the RF deck). I get
> > > about
> > > > > 1.8M
> > > > > here. Those two resistances in parallel yeild what I'm
> seeing
> > > at
> > > > > Pin 6 of
> > > > > V506 when everything is connected.
> > > > >
> > > > > I haven't traced it out, but I assume if I'm supposed to
> be
> > > seeing
> > > > > 500K at
> > > > > Pin 6 of V506, then the resistance at Pin "E" on the RF
> deck
> > > > > should be
> > > > > nearly infinite. Any comments on this?
> > > > >
> > > > > Barry - N4BUQ
> > > > >
> > > > > > I got the resistors replaced and all seems fine. I need
> to
> > > do a
> > > > > full IF
> > > > > > alignment, but it seems to be behaving just a slight bit
> > > better.
> > > > > I'm not
> > > > > > sure, but I think it doesn't take quite as long for the
> > > system
> > > > > to dump the
> > > > > > charge from SLOW AGC as it did. Is that perhaps a
> function
> > > of
> > > > > that 2.7M
> > > > > > resistor?
> > > > > >
> > > > > > I wanted to check the resistances and voltages around
> where
> > > I
> > > > > had worked.
> > > > > > Everything looks pretty good except for pin 2 of V506.
> > > > > According to the
> > > > > > manual, I should have about -0.4V, but I'm getting
> nothing
> > > > > (HP410B used
> > > > > > here). Checking the resistance, the manual states I
> should
> > > be
> > > > > seeing 500K
> > > > > > in AGC mode and I'm only seeing 400K here. There are
> three
> > > > > resistors to
> > > > > GND
> > > > > > from V506, pin 2 and they add up to 500K. Apparently I
> have
> > > > > something
> > > > > else
> > > > > > parallel to this that's pulling the resistance down a
> bit.
> > > It's
> > > > > always> something. Any guesses why I don't see -0.4V here?
> > > > > >
> > > > > > Thanks,
> > > > > >
> > > > > > Barry - N4BUQ
> > > > > >
> > > > > >
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