[R-390] A bit more on my IF deck
n4buq at knology.net
n4buq at knology.net
Tue Mar 7 18:15:42 EST 2006
Dennis,
If you selected the "good" filter positions, did the resistance return to
its correct value? I see where the AGC line ties to the filters, so that
is a good possibility.
Thanks!
Barry - N4BUQ
> Barry,
>
> I am at somewhat of a disadvantage being at work with none of my
> diagrams, but your problem sounds very much like the one I was having
> with my Moto IF deck when I was trying to track down my AGC problems.
>
> Where I got to was a leaky mechanical filter. I got there by
> lifting the ground side of the filter and watching the resistance to
> ground go right back to where it needed to be. I know its kind of
> vague at the moment..but when I get home I hope I can give you more
> detail. But in the meantime, don't over look the possiblility of a
> leaky filter(s). I ended up with two.
>
> Dennis
>
> On 3/7/06, JMILLER1706 at cfl.rr.com <JMILLER1706 at cfl.rr.com> wrote:
> > Barry
> > I just noted you are using a 410B VTVM which should be fine. So forget
> > that part of my comment. I can suggest no other reasons you are not
> > getting -0.4... but just be sure you are reproducing the exact test
> > configuration the manual calls for. Oh, another thing that can affect
> > B+ voltage (and operating points) is a conversion to solid state
> > rectifiers, unless you have used a 220 ohm dropping resistor.
> >
> > ----- Original Message -----
> > From: JMILLER1706 at cfl.rr.com
> > Date: Tuesday, March 7, 2006 1:41 pm
> > Subject: Re: [R-390] A bit more on my IF deck
> > To: "n4buq at knology.net" <n4buq at knology.net>
> > Cc: R-390 at mailman.qth.net
> >
> > > 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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>
> --
> Dennis L. Wade
> KG6ZI
> Carmichael, CA
>
>
>
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