[R-390] A bit more on my IF deck

Wed Mar 8 08:46:36 EST 2006

Sorry .. I was going to post a reply on the missing -0.4 volt thing sooner, 
but assumed you had already checked for grid emissions.

Also, my knowledge is a bit spotty in this area - I'm going mostly from what 
I've learned on this reflector and I usually wait for someone more 
experienced to post a reply.  So, someone please correct/amplify on this in 
case I got it wrong:

A tube may be good in terms of no shorts and good quality, yet have a case 
of "grid emissions" - usually synonomous with "gassy".  In some 
sockets/functions, it may have little adverse effect, however it was cited 
as one of the possible causes of fouled up AGC.  As far as I understand, 
tubes are not supposed to have anything emitting from the grids at all --  
they're inputs for control voltages.  So, most likely, the gassy tube was 
putting out an errant positive voltage, cancelling out the small negative 
voltage you were supposed to be getting there.  I imagine, grid emissions 
could be caused by internal defects in the mechanics of the tube, but (I 
think) are mostly the result of the gas conducting some current out through 
the grid(s).

Practically all tube testers have a test mode for this.  Some have dedicated 
setups for testing grid emissions, others have a gas test.  Some have a 
"gas" button and you're supposed to press it during the quality test to see 
if there is any movement of the meter.  Others may have some kind of 
indicator bulb.

There was a thread way back where someone was having problems with AGC not 
working and after a lot of checking of nearly everything else, found that it 
was due to grid emissions from one of the tubes in the IF deck.

Barry

----- Original Message ----- 
From: "Barry" <n4buq at knology.net>
To: <R-390 at mailman.qth.net>
Sent: Tuesday, March 07, 2006 11:22 PM
Subject: Re: [R-390] A bit more on my IF deck

> Paydirt.  I think I have some weak/gassy/whatever, tubes.  I swapped V506
> with another 5814 in the RF deck and I got about -0.1V at pin 2.  I then
> swapped it with one of the other 5814's in the IF deck.  This tube yields
> about -0.5V on pin 2.  I seem to recall that gassy tubes can cause low 
> grid
> voltages, but not sure about that.  Also, it seems that the radio is a bit
> "hotter" with this tube arrangement.
>
> I know Roger created a writeup on swapping tubes for maximum performance.
> Maybe I'll give that a look-see.  I didn't put these tubes on the tester
> before firing this thing up.  Maybe that will reveal something about them
> too (yes, I know the *best* tube tester is the circuit itself, but I want 
> to
> see if the Hickock will tell me anything obvious).
>
> Oh well, this has been a fun exercise.
>
> Barry - N4BUQ
>
> ----- Original Message -----
> From: <n4buq at knology.net>
> To: <JMILLER1706 at cfl.rr.com>; <R-390 at mailman.qth.net>
> Sent: Tuesday, March 07, 2006 10:44 AM
> Subject: Re: [R-390] A bit more on my IF deck
>
>
>> 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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