[R-390] 60 Hz on Audio - Cause Begins to Reveal Itself

[Jim M.]

This is a stab... check for poorly soldered or unsoldered connections
especially on tube sockets.  I once fought with an IF deck misbehaving and
discovered several "forgotten" solder joins in the deck - wires were crimped
onto the lug but never soldered! Either poor QC or poor eyesight on the part
of the mfgr (which was non Collins).

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of w9ya
Sent: Tuesday, June 16, 2009 11:46 PM
To: Jon Schlegel
Cc: r-390 at mailman.qth.net
Subject: Re: [R-390] 60 Hz on Audio - Cause Begins to Reveal Itself

I *really* think you have a ground loop issue. To 'see' this, use your scope
probe to look at signals between various measuring points ALL of which are
suppose to be at ground potential. When you see signal between two parts of
the cirucuit (and/or chassis/chassis connected parts) that are suppose to
have the same signal, you are 'looking at' a ground loop. And you are
propagating this signal just like it was an intended part of the signal
path.

And go from there.

Vy 73;

- Bob
w9ya


On Tue, Jun 16, 2009 at 8:22 PM, Jon Schlegel
<ews265 at rochester.rr.com>wrote:

> Hi All,
>
> After much experimentation,  it's beginning to appear as though a
> large part of the 60 Hz I see on my IF module is related to coupling
> of AC heater power into the signal path within the
> tubes.  Heater-Cathode shorts don't seem to be at issue.  It also may
> not be simple Heater-Cathode leakage either.  I don't have enough
> experience to make this kind of call.  All I can say is that of the
> tubes I have, none showed Heater-Cathode problems when I tested them.
>
> DC Heater power without doubt eliminates the 60 Hz problem so AC
> Heater power is clearly at issue. Generally, what I've observed
> beyond that is that the 60 Hz level stays reasonably low until the IF
> module starts to go into AGC.  Nevertheless, I haven't been able to
> find any 60 Hz on the AGC bus.  Under these conditions then, the 60
> Hz can increase to a level as high as almost 30 dB below a 30%
> modulated tone. It's clearly visible riding on top of the tone as
> shown in the example scope photo.
>
> <http://www.vecnav.com/R-390A_IFmodule/DiodeLoadOut_8kHzMechFilter.jpg>
>
> After going through a lot of effort to eliminate possible heater
> wiring issues; return current in the chassis, heater supply wires in
> wiring harness bundles, apparent sensitivity of mech filters to AC
> heater power, etc, I realized that 60 Hz level was really dependent
> on the specific 6BA6 tube in V502 and V503.  My setup at this point
> had evolved as described and I used it to gauge the sensitivity of
> the 6BA6 tubes that I have on-hand to AC heater power .  All on these
> tubes are old and no doubt well used.  Two checked poor on
> transconductance but the remaining seven checked ok.
>
> Here's the setup.
>
> (1)   IF module run separately from receiver via test fixture and
> regulated B+ supply.
>
> (2)   30% modulated 455 kHz signal injected into V502 Grid via a
> network that simulates the mech filter normally driving the grid.
> Network also allows for  injection of normal AGC voltage into V502
> Grid.  This eliminates the mech filters, switching/grounding
> circuitry and any influence they may have.  Set 455 kHz RF level for
> about negative 5 VDC on the AGC bus.
>
> (3)   DC Heater power supplied to V502 only.  Normal AC heater power
> supplied to all other tubes in module.  This hopefully eliminated any
> 60 Hz contribution by the V502 stage.
>
> (4)   V503 socket on IF module used to test each of the nine on-hand
> 6BA6 tubes for sensitivity to AC heater power.  Each 6BA6 was plugged
> into V503 socket and then a relative measurement was made of the 60
> Hz level referenced to the 30% modulated tone.
>
>
> Here's the list of each tube's 60 Hz level relative to the tone.
>
> -59 dB
> -62
> -43
> -42
> -39
> -33
> -42       (Poor Gm)
> -38
> -44       (Poor Gm)
>
>
> Seven tube samples are grouped in the -33 to -44 range while two of
> the samples are around -60.  On the average, these two are 20 dB
> better than higher valued group.  (Surprisingly, the best of the
> bunch at -62 has the name "Delco" printed on it!)
>
>
> *******************
> Can Heater/Cathode leakage be blamed on the seven poor performers or
> is something else happening?  Is there some sort of cross-modulation
> thing going on?  A clue here may be that the 60 Hz really only shows
> up when the module goes into AGC despite the AGC bus being
> clean.  Can NOS tubes be had that aren't beat up like all of my samples?
>
> *******************
>
> Thanks for the effort of reading through all this gobbledy goop .  I
> did my best to keep it as short as possible but still make sense (I
hope!).
>
> Regards,
> Jon
>
>
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