[Boatanchors] AVR-20 output transformer wiring?

[Ian Wilson]

There are no connections to the plate or screen of the 6B8
other than the 220k to B+ and 1M to B+/0.033uF to ground,
respectively.

'Scoping the plate of the 6B8 I see healthy signal levels
(up to 20v peak). There are some tens of mV on the phones,
so I think this is just capacitive pickup.

I can't find the primary leads from the output transformer;
the secondary leads are still connected to the phone jacks.
C16 (should be across the primary of the o/p xfmr) is gone.

Will try Robert's suggestion (cap from plate resistor to high
impedance phones,  remove 1M screen resistor). I will
reduce the plate resistor also, since the cathode voltage of
the 6B8 is used for AGC delay.

There's no room in here to add a different output transformer.
Would have to remove the existing one (and getting to the
fixings isn't trivial).


73, ian K3IMW

On Sun, May 17, 2015 at 11:04 PM, WA5CAB--- via Boatanchors <
boatanchors at mailman.qth.net> wrote:

> I have a manual that includes the AVR-20A.  I don't know what the
> differences are between an AVR-20 and an AVR-20A.  One thing that I
> noticed from the
> schematic is that the AVR-20A may be wired for 6VDC or 12VDC.  The photos
> in
> the manual won't answer your question.  Disconnect one end of the 220K and
> check resistance between power plug pin 2 and 6B8 plate (that's either pin
> 3
> or pin 6).  If you get an open reading, the transformer primary may be your
> problem.  However, if that's the case, reconnect the 220K and jumper out
> the 1 MEG and it should work much better.  You'll need a HI-Z headset
> (HS-23)
> or loudspeaker (LS-3 or LS-7).  Oh, and a 0.1 ufd from plate to headset
> jack
> (which is shown dotted on the schematic).
>
> Robert Downs - Houston
> wa5cab dot com (Web Store)
> MVPA 9480
>
> In a message dated 05/17/2015 22:40:55 PM Central Daylight Time,
> ianmwilson73 at gmail.com writes:
> > I picked up an AVR-20 (OK, this is an anchor for a tiny boat).
> > Electrically this is in good shape except that someone has
> > performed magic rites on the output circuit. There is some
> > audio output but it is very low and has a pretty high LF
> > cutoff.
> >
> > The phone sockets show reasonable DC resistances to ground.
> >
> > The plate/screen grid circuit of the 6B8 has been hacked; there
> > is a 220k resistor from the plate to B+ and a 1M resistor from
> > the screen to B+.
> >
> > I cannot identify the connections to the primary of the output
> > transformer underneath the chassis. Just too many wires. And
> > the regulator subchassis blocks some of the view.
> >
> > If the transformer is o/c, I may have a replacement. But I
> > don't want to start on large disassembly jobs until I can take
> > some measurements.
> >
> > Does anyone have a picture/description of where the
> > primary leads run?
> >
> > Thanks for any info,
> >
> > 73, ian K3IMW
> >
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