----- Original Message -----
From: "Chuck Grandgent" <[email protected]>
To: "drakelist" <[email protected]>
Sent: Monday, December 20, 2010 11:13 AM
Subject: Re: [Drakelist] R4A no BFO
Well, since I couldn't make out the markings on the VR
tube in both my R4A
and R4B, I just drove over to vacuumtubes.net (next town
over) and got a
couple 0B2's, these with clear markings.
But, voltages in my R4A are still high, just as described
below.
So anyway, they weren't 0A2's in there, they were the
correct 0B2.
Chuck, K1OM
On Sat, Dec 18, 2010 at 6:40 PM, Chuck Grandgent
<[email protected]> wrote:
I dusted off the R4A that I retired maybe 3 years ago
when i got an R4B, as
I recall it WAS working fine when I put it on the shelf,
but I could be
wrong.
Signals seem plenty strong. but no BFO though signals
seem plenty strong,
sounds like it's ALMOST oscillating.
It's the 11-tube mode, serial 6987G.
Other notes: My DVM is cheapo, so not high impedance. My
scope is on the
fritz.
I run the Drakes off an autotransformer, so 110VAC not
120VAC.
I have Garey's new R4A CD.
Took base voltage readings (see below), plate on V5 and
V12 were quite a
bit higher than they should be,
so I thought to myself, whoa, maybe an OA2 is in there
instead of an OB2.
So, I took the OB2 from my R4B, but this didn't change my
readings, but
actually, can't make out any markings on either of the
tubes.
Now I'm worried both receivers had OA2 in there instead
of OB2, but I find
it doubtful that both would be wrong, and the R4B has
been working fine for
several years.
I have the voltage readings from when I worked on the AGC
back in 2008, and
they were fine then.
V5 pin 1 s/b -1, was -18
pin 5 s/b 110, was 146
pin 6 s/b 70was 95
V8 - haven't been able to check that one yet, hard to get
the probes in
there
V9 pin 1 s/b -1 was -21
pin 5 s/b 128 was 144
pin 6 s/b 110 was 120
V11 (OB2) - haven't been able to check that one yet, hard
to get the probes
in there
V12 pin 1 s/b -44 was zero
pin 5 s/b 55 was 128
pin 6 s/b 55 was 128
Besides these voltage anomalies, what else might I check,
maybe D11, D12
for the BFO Q6 2N3394 ?
Chuck, K1OM
You might want to borrow another voltmeter. Its just
possible the meter is out of calibration, if it is you are
just chasing your tail.
Most modern meters, either electronic or passive, have
high enough impedance for DC not to give false readings and,
in any case, if the impedance (or rather resistance) is too
low the voltages will read low, not high.
For the most part gas regulator tubes are pretty close
to the nominal value, within a volt or two. If you read high
right at the tube plate the meter is wrong. The voltage
might be high if the tube is not conducting but that would
be very obvious because it would not be glowing. You can
double check by pulling the tube out and measuring again. If
the tube is applied correctly there should be _no_ voltage
on the regulated line when it is removed because the tube
has a built-in jumper for just that purpose (i.e., to
prevent excessively high voltage from being applied to the
regulated stages. If the tube is glowing its working.
--
Richard Knoppow
Los Angeles
WB6KBL
[email protected]
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