Steve -
Did you reset RCVR SENS to -1.35 VDC at TP2 AFTER changing V1?? The bridge should balance well
within range with about 800 ohms in the two resistive legs.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Steve Wedge wrote:
It just wasn't one of the tubes in the bridge.
Garey's recommendation to change V1 did the trick. My tube tester had passed this, but the
minimum transconductance allowable bu the tester chart does NOT get you optimum performance!
As an example here, the 6BZ6 has a minimum Gm of 1575 uMhos in the tester rollchart, but a new one
is spec'ed with a Gm of 6100 uMhos.
Obviously, there's a major difference here, and it's apparent that the RF amp that was in this
receiver wasn't performing to new specs. In fact, even before re-doing the S-meter calibration
again, I could hear and see (on the meter) the difference in performance.
The meter's calibration is still near the edge, but it's no longer at the edge. The receiver is
also "hotter" now and the meter is certainly more "lively". I don't know why it hadn't occurred
to me that if I had less signal coming in that this could affect the meter's calibration. Thanks
for setting my head straight.
73,
Steve Wedge, W1ES/4
Live free or die: Death is not the worst of evils.
John Stark.
All my computers have my signature with various pearls of wisdom appended
thereto.
--------------------------------------------------
From: "Garey Barrell" <[email protected]>
Sent: Thursday, January 12, 2012 9:55 PM
To: "Drake List" <[email protected]>
Subject: Re: [Drakelist] S-meter balance: ain't the tubes.
> Couple more possibilities....
>
> Since 'someone' has been 'fixing' the meter, is it still ~ 3mA full scale & ~
700 ohms resistance?
>
> Also, try a new 6BZ6. I have seen cases where a gassy 6BZ6 will drag down
the AGC2 buss, screwing
> up the RCVR SENS (R87), which screws up the AGC buss, which ... you get the
picture. Each time you
> swap the 6BZ6 recheck for -1.35V at TP2. It may take a couple of iterations.
>
> 73, Garey - K4OAH
> Glen Allen, VA
>
> Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
> and TR-4/C Service Supplement CDs
> <www.k4oah.com>
>
>
> Steve Wedge wrote:
>> R-4B, s/n in the 9000's (I'm too lazy to peek around the back). Had
previously fried R89 and the
>> S-METER ZERO pot and had to replace the pot with a 200-ohm padded with a
390-ohm CC. It barely
>> reaches S-1 with the pot maxed still. The S-METER SENSITIVITY pot which is
supposed to be 1k was
>> reading 600 ohms, so I padded it with 390 ohms. I can't get the needle to
reach 60-over with the
>> RF GAIN fully CCW.
>> I tried replacing V4 an V5 with NOS tubes and there was no change. V2 and
V3 were likewise
>> replaced with no improvement. (V2 was already a new 6HS6 anyway but I
swapped to be sure). Put
>> all the original tubes back in.
>> It's a simple bridge, fercryinoutloud! What am I not seeing?
>
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