[Boatanchors] RCA AR-88 problems
Richard Knoppow
1oldlens1 at ix.netcom.com
Tue Jun 4 22:22:44 EDT 2013
----- Original Message -----
From: "mac" <w7qho at aol.com>
To: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
Cc: "Boat Anchors List" <boatanchors at mailman.qth.net>
Sent: Tuesday, June 04, 2013 6:05 PM
Subject: Re: [Boatanchors] RCA AR-88 problems
> Richard,
>
> I've had an AR88/USN RDQ for something like 30 years now.
> Not "mint" at the beginning by any means and I've had to
> dig into it several times over the years. Finally did a
> full re-cap about 3 years ago when the seals on the bath
> tubs started to let go big time. Did away with most of
> the long leads necessitated by the placement of the
> bathtubs around the periphery of the chassis by mounting
> the new components directly on the screen and cathode
> pins of the relevant tube sockets, transformer and coil
> lugs, etc., with excellent results.
>
> Have not experienced instability of the kind you report
> either prior to or following any of my forays into the
> radio, and have not given any particular attention to
> grounding considerations along the way but Dave's points
> are well taken. Your observation concerning changes when
> the (presumably metal) second RF and mixer tubes are
> touched certainly suggests ground related issues. You
> might take a big iron and try soldering the grounding
> points in that corner of the radio directly to the
> chassis as opposed to depending on bolts and star washers
> and see if that helps.
>
> Good luck....
>
> Dennis D. W7QHO
> Glendale, CA
I've just been working on it. On the top of band 4 it
breaks into white noise, not affected by either of the
trimming caps. This clears up when I put the foil around the
2nd RF. So far I've worked the mounting nuts for the trimmer
caps involved and worked about every screw I could get to
from the top of the chassis. None of this helped. I am
beginning to wonder if the new caps may be causing the
trouble. I bridged a couple in the RF compartment, one at a
time, with a .01 pf ceramic with no change but I haven not
checked them all. I don't remember if it had this problem
with the original Micamold caps. I may lay in some mica
caps and see if they make any difference.
BTW, the handbook calls for 4700 pf caps but all the
Micamold caps were 6800 pf. Perhaps for a reason. The caps
in the photographs are NOT Micamold but are RCA made caps
which I am pretty sure were mica. My guess is that the RCA
caps were in short supply.
I thought of going point to point with the new caps and
did in some places, but I didn't want to rip out the
existing wiring. I may still do this. I think the
oil-filled bathtubs were the most reliable caps available at
the time and are too big for point to point construction
although there are a couple in the center of the chassis
that are certainly there to mimimize lead length. In the RF
compartment all caps are point to point. I followed the
original lead dress as closely as I could.
I will try to get a handfull of 0.01 mica or ceramic
caps at All in the next few days and see if they make any
difference.
Just so no one is mislead I've made two modifications to
this receiver.
1, Installing a meter. I was lucky enough to obtain an
original RCA meter and mounting hardware from someone on the
RCA list.
2, I've changed the bias resistors on the first and second
IF tubes to the values in the D version and disabled the IF
gain trim pot on the front panel although its still there.
This is an F model made to go into the DR-89 diversity
receiving system. The F is essentially indentical to the D
except for:
AVC has a small positive voltage on it to overcome most of
the AVC delay voltage. Easy to disconnect and I've tried it
both ways.
First and second IF stages have their cathode returns
connected to a pot on the front panel to trim the receiver
gain.
2a, as part of the above, the first IF has a 100 ohm cathode
resistor in place of the 150 ohm in the D and the second IF
has only a 15 ohm resistor in series with the cathode and
large value cathode by-pass capacitor to ground. In the A
and D the second IF has a 180 ohm cathode resistor which is
unbypassed. I think the reason for the non-bypassed cathode
resistors in both IF stages is to compensate for Miller
effect which can detune the IF under AVC action.
3, the F also has an IF output tapped into the transformer
between the first and second IF. In the DR-89 this goes to a
"monitor" panel which is essentially a complete IF strip and
detector fixed on the IF frequency. It has a BFO which can
be crystal controlled at the IF frequency to exactly zero
beat incoming signals, or have a variable frequency for a CW
beat note. This thing also has detector current meters for
each receiver on it which is why the receivers do not need
meters.
The mods I made improve the strong signal handling
ability of the receiver and make it a bit more pleasant to
listen to because the AVC is not so agressive. All this can
be resored in about half an hour.
This is off the original subject but I measured the
sensitivity with and without the positive bias. Without the
bias, that is configured like the A and D, sensitivity in
position 3 of the bandwidth switch is just over a microvolt
at most frequecies. With the bias it drops to about 0.8 uV
for a 10db Snr. Either is very good.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com
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