[MRCG] GRC-9 repair

[Tim]

Hi Craig - A few thoughts....
Receiver throughput - is the bias battery in place but possibly shorted or
otherwise presenting a low impedance path to ground for signals on the AF
amp grid?  If you remove it, do you see any change in audio output?  (For
low volume levels the bias battery is not needed but with high volume
levels proper bias will reduce AF amp distortion).  Other than that, I
would just trace signal flow through the receiver..  A simple "bush trick"
is just touch a random wire antenna to the grid of the first IF amplifier
and see if you get lots of noise downstream via subsequent stages (thus
ruling them out somewhat).
Another thing I have encountered is "tin whiskers" growing between the
tuning cap plates, shorting them together.  On mine, a whisker(s) shorted
out the local oscillator tuning cap section - no LO, no receiver...Try
running a business card between all the plate gaps to clear them - they can
be practically invisible..compressed air may, or may not knock them off...

Transmitter:  That neon lamp could glow (off resonance) due to the high
voltage "keep alive" bias that is kept on the lamp to make it more
sensitive to tuning when RF is also present.  Gets much brighter when
resonance is achieved..

Also, and this may sound stupid, but verify that the TX band switch *knob*
has been installed correctly.  There is a possible 180 degree ambiguity in
its rotational position on the shaft.  If it was installed backwards, the
transmitter will drive you nuts - nothing will work, or even make sense.
I've see that problem on 2 different sets.  A real hair-puller...

On your previous question on the RX filament voltage from the DY88:  I have
not measured the "system operating" voltage on the DY88 RX filament output
but you have seen 1.8 VDC on 3 different DY88's.  That may be normal, and
as you suspected, the RX filament selenium rectifier "regulator" may need
that somewhat higher source voltage to conduct properly and then dropping
1.4 VDC for the filaments...  I'm thinking of changing my selenium
rectifier to silicon just for peace of mind....

Have you tried powering the receiver from a D cell and 10 nine volt
batteries? To rule out the DY88?  Otherwise, are the voltages at various
key points in the RX proper?  Shorted or leaky bypass caps will of course
pull that down somewhat...Leaky coupling caps will raise havoc as well. (I
have 2 GRC-9's with all the original caps installed and they both work
great...not seen any cap problems in these 2 sets, probably just
lucky..).....Keep us posted!

Tim
N6CC

On Mon, Jun 15, 2015 at 12:15 PM, Craig VonIlten <
craig.vonIlten at trustautomation.com> wrote:

> All,
> I am going through a GRC-9 for a relatively new ham with his General
> ticket (Bryce Dilger, KK6PTR).  He is getting hooked on old military gear
> (N6CC site is like heroin).  I'd like some repair pointers from some of you
> who know this radio better than I do.  I know they're tough...so there may
> be a common failure mode.  I have checked all of the tubes with an
> I-177...and they are all good.  Here are the symptoms when powered by a
> DY-88:
>
> Receiver
> It produces some very mild hiss and pop when switching bands and adjusting
> RF and AF knobs, but it isn't passing RF.  The output volume (hiss) into an
> LS-7 is VERY week with RF and AF cranked all the way up.
>
> Transmitter
> Keying up the transmitter in AM phone, lights up the antenna load
> (resonance) indicator, but it isn't really loading the antenna...I didn't
> have a scanner...but went up and down the dial on another radio near the
> intended xmit frequency and did not pick up any signal.  If operating
> normally, you can hear the radio load up the DY-88 when the antenna is
> resonant and you key the transmitter.  In this case, the DY-88 is not
> getting loaded down and the antenna resonant indicator is illuminated
> regardless of the antenna tuning knob position (yes...the knob is connected
> and moving the inductive matching coil/core).
>
> Any ideas?  Specific capacitors to check first on the hit list?
>
> Also...if any of you would be willing to take a look at this radio and
> identify the work needed, Bryce would be willing to pay you for your time.
>
> Thanks!
>
> Craig
> N6CAV
>
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