[R-390] K601

[Roy Morgan]

At 03:07 PM 9/11/2006, Tim  Shoppa wrote:
>Rick writes:
> > When I connect the loose black and white wire to the terminal on K602
> > with the other black and white wire and press PTT I get a buzzing
>sound
> > similar to a DC relay on AC current.  It appears to be coming from
> > K601.  This does quiet the audio as it is supposed to, but there is
> > obviously a problem with the relay (K601) or the associated circuitry.
>
>K601 is supposed to be an AC coil relay.... one side is 6.3VAC from the 
>filament (transformer) winding, and the other can be grounded by the 
>transmitter through pin 9 of TB103 if and only if the break-in switch is on.

Tim and others,

Tim has it right. The relay operates on the 6.3 volt AC supply. It draws 
about 40 ma of current, AC.  Presumably, it would work on DC if rewired, 
but I don't know what voltage/current would run it correctly. (Anyone 
experimenting with this may realize that a relay pulls in at about 80 
percent of it's rated voltage. Also, running an AC relay on DC may create 
more heat in the coil than is desired.)

If you turn the front panel "Break-In" switch to the ON  position, and 
ground the correct rear terminal, the receiver should go quiet: the 
break-in relay releases the antenna relay (which action shorts both antenna 
connectors to ground) and shorts the audio from the first audio amplifier 
tube to ground.

I, also, don't know the colors of wires in there, however:

If your radio does NOT have the Squelch modification (most or all 
R-390/URR's do have it, most R-390A/URR's do not), there are some unused 
wires in that vicinity.  At least one comes from the Function Switch, which 
has an unused position, and is equipped with the needed wire(s) to the 
audio module to put in the Squelch functions if needed.  You may have 
gotten ahold of one of these wires.

By the way, the two parts hard to find for this modification are the high 
coil resistance plate relay (possibly 10 K ohms, it operates on a 12AU7 
plate current) and the plate to add to the front panel with the extra 
"SQUELCH" marking.  The switch itself is simply locked out from the last 
position by a re-position-able stop plate.

>Often when AC coil relays "go bad" they exhibit the buzzing you
>describe.

Right - if they have some shorted turns, or a poor connection to the supply 
causing low operating voltage, they will buzz. That black/white wire may go 
not to ground but somewhere else associated with the squelch 
circuitry.  Try grounding the coil terminal of the relay that does NOT go 
to the filament supply.

Re: the break-in relay coil wires:  One goes to the 6.3 volt filament line, 
and should have that voltage on it at all times (more likely up towards 7 
volts.)  The other one should show connectivity to the rear panel barrier 
strip terminal (which Tim says is TB-103 terminal 9) IF the break in switch 
is ON.

Running the radio in break-in mode requires a set of relay or switch 
contacts that are open in receive and closed in transmit.  It may be 
preferable that the contacts be isolated from the transmitter or control 
panel ground to avoid ground loops.

Roy


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing
13033 Downey Mill Road, Lovettsville, VA 20180
Phone 540-822-5911   Cell 301-928-7794
Work: Voice: 301-975-3254,  Fax: 301-975-6097
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