[ARC5] Re: Question about ARC-5 mods

Sun Sep 12 23:27:11 EDT 2004

Neil, Tom and the list,

Thanks for your input on this matter. I should point out that the receiver 
that I am trying to salvage is one that seems to have been modified a number 
of times and perhaps by multiple owners. It was acquired in an appalling 
state of filth, bodily injury and corrosion. At first I thought it was worth 
its cost for parts but curiosity got me wondering a) what ARE all these mods 
and b) is it still possible to get functioning.

The mods included the following that I have been able to determine and 
discover: the conversion of the rear connector to a 5-pin socket, the 
conversion of the dynamotor port to an octal plug (which I believe is meant 
to power an outboard complementary device (converter?), installation of a 
crystal socket in the left front side which leads to a 16-turn winding on 
the coil in the antenna coil can, installation of an RCA jack in the side, 
rearward but forward of the 3X.22 uF tub which leads through a small pF cap 
to nowhere, the rewiring of the heaters for 12.6 V operation, the 
introduction of a phone jack in the front panel as well as a BFO switch and 
gain control (with power switch that turns on heater power to the octal 
plug), conversion of the antenna terminal to "UHF" coaxial connector, 
conversion of the tuning spline to 1/4" dia. shaft, and probably some other 
things not discovered or were mentioned in my previous post (or not coming 
to mind just now).

I checked the output transformer and found that it is still functional 
despite being possibly stressed by the new bias conditions. The transformer 
is marked "ES-691027" and has 1.16K resistance from pins 1-6 and the hi-Z 
pins 3-GND (304 ohms DC resistance) are used for the phones output so 
despite the new bias conditions, a speaker output transformer was NOT 
swapped in, in this case. I have no intention to put in a new output 
transformer so I suppose that I will unmod the cathode and G1 resistor 
changes.

I timidly fired up the receiver to see whether it would work. First I fed 
the heaters with a 12.6V storage battery and verified the current at about 1 
amp. Then I used my bank of 12.6V gel cells to step up the B+ until I could 
hear anything, if anything. With 4 cells the audio began to come to life ... 
but it was intermittent. The sound would build a bit, then a "pop" and a 
pause before the sound came back. I suppose some caps are not in very 
healthy condition. The lower panel had crud on it below the 3X.22uF tub when 
I first open the receiver.

Interestingly, the gain control is wired so that one side of the 10K unit 
goes to the 3 uF forward tub cap, the wiper to ground and the other side to 
the "hot end" of the "ALIGN INPUT" trimmer cap.

Thanks again for the input.

Brad
KG6IOE

>From: Neil Barnett <neilba at clear.net.nz>
>To: ARC5 at mailman.qth.net
>Subject: [ARC5] Re: Question about ARC-5 mods
>Date: Sun, 12 Sep 2004 22:36:56 +1200
>
>
>   ----- Original Message -----
>   From: Tom Bridgers
>   To: neilba at clear.net.nz
>   Sent: Sunday, September 12, 2004 1:25 AM
>   Subject: Question about ARC-5 mods
>
>
>   >Neil...
>   >The analysis you gave of the ARC-5 mods seems to suggest that the 12A6 
>audio stage can be tweaked enough to provide good listening levels >through 
>a loudspeaker.  I believe that also meant one needed to change the output 
>transformer to one having a 4 ohm or 8 ohm speaker secondary.
>
>   Tom, you're correct in that assumption.
>   Under the biasing conditions for loudspeaker operation given in
>   my earlier post, the 12A6 will be properly matched if it sees a
>   reflected load impedance of 7.5 K ohm.
>
>   Any single-ended (not push-pull) tube type of output transformer
>   having a 5 K to 7 K primary and designed to operate into a 4 ohm
>   or 8 ohm 'speaker will be satisfactory.
>
>
>   >Now my question is this:  What about going on step further and changing 
>the 12SQ7 triode section from being the BFO oscillator to one being the 1st 
> >preamp of audio driving the 12A6?  How does the 12SQ7 audio mod improve 
>the audio output and how good or worthwhile would that mod be?
>
>   >I've been torn between doing the 12SQ7 audio 1st stage mod, and then 
>adding a transistor circuit to do the work of the former BFO stage.  
>Because >you do need the BFO to receive SSB.
>
>   >And of course, there is the reverse of the above scenario.  Keep the 
>12SQ7 triode section as a BFO, but add a one or two stage transistor preamp 
> >audio stage to drive the 12A6.  I don't know if such is possible, but the 
>thought does rattle around in my brain!
>
>   >Thanks in advance for any help on this or reference material you might 
>point me to.
>   >-Tom
>
>   Firstly, I think modifications should be kept to a minimum,
>   unless they are essential for putting a piece of equipment into
>   running condition.
>
>   Getting rid of the BFO is a step you'll regret. As you've pointed
>   out, the BFO is essential for receiving SSB (and CW) and it is
>   also a useful zero-beater for tuning AM signals accurately.
>
>   I'm not convinced that its necessary to turn the triode section
>   of the 12SR7/12SQ7 into an audio pre-amp. Its been a long
>   time since I owned an ARC-5 receiver, but as I recall, the 12A6
>   was capable of delivering plenty of audio, and didn't need a
>   pre-amp.
>
>   I think you'll find the audio output to be more than adequate if you
>   do the following:
>
>   1. change the 12A6 cathode resistor to 390 ohm, 5W.
>   2. disconnect the 12A6 cathode bypass capacitor, and solder in
>       a 47uF,  25 volt electrolytic instead.
>   3. replace the original output transformer with a 5 - 7 K to 4 ohm
>      or 8 ohm 'speaker version.
>
>   If there's still not enough audio, you probably have an unrelated
>   problem in some other stage, or the 12A6 is bad.
>
>   In my receiver years ago, I replaced the 12A6 grid resistor (2 Meg)
>   with a 2 Meg volume control pot mounted in the front panel control
>   box. An even better arrangement worth trying is a 1.5 Meg resistor
>   in series with a 470 K pot, with the 12A6 grid connected to the pot
>   slider.
>
>   Those values retain the original grid loading of 2 Meg, and mean
>   that the 12A6 grid will never be more than 470 K above ground, but
>   there may not be enough audio appearing at the 12A6 grid in this
>   setup. Its worth trying though. How sensitive is your sixty-five year
>   old 12A6 ?
>
>   73 de Neil, ZL1ANM
>   in Auckland.
>
>
>
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