[ARC5] Re: Question about ARC-5 mods

[k0ng at inebraska.com]

Brad et al: The crystal socket could be for a "marker crystal" which would
be excited by external RF. When the RF is on the crystal frequency, you
know where you are!! Could also be a Q multiplier (RF) input. The RCA jack
could be either an "IF take-off" used to feed other receivers like for dual
conversion or a Q multiplier input (IF). Pure speculation but free.
73 DE Charlie, K0NG  ..

Quoting Brad Hernlem <alihernlem at hotmail.com>:

> 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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