[ARC5] a few questions for the both of you.

[Kenneth G. Gordon]

On 8 Jan 2014 at 18:12, Joel R Roberts wrote:

> Well now that the holidays are gone for a while I am getting back to
> the rigs. I have a R-23A that is actually working after I recapped it.
> I have a question on it though. The larger Electrolytic cap C-30,
> according to the rig before I was into it, was left floating from
> ground or was insulated from direct ground and instead connected to a
> black wire over to the resistor R-27 on the resistor string mount. Is
> there a reason this was done?

Yes. That was common in some models.

C-30, 15 mfd 35 WVDC electrolytic, is the cathode bypass capacitor for the 
12A6 audio output tube. R-27 is 100 Kohms, but at this point I am not sure of 
its purpose. In any case, that isn't important. Then a lead runs from R-27 to a 
connection on the front panel connector, which is bypassed to ground with a 
.35 mfd cap.

This connection was used, as I recall, for an early version of the ILS, which, 
again as I recall, was eventually done away with.

> I may have to go back into the others
> since I don't think I left it floating.

Well, it is not really left floating when the external equipment is connected. In 
actual fact, whether or not C-30 is grounded or not makes very little 
difference in the audio output power of the 12A6. Leaving it ungrounded 
introduces a small amount of negative feedback in the audio amp, which 
enhances fidelity....which is of no real use in that receiver anyway.

If you are replacing bad capacitors, you can substitute a small modern 
electrolytic of 10 MFD or more at 35 VDC or so for it. Simply solder the + end 
to pin 8 of the 12A6, and the - end to the nearest ground.

> Next question is on the
> CBY-46106. I am still only getting Dynamotor whine from the headset,
> that doesn't change with the volume pot, and it only give a click when
> the BFO switch is flipped on and off.

Well, then you have a problem....or more than one.

> You had mentioned that the AF
> Transformer may be bad once before. Is there one that can be exchanged
> with that transformer, a modern one, or should it have the original
> ARC-5 series transformer to replace it?

It doesn't really matter,  if your receiver has been badly hacked and you have 
no plans to restore it. In fact, the small audio output transformer from an old 
tube-type AC/DC table radio will work quite well in that receiver. You can 
usually find complete All-American Five (AA5) BC band receivers in junk 
shops for less than $5.

However, any "ARC-5" audio output transformer will work in the receiver, 
although they may not be necessarily the "right" one.

Robert Downs recently posted an e-mail in which he listed the different LF 
receivers and their various audio outputs. Input impedances for all the 
transformers are close enough to the same value that the only thing you 
have to be concerned about is OUTPUT impedance.

> Would you know where to get
> one that is not already bad to begin with? I know the NOS stock is
> almost impossible to get so that is why I was wondering if a modern
> one will work.

You can still buy ARC-5 audio output transformers from Fair Radio Sales, if 
none of us has a spare one. Fair's price for one is not cheap. 

It is not unusual for AF output transformers to be bad. If someone cranks the 
gain control up against the stop without a load of some kind connected to the 
output transformer, that CAN result in a shorted or blown transformer 

>     I have an RF generator on it's way to my house soon, and I will
> probably need to rebuild a portion of it to make sure it is on the
> mark. After I am sure of that I will start retuning the BFO in the
> R-26 that I had to replace the BFO on, and then I can tune the IF cans
> after that.

I think you are working at that backwards. 

When you get your RF generator, feed a modulated signal to the receiver on 
something "reachable" like 500 Khz. with the BFO OFF. Tune the receiver to 
it.

Then connect a sensitive meter either across R-20 (2 megohms), or in 
parallel with the speaker or headphones, then having tuned the receiver to 
500 KHz or thereabouts, peak the signal by ear with the receiver's tuning 
dial.

Then having set your audio output level and your meter to about 1/2 scale, 
first peak the receiver's front panel tuning, then adjust each IF tuning slot in 
turn, beginning with the REAR one, working forward. There are two IF 
adjustments in each can under the cover, which screws off.

Keep adjusting your audio output and/or signal generator RF input to the 
receiver lower and lower to keep the meter "about" centered.

You may have to go through all the IF transformer adjustments with the 
power off with a metal screwdriver first in order to "break them loose".

They should need very little adjustment to get them right on.

There are at least two of those IF adjustments that have DC on them, so 
shorting those to the IF can covers will stop the receiver from receiving, 
although it won't hurt anything if you don't leave them shorted for very long..

Do this adjustment procedure repeatedly, without moving the tuning of either 
the signal generator OR the receiver, until there is no more improvement. 
You will probably have to do it at least 5 times, but keep at it.

After you have the IFs peaked, THEN you turn off the modulation on the 
signal generator, and turn ON the BFO. Do not touch either the receiver's 
tuning or the signal generator tuning.

There is a small hole on the left side of the receiver near the rear through 
which you can see a small slotted adjustment. There is no DC on this 
adjustment.

Using a small screwdriver, and if you don't hear the BFO, turn this 
adjustment slowly until you hear the BFO signal. Adjust it for about a 750 to 
1 Khz audio tone. This will put the BFO on one side or the other of the IF 
passband providing some "single signal" (one side of zero beat much 
stronger than the other) action.

If you tune the BFO for zero-beat, it will be centered in the IF passband, and 
there will be no, or very little, "single-signal" action.

Lastly, there are three adjustments under the outer top cover at the very front 
under the smaller cover for the tuning capacitor. Usually, these need very 
little or no "tweaking". Do not remove the small cover.

The left hole is the RF amp output or mixer input tuning, the center one is to 
adjust the frequency of the dial at the HIGH end of the tuning range, and the 
far right hole is for adjusting the LOW end of the tuning range.

However, unless you have a VERY accurately calibrated signal generator, 
you would do best to leave the center and right adjustments strictly alone.

To adjust the left one, again use your meter on the audio output, set the 
signal generator and receiver to about 500 Khz, and after first peaking the 
front panel "Align Input" control for maximum on your meter, adjust that left 
adjustment for maximum.

After all this, you are finished. 

> Lots of fun in store for me. I saw the string about
> questions on the BC-221 Frequency meter and I looked at a few. Are
> they an earlier version of the RF generator we use now?

Well, not really. They have no calibrated output level as any real RF signal 
generator would, and most do not have a choice of modulated output. They 
are called a "Frequency Meter". They can output an accurately calibrated 
signal and can also RECEIVE a signal which you can tune to and read off 
the actually frequency.

I have two of the -AK model, and find them very useful and still accurate 
(frequency wise) after all these years.

As I have said before,there are many different Army BC-221 models. Only 
the AJ, AK, and AL models have 400 Hz modulation of the RF output 
available. None of the other models have that choice.

However, ALL of the Navy's LM models to have it. Those are much smaller 
than the BC-221, although they have the exact same electronics. So, if you 
see an LM available for a decent price, grab it.

Ken W7EKB