[ARC5] BC-453 B dynamotor hum

[Kenneth G. Gordon]

On 31 Oct 2015 at 23:52, J Mcvey via ARC5 wrote:

> Hello all,
> Been tweaking this beacon receiver in preparation for the 600m event in
> November. Peaked the IF's , but the dial is off about 2khz or so.Is this going
> to be hard to calibrate/?

No, but you will have to remove the outer case to get to the calibration 
trimmers. Remove the silver screws, and loosen the black ones. Do you 
have a manual, and a signal generator? If not, I can help with the manual.

> Maybe I should leave it alone?

You could, but it isn't all that difficult to "tweak" it. Besides, once you get it 
calibrated, it will stay for the rest of your life.

>  I 'm wondering if
> it's worth the trouble. This radio is running on the original 24 V config. Also
> matching the the output for best results using a 70V audio line transformer
> which seemed to be a toss-up between the 0.625W and 1.25W taps on this unit. Now
> the only thing that REALLY bugs me is the constant dynamotor tone in the audio
> it sounds like 120 to 200 hz ( its really loud when the gain is turned down).
> Whats the most likely culprit?

There are several:

1) there are bypass caps inside the end-bells of the dynamotor. You should 
check those for condition and value. I have found several which were bad.

2) ARC issued an MWO near the end of WWII which addressed this issue: 
they mandated two pieces of copper strap which were to be connected 
solidly to the receiver-chassis on one end, and were to make solid contact 
with two of the dynamotor's snap-slides on the other. They were generally 
placed on diagonally opposite corners of the base.

The idea was to solidly connect the dynamotor and its base to the receiver 
chassis. That small ground wire doesn't do an adequate job.

3) Make sure the end-bells make good contact with the rest of the 
dynamotor. I use a bit of DeOxit to make sure.

> I had already replaced the potted caps with
> modern ARC pots from the 60's (?) which have white plastic tops. They tested
> good with very low ESR so I assumed that these are probably ceramic inside. Was
> this an error?

Probably not.

> Any suggestions before I start hunting and pecking?

See above. :-)

> BTW, this radio works extremely well with my 160 M dipole> I have stations all
> over the dial. Picked up WD2XSH/38 which is reputedly only running 0.25 W in NH
> and I'm in southern NY.It doesnt like the shorter ones as much which kind of
> surprised me because an airplane is just so long...Didn't they work all these
> ARC5 radios off of the same antenna?

Yes. At the same time too.

BTW, I posted a youtube video of a BC-453 I overhauled. My HP-8640B 
showed the sensitivity of 1 microvolt in CW. Go check out the videos.

Here is the link:

https://www.youtube.com/watch?v=vlz83QXM66A

Ken W7EKB