[ARC5] Restoring a BC-455-b, request advices

[Kenneth G. Gordon]

On 1 May 2016 at 10:09, Paolo Gramigna wrote:

> I have seen several posts in the web suggesting to replace the 
> capacitors, especially the electrolytic ones, with modern ones. That 
> will cause big detriment to the authenticity, but probably will be 
> beneficial to the functionality.
> 
> What is your wise advice?

Hello, Paolo: It is nice to see you here.

My advice is to completely replace every capacitor in the receiver. Any which are not 
already shorted, will short out as soon as you apply any normal operating voltage. The 
originals were rated at no more than 300 VDC, were paper (other than the three 
electrolytics) and there are several reasons why now, some 70 years later, they fail 
repeatedly.

As Glenn mentioned, replacing the capacitors with modern, good, ones will bring the 
receivers back to their normal very high performance. You can see my work with at least two 
such receivers on youtube, beginning here:

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

I have other videos of the BC-453 and of a BC-454 also.

My receivers were in very bad condition when I began my work, so I saw no reason to 
attempt to put them back to factory condition.

As Ian mentioned, it is certainly possible to operate the receivers at a +HV level well below 
the normal +250 VDC which is output by the dynamotor. Some users have operated them at 
voltages as low as +24 VDC. At that low voltage, however, the audio output level is quite 
low, and it takes the receivers a much longer time to warm up.

At that low voltage level, it is advisable to move the screen voltage feed from the junction of 
the two black screen-voltage divider resistors mounted vertically in the rear to the high 
voltage connection point in order to increase the screen voltage on the tubes to a more 
normal level.

I have found in my tests that a voltage of about +180 VDC seems to be the best 
compromise, though, and when I use that voltage level, I also move the screen voltage feed 
to the high-voltage end of the divider-string.

In order to preserve the original appearance of the receivers while still obtaining the superb 
performance of which they are still capable,one should, as Robert stated, "re-stuff" the 
"can-caps", or "flower-pots", then carefully reinstall them.

Here is a web-page I published some time ago of my version of how to do the restuffing:

http://www.w7ekb.com/glowbugs/Military/PDF%20files/ARC5/Restuffing.html

In addition, there were two recent articles in Electric Radio Magazine on how to do this also, 
using slightly different methods.

Oddly, I have found that the three electroytic capacitors in almost all of the recievers I have 
refurbished have been still good. Even so, I do replace or restuff those with modern ones in 
the interests of reliability. These electroytics are 1) C-5, the 3 mfd RF gain control buffer 
capacitor, 2) C-30 the 15 mfd audio amplifier's cathode bypass capacitor, and 3) C-32, the 
screen voltage filter capacitor.

Some folks have thought that the audio output level from the restored receivers is "low", but 
I and others have found that if the output impedances of the receivers are properly matched, 
the audio output level is quite enough to rattle the windows.

The audio output transformers have become a "problem" in many cases in recent years, and 
have begun to fail if not used properly, so that is something you may wish to watch for.

There are inexpensive modern transformers which can be used to replace a dead original, 
but they are not the correct "form factor" and mounting them can be a problem.

I connect my external home-brew power supplies to the recievers via an old dynamotor 
bottom plate, which preserves the recievers from being butchered.

Fair Radio Sales made and sold in the 1970s two items which if you can find one would 
make your use of the receivers considerably easier. 

One such was a complete power supply mounted on a dynamotor base, which simply 
plugged on to the dynamotor deck using the original clamps. This used a special 
transformer which Fair had made especially for them. 

Secondly, and the best solution yet, was a box containing the power supply, a speaker with 
impedance matching transformer, and all the controls for volume and BFO control which 
connected to the receiver thorough a patch cable ending in the proper ARC connector to 
plug into the power/control connector on the rear of rhe receiver. This prevented having to 
make any modifications to the receiver at all.

I have two of those here, and I am sure there are others available other places.

Lastly, be aware that all the resistors in the receivers are NOT covered with a layer of 
phenolic as most other resistors of the period were. This means that any resistor which has 
HV on it can be damaged if the BODY of the resistor is accidentally shorted to ground, so 
be careful. In addition, if any of the capacitors is shorted when HV is applied to them, an 
associated resistor can be damaged, so check them as necessary.

Anyway, good luck with one of our favorite receivers.

vy 73 for now,

Ken W7EKB