BC 221 Frequency Meter tidy up

[Rod Salter]

Part one

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BC 221- J Frequency Meter Serial No 3156

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Cabinet # 17024
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Calibration book for MC177 – AK serial no 70
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I recently attempted getting a crystal calibrator for a Wireless set 19, but the cost of postage from the US to AU was extraordinarily expensive. More than the unit itself.

Then I thought that the crystal calibrator would probably only work with the 19 set anyway, this got me to think about a BC 221 Frequency Meter I had.

I personally can not understand the naming of the unit as there is not a 'Meter' on it. So be it.

I dusted the unit off and removed the chassis, it was reasonably clean although, like most of the radio gear I have, it had been under in a flood.

Using a compressor I brushed and blew out the dried dirt.

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The cabinet was hosed out and then lightly sandblasted after which a thin touch-up spray coat of crinkle black paint was applied.

[Rod Salter]

Next came the testing of the unit itself, the headphones must be plugged in to 'Switch on' the unit.

Armed with a circuit I checked the filament circuit connections and applied 6 volts from a truck battery.

The leads soon started to give off “The Smoke”

After some poking around I found pins 2 and 7 of one of the valves were bridged together, snipping that allowed the valves to warm up. Good so far.

Next was the B+, I used a dropping resistor connected to a mantle set as the power source.

Some testing with a shortwave radio and a Digital frequency counter, I found - 'nothing'

There is re-soldering on the output valve's socket, the cathode resistor had come adrift.

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This resistor and some others are not originals.

Resolder and more testing, Bingo, a signal although it seemed weak.

I left the valves on without the B+ while I had dinner when I got back I sensed the output valve was barely warm compared to the other metal valve.

It was a 12 SJ 7 for a 12 volt system, swapping it for 6 SJ 7 brought a hot valve and a good signal.

I manufactured an aerial connection from a .22 brass shell and a spare screw.
Putting it back in the case and powering up, it works well.

Some of you may have noticed the calibration book was not for this unit, believe it or not, the calibration sequences are spot on!

A power supply - in the next post.

[Rod Salter]

Next for my attention was a more permanent power supply, batteries not being available any-more.

I could make up a battery bank of sorts but with infrequent use, this would be waste of money and there is always the risk of them leaking and therefore corroding everything.
I can do without that.

I suppose my Dad's radio room alive with the buzz of battery vibrators is the cause of my obsession with the things. I have collected many rotary power supplies and vibrators over the years.

I even have a supply for a Spitfire, but as I see two more coming up in an auction, there goes the value for that up in the air. There is one for £2000 in England, but I don't suppose there are many Spitfires left needing a genuine war radio.

I digress.

I have a few power supply/audio amp chassis from car radios where the tuner and volume control is mounted in or under the dash but the supply, amp and speaker are mounted on the firewall.
I selected a 6 volt version. The vibrator had already been dismantled in the past. (Not a good sign)

I spent ages getting it to vibrate as the contacts had been “adjusted” by someone without much knowledge of their operation.
I had no more of 4 pins 6 volt vibrator units.

Now a perplexing situation occurred.

Applying the battery would see the voltage from the transformer come on immediately and as the rectifier valve warmed up, so too the B+ of around 225 volts, probably OK, BUT after a few minutes the voltage would fade to around 35 volts and the leads from the battery became exceptionally hot, the vibrator would slow and stop. Something was wrong.
Then after many starts, I could smell the transformer.
I concluded there was/were a shorted turn/s inside. Next, please.

I had only 12 volt versions now, but as the BC221 needed 135 volts, not the full 275 or so that a car radio would use.

I wonder?

Plugging the 6 volt vibrator can into the 12 volt chassis, I connected the truck battery (with a fuse) and was greeted with the steady hum of the vibrator.

The rectifier valve was rewired for 6 volts and the audio amp valve removed.

The voltage came up around 150 volts DC with no load, unbelievable!

I had replaced every condenser in the 6 volt chassis as a precaution before applying the battery, this 12 volt chassis I did nothing to at all before testing it.

Connected to the BC221 the voltage fell to around 100, near enough to a slightly flat battery pack.
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I now changed a few condensers and fitted a new braided metal shielded battery lead and two old clips.
I gave the shiny woven braid a spray with flat black, after all, I don't want the enemy seeing the sun glinting off it, do I?

I slid the vibrator power chassis into the battery compartment and wired it.
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The Frequency Meter works well with the added bonus of the vibrator hum.

The headphone repair will be in the next post.

[Rod Salter]

Headphones.

I do not have the exact headphones for the BC 221 frequency meter so I selected a set made in America.

I believe they should be P18 or P20 style, but I do not know what they look like.

The rubber wires have perished and I think unable to be repaired.
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I had seen three wire mains power cloth covered cabling on eBay, so I ordered some.

I was able to unwind one wire from the twist.
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I selected a reasonable length and fashioned a new lead.

I bound the ends and junction with black industrial sewing thread.
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Looks almost as good as new, anyway they work.

[Rod Salter]

The output valve should have been a VT 76.

I am unable to find exactly what that is.

The unit has been rewired for a 6 SJ 7

There are extra resistors in the cathode circuits, they appear not to hinder the unit so I left them there.

Connected to a dipole aerial it has a range of around 100 yards.

[Mike Kelly]

A nice job !

I have a black one similar to yours, it has a 1950's era Aust. army power supply inside the little battery cabinet. The power unit is switchable from either 240 mains or the battery/vibrator.

I've seen the BC 221 in a different style of case, i.e. finished in a olive green colour. There were many models and the final number made would have been mind blowing.

These were a nice bit of kit, mine is still a accurate instrument . I think each unit was tuned at the factory to match the reference xtal and if you swap the xtal over from another set , the calibration book will not be accurate . I have read that quartz xtal's deteriorate over time and the reference xtal might change frequency slightly , don't know if that will have any impact on the accuracy.