BC 221 Frequency Meter tidy up

[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.
6.jpg

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.
7.jpg

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.
9.jpg

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.
10.jpg

I selected a reasonable length and fashioned a new lead.

I bound the ends and junction with black industrial sewing thread.
11.jpg

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 K]

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.

[Rod Salter]

Mike,

I am pleased you have the power supply for yours.
I may use mine in a field display sometime, so I went for a battery version.

I agree with you that crystals drift off frequency,
In fact, I think this drift is why the Frequency Meter was invented perhaps?

scan bc2210001.jpg
Mine has no crystal in it.
The switch is set to xtal off, which bypasses the inner grid circuit to ground.
I, therefore, am unable to check the calibration, however, it seems accurate with this book.

Considering their age our units are doing well.

Somebody was very clever to find a book to match this unit.
Even the case is a mismatch.

I would like to know or see a photo of the 1000kc crystal if anyone has one? Please?

[Rod Salter]

BC221 PSU-9160 sm.jpg

I found a picture of a power supply

This brings me to another question seeing the circuit attached inside the battery compartment.

Mine does not have that one except there are holes where it could be attached.

My question, -- There are holes in the front lid, what are they for?
Are they for a card with instructions for use?

I have scoured the WWW for an open photo, but always they have the book displayed, covering up any writings that may be attached there

[Bruce MacMillan]

Nice bit of work!

The crystal in these units looks identical to the black valves. It has the same base as the valves.

If the holes are in the outside centre of the fold down lid they are for a data plate. It enabled the set to be identified without opening it up.

The term "meter" applies to a device that measures. Instead of a panel meter displaying visual info this unit gives aural output via the headphones.

[Jesse Browning]

The crystal unit looks identical to the other metal cased tubes except for the markings. What kind of holes are in the lid? Jesse.