BC 221 Frequency Meter tidy up
 

Part one

Attachment 97999

BC 221- J Frequency Meter Serial No 3156

Attachment 98000

Cabinet # 17024
Attachment 98001

Calibration book for MC177 – AK serial no 70
Attachment 98002

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.

Attachment 98004

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

part two
 

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.

Attachment 98005

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.

A Power supply
 

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.
Attachment 98006

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.
Attachment 98007

The Frequency Meter works well with the added bonus of the vibrator hum.

The headphone repair will be in the next post.

Headphones
 

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.
Attachment 98008

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.
Attachment 98009

I selected a reasonable length and fashioned a new lead.

I bound the ends and junction with black industrial sewing thread.
Attachment 98010

Looks almost as good as new, anyway they work.

Notes
 

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.

Bc221
 

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.

xtal
 

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?

Attachment 98011
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?

Attachment 98012

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

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.

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

Attachment 98078

The front has 6 holes

I do not know what they held either inside or out.

Thanks for the information on the crystals, fellows,

Please, Anyone, have a photo of those crystals markings?

.

Rod.

On my meter, the 6 holes you asked about are where a set of solid head, hollow rivets secure the metal backing plate for the calibration chart booklet to the front cover.

You mention your charts are not matched to the meter so I am guessing somebody at some point went to a lot of trouble to remove the original chart book from the meter.

Hope that solves your mystery.

David

The 6 holes should line up with the aluminum backing plate to the calibration book, BC221a. BC221b is the crystal and it is mounted underneath at a right angle, much like the valve on the other side.

Thanks
 

Thanks, David,
Thanks, Bruce, I would have never dreamed that a crystal could be in a metal valve.
I have some old metal valves that have lost their markings and maybe with open filament
I may think of using one to make a look-a-like crystal or maybe they are the crystal
I will plug them in and see if they oscillate before commencing surgery

Visitor here this morning can make decals to apply to it

He can print, redo dashes, radio fronts and who knows what else

I had forgotten about his business as I hadn't seen him for 2 years
Apparently, he and his motorbike found a landcrusier running a red light, He got banged up pretty bad and only just getting about again

Some of the frequency meters had a full complement of spare valves in spring clips inside the case. A TS-174/U (the VHF version of the BC-221) I used to own came with a full set of spare valves and a spare crystal. (That was a big surprise!)

There was also a post-WW2 official power supply with a stabilised output that was a drop-in replacement for the battery cage.

Chris.

V T 7 6 valve
 

Hi Rod, VT76 is the same as 76 which has an octal equivelant 6 P 5 G also a rare beast. Good to see you have got it going. Re vibrator power supply problems, dont overlook thr buffer cap across the secondary of the transformer and sometims a second one across the primary as well.

Here it is acouple of weeks (years) since I did this job
I got the 1000 k crystal in the last week
all good things come to those who wait ;)