BC-221 Frequency Meter restoration.

[mpegjohn]

Thank you everyone for you remarks and help.

Last might I removed the PSU, and noted it had a VR tube.
The wiring on the PSU chassis was all cotton covered, and one wire was broken. Could have been done while wrestling the thing out of the BC-221.
I replaced the wire (One side of the transformer to the rectifier).
The dual electrolytic looked in good nick, not bulging or leaking. So I plugged it in and, standing well back, switched it on.
My AVO read 150V and the VR tube was glowing

I wired up the BC-221 to the PSU, and switched on.
I connected a freq counter to the aerial, but nothing

Not sure what to do now. The tubes are all in place and the inside looks very clean, even if the exterior is grubby.

[linescan87]

On my one the head phone jack is switched, so if no 'phones plugged in it wont come on. Apperently this is to save the batteries. Just a thought.

[mpegjohn]

Ah yes I see from the circuit you are right.
Thank you.
I will give that a go tonight.

[mpegjohn]

Linscan87's suggestion was correct, of course, I needed the phones plugged in. I ain't got none at the moment, so I used a plastic rod, just to see if anything happened, it didn't.

I pulled the unit out of the box, and used some croc clips to connect the PSU.

I soldered a link across the headphone switch, as I don't see any need for this with no batteries.

Still no heater volts on any valve.

Turned out to be the power switch, doesn't matter what I do to it, there is no continuity either side.

Any ideas how I could fix the existing toggle switch, or do I need a new one?

Anyway, I shorted the switch, and low and behold the beast sprung into life.

A frequency counter on the aerial showed 3.5MHz, and quite stable.

There is a problem on the low frequency range. Things aren't that stable. Any ideas where I can start?

Thanks,
John

[G3VKM_Roger]

Originally Posted by mpegjohn

Linscan87's suggestion was correct, of course, I needed the phones plugged in. I ain't got none at the moment, so I used a plastic rod, just to see if anything happened, it didn't.

There is a problem on the low frequency range. Things aren't that stable. Any ideas where I can start?

John,

The unstable LF range problem may be your counter, does it have a separate LF input? My old Thandar 200MHz counter is picky about anything below 1MHz but works fine if I use the LF socket.

If you'll post or send me via P/M a photo of the duff switch I will look through my box of switches.

Roger

[Chaparal]

To determine whether your BC-221 is in good condition,
two simple tests are available.
However, the first thing to do in checking your BC-221
is to remove the nameplate, carefully putting aside the screws and lock washers.
Behind the name plate there should be chalked or crayoned a number.
This was put on by the original manufacturer and this number subsequently became
the serial number on the name plate and on the frontispiece of the calibration book.
If your BC-221 calibration book number does not match the plate or the number behind
the plate, you are in serious difficulty.
Many plate changes were employed by disreputable dealers in an effort to sell BC-221's.
I have noted in examining some thousands of instruments that, at various times, the manufacturer omitted marking his serial number behind the nameplate, so this omission by itself shouldn't be considered too serious. If the book does not match your instrument it is still possible to use the frequency meter and calibrate it with its own harmonic markers and sub-harmonic markers and with the aid of a slide rule, provided
that the instrument is otherwise in excellent condition and complete. Such a process
of calibration involves a great deal of work and careful concentration to avoid errors and
was thoroughly covered in a previous magazine article. (QST)

Checking for Accuracy
To check the frequency meter for accuracy the following procedure may be used.
Set the function switch to the HETERODYNE OSCILLATOR or OPERATE position.
Set the RANGE switch to HIGH.
Set the MAIN TUNING dial somewhere in the 1000-2500 kc region.
A suitable spot would be 2333 .333 or 2250 kc.
Now switch back to the CRYSTAL CHECK position and observe the resulting beat note heard in the earphones. The note should not exceed 150 cycles.

Another and somewhat more suitable test is to set the frequency meter to any crystal check point in the HIGH range. Zero in with the corrector in the prescribed manner.
Set the function switch to the HETERODYNE or OPERATE position.
Do not disturb the corrector setting.
Now, move the MAIN TUNING dial to the next check point listed in the calibration book.
Set the function switch back to the CRYSTAL CHECK position:
a tone will be heard in the phones.
Note the MAIN TUNING dial reading and tune the dial for an exact zero heat.
If the difference in the two dial readings exceeds 1.2 divisions,
the calibration is not good.

On the low band this same test should indicate a maximum error
not greater than 1.8 dial divisions.
If the error is greater than this, your instrument is bad.
The smaller the error the better the condition of your instrument.

Maximum Frequency Error
--------------------------
Since the principal application of the BC-221 is to measure radio frequencies
so as to determine edge of band positions in compliance with tolerances imposed by the FCC, it follows that the ordinary error found in the BC-221 should be both understood and rectified.
The technical manual TM-11-300 is the source of the following statistics on possible frequency errors.

Cause............................................. ......................... Error in Cycles
----------------------------------------------------------------------
Small shocks
(caused by handling and thrust on the dial and panel)...............100
Action of locking the dial.............................................. .........30
Warming up................................................ .......................100
Change of load on antenna post........................................... 50
A drop of 10 % in voltage or of 5° C in temperature..................325
Error in calibration....................................... .......................500
Error in crystal frequency......................................... ............250
------------------------------------------------------------------
Total Error............................................. ........................1.355 C.p.s.

This represents 0.034 % error at 4000 kc and is the theoretical maximum.

------------
from QST 1962

[Stockden]

Originally Posted by FrankB

You will note all of the entries in the calibration manual are hand typed in. (Some poor secretaries spent several lifetimes doing all that typing & verifying on those units. VERY labor intensive to make the calibration manuals).

I think that this was only true very early on. As I understand it, the majority were calibrated with a purpose built rig that printed the books directly. The file at the end of this link explains how it worked (Warning: It's about 6.5Mb).

Hugh