The Mississippi Historical Radio and Broadcasting Society

Vol 3 #6 December 8 1993 / January 18, 1994

News Letter

Due to illness and an unusually high demand upon my time, several months have passed since the last newsletter. For this I offer my deepest apology - but it has been something that could not be helped. I appreciate your understanding - and I hope the length of this newsletter will help to make up for the missing ones.

Much has happened over the past couple of months. The regular meetings of the club were held September 14 and October 12. Our Show & Tell at the Village Fair Mall was held on Saturday, November 6th. It was a good success as it garnered a couple of new members. The Meeting scheduled for November 9 was cancelled.

Due to personal reasons, Randy Guttery resigned as club President on November 9th. The Vice President resigned on November 13; no reason was given. In accordance with the Club's Bylaws, the Offices have been filled by succession. Sherry Guttery is to serve as President until new officers are elected at the next annual business and member's meeting (March 12, 1994). Randy Guttery will continue to contribute to / and edit the Newsletter. Regular meetings were held December 13th and January 11th. Remember: the regular meeting is now held on the second Tuesday night of each month at 7:00PM at 2412 C street, Meridian.
Some dates to mark on your calendar:
Feb. 8:Regular Monthly Meeting
Mar. 8:Regular Monthly Meeting
Mar. 12:Annual Meet & Show
Apr. 12:Regular Monthly Meeting
The meeting on January 11th had an interesting project: the completion of a Zenith console restoration. The Cabinet was refinished by it's owner, Pastor Frank Trible, and the chassis had been restored by Randy Guttery. Several "last minute" problems were encountered: such as dial glass gasketing, etc. These problems were solved and by the end of the evening, the Zenith was completely assembled and entertaining us all with broadcasts coming in from all over the world.
Many people have requested that the club publish a directory of the membership. After talking with a majority of the club members, it has been decided to publish such a directory. Members that we have been able to contact and determine their wishes are included. If you have been left out and wish to be included - and / or any changes that need to be made - please let us know.

Radio Collecting Today

We left off last time dealing with figuring out unknown values. The project at hand was a Philco multi-meter made in 1936. First let me correct a typo -- in the section on measuring the current of the meter movement, the correct current is .001128 not .01128 as stated in one place. The passage should read:
"...then 700/467K = 1.5 ma. -- real close to what we measured on the meter - but it shows that the range is calibrated by a shunt across the meter to increase the current from .001128 to .0015 amps."
That out of the way, let's move on.

As we noted in the last article, the design value for this meter was .0015 amp for full scale. Using this value, we can quickly calculate the rest of the values needed:
DC Voltage dividers based on the ranges of 1000V, 300V, 100V, 30V, 10V
1000V = (1000-300=700) then 700/.0015 = 466K
300 = (300-100=200) then 200/.0015 = 133K
100 = (100-30=70) then 70/.0015 = 46K
30 = (30-10=20) then 20/.0015 = 13K
10 = 10/.0015= 4.6K - meter system (200 Ohms)
Pretty weird values but look a little closer -
The 466K & 46.6K can be replaced by a 470K and 47K respectively - and still be within a percent of design value. And those ARE common values. The 133K and 13K are almost as easy - Two 270K resistors in parallel are 135K - again within a percent of design value (two 27K's for the 13K, of course). In the case of the last resistor, 4.3K 2% will due here. Our divider chain is now added as:
470K+133K+46K+13K+4.6K+200=666800
Then 1000/666800=.001499amps - very close to the design .0015. in fact .001499/.0015=.067% - closer than any meter movement might be capable of!
Taking the design step further:
Meter movement resistance times system current is: 200*.0015=.3Volts for full scale.
Current shunts are then calculated:
10ma = .3/.01=30 Ohms
Remember, this is the value of the shunt AND meter system in parallel - so the current shunt must be higher so the total comes out at 30 Ohms.
So here is a rough but simple way to calculate the value:
30/200=.15 30*1.15=34.5 or roughly 35 Ohms
(200*35)/(200+35)=29.78 - within .8%
100ma = .3/.1=3 Ohms
10Amps = .3/10=.03 Ohms
Note that these last two calculations don't take into account the parallel resistance of the meter system. Since this value is around 67 and 6700 times the resistance of each shunt, it produces an error of some 1.5%and .015% respectively - obviously - we just ignore it.
Last time we covered the ohm meter portion - those numbers are still valid since the system current is dependent on battery (and the variable shunt across the meter) rather than dividers as we noted then.

Now - on to practical matters. Let's say you have a screen or plate resistor that is burned. No schematic available. Measurements? sure. we could - but most of the data we need has already been figured - and published - in a tube manual.
Let's take, as an example, a radio that has a 6K7 IF amp, and the screen resistor is burned up. Doing some quick checking we find the decoupling capacitor shorted - so it is replaced with the same value as marked. No schematic available - so how do we figure this out? First pull out the old tube manual. It lists "typical conditions" as 250V plate & 125V screen, and at that voltage the screen current should be about 2.6ma. We now turn on the radio and measure the voltage on both the plate and the supply the screen resistor is sourced by. In this case the plate is 240 and B+ is 240. Taking 125 from 240 leaves 115V. 115/2.6ma = 44K ohms - we'll use 47K as a test. 115*.0026=.3W so we'll use a 1/2W resistor. Turn off the set and solder in the resistor. Turn the set back on and check our screen voltage - about 35 volts - clearly we've missed something... Looking closer we note that the screen of this tube is tied to the screen of the RF amp! which is also a 6K7... no wonder... that means we need about 1/2 of the value we originally calculated. Installing a 27K resistor gives us a voltage of 105 volts - more like what we expected. Most radios are designed around a 20% tolerance. So this puts us within that limit. So why is that resistor getting hot???? Oh yeah - we kept our voltage drop from our earlier calculations - but we doubled the current! 105V * .0052A = .546W through a 1/2 watt resistor! No wonder it's hot. A one watt will solve that problem... and that points out an easy thing to over look - when you change a value be sure and do all of the calculations - they may help limit the smoke!

A second method of finding that unknown value is to look at the schematics of similar radios using the same tube. In using the 6K7, Zenith tended to use larger values, keeping the screen voltage down - GE and others tended to use screens closer to what we calculated, but most of them run within about 20%. And that gives us more confidence that we have replaced our burnt part with one that will work correctly and last a long time!
Next Time: Hard decisions: What if you find a set that isn't what it used to be: finding a transformer where the battery plug used to be!

P.S. Don't forget our meetings on the second Tuesday night of each month at 7 PM!!!!! The next two meetings will be very important as our annual meeting is only a month away!
The Mississippi Historical Radio and Broadcasting Society Newsletter is published monthly by:
The Mississippi Historical Radio and Broadcasting Society
2412 C Street
Meridian, MS 39301
601 693-5958

© 1994, The Mississippi Historical Radio and Broadcasting Society.

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