Download HVRA Basic Electronics Class I April 2015

HVRA Basic Electronics Class I
April 2015
Introduction - Voltage, Amps, Power, & Resistance
A series of basic electronics classes will be made monthly, on line, to both members and non members
alike. The goal is to teach both theory and troubleshooting of vacuum tube equipment at the same time.
In order to successfully troubleshoot electronic equipment, one must be able to understand basic theory
and how to apply it in repairing, modifying, and in some instances designing circuitry. In dealing with
vacuum tube electronics, the first thing that comes to mind is vintage radios. Vintage radios bring back
nostalgic memories of bygone years and many have beautiful enclosures made of wood or art deco
materials. Some are collectable antiques and there is a market for their restoration and repair to make
them functional again. This is where understanding basic electronic theory is essential in that often
times modifications must be made in the circuitry as direct replacement components are no longer
available. This is why it is advisable to keep a stock of parts, and vintage parts sets, referred to as
“junkers” on hand for future repairs. In recent years there has been a renaissance in vacuum tube
amplifiers especially among musicians who play string instruments. Interest in vintage car restoration
continues, and many owners prefer to have that car’s radio functioning once again. Some transistorized
radios and amplifiers dating back to the 1950’s and 60’s are also becoming collectable and servicing
these will be included in future classes. What you learn in the first series of classes will be applied to the
actual troubleshooting of a basic five tube radio with the schematic shown on the last page of the
preliminary lessons.
Let’s start with electrical power sources, DC and AC. There are two basic types of cells; primary (zinc
carbon flashlight cells) and secondary cells: (lead acid auto battery, nickel cadmium, lithium ion, etc.)
these can be recharged and used over and over, but the primary cell or battery cannot and must be
discarded when discharged. When one or more cells are connected in series, their voltage adds, and it is
called a battery. DC is of course the current that we can convert to chemical energy to store as potential
energy in batteries. Batteries are comprised of two or more cells, for example a 12.6 volt automobile
battery contains six 2.1 volt lead acid cells connected + to -, + to –.
Alternating current (AC) does not have fixed positive or negative polarity, it alternates plus and minus at
some frequency such as our household power source at 60 times per second, expressed as 60 Hertz per
second. The human ear can hear from 20 to 20,000 Hertz per second. An AM station transmitting at 700
on the dial is 700,000 Hertz (Hz).
One of the essentials in getting started is basic math as applied to simple resistance circuits. There are
exercises page that you can complete and bring to the next club meeting to be reviewed, checked over,
and discussed. If you are unable to attend club meetings, correct answers for exercises 1 – 4 will be
made available on line April 23. Class II will be on line around May 8, 2015.
Here are a few publications that I recommend for getting started:
Allied’s Electronics Data Handbook, Allied Radio Corporation, Chicago 1956 – 1967 editions
RCA Receiving Tube Manual (pre 1966) HIGHLY RECOMMENDED!
Elements of Radio Servicing, third edition, by Marcus and Levy, Mc Graw-Hill Book Co. 1967
Understanding Radio, by Watson, Welch, and Eby, Mc Graw-Hill Book Co. 1951
Rider’s “Perpetual Troubleshooter Vol. I thru XXII, (also available at www.nostalgiaair.com)
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Basic Math
No you don’t need more than simple algebra for volts, amps, ohms & watts!
Just remember: Ammeters or are always connected in series with the load (R).
Voltmeters are always connected in parallel with the load (R).
Let’s begin with Ohm’s Law E (voltage) = I (current in amps) X R (resistance in Ohms). If a 100 Ohm
resistor passes 0.10 Amp (this is 100 milliamps), then a voltmeter across the resistor will read 10
volts. Now, let’s say we have a resistor in a radio that has no markings or color code, a voltmeter
across it shows 10 volts, and an in line ammeter reads 100 milliamps (mA). Now, R = E/I , so
R = 10.0 / 0.100 and R = 100 Ohms. In parallel resistance circuits, the formula for calculating
Resistance is R1 times R2 divided by R1 + R2.
Now for a real life problem: Felix Faraday’s 1931 Atwater Kent has a resistor that is broken in two.
The schematic identifies it as a 250 Ohm resistor, but what size (wattage) resistor must you replace
it with? Resistors come in all shapes and sizes rated in watts. From the schematic we find R drops
10 volts. We now know E and R, so I = E/R, I = 10/250, divide 10 by 250, I = 0.04 amps. Since watts
(W) = E X I , then 0.04 X 10 = 0.40 watts. A one, or better yet, two watt 250 ohm resistor will work.
As a rule of thumb, it is best to use a resistor 3 times the wattage you have calculated. Oops! Felix
did not have a 250 ohm resistor so he wired two 500 ohm resistors in parallel for 250 ohms. Two
one half watt resistors in parallel will also equal one watt in size.
[2] Resistor color code – see attachment [A]
Resistors are commonly identified in ohmic value by colored bands. Some will have the value
stamped on the resistor itself, older types may have a simple colored dot, or no markings
at all. This is where a schematic diagram will be required for proper identification. A great on
line source of schematic diagrams for older equipment can be found at www.nostalgiaair.com
as mentioned on the previous page.
[3] Basic resistor circuits you will encounter – Let’s begin!
Begin by completing exercises #1 and #2 – pretty simple stuff - basic Ohm’s Law. A very important
thing to remember: When measuring resistances with an ohmmeter – all power must be off in
the circuit. Otherwise you may sadly find yourself in the market for a new meter!
In exercise I, problem 1, we have resistor ( R ) wired in series with a battery as in the diagram.
In solving the problem, where do you start? Well, the unknown is the current (I). So with I = E/R ,
the first answer is 0.15 amp, or expressed as 150 milliamps
Remember this rule: In a series circuit, the current will always be the same no matter where you
Place the ammeter. This is known in physics as Kirchhoff’s Law. In series circuits, current remains
constant, voltage divides! In parallel resistance circuits, voltage is constant, but current divides!
Now let’s go to exercise #3 and #4 and apply what we have learned. Answers will be posted on
this web site April 23, 2015. You may also bring printed copies of exercises 1 – 4 to our meeting
Saturday, April 25 at 9 AM in the Bayland Park Community Center for questions and answers.
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Correction: Exercise #1
II Calculate the resistance of the unknown
resistor in circuit B in Ohms and the amount
of current in Amps when:
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Exercise # 2
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Exercise #3
Measuring Resistors with an Ohm meter
Old carbon resistors tend to change in value as they age. When troubleshooting vintage radios or other
vintage electronics, it is a good idea to test the fixed resistors as well as the variable resistors such as the
volume control and bass – treble controls if present. For accurate measurements it is best to un- solder
one lead to isolate the resistor you are measuring from the adjacent circuitry. Refer to the above table
in answering the following:
Sarah Silberfarb is testing some carbon resistors in her 1960 Mc Intosh amplifier that her father gave her
when she graduated from college in 1965. Should Sarah keep or replace these resistors: [1] a 2 watt
colors BROWN BLACK RED SILVER reads 1250 ohms. [2] a 1 watt colors RED VIOLET RED SILVER reads
1800 ohms. [3] two ½ watt resistors YELLOW VIOLET YELLOW GOLD read 650,000 ohms each (expressed
as 650 K ohms). [4] a ½ watt resistor BROWN BLACK GREEN SILVER reads 1,050,000 ohms (expressed as
1.05 meg ohms. [5] a 2 watt resistor with colors GREEN BLACK BROWN reads 520 ohms.
List the resistors that are out of spec here:
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Exercise #3 (Continued)
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(Diode)
“Allied’s Electronics Data Handbook”, Second Edition, Allied Radio Corporation, Chicago, 1956, p.54
Try to obtain a copy of this publication; it contains a wealth of information – HIGHLY RECOMMENDED!
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Exercise #4
The above schematic is that of a Westinghouse Model H-188 five tube AM radio from the year 1948.
We will refer to this radio diagram throughout the introductory lessons, introducing the various
components; and how they interact to make a functioning radio. Study this diagram and become
familiar with the symbols representing tubes, coils, transformers, resistors, capacitors, etc. In this radio
the filaments are in series, the voltage drop is: 12.6+12.6+12.6+50+35= approx. 123 volts at 0.15 amp.
Troubleshooting with your volt ohmmeter: What could cause the following to occur?
[1] When plugged in, 117 VAC is measured across C 11 at the switch when turned on, but the tubes do
not light up. [A] 50B5 filament open. [B] Dial lamp burned out. [C] Bad SW 1 switch. [D] R 11 open
[2] A new 6 volt No. 47 dial lamp burned out as soon as the set was turned on. [A] Short between
cathode pin 7 and filament in 35W4. [B] R 11 is open. [C] Filament open between 35W4 pins 4 & 6.
[3] True or False Operating the radio without the dial lamp will strain the 35W4 filament.
[4] How many watts does this radio require to light the tubes?
[5] Felix Faraday found a junker radio like this one in a garage sale to use as a bench radio in his shop.
Someone had removed the dial lamp socket so Felix decided to replace it with a resistor. If the #47
lamp operated on 6.3 volts at 0.15 amp, what value (ohms) resistor will he need and size (watts) ?
[6] When unplugged from the outlet, but turned on, a very low resistance (15 ohms) is measured across
one side of the line plug and metal chassis. The set is unsafe to operate True or False.
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