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CK722 Forensics

Below is a great article by Steven Coles on protecting transistors when building CK722 and other vintage transistor projects from the vintage hobbyist publications. He also comments on the coils and transformers specified in these vintage projects which may be very useful for component substitutions.

CK722 Forensics

Power dissipation:  The specifications sheet in Raytheon Transistor Applications gives the CK722's maximum collector power dissipations as P C_MAX = 4(70ºC – T A) in milliwatts where T A is the ambient temperature.  Assuming room temperature or slightly above, T A = 25C.

P C_MAX = 4(70ºC – T A)

P C_MAX = 4(70ºC – 25 C)

P C_MAX = 180 mW

The enclosure creates the transistor's ambient environment.  When intended for outdoor use, a very hot day might drive a simple applications enclosure interior to 110ºF.  In this circumstance

T A = ( 110ºF – 32º F) × 5/9

T A = 43 C

Then P C_MAX drops to

P C_MAX = 4(70 C – 43 C)

P C_MAX = 108 mW

Simple biasing preventing damage may suffice for hobby projects.  According to the power-transfer theorem, a transistor in a common-emitter configuration draws maximum power when its collector voltage equals half the supply voltage.  This gives the messy-looking, but conceptually simple formula

Pc = Vc*Ic = (Vcc/2)*((Vcc-0.5*Vcc)/Rc))

or

Pc = (Vcc^2)/(4*Rc)

Which rearranges to

Rc >= (Vcc^2)/(4*Pc)

To protect a P C_MAX = 180 mW transistor from damage, the series resistance must be equal to or larger than values given below.

Battery            Resistance                Resistance

Rating                        at 25 C                       at 43 C

1.5V                3 ohms                       5 ohms

3.0V                13 ohms                     21 ohms

4.5V                28 ohms                     47 ohms

6.0V                50 ohms                     83 ohms

9.0V                113 ohms                   188 ohms

15V                 313 ohms                  521 ohms

22.5V              703 ohms                  1172 ohms

A large collector or emitter resistor does not prevent thermal runaway.  The circuit may still lock up in a “soft” failure.  However, the transistor will remain within its dissipation limits.  Remaining within dissipation limits does not guarantee a transistor will remain within its voltage and current limits.

Where a load provides too little resistance (frequently occurring with lamps, relays, solenoids, speakers, transformers, . . . ) a series resistor can sometimes be added.

RF Coils and IF Transformers:  Some Raytheon Transistor Applications projects with an RF coils and IF transformers:

Project            Supply           Coil                             Collector Winding    Comments

Name             Voltage           Type                            Resistance (Ω)

Tr Regen Rx  22.5                Loopstick                   10                                Check KVL resistance.

Tr HF Osc      15                    IF or Loopstick          10                                Tr would fail – Note 1

Subharmonics  1.5              IF Transformer          20                                Safe

Band Spotter  1.5                 Utah 9280                                                     No kickback protection

Varistor Modulator 24          loopstick                    Re = 9.7K                  Safe

Am Test Osc 22.5                 loopstick                    20                                Tr would fail – Note 1

Freqmeter      15                    Miller 20 A                 20?                             Tr would fail

Relays and Solenoids:  Some Raytheon Transistor Applications projects with a relay or solenoid:

Project            Supply           Relay                          Collector Winding

Name             Voltage           Type                            Resistance (Ω)

Counter         6                      Advance 1200          5500                           No kickback protection

Photo Rly      15                    Advance 850            2500                           No kickback protection

Control Rly    24                    BK 35                        10,000                        No kickback protection

Timer              6                      Advance 1200          5500                           No kickback protection

Radio Cntl     22.5                                                    5000                           No kickback protection

Moisture Det 6                      ?                                 1500                           No kickback protection

Commercial   15                    BK-7-B                      ? Note 2                     No kickback protection       

Transformers:  Some Raytheon Transistor Applications projects with an audio transformer:

Project            Supply           Transformer               Collector Winding    Comments

Name             Voltage           Type                            Resistance (Ω)

Regen Tr Rx    4.5                   UTC SSO-2                3150                            Safe

Guitar Amp     6.0                   UTC SO-2                   215                              Safe

Guitar Amp     6.0                   Stancor A-3856           Unspecified                 Unknown

Hearing Aid    15                    UTC SSO-2                3150                            Safe

Hearing Aid    15                    UTC SSO-6                3500                            Safe

Signal Tracer   22.5                 Stancor A-3332           Unspecified                 Unknown

Keying Monitors:  Two Keying monitors have no protection from excess radio-frequency energy.

Note 1:  A Sylvania version uses a 3-volt supply and a 750K base resistor.  Second and third versions use 2700 and 750-ohm emitter resistors.  How to Make a Transistorized Portable Radio, 1956, pages 4-7 (pdf pages 11-14).

Note 2:   A maybe thing:  In the “Commercial Killer” the greatest concern arises with with the 50-KΩ rheostat inadvertently set to zero.  A resistor in series with the 50-KΩ rheostat might limit kickback current to values causing minimal damage. 

Out of the 50+ circuits 2 intended for near-transmitter operation lack input protection, 3 allow thermal runaway continuation to transistor destruction, and 8 lack inductive-kickback protection.

Original designers mostly used the relay-driver circuits in applications with slowly changing voltages (by electronics standards).  Photocells and RC tine-constant circuits provide examples.  In these cases the current ramp up and ramp down proceeded to slowly to generate a strong kickback voltage.  Such circuit would suffer transistor destruction when responding to rapidly-changing inputs.

Transistor transformers and relays introduced in following years often had much lower DC resistances than their predecessors.  Building the circuits with the newer type numbers would result in thermal runaway continuing to transistor destruction.

Preventive Actions:  A snubber will absorb and dissipate sufficient inductive kickback to protect the transistor.  A snubber usually consists of 10 ohms in series with 0.22 μF.  This series combination connects in parallel with the relay or pulse-transformer coil.  Snubber circuits win the period-authenticity vote.  Alternatively, a rectifier diode connected back-biased for normal relay voltage will also protect the transistor.

Usually, a series resistor in the input lead combined with diode clamps preventing excessive voltage transitions protects a transistor from excess radio-frequency input.   This method works for many situation like protecting a sound-card oscilloscope from over voltage.

An emitter resistor of sufficient value prevents thermal runaway from continuing to transistor destruction.  Usually, the emitter resistor must be capacitor bypassed for the lowest audio or radio frequency in the stage.

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