Heathkit HD-10 Manuale utente

HEATHKIT

ELECTRONIC KEYER HD-10

CIRCUIT DESCRIPTION

SCHEMATIC DIAGRAM

The letter-number designations on the Sche-

matic Diagram are used to identify resistors,

capacitors and diodes. Each designation is re-

lated by the first number to the transistor stage

in which it is used. For example, the resistors in

transistor stage Q are designated R 0, R ,

etc. In transistor stage Q7 they are designated

R70, R7 , etc.

Letter-number designations using numbers

through 9 are not directly related to any tran-

sistor stage. This system of circuit component

designations is used throughout the Schematic.

Circled letter designations on the Schematic Di-

agram indicate wire connecting locations on the

circuit board.

POWER SUPPLY

With slide switch S3 in the Operate or Hold po-

sition, the AC line voltage is applied across the

primary winding of power transformer T . Re-

sistor R8 drops the line voltage that is applied

to the neon pilot lamp, which indicates when

the keyer is on. The fuse provides protection in

case of a short circuit.

The output of power transformer T is rectified

by silicon diode D2 and filtered by capacitor C2

to produce a positive (+) 9 volts DC. Silicon

diode D3 and filter network consisting of ca-

pacitors C3 and C4 and resistor R9 provides a

negative (–) 6 volts DC. These voltages are

the operating and bias voltages for the transis-

tor stages.

KEY SWITCHING

The key lever is moved in one direction or the

other to actuate switch S or S2. Switch S

turns on the dot generator multivibrator circuit

to produce dots. Switch S2 turns on the flip-

flop and dot generator multivibrators to pro-

duce dashes. The operation of these circuits

will be explained later.

BLOCK DIAGRAM

DOTS

Dot Clamp Circuit

When the key lever is in the center (neutral)

position, a positive voltage is present at the

base of dot clamp transistor Q3. This voltage

which is applied from the power supply through

resistors R , R5 and R6, biases Q3 in an on

condition (conducting). When Q3 is on, a posi-

tive voltage is applied to the base of Q2, one

of the dot generator multivibrator transistors,

keeping it turned off (not conducting).

When the key lever is moved to the dot posi-

tion, switch S closes. This removes the posi-

tive supply voltage present at the base of Q3

by directing it to ground through resistor R6. A

negative supply voltage from the voltage divid-

er consisting of resistor R , R2 and R5, is ap-

plied to the base of transistor Q3. The negative

voltage at the base turns Q3 off. This removes

the positive bias voltage at the base of transis-

tor Q2. Diode D30 isolates Q3 from Q2.

DOT GENERATOR MULTIVIBRATOR

Transistors Q and Q2 and their associated cir-

cuitry make up the basic timing dot generator

which is a free running multivibrator. Capaci-

tors C 0 and C20 with Speed control R and

R2 control the switching speed of the multivi-

brator. Control R2 (screwdriver adjust) is ad-

justed so that the spaces are the same length

in duration as the dots. Since the dot and

space ratio must be maintained for all settings

of the Speed control, the two sections of the

control are clutched. Turning the Speed knob

turns both sections of the control a like amount

without changing the space and dot duration

ratio.

The positive bias voltage from Q3 that is

present at the base of Q2, keeps the dot gener-

ator multivibrator turned off. In the off condi-

tion, Q2 will not conduct or allow current to

flow through it. This allows the full negative

supply voltage to be present at its collector by

way of resistor R20. However, transistor Q has

a negative voltage (with respect to the emitter)

present at its base through resistor R 2, R2

and R22. This causes Q to conduct to a satu-

rated condition. Therefore, its collector voltage

is practically zero or very near ground potential.

At the positive bias voltage is removed from

the base of Q2, it immediately becomes less

negative (more positive) as shown at point T0

on Figure 0. (T0 is at the start of the first dot;

T ia at the start of the following space.)

Figure 0

This positive rise is coupled through capacitor

C20 to the base of Q . This drives the base of

Q to collector cutoff. With the collector current

of Q cutoff, the collector voltage increases to

its maximum (saturation). The result is to turn

transistor Q2 fully on and Q off.

Although this on and off (switching) action is

fast, it is not repeated instantly, since the volt-

age across C 0 requires time to change. When

the collector voltage of Q goes more negative,

capacitor C 0 must charge to this higher collec-

tor voltage. The charge path is trough the base

of conducting transistor Q2 an through collec-

tor load resistor R 0. The collector voltage of

Q will rise with a slight rounding off. This is

shown between points T0 and T on the lead-

ing curved line in Figure .

Figure

Although Q is now off and Q2 is on, the circuit

cannot remain in this inactive (quiescent) con-

dition indefinitely because of the action of ca-

pacitor C20. This capacitor has previously been

charged to the Q2 collector voltage. Since the

Q2 collector voltage has very rapidly become

less negative C20 must now discharge. This is

shown by the flat top line in Figure 0. The dis-

charge path is through control R2 . The time

constant of the discharge curve is determined

by capacitor C20 and control R2 . The charging

time of C20 is very short with respect to the

discharge time. This is due to the high resist-

ance value of control R2 as compared with re-

sistor R20. As C20 discharges, the voltage at

the base of Q becomes less positive (ore neg-

ative).

When the conducting level of Q is reached

and the transistor starts to conduct as shown

at T , the complete cycle is now reversed, with

Q conducting to saturation and turning off Q2.

The circuit then reaches its second inactive

(quiescent) condition. Capacitor C having pre-

viously been charged, now discharges at a time

constant determined by the value of C 0 and

control R . As long as the key lever is held in

the dot position, the dot generator multivibra-

tor continues to run free. Each stage is then al-

ternately on and off for relatively long periods

(determined by the value of C 0 and R , and

C20 and R2 ), followed by a very rapid switch-

over to the opposite stage.

The collector output signals of Q and Q are

fed to other sections of the circuit. The output

from the collector of Q is fed through R30 to

the base of Q3 and through R79 to the base of

Q7. The output is fed to Q3 to make the multi-

vibrator action self-completing. Once the multi-

vibrator is started, any movement of the key

lever will not have any effect until the dot and

following space have been completed. The op-

eration of Q7 will explained in the following

section.

Driver Follower And Switch

The base of switch transistor Q8 draws more

current than can be supplied by the dot gener-

ator transistors. Therefore driver transistor Q7

is used as an emitter follower to supply the

necessary drive for Q8. Resistors R73, R74,

R75, R76 and R78 divide the power supply vol-

tages to set the proper bias voltages to fully

control Q7 and Q8.

Transistors Q7 and Q8 have a positive base bias

voltage which keeps them cut off (no current

flow). Each time the collector voltage of Q is

at – 6 volts, transistor Q7 is biased to conduc-

tion. This action then biases Q8 to a point

where it will also conduct. The output from Q7

is fed through resistor R to the base of

audio clamp Q . The operation of Q will ex-

plained later. The collector output of Q8 is used

to key the transmitter. This waveform is shown

in Figure 2.

Figure 2

Manual keying may be accomplished by con-

necting a hand key between the Hand Key and

GND terminals. This operates Q7 and Q8 inde-

pendently of the Keyer operation by shifting

the bias voltage on Q7. The Hand Key terminal

is also grounded when the slide switch is

placed in the Hold position. This allows continu-

ous transmitter operation for tuning or adjust-

ment purposes.

DASHES

Dash Clamp

Dash clamp Q6 operates the same as did the

dot clamp. Transistor Q6 keeps the flip-flop

multivibrator circuit turned off by applying a

positive bias voltage to the base of Q4. This

bias voltage will cause Q4 to be cut off.

Flip-Flop Multivibrator

The flip-flop multivibrator consisting of transis-

tor Q4 and Q5 and their associated circuit are

similar in construction and appearance to the

dot generator multivibrator. However, instead

of having cross-coupling (collector of one to the

base of the other) capacitors, it uses resistors.

Therefore, it cannot oscillate as did the dot

generator multivibrator. This circuit will stay in

either of two states, where one transistor is cut

off (fully off) and the other is saturated (fully

on).

When the base of Q4 is positive with respect to

emitter, it cannot conduct. The collector volt-

age of Q4 is then maximum negative and is

coupled through resistor R4 to the base of Q5.

This negative voltage on the base causes Q5 to

conduct, thus keeping it in saturation. In this

condition, the multivibrator cannot switch. It

will stay in this condition until a positive-going

pulse is fed to the base of the transistor that is

in the saturated condition. In this case it is Q5.

A positive pulse on the base of Q5 will turn it

off, causing its collector current to decrease

and its collector voltage to become more nega-

tive. This increase in negative voltage at the

collector of Q5 is coupled through resistor R5

to the base of Q4 to turn it on. The negative

voltage will continue until Q4 is driven to maxi-

mum collector current (fully conducting), while

the collector current of Q5 is cut off (noncon-

ducting). Nothing further can happen until Q4

gets a positive pulse at its base to turn it off,

causing the cycle to repeat itself in the oppo-

site direction. This switching action takes place

very rapidly.

As mentioned before, the flip-flop multivibrator

cannot switch as long as the bias voltage from

clamp Q6 is present at the base of Q4. Moving

the key lever to the dash position removes the

bias voltage from Q6, letting Q4 operate. Si-

multaneously, diode D turns off dot clamp Q3,

allowing the dot generator multivibrator to run

free. In its starting condition it will be remem-

bered that Q2 was off (maximum negative volt-

age at the collector). As soon as the dot multi-

vibrator started to operate, the Q2 collector

voltage decreased rapidly from maximum to

near zero. This produces a positive (less nega-

tive) going pulse from the collector of Q2 to the

junction at the bottom of capacitors C4 and

C5 . This waveform is shown at pints T0 and

T2 in figure 3.

Figure 3

These positive going pulses trigger the flip-flop

multivibrator. they are coupled through capaci-

tor C4 and diode D40 to the base of Q4.

These pulses are also coupled through capaci-

tor C5 and diode D50 to the base of Q5.

These pulses affect only the transistor that is in

the saturated condition. In this instance, it is

Q5. The waveform of the pulse at the junction

of C5 and the diode D50 is shown in Figure

Figure 4

As Q5 turns off, a negative going pulse is pro-

duced. This is because Q5 stops drawing cur-

rent, permitting the collector voltage to become

more negative. Capacitor C50 is used to pass

this pulse to the base of Q4 to speed up the

switching action. Capacitor C40 performs the

same task in the opposite cycle.

The collector output of Q5, as shown in Figure

5, is coupled to the junction of resistors R33

and R6 . Resistor R33, diode D3 , capacitor

C3, resistors R6 and R62 and capacitor C60

form filters. These filters direct voltage from Q5

to clamp Q3 and Q6 to make the dashes self-

completing. They also keep the control voltages

from interacting between dots and dashes.

The output from the collector of Q5 is also

coupled through a delay line consisting of resis-

tors R70, R7 , R72, and capacitors C70 and

C7 to the base of Q7. This delay is necessary

to insure that the output switching caused by

the flip-flop multivibrator slightly overlaps that

of the dot generator, while making dashes. Fail-

ure to overlap may leave holes in the dashes

similar to that caused by the bouncing contacts

of relays.

When the collector output of Q5 remains at its

maximum negative voltage level, it produces a

negative voltage to turn on transistor Q7 which

drives Q8. Before Q5 can switch to on, the flip-

flop must be triggered again by a positive

going pulse from the collector of Q2, The pulse

that just triggered Q5 was a positive going

pulse from Q2. The output from the dot gener-

ator multivibrator is now a negative voltage

from the collector of Q . This is fed to the base

of Q7, which is already on because of the nega-

tive voltage at Q5. Therefore, the output from

Q has no additional effect. The next pulse

from Q2 will be a negative going pulse, as Q2

turns off. This negative pulse from Q2 has no

effect on the flip-flop multivibrator, as only pos-

itive going pulses can reset it. After the dot

generator multivibrator goes through another

half cycle, a positive going pulse from Q2 will

result. This positive pulse is coupled to the

base of Q4 (saturated transistor), resetting the

flip-flop multivibrator.

Note in Figure 4 that the flat portion of th

waveform is equal to the length of a dot an a

space. This is because Q2 produces a positive

going pulse every time it switches on. Since the

positive pulses switch the flip-flop transistors

alternately, they operate at half the rate of the

dot generator multivibrator. The output from

the dot generator and flip-flop multivibrators

add at the base of Q7. This produces the first

third of a dash signal from the collectors of Q

and Q5, as shown in Figure 5. The middle

third of the dash is produced by the collector

output of Q5 alone.

Figure 5

The last third of the dash is filled in by the next

dot signal from Q alone as shown in Figure

Figure 6

The result of these two signals at he base of

Q8 is the waveform shown in Figure 7.

Figure 7

At this instant both Q and Q5 are on and a

space results. This completes a dash and a

space at the collector of Q8 to key the trans-

mitter. This waveform is shown in Figure 8.

Figure 8

As the key lever is moved to the center or dot

position, clamp Q2 is turned on and applies a

positive bias voltage to Q4. This voltage turns

Q4 off an Q5 on to set the flip-flop multivibra-

tor. Thus, it is ready to start a dash the next

time the key lever is moved to the dash posi-

tion.

MONITOR CIRCUIT

Audio Clamp

Each time Q7 is turned on by a dot or dash sig-

nal, a negative bias voltage is applied through

R to the base of audio clamp Q . This in

turn starts the free running audio multivibrator.

The audio multivibrator consists of transistors

Q9, Q 0 and their associated circuitry. It is

identical to the dot generator multivibrator, ex-

cept for component values. Both of these multi-

vibrator circuits works in the same way. The

only difference being the audio multivibrator

oscillates at an audio rate. This rate is deter-

mined by the time constant of resistors R90

and R 00, with capacitors C90 and C 00.

The output signals from the collectors of Q9

and Q 0 are fed to the primary winding of

audio output transformer T2. Capacitor C 0

prevents any DC voltage from reaching the

transformer. The signal from the secondary

winding of T2 is connected across volume con-

trol R 02. The wiper of the volume control is

connected to the speaker. The control my be

adjusted for the desired listening level.

If desired, the receiver output may be con-

nected between GND and the Receiver Audio

terminals. This allows the audio to be heard in

headphones connected to the Phone jack of

the Electronic Keyer. The speaker of the Elec-

tronic Keyer cannot be used when a receiver is

connected, as it is of too high an impedance

and cannot handle the output level of the re-

ceiver. If speaker operation is desired, the indi-

vidual speakers of the Electronic Keyer and the

receiver must be used.

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