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The Tektronix 321 is a very compact single-trace, single-timebase portable oscilloscope that is almost completely solid state. Design work took place in 1959 and it was introduced on April 8th, 1960. It was designed by Oz Svehaug, Oliver Dalton, and Hiro Moriyasu.

The 321 includes several germanium bipolar transistors, which are a known reliability problem.

The 321A was introduced in 1964. The 321 and 321A have slightly different circuits. Early instances of the 321 (sn pre-720) use a 5718 peanut tube as the front-end cathode follower.

Later 321 and 321A use a 7586 Nuvistor for that role. Latest 321A (SN6000 up) use a FET 151-1011-01. In several circuits, the 321 uses OC170 transistors where the 321A uses 2N2207 transistors.

Power Supply

The 321 uses a DC-DC converter to generate the DC voltages needed for the scope. Prior to serial number 720, 321s used the 120-153 transformer along with a separate high voltage power supply with its own transformer the +3350 V CRT anode voltage. Starting at serial number 720, all DC voltages (including the CRT anode voltage, via a voltage multiplier) are generated by a single power supply based on the 120-210 transformer.

Key Specifications

Bandwidth 5 MHz (321) / 6 MHz (321A)
Rise time 70 ns (321) / 64 ns (321A)
Sweep 0.5 μs/Div to 0.5 s/Div, 1–2–5, ×5 magnifier (i.e. to 50 ns/Div)
Deflection 10 mV/Div to 20 V/Div
Input impedance 1 MΩ // 35 pF
Calibrator 0.5 Vp-p, about 2 kHz
CRT T3210 or T3211-31 or 154-433, P31 phosphor, 10 × 6 Div. @ 6.25 mm (0.25"), 4 kV acceleration
AC Power 90–136 V or 180–272 V, 20 W
DC Power 11.5–35 V, 8.5 W min., 24.5 W max., 700 mA current typ.
Battery Ten size "D" NiCd cells, runtime 5 h (4 Ah cells), 3 h (2.5 Ah cells), charge time 16 h
Dimensions 21.67 cm × 14.6 cm × 40 cm (8.5" × 5.75" × 16")
Weight unknown, about 5 kg
  • Ext Trig / X input (supports X-Y displays)
  • External blanking input (Z modulation)
  • Built-in battery

External Power Connector

Douglas W Jones wrote:

I've got a Tektronix 321 scope (serial number 001917) that I need to run from an external DC supply. The problem is, I have only the line cord, and not the DC cord. The power input plugs on the scope are obviously custom, long predating the standard IEC plug everyone uses today.

So I improvised. The pin spacing for the 3-pin DC input plug is very close to the spacing of Molex 6, 9 and 12-pin plugs, and the pin diameter is not too much larger than Molex uses. So, I picked up a 6-pin Molex housing and 3 female molex pins, and then cut the housing down to 4 pins (using my pocket knife).

I used an old IEC cord for the DC input wires, with the plugs cut off both ends. I attached the wires on one end to the molex pins and pushed them into the housing, then jammed to housing down onto the Tektronix connector (this flared the female Molex pins to fit the Tektronix pins. The other end of the DC power cord got banana plugs to fit the output posts of my DC power supply.

The net result works fine, plugs and unplugs nicely, and unlike other power supply solutions I've seen, involves no changes to the scope.

Added note: My bench supply has an analog Ammeter on it. The starting surge of the 321 is about 1.5 amps, perhaps 2, tapering down to 0.7 amps in under a second. It seems to draw a steady 0.7A regardless of what you're doing with it. The current drain doesn't change when you vary the supply from 12 to 18V — this is because the 321 uses a linear regulator.



Some Parts Used in the 321

Part Part Number(s) Class Description Used in
120-153 120-153 Discrete component power transformer 321
120-210 120-210 Discrete component power transformer 321 321A
5718 154-0053-00 Vacuum Tube (Triode) subminiature high-frequency triode 321 P500CF P170CF
7586 154-0306-00 Vacuum Tube (Triode) Nuvistor triode M 1A1 1A2 1A5 10A2 10A2A 11B1 11B2A 321 321A 3A1 3A1S 3A3 3A5 3A6 3A7 3A8 3A74 3S76 3T77 3T77A 3B5 4S1 4S2 6R1 6R1A 9A1 9A2 82 86 S-311
OC170 Discrete component alloy-diffused Germanium PNP transistor 321 661
T3210 154-0226-00 154-0293-00 154-0294-00 154-0295-00 154-0347-00 154-0385-00 CRT 3" CRT 321
T3211 154-0442-00 154-0443-00 154-0444-00 154-0433-00 CRT 3" CRT 321

Some Parts Used in the 321A

Part Part Number(s) Class Description Used in
120-210 120-210 Discrete component power transformer 321 321A
2N2207 151-063 151-0063-00 Discrete component germanium PNP transistor 3B1 3B3 3S76 321A 547
7586 154-0306-00 Vacuum Tube (Triode) Nuvistor triode M 1A1 1A2 1A5 10A2 10A2A 11B1 11B2A 321 321A 3A1 3A1S 3A3 3A5 3A6 3A7 3A8 3A74 3S76 3T77 3T77A 3B5 4S1 4S2 6R1 6R1A 9A1 9A2 82 86 S-311