DM502

Manuals – Specifications – Links – Pictures

The Tektronix DM502 is a basic 3½ digit digital multimeter plug-in for the TM500 system.

It was introduced in 1975 and superseded by the DM502A in 1980.

It has an Int/Ext switch that allows alternate input connections via the rear interface to be selected.

The DM502 is available with an input for temperature measurement with a transistor temperature probe (P6430) using an NPN transistor, e.g. a 2N2484 (any NPN-Transistor with h_FE >150 can be used).

Key Specifications

Voltage ranges	200 mV to 200 V in decade steps; 1 kV (DC) / 500 V (AC) top range
Input impedance DM502A	10 MΩ
Current ranges	200 μA to 2 A in decade steps
Resistance ranges	200 Ω to 20 MΩ in decade steps

An "Option 2" instrument deletes the temperature measurement function,

Links

• Tektronix DM502 Multimeter Repair by NFM @ YouTube
• Tektronix DM 502A Restoration and Calibration for a Viewer @ Zenwizard Studios YouTube

Internals

The DM502 circuit is built around a Siliconix LD110/111 DMM chipset.

The entire unit with exception of the temperature circuit can be elevated up to 1 kV above chassis ground. For this purpose, it is supplied through a mains-frequency isolation transformer with a 50 V primary that is supplied from the two 25 V_AC windings in the mainframe connected in series. This transformer feeds a a discrete +5 V regulator, and an RC4194D regulator for +/–12 V.

The rear interface supports a digit-serial BCD output but the manual cautions that only the Temp Out, Temp GND and HI and LO inputs are factory-wired, the BCD connections must be soldered in by the user if needed – presumably because the digital section is not galvanically separated from the floating input ground. In Temperature mode, the floating ground is connected to the chassis ground (because the temperature circuit uses the mainframe's ±33 V supplies) but the V/A inputs are disconnected.

Pictures

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  DM502 Opt.2 (no temperature function)

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  DM502 switch with cover off. The cover removes easily using a Pozidriv screwdriver.

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  DM502 rear interface – see note above