Monday, October 05, 2020

Telequipment D54 oscilloscope bad MPS6518 transistor in sweep-gating bistable

My Telequipment D54 oscilloscope didn't show a trace. Instead, if I turned up brightness all the way there were some very faint blobs which changed a bit with the horizontal position adjustment. If I pulled the plug, the spot appeared coming from the left of the screen. This is similar to when trigger stability has been turned up and it is not being triggered, but stability couldn't make it work and even external X input mode did not work.

The service manual (available at contains schematics and a detailed circuit description.

Clearly the horizontal deflection circuit was pushing the beam all the way to the left. I first removed JFET TR107 and connected a resistor to ground from the sawtooth output. This affected the voltage at the collector of TR108, and the horizontal deflection amplifier worked as expected. But there was still no spot on the screen. I assumed that was due to blanking.

The fact the CRT was blanked and even external X input didn't work made me focus on the sweep-gating bistable built around TR105 and TR106 transistors. After probing it a bit I unplugged TR105, an MPS6518 transistor, and to my surprise the base seemed open circuit. I don't know what made it fail there. Putting in a 2N2907A made it work. This replacement was very easy thanks to the fact all the transistors are socketed. Here's the right side board with the timebase circuitry, a closeup on TR105, and the bad MPS6518 transistor:

For completeness, here's the left side board, with the two channel inputs, vertical amplifiers, and trigger circuitry which provides an input to that sweep-gating bistable:

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