Re: New member and owner of a dead 2225


A shorted Q913 should have protected driver Q912. Q911 should have
protected TL594 U910.

The linear series regulator would have tried to control the output
voltages until Q923 overheated and likely failed shorted. Switching
regulator controller U940, another TL594, operates open loop to drive
the inverter but uses one of its error amplifiers to detect the supply
voltage to the inverter and if it rises above 42.5 volts, it shuts
down so the rest of the oscilloscope circuits should be protected.

I am not sure about how the +40 volt supply rising to +67.5 volts is
going to affect the sweep circuit on schematic 5 and the vertical CRT
amplifier on schematic 5. Normally both circuits should be able to
protect themselves until they overheat but when the low voltage
outputs shut off, they may not be able to. I am inclined to believe
Tektronix took this into account but the theory section says nothing
about Q913 failing as a short.

On Mon, 13 Nov 2017 01:01:56 -0500, you wrote:

That's not good. The Q913 is a TIP30C and is available pretty cheap. The analog pass xistor Q923 is also on ebay. You should also check the driver to Q913, Q912. Probably pull Q913 to double check it and follow up with some other measurements.

The switching regulator is a TL594. They are common. Two are used. probably both are now toast.

You will need to check everything that is supplied the pre-regulated voltage (38 volts) as it would have exceeded many ratings. Besides the power supply schematic #7, check schematics 2 and 5 as this overvoltage goes to those sections.

Good luck,
Tom M

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