Re: SG502 Schematic, Component Layout

 

On 25 Jul 2017 18:19:27 +0000, you wrote:

I apologize for the somewhat off topic question, but this group seems to be the nexus of Tektronix knowledge in general, not just oscilloscopes. I'm attempting to troubleshoot the square wave output of an SG502 sine wave generator plug in, but I'm having trouble correlating the schematic and board component layout from the manuals I have with what's actually in front of me on the board.
Discussion of the SG502 and other non-oscilloscope Tektronix
instruments is not off topic at all.

I have two copies of the manual, and in both, the component layout is Rev C, Nov 1976 and the schematic is Rev D, Nov 1976. The most obvious disconnect is that on the physical board I see two ICs in the lower left corner of the board, while the layout shows only one - plus lots of other differences in the details. Another disconnect is that the physical board has a 600 ohm, 1% resistor in series with the square wave output, while the schematics show it as parallelled to ground.
That is the service manual and schematic that I have also.

The date on the circuit board is around 1971 or so (don't have the board handy to look at as I write this). No serial number that I can find anywhere on the unit itself.
That is odd. There should be a serial number somewhere on the frame
either as part of a sticker or written on the aluminum.

Instead of looking at the date on the circuit board, what is the
circuit board's Tektronix part number and what are the date codes on
the components?

Now, I don't REALLY need the square wave output to work. But my OCD is driving me to fix it anyway. Would sure appreciate anyone who might have another version of the schematic / component layout or some ideas of how I might proceed.

By the way, the actual problem is that the high side of the square wave is rounded and exceeds the 5v open circuit value by a volt or two. Perhaps a bad diode? Would be a lot easier to troubleshoot if I had confidence that the schematic matched the circuit board...
Unless there was a major change in the design, the 600 ohm resistor
should be going from the output to ground and the output should switch
between ground and +5 volts with a sharp and clean edge.

If the voltage is high, then the transistor current source and
switching is going to saturate causing rounding. It is not marked on
the schematic but make sure the voltage at the base of Q235 is +15
volts. Q235 provides the constant current of 8.6 milliamps which
across the 600 ohm output resistor produces +5 volts.

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