I got a free, but dead Tek 475 scope. When I opened it up, I found the previous owner had unsoldered CR1462. He was a smart guy, so apparently he concluded that part was bad, but never got around to replacing it. It is a Motorola MO960-3 200V, 2.5A bridge rectifier

Does anyone have a good substitution? I can wander around in the parts sites looking for such a part. I am hoping someone has hit this problem and can speed things along

thanks

Ed

Thanks Harvey for your reply.

I'd like to ask questions about 468 triggering:

I analog mode (same as 465B), where is the triggering point in the WF on the screen? It seems to me that it must be near the beginning of the WF because the trigger generator <6> starts the sweep. The 468 uses a delay line for the vertical signal to the vertical amplifier but not to the trigger circuit. This allows the trigger circuit to determine the trigger point and start the sweep before the vertical signal arrives at the vertical amplifier. But the delay line provides a fixed delay and the trigger delay is variable so the scope cannot start the sweep at the exact moment the vertical signal arrives.
I understand the vertical position of the triggering point is determined by the Trigger potentiometer, going up or down as you turn that knob (unfortunately, not visible).

In digital (STORAGE) mode, the 468 starts a new Waveform acquisition by strobing address 0xE200, which triggers the RSTACQ signal. From there on the 468 acquires samples continuously, storing them in the 512 byte acquisition memory in a rotating fashion, until a trigger signal (from the analog side of the scope) arrives.
The 468 has a "STORAGE WINDOW" push button. When pushed, PRE-TRIG mode is selected. In that mode, the 468 receives 64 more samples before it declares that a new WF is available for processing. The wait for 64 more samples is done in hardware (U536 and U540 in <19>).? This implies that at least 512-64=448 samples must have been acquired before the trigger signal arrives to have a complete WF in the acquisition memory (which always requires 512 samples). The 468 inhibits the analog trigger signal until enough samples have been acquired using the BHOF signal.??In POST-TRIG mode, the numbers are mirrored but the idea is the same.

There is no on-screen indication where the trigger point is, AFAIK.
Other digital scopes that I have seen set their trigger point in the middle of the screen (horizontally) by default, though that point can often be adjusted.

My question: what is the advantage, if any, of providing trigger points at location 64 or 448 when acquiring WFs in digital mode?

Related to this is the discussion on page 75 of the Operator's Manual about "EXPANSION OF NORM STORAGE DISPLAYS". on page 76 it talks about adjusting the DELAY TIME POSITIONING control to hold a trigger point at a certain position but the trigger point is invisible! How can you make such an adjustment?

I posted this message in the TekScopes group but apparently there is no interest, so I'm trying here. Also, does anyone know a group discussing digital electronics for older devices? Thanks -Jan- While studying the digital internals of the 468 and experimenting with various settings I came across the following 2 issues:

- Sometimes a small dot appears on the left side of the display
- While showing two WFs on the screen while the Time cursor is active, the 468 shows the Time Cursor dots only on the primary WF. When freezing the display (SAVE) in that state, the Time cursor dots
appears on both WFs.

At first, these looked like anomalies, but on further inspection, it turns out these are features of the 468!

The Operators Manual on page 62 describes a procedure to produce a "Ground Reference Dot" (GRD) on the screen, which is the small dot from issue #1. This dot can be used to make DC voltage references. Any change in the vertical mode will clear this GRD. This GRD will not move when moving the WF with the position control knob, unless SAVE mode is selected. In that case both the WF and the GRD will both move in tandem.

The Operators Manual on page 71, item 14 mentions that the Time cursor dots will be present on both WFs if the SAVE button is pushed. This behavior make measuring phase differences on sinusoidal signals easier.

While looking into this, I found an barely documented feature of the 468: LINE CURSOR SNAPPING:
If a Ground Reference Dot was set previously AND
the cursor mode is "Coupled Volts Time" AND
one of the Time cursor dots is moved all the way to the left in fast cursor mode AND
the cursor dot stays there for over 512 mSecs, THEN

The cursor line associated with this cursor dot (coupled cursor mode) snaps to the GRD's vertical position.
This snapped cursor line cannot be moved by the Cursor Potentiometer unless the potentiometer is held to the right in fast cursor mode. In that case, the cursor dot and line will unsnap and move along the WF again. It is possible to select the other cursor dot (and its line) by pushing the Cursor Select button: the other cursor/line can now be freely moved along the WF.
The WF and the non-snapped dot/cursor can be moved using the position control know but the snapped cursor will stay in place, unless SAVE mode is selected.
This feature allows for easier DC voltage measurements than described in the Operators Manual pages 61 and 62.

This feature is documented in the Tektronix Service Manual Volume 1, page 2-12 but not in the Operators Manual (May 1981).

I also discovered a minor problem:

Feed the scope a sine wave of 10 mV amplitude, any frequency. Setup for 1 channel, 5 mV/div. Turn on the coupled VT cursor and move the cursors to the top and bottom of the sine wave. The numeric window should read about 10 mV. Now turn on average mode (AVG). The window still reads 10 mV. Move the Volts knob for the selected channel to 2 mV/div. The numeric window still reads 10 mV. Move the volts knob one more position to 1 mV/Div. The numeric window now suddenly reads about 5 mV/div and, if you turn up the intensity, you'll see that the WF has lots of spikes going to the top of the display. The same effect can be achieved in 2 mV/div by positioning more than 50% of the WF below the bottom of the display.

As a computer person, who learned some electronics later in life, I am intrigued by the digital internals of the Tektronix 468. I'm curious if anyone else is.

Thanks

I get Textronix spares from Qservice in Greece.



Hope this helps,
Richard