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have a trio of Tek 453's. Two are MOD 703K and the other is MOD 703H.
I have had one of the 703K's on my bench for many years. Of late, it has developed trigger issues. I can only get a stable waveform if feed it a square wave. It will not trigger on a sine wave.
The other 703K triggers fine, but has a few other issues, chief among them it is missing the center control knob for the sweep control. Not just the knob, but the center shaft. Not clear what is actually missing internally since that control is fairly well buried (as is everything else on these).
The 703H has many other issues, stability wise, so it may become a donor for missing knobs/controls, feet etc.
Thoughts on the trigger issue?

Ray,

Check the tunnel diode circuit to see if it is working right. There are internal adjustments that may need to be adjusted. This circuit is sensitive to get adjusted. In mine, I replaced a number of resistors in the piece to 1% because that is what I have have in stock and they are very low drift. R497 should be 1/2W. Resistors R473 and R674 should be 1W and R544, R412 and R483 should be 2W. The originals are 1/4 and 1/2W carbons. Likely they are out of tolerance. They were in mine and bodies were dark. Some other resistors in the scope should also be increased in wattage. Check C485 to check its condition. V443 could be weak/leaky. Check the other Nuvistors as well.

R343 and R353 are problematic. If vertical is not right for known signal and v/div, these resistors are high in value. Mine were several times the value. Using a 510 and 560 ohm 1W 1% in parallel will give the correct resistance.

Mark

Mark, I think these are late enough to have FET's instead of Nuvistors.

Are your part designations particular to a specific model? It looks like the 453 MOD 703K and 7034H borrowed parts (boards) from the 453A.

Ray,

I used the 453 schematic.

Mark

Mark,

I did verify the A-Sweep board in the MOD 703K matches the part number of the board used in the 453A-5, part no 670-0417-06. It looks like the main change may be the calibrator, so hopefully the parts you recommend looking at align with the other revs. The change doc indicates changes for serial numbers > B02100 and the ones I have are B021516 and B021836

In comparison, the MOD 703H has a 670-0417-02 board. The change doc states it should have the same board as above, for serial number >60770 and this one is 60794.

I have an original manual coming so hopefully the board layout photos will be easier to decipher.

I uploaded some pictures to an album I created showing the two boards.


Tek 435 MOD 703H and MOD 703K