Tektronix 475 - Ok I am calling it fixed!

Mixed sweep rates problem

Well to be honest I still am not sure of the cause however this one has kind-of gone away. The problem was that for some B sweep rates the B sweep wouldn't start.

The really weird part is that the same sweep generator is used when in A mode and works fine for all sweep rates.

I looked at the gate signal coming in and found there is a B gate even when the B sweep doesn't start. I thought maybe the sweep disconnect amp Q974 was marginal and not passing the signal through or that something was going wrong on the timing board and the disconnect amp was being turned off.

Initially I thought the signal was not there after Q974 when in the broken B sweep rate but later I realized this was just a triggering problem on my test scope.

I had the thought to compare the signal at the output of Q974 with the output when in A mode and actually the B mode start signal was much smaller. I followed this back and it is much lower after the B gate amplifier (Q996) also.

Unfortunately Q996 is an old transistor that is not easy to replace. I decided to swap Q992 and Q996 to see what would happen (assuming the problem would just move) but actually the problem just went away! Weirdly it now works fine on all B sweep rates. 

For now I think this is near enough and I'll look at it again later if it is a problem.

A Start Problem

The next problem is that for the 0.1ms/div  and 0.2ms/div A sweep rates (when in MIX, A INTEN etc) mode is on the signal starts about half way along the screen and jitters a lot.

Looking at the sweep signal at CR985 and while also looking at the B gate signal you can see what is happening. Basically the sweep signal is not making it back to its initial state before the sweep start is going off. The B gate goes low (into hold-off) at the end of sweep but then goes high again before it has returned where it should.

The slope of the sweep as it returns looks low although not lower than the other slower sweep rates. The other slower sweep rates however wait longer before hold-off ends. At the faster sweep rates the slope of the sweep signal is high enough that the sweep makes it back to the initial level with enough time for some hold-off before the sweep start signal occurs.

If I adjust the sweep hold-off the signal looks fine on the display.

I give up

Given I have just one problem left and I can work around this easily enough I am giving up for now. The scope is very useable at this point. I might come back to this later and have another go but for now I've installed the unit into my test bench and it is up and running and actually pretty useful!