Tektronix 475 - -8V still wonky

No Sweep

Ok so the scope has a spot now. I can move it around (a bit) with the vertical and horizontal position controls. This tells me the deflection is Ok but there is no sweep. No matter what I can't get the trigger or ready light to come on so there is a problem with the triggering also.

There has to be something more fundamental that is causing this so I go back to the PSU as I recall the -8V still wasn't quite right.

-8V

The -8V line is now sort-of working after I changed out the bridge rectifier. The odd thing is that it isn't -8V but -8.4V which isn't catastrophic (I don't think) might be causing other things to go wrong.

So I go back to the power supply circuit diagram. I get the transistors being driven by the opamp - the opamp is an error amplifier and then causes the transistors to adjust the voltage until it matches the reference. The transistor with the base pointing towards the right initially stumped me (Q1464) but it turns out this is a 'foldback' protection circuit. It reduces the drive on the main pass transistor if the current is too high. When the current increases then the voltage across R1467 increases with switches on Q1464 which turns down the drive to Q1466.

There is a tonne of current put out by the circuit so maybe this is it. But then I would have thought this would make the output more positive (i.e. less negative) rather than more negative. Doing some voltage measurements the circuit doesn't look like it is in foldback. Hmm

The -8V reference works using a volrage dividor composed of a 50K and an 8K resistor. It uses 50V to create a reference for -8. The voltage between the two resistors is 0V. When I test this with my meter the resistors have 50.0V and 8.4V over them respectively which makes no sense.

I tried measure the resistors but as they are in circuit the numbers don't look right. The 50K looks like 12 and the 8K looks like about 7.6.

Someone suggested the opamps can get flakey over time so I tried swapping this out (they are easy enough to get) but this made no difference. I also swapped the foldback and the smaller pass transistor with some garden variety BC547s but again this had no effect.

The resistors are 0.1% tolerance types and these aren't too easy to get. I ordered a 50K as the measurement on this one was way off.

While waiting for this in the post I gave in one day and decided to lift one leg of the 50K resistor from the circuit and test it (when you have excluded all the likely stuff eh?). The resistor was bang-on unfortunately. I decided I may as well keep going and lifted one leg of the 8K. The weird thing was that it measured 8.4K. Umm hang on - that is my voltage error? Seriously a resistor went *slightly* high resistance? Apparently!

I could get 8.06K 0.1% resistors from element14 so I ordered some and sure enough my -8V supply is now -8.01V. I found an ebay vendor that cheaply sells the 8.0K 0.1% resistors but they aren't here yet. Still -8.01 is well within tolerance. I call that a win!