Re: 475 questions


VK1GVC
 

Bruce, good to hear of your fault-finding success so far. Extracting all the power supply capacitors out is a bit adventurous but you've done it successfully and it certainly allows you to test each one without the uncertainties of testing in-circuit.  Now that you've gone that far, replacing them all is by far the best plan even tho' C1442 to C1472 are probably OK.  The cost of new components should not be significant and by replacing them all now you can be confident that they're all good for years to come.

While you have the capacitors out I suggest you take the chance to test the rectifiers.  A quick scan of the service manual doesn't reveal whether you can easily remove the A9 board or not - my guess is not.  So you'll need to at least partly disconnect each of the transformer secondary windings as the winding resistances will all be close to zero ohms and will interfere with rectifier testing. For the +50V supply, disconnect 2 of the transformer connections as this winding has 3 connections.  The rest of the regulator circuits *might* affect in-circuit testing of the rectifiers but I doubt it.

Testing a 4 terminal bridge rectifier is straighforward with an ohmmeter as diodes usually fail to a dead short circuit and shorts are easy to find.  But if you're unfamiliar with what to expect, and different meters behave in different ways, I suggest you find or even buy any cheap bridge rectifier and do some tests on it so you gain confidence in what your ohmmeter will read as you do the various readings.  Or if you have 4 rectifier diodes on hand, join them up to make a bridge and use that to practise on as the rectifier modules are just 4 diodes in a neat package.

I agree with your comment about not turning on the CRO with all 6 caps out - imho DON'T do it!  Hitting all those regulator circuits with unfiltered rectified volts is a bit brutal and might create more faults, and I don't think it will be of any diagnositic value anyway.

And since we're in the prediction business :-) I'll stick my neck out and say that I *think* your rectifiers will all be OK, even the +50V one.  My reasoning is as follows:

1. There is a known fault in the +50V and it's very likely that other low volt outputs result from the +50V being low
2. You've found 2 faulty components in/near the +50V supply already which could explain the known +50V problem
3. Diodes are usually reliable and a failed (shorted) diode in one of those rectifiers would probably have resulted in high fault currents, heat and smoke, and maybe a blown fuse - not observed.

You're doing really well, keep at it!

Graham

On 26/05/2020 10:32 am, ciclista41 via groups.io wrote:
Hi Albert,

Yup, 2.22 is ~ pi/√ 2! And I did not know that, but I thought the consistancy of the ratios between those measurements on each rail might indicate something to somebody, and you were the one! Very interesting observation.

My trigonometry is rusty, but maybe I'll look into the math behind this more deeply when I don't have such a steep learning curve going, anyway. My guess is that this has something to do with the sine wave and the fact that pi is derived from the circumference of a circle and 1/√ 2 is the sine of the unit circle at 45° (1/(4pi) radians). Since the sine wave is what we use to describe the oscillation of voltage with respect to time in AC and is also the graph of the values of the sine of an angle as it goes through the 360° (2pi radians) of a full circle (1 Hz), it has to make sense there, somewhere! My calculus is incalculably rustier than my trigonometry, given that I never had the opportunity to teach calculus in my entire career, but it could also derive from the derivative of the sine function. ;-)

Bruce



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