Re: 2235 AN/USM-488 power supply low voltages


 

On Fri, Mar 5, 2021 at 02:18 PM, <richardallred@me.com> wrote:


Yup, I knew about that procedure, but was trying to avoid that until I was
sure that was it. It's my understanding once you do that you have to calibrate
the entire scope. Is that correct?

Yes, after adjusting the -8.6V you'll have to calibrate/adjust the 'scope for it to be as accurate as it is capable of.

Be aware that *everything* is connected directly to the mains, until after T948!

Since none of the voltages currently are ok, calibration will be off - if the 'scope works at all. The error is around 15%, which is far too much.

I don't think your problem simply is just a matter of adjusting the -8.6V, *unless* someone misadjusted it or any of the voltage-determining components have drifted. I'd start by checking R937, R938, R939, and the voltage across VR943.

Repeat: Be aware that *everything* is connected directly to the mains, until after T948!

It's a very good idea to fully calibrate/adjust a 'scope which hasn't been calibrated for a long time.

You may consider noting as exactly as you can the current -7.3V of the -8.6V supply to be able to see if the voltage changes by slightly adjusting R938 and carefully readjusting it to the current voltage afterward. The last calibration may have been done with a slightly different voltage than exactly -8.6V and this way, you may be able to restore that. If you find a problem and repair that, the -8.6V hopefully will revert to a voltage close to what it has been adjusted to during the last calibration.

Raymond

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