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I've begun to go through the calibration process on the Tek 570 I picked up last summer. It's an early model, S/N 169. It works well but I wanted to give it a once over since I have no idea how long ago it was last calibrated.

For some reason the floating supplies, +400v and -300v are both reading high; +446v and -350v

The manual says the must be within 5% of the proper voltage, so these are out of spec. Im performing my tests with line voltage set to 117vac per the manual. I confirmed that the transformer is properly wired for 117v.

I have the front panel controls set per the manual. I tested the 6ax5 and 6x4 rectifiers, both sections strong on both. The 80uf electrolytics were already replaced with modern parts by the previous owner.

Any ideas? You can see in my pic (assuming I can figure out how to attach it) that the secondaries on the transformer are reading a bit high for both sections in question - but not too bad. (the 325-0-325 for the +400v 6ax5 is reading 342-0-342 and the 250-0-250 for the -300v 6x4 is reading 265-0-265)

I checked the diagram for the unregulated transformer at VintageTek, p/n 120-0069-00 to confirm that it was indeed made for 117v. Anyways, lowering to 115v doesn't bring it into spec - have to go all the way down below 110v.

I plan to start looking downstream and see if something's not pulling enough current, but figured I'd get this post started now.

Thanks,
Phil

I think that is normal, is unreg voltage from a transfomer connected to mains

I no longer have a 575 or manual but the power supplies will be similar to other vacuum tube Tek scopes so these comments are generic but I can't quote part designations for the 575.

As others have said, don't worry too much about the unregulated voltages. They should be a bit higher than the regulated voltages as the regulators need some headroom to operate in. What you really need to do is make sure the regulated voltages are correct.

Firstly the -150 supply needs to be correct before you do anything else. The -150 uses a gas discharge reference tube type 5651 (0G3, 82A2) and that needs to working properly. You can check that by carefully measuring the voltage across it (pins 7 and 1) which should be 85 volts. There's a preset pot to set the -150 .

All the other supplies use the -150 as a reference. They will all have a voltage divider made up of unusual value precision resistors that set the various voltages and there are 0.01 uF bypass caps on the dividers that need to be free of leakage. If those resistors have drifted or the caps are leaky that will affect the regulated voltages. And of course, the tubes in the regulators need to be good too. If you need to replace any of the resistors you will have to make up the exact value by series/parallel arrangements of 1% metal film resistors. 630 volt polyester caps are fine to replace the originals.

So there are 4 possible causes, weak tubes, a faulty voltage reference, drifted resistors and leaky caps. I have seen all 4 at various times in Tek vacuum tube scopes.

Good luck!

Thanks for the feedback. Just to clarify, this is a 570, not a 575, although there are a few similarities.

I checked with an acquaintance who also has an early 570 like mine and he said that his floating unregulated voltages were high as well. He reminded me that Tektronix mentioned upgrading the caps in the floating unregulated supply from 450v to 500v in the modification summary document. They say that the caps on the floating unregulated supplies would exceed their rating at high line. I haven't tried setting the line to high line yet (125vac) - I have a feeling it will exceed the 500v rating on those caps. He mention upgrading his to 600v. It just bothers me that the manual states they should be within 5% of -300v and +400v, and I'm sitting at 16% high on the -300v supply and 12% high on the +400v supply, when testing at their specified 117vac line voltage. All of the regulated supply voltages are spot on, as are the non-floating unregulated supplies.

-150v = -150.66
+100v = +100.03
+300v = +305v (they allow +/- 6v)
+400v unregulated = +411 (they allow +/- 32v)
-300v unregulated = -310 (they allow +/- 24v)

so, it's just the two additional FLOATING unregulated supplies that are reading high.
+400v floating unregulated = +446v
-300v floating unregulated = -350v

I think at this point I will proceed with the rest of the calibration procedure, maybe I'll run into other issues that will give me some clues.