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My 575 mod 122C arrived at the weekend. It did basically work when brought up gently on a variac. I've been through most of the valves and replaced a few which were well down on emmision, including the 6AQ5 /V810.
Similarly, most of the paper caps have been replaced with polyester 600V. I've just been through the cal. procedure and everything seems fine, but there is now a slight dot-pattern on the trace which I don't recall noticing when it had the "old" caps and "old" valves in place. As there doesn't appear to be any Z-mod capability, I'm wondering what might cause this? Or is it normal? I've checked the -1700V with a P6015 probe, and it looks clean. There's small amount of ripple on the +2300V anode, but only c 20V p-p.
Photo album uploaded under "575 dot-pattern trace"
John

How about some pictures

Hi John,

Lack of shielding, maybe (RFI from HV circuit)? I think the '575 doesn't have a shield lid in the HV area, but you can try again with the bottom lid in its place.

Regards,
Sebastian.

PS: Pete, please note that John provided pictures in the Photo files area.


Pete Lancashire 10:38am #149306
How about some pictures
. . .

On Fri, 29 Jun 2018 at 13:38 Pete Lancashire <xyzzypdx@...> wrote:

How about some pictures

Here's </g/TekScopes/album?id=61711> the album John

mentioned.

John - Groups.IO doesn't have any sane searching of photo albums, so best
to link the album in your messages.

On Fri, Jun 29, 2018 at 11:58 am, Siggi wrote:

/g/TekScopes/album?id=61711

Thanks guys. The 575 in my possession has no "bottom" plate, either over the EHT box nor the 'scope main chassis. The second photo is of the underside "as-is". I can't see how ether could be attached, and there's no exploded mechanical drawing in my manual?

John

John,

I had the exact same pattern and it turned out to be noise on the AC mains. Turning off fluorescent lights did wonders to reduce it. The collector sweep comes directly from the AC through a transformer, variac and diodes and applied directly to the device under test.

George

Many thanks George. I did wonder if the pattern might actually be due to velocity mod on the X-deflection, which is tantamount to the same thing. I'll investigate and report back.

John

Resurrecting this old thread since it's exactly the issue I'm having. I'm seeing the same artifact that's described here, and it's due to ripple from the HV oscillator that shows up on the Collector Sweep.
I had to replace the HV caps, C812, 813 & 818, as it wasn't getting anywhere near full voltage with the old disc caps. All of the paper caps test good up to 600V; they're not bumble-bees or black beauties, but newer red ones (dunno what you'd call them).
Are others experiencing the same thing?
thanks,
Paul

Paul,

Replace the red caps with Sprague Orange Drops, good Panasonic, etc. rated at 600 or 630V. If they are in the main power supply, replace them also. You can increase C811 to a ,1mfd since that is a decoupling cap. It may be as simple as these caps to fix your problem.

Mark

Thanks Mark - As a part of my initial trouble-shooting I did change out C811 with a polyester film cap of the same value and saw no change, but I'll try increasing it's value.
I'm generally stocking 600V axial film caps (CDE or Vishay, usually) for paper cap replacements - would one expect to see any performance difference in using Orange Drops over those in filtering applications?
thanks,
Paul

In my case the dot pattern on the collector sweep was due noise (from compact flourescents /smps etc) on the mains supply. This has been likewise identified as a common problem by others.
John

Paul,

Increasing C811 will not hurt. Higher capacitance will reduce the noise at that circuit. I mentioned Orange Drops since those are well known. The ones you have in stock are fine. Replacing an axial to radial is fine. What John said is correct about noise sources! Invest in a Isobar power strip. Those are worth the cost. A shielded power cord will also help.

If you replace any of the electrolytics, use Nichcion LGR, LGZ (for 82mfd or larger 450V), ULD, UCY or UHE. These are low ESR which filter high frequency noise better than the originals or new general purpose electrolytics either 85 or 105C that are not low ESR. These ones I listed are also long life, 5000-20000 hours. Adding a ,01mfd 1kV ceramic across electrolytics will help with noise reduction. A Corcom line filter may be needed underside if the noise is still there. I would mount it as to not drill any holes to keep the piece original. This type would be with flying leads on each end.

You can change the light source if that is a noise generator. Using a lamp with a filament will not generate noise, e.g. carbon filament, tungsten or halogen. A small VARIAC can be used to dim the lamp when desired. That will be a linear way that does not generate noise like a standard triac lamp/motor dimmer.

Mark

Mark