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564 AC ripple in beam intensity


Paul
 

Hi Folks -

I'm seeing an intermittent intensity modulation at line frequency on my 564, and have been unable to measure the source. I've tried both a 3B3 timebase, and a 3A72 in the horizontal slot with an external ramp to try to confirm that the problem is not from one of the plug-ins. In either case, I see the same thing. Given the depth of modulation I'm seeing, I'd expect there to be dozens of volts of ripple somewhere in the beam's path, but it's proving to be illusive.
There was some out-of-spec ripple in the low voltage supplies, but I've all but eliminated that by changing out some of the electrolytics (there's still a little more than spec on the -100V, but only by a few mV).
I can't measure any appreciable ripple on either side of the pair of neon bulbs between the cathode and the grid, and about half the time, when I touch my 1000x probe to those points, the problem disappears. On the storage targets of the cathodes of V913, I'm getting a ramp of about 400-500 mV, which, while I'm not sure what's up with that, it doesn't seem large enough (or the right shape) to be the source of this problem.
The external CRT cathode on the rear is grounded, and the switch has been cleaned and is in the chopped blanking position.

Any ideas?
thanks!
Paul


Harvey White
 

Not familiar with the circuit, but I have had times when touching with a probe made something go away.  The probe shows up as a capacitance and resistance to ground, thus loading the circuit.

If, for instance, you measure across a capacitor and the problem goes away, I'd suspect that capacitor of being low in value, unless it were a capacitor more or less equal to the probe loading.

I'd look at the places where the probe makes things look nice, and perhaps that will give you a clue to what may be going on.

(suspecting an open high voltage capacitor, just by the symptoms).

Harvey

On 1/16/2021 12:06 PM, Paul wrote:
Hi Folks -

I'm seeing an intermittent intensity modulation at line frequency on my 564, and have been unable to measure the source. I've tried both a 3B3 timebase, and a 3A72 in the horizontal slot with an external ramp to try to confirm that the problem is not from one of the plug-ins. In either case, I see the same thing. Given the depth of modulation I'm seeing, I'd expect there to be dozens of volts of ripple somewhere in the beam's path, but it's proving to be illusive.
There was some out-of-spec ripple in the low voltage supplies, but I've all but eliminated that by changing out some of the electrolytics (there's still a little more than spec on the -100V, but only by a few mV).
I can't measure any appreciable ripple on either side of the pair of neon bulbs between the cathode and the grid, and about half the time, when I touch my 1000x probe to those points, the problem disappears. On the storage targets of the cathodes of V913, I'm getting a ramp of about 400-500 mV, which, while I'm not sure what's up with that, it doesn't seem large enough (or the right shape) to be the source of this problem.
The external CRT cathode on the rear is grounded, and the switch has been cleaned and is in the chopped blanking position.

Any ideas?
thanks!
Paul





Paul
 

Thanks Harvey -
I think my next step is to change out those few HV caps in the neighborhood. They're all pretty low value disc caps, which tend not to be components I suspect when looking for 60Hz ripple, but I'm running out of better ideas.

Paul


Harvey White
 

The ripple shouldn't be 60 Hz, more like 30 to 60 Khz or so.  Tek used oscillators to minimize HV transformer weight and size.  I've had issues with parts where the probe touching the circuit makes it work beautifully.  That turned out to be a floating lead that I hadn't connected (IIRC).  probe loading would make it look like ground for a bit.  Those would be the HV filter capacitors, so looking at them with an HV probe could be helpful.

The higher the frequency, the more effective a smaller value of cap will be.  Some old scopes that ran the CRT off 60 Hz high voltage used some very large (and nasty) filter capacitors to keep ripple down.

Harvey

On 1/17/2021 4:01 PM, Paul wrote:
Thanks Harvey -
I think my next step is to change out those few HV caps in the neighborhood. They're all pretty low value disc caps, which tend not to be components I suspect when looking for 60Hz ripple, but I'm running out of better ideas.

Paul





Paul
 

Yeah, the HV supply works around 45kHz on this one, but this problem is definitely happening at line frequency. If I line sync the sweep the intensity banding is stationary.


Harvey White
 

Ok, then it's not the HV supply unless it's the power going into it.

This suggests two possibilities, one is that there's ripple somewhere on a supply line, (which you'd expect to be 120 Hz, not 60).  The second is ungrounded shielding somewhere, or a ground off, and inducing hum.  Does it get worse if you (a nice ungrounded 60 Hz antenna/source) get near (but not touching nasty stuff!) the circuit?

Harvey

On 1/17/2021 4:57 PM, Paul wrote:
Yeah, the HV supply works around 45kHz on this one, but this problem is definitely happening at line frequency. If I line sync the sweep the intensity banding is stationary.