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555 restoration


Sparky99
 

Hi everyone, I recently purchased a 555 - it arrived in very dusty and grubby, but has cleaned up to be in almost mint condition. Despite very clearly not being powered up for a very, very, long time (both fans were immobile ...) it more or less works in the more general sense of the word - a tribute to the quality and robustness of the engineering !

There is one problem however, which is why I'm posting here. On power up, both the upper and lower beam traces are displayed with good brightness. About 3 minutes later, the upper beam fades and then disappears. Turning up the intensity temporarily brings the trace back for a few seconds, but it then permanently disappears. The lower beam is still fine.

Powering the instrument off/on temporarily brings the upper beam trace back for a few seconds only, which seems to indicate that the problem is temperature related. Does anyone have any ideas where I should be looking of what the problem might be?

All the best

Julian


John Williams
 

Hi Julian. I would start as always by checking all the power supply voltages per the manual. But I think your problem is loss of high voltage, either because of a heat sensitive hv transformer or capacitor. You could remove the shield over the hv power supply for the beam that is failing. You might then cool that area with a fan or Freon spray. If the beam returns that might point to a component. If you have an old analog volt meter you can also measure some voltages. Be careful using a DMM though, this much voltage will eat most DMMs.


John Williams
 

One other thing to try. You could swap the hv oscillator tubes around. If one is failing it should move the fault to the other beam. Same with the hv regulators.


Richard Knoppow
 

Did you get the fan or fans going? If not that could be the problem. If so my guess is tired capacitors in the PS.

On 9/3/2019 2:55 PM, Sparky99 wrote:
Hi everyone, I recently purchased a 555 - it arrived in very dusty and grubby, but has cleaned up to be in almost mint condition. Despite very clearly not being powered up for a very, very, long time (both fans were immobile ...) it more or less works in the more general sense of the word - a tribute to the quality and robustness of the engineering !
There is one problem however, which is why I'm posting here. On power up, both the upper and lower beam traces are displayed with good brightness. About 3 minutes later, the upper beam fades and then disappears. Turning up the intensity temporarily brings the trace back for a few seconds, but it then permanently disappears. The lower beam is still fine.
Powering the instrument off/on temporarily brings the upper beam trace back for a few seconds only, which seems to indicate that the problem is temperature related. Does anyone have any ideas where I should be looking of what the problem might be?
All the best
Julian
--
Richard Knoppow
dickburk@ix.netcom.com
WB6KBL


Morris Odell
 

AFAIK the 555 does not have the HV transformer thermal runaway problem that plagues later models. One other component that sometimes causes these symptoms is the 1 nf resonating capacitor across the HV transformer primary. Try replacing it with a 630 or 1000 volt modern cap as a starter.

Morris


John Williams
 

Good point Morris. There are ways to tell if the hv is failing on the one beam side. You can turn out the lights in the room and check the hv rectifiers for the familiar red glow. If the hv has failed all of them could also be out. Or you can hold neon bulb close to the tubes in the hv secondary side and see if it glows.


Sparky99
 

Hi everyone, thanks for the replies - I investigated further tonight and indeed the red glow from both upper beam HV rectifiers disappears when the trace fades. I will investigate more tomorrow and post my findings!

Julian