Fluke/Philips PM3390A "Discharge/arch sounds" symptoms.

[Vtile]

Hi

I need expert advice now with this oscilloscope. For out of nowhere it have started to give a faint arc over kind of sounds, like while it is powered on.

The sound is not repetitive in nature, but constant (still random) I did open the cover, but can not visually detect any defects. Only thing is that around HV transformer it had gathered some minor black dust, which I did blow out with canned air. This didn't take the sounds out.

The sound is like a butterfly hitting its wing to some twig, that kind of low energy arc over kind of sound. I recal that old CRT TVs at times also did give these, I'm old enough to rember those, but unfortunately too young to have any technical exposure working with these CRTs.

Any ideas, should I just ignore it (nope) or stop using this (nice) scope until it is fixed. There is no other symptoms, ie
distortion etc. on screen.

[shakalnokturn]

Without getting your nose too close try smelling for ozone and looking at the HV areas in the dark for a faint purple glow.
If this scope uses a voltage multiplier that's external to the HV transformer that's the first place I'd search, it was quite common to have the insulation deteriorate on these "triplers" giving regular ticking sounds.

[HighVoltage]

Check the long high voltage cable. I had one before that was cracked on sparked once in a while to a ground clip.
It could also be the high voltage transformer, I had a broken one on a PM3380B.
I would also see if you have dirt on the CRT that carries to discharge the high voltage.
It should not be too complicated to find the problem.

BUT... be careful with the high voltage !!!!

[m k]

Only thing is that around HV transformer it had gathered some minor black dust, which I did blow out with canned air. This didn't take the sounds out.

Arcs have force, dust may have shapes.

[Vtile]

Ok, thanks all the ideas and advices. I think the first thing is to power it up at dark space to see if there is visible corona discharge to be observed.

The CRT have partial aluminium sheet metal shielding around it in this model / scope family. Is there high-voltage part(s) under it that may arc over and should I use compressed air (from compressor this time ..should be clean enough ) to clean the surface of CRT at both visible (around HT rubber connector) and unvisible surfaces under the sheet metal shield.

Was the dark coating in CRT outer surface a conducting material? Shielding of some form? I would assume one want not to have a dead short from HT cable to that surface when discharging the HT circuit.

I must do somekind of pokestick to safely discharge (a few 500V 3W 600k resistors in series should be sufficient) and then short the HT parts with pure copper.

If the trouble is HV tranny is the whole scope dead or are those so much standard parts that one could source a new one somewhere? I might try to impregnate it to epoxy or wax of some type anyway (or similar hack job).

How many kilovolts these scope CRTs usually have ... below 20kV? I just did remember I have my 120kV / 250MOhm voltage divider hanging on the wall under all posters, maybe I actually could measure the charge on CRT ... oh no!

[EHT]

The CRT have partial aluminium sheet metal shielding around it in this model / scope family

I suppose this is a magnetic shield, not aluminium but Mu-Metal or similar. Maybe you can remove it for this test?

[Vtile]

The CRT have partial aluminium sheet metal shielding around it in this model / scope family

I suppose this is a magnetic shield, not aluminium but Mu-Metal or similar. Maybe you can remove it for this test?

That Mu-metal magnetic shield it must be and that would make much more sense. I will see if I need / can remove it when I get space to my workbench to open the scope again. At now I'm middle of  project which takes all the room. I somehow don't want to touch the CRT only adjustment I have done for such was throwing a stone to one as a kid. 

[Vtile]

Ok. I did open the scope, did turn lights off from the room (no windows) and no nothing, only the sound is there can not visually locate it.

It does feel that the sound would originate from mains PSU, but again no visual anything. Can't see any corona discharge or arch over there or anywhere else. The original big mains electrolytes are looking just electrolytics of the era, no visual damage, no excessive heat (any above room temp by finger thermometer).

Annoying as ... tried even use a microphone and amplifier to pinpoint the source, but the fan is too loud (and the plug is somewhere under the tube) for this aproach.

Anyway sound is coming from the back portion of the scope and not from the front where the HT-transformer is.

[inse]

Block the blades with a screwdriver before turning on, usually no problem for brushless motors.

[Vtile]

I did make another round of noise finding, with the microphone and amplifier (another with higher low-pass frequency)... It seems the source is the IEC-plug or plug holder + mainswitch assembly. 

Oh, well hopefully on that part, would be a quick repair I hope. Need to get it disassembled in becoming day, when I have time again for this.

[m k]

If you can find a long enough stump of small plastic hose you can use it as a stethoscope.
Sound should be a bit different around the jack.

[SeanB]

RIFA strikes again.

[ResR]

I did make another round of noise finding, with the microphone and amplifier (another with higher low-pass frequency)... It seems the source is the IEC-plug or plug holder + mainswitch assembly. 

It could be that the female plug on the power cord is starting to get weak, a soon-to-be-faulty filter capacitor isn't out of the question either._

[Vtile]

If you can find a long enough stump of small plastic hose you can use it as a stethoscope.
Sound should be a bit different around the jack.

Oh damn. That is so genious old school trick! I actually used A Electronic stethoscope, but A non conducting hose is much more safe around high voltage circuits.

[Vtile]

RIFA strikes again.

Yes, it is on top3 of the source, it just seems that much small package with integrated fuse holder. Let See.

[Vtile]

I did make another round of noise finding, with the microphone and amplifier (another with higher low-pass frequency)... It seems the source is the IEC-plug or plug holder + mainswitch assembly. 

It could be that the female plug on the power cord is starting to get weak, a soon-to-be-faulty filter capacitor isn't out of the question either._

Now when I did locate the source as something around IEC connector assembly I did start also wonder if it could be the cords plug itself. Although it is not cheap New Wang hung low brand. Need to investigate when I have a few moments to spend on this.

Should I open some Bet-Service on one of those net Betting SITES.

[Vtile]

A new problem. I do not have a small dentist mirror.

Have anyone disconnected these Alpps "remote" switches soldered to this IEC-socket? It seems to have some form of miniature "energy chain" hausing a sheet metal strip between front panel push button to backpanel mainswitch. I have no idea how this would disconnect (need that mirror to see under the switch assembly)..

Oh well, some other day.

[Vtile]

I found no way to disconnect the "cord" from Allps, but I then did cut out the soldered links. It seems the alps switch is just hanging there semi loosely mostly by wiring.

Anyway, bypassed the IEC socket & line filter assy. No cracking sounds at all.. Now I still need to test the alpps switch if it is the source. IEC socket didn't make a sound either when I did try it out without a load.

[Jeff eelcr]

That was my first thought. Althought I did have a power switch do that also.
Jeff