Cossor CDU150 Repair Help

[jimmy1975]

Hello,

I have an old Cossor CDU150 35Mhz CRO, it had recently become a bit temperamental; erratic triggering, three or so of the timebases settings not sweeping, all of which I could tolerate until I brought a DSO to replace it.

However when setting down from its flip down front stand it (possible knock) it gave an erratic trace, and the sound of arcing. I turned it off and then eventually turned it on again to check, the arcing sound remained but with no trace and a smell of ozone from the arc.

 I removed the back panel to asses the voltage multiplier and transformer, there was a minute (very difficult to see even in the dark) arc on a solder joint of a HV diode of the voltage multiplier.
Strange thing is, it doesn't seem to arc to anything. The arc is as long as it is wide, only a small fraction of a millimeter or less.

If I tap the scope, occasionally, it will give a centered dot on the screen, which makes me think there is a loose connection. I though the problem was with the voltage multiplier but if the anode lead is placed near the chassis and the scope powered up, it readily produces a large arc.

I believe I have a pdf of the service manual somewhere, but I'm not that knowledgeable of scopes.

Any Ideas?

Thank you

[gilbjd]

Try re-soldering the arcing joint. It might be dry.

[jimmy1975]

Cheers, I re-soldered the joint and checked the resistance with a multimeter, resistance was negligible. It is still making an arcing sound though.

[BurningTantalum]

A bit late with this but just found the thread:

I have had my Cossor CDU150 for 34 odd years and I still use it regularly. It had the same issue when new to me. I seem to recall that after much reflowing and messing around with the EHT stack in the back cover, I 'potted' it in clear silicone rubber using a card mould lined with polythene (removed afterwards...). I have never had any grief with it since.

Regards,  Burning Tantalum

[Zepnat]

I bought one of these from a radio rally years ago the guy said they were good scopes but could do with a fan adding as something gets a bit warm inside, hope you get it fixed, mine has channel 2 faulty is there any service information online?

[N2IXK]

An "arc" that doesn't appear to be between contacts or from contact to ground could be a corona discharge into the surrounding air.

Clean everything well, make sure the solder joint is smooth and rounded (no sharp points), and if the discharge is still happening, cover the terminal with insulating varnish or silicone rubber. Back when CRT TV sets were in common use, service shops used a brush-on goop called "corona dope" when this problem happened in CRT high voltage supplies.