Electronics > Repair

Cossor CDU150 Repair Help

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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?

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