Please help me fix my oscilloscope....

[JaMin]

***n00b alert***

A big hello to everyone!!, I'm new here so please go easy on me.

I'm hoping your collective knowledge can help me with a little problem...
A collegue has recently given me a 20Mhz Gould 1604 DSO. When he gave it to me I quickly powered it up to see if it worked, and it did
However, after I had cleared a space on my bench to give it pride of place   After the smoke had cleared, it still seemed to work (self test on power on) but nothing on the screen.
I opened it up and found a suspiciously charred .01uF 5kv capacitor which I think is on the driver for the CRT. That must be the culprit I thought.
After a lot of searching the only place I could find a cap with the same rating was from LCR (the company who made the burnt one back in 1989 when the scope was born). Trouble is... they want £25 for a new one!! £25!!
I didn't really want to gamble that kinda money on a guess as I don't have the skills of equipment to prove if that was the fault.
Now, I found on RS a 10nF 3kv capcitor.  I am aware that .01uF is 10nF. I am also aware that 3kv is most certainly not 5kv.
So, after some deep contemplation and a couple of pints of dutch courage, I decided to replace the capacitor.
Nervously I hit the power button.........ya know what?.............it only bloody works!!!!   QUICK turn it off!!!

My question to the collective is: WHAT NOW?
A: forget it ever happened, use it, job done! (it didn't need a 5kv anyway, thats all they had on the shelf)
B: pay £25 and replace it with the correct part
C: replace it with a ceramic cap of the correct rating (they are MUCH smaller, must be a reason for that?)

Any thoughts, suggestions or ideas would be great

Thanks

James

[Andy Watson]

Find a copy of the service manual and schematic. Identify the component and assess what its working voltage is. It could be that 5kV was the only rating available at the time of manufacture, or, more likely, your 3kV part is sitting there quietly smouldering and waiting for the most inopportune moment to release its magic smoke.

[PA0PBZ]

Get 2 20nF 3KV ones, put in series.

[JaMin]

Thanks for the quick reply :-)
I've searched for a service manual but the only one I can find is incomplete. Would you believe my luck, it ends a few pages before the section relating to the crt.
I have found a schematic and have identified the capacitor on there, but to be honest I don't really understand what I'm looking at... It's beyond my level of knowledge as I'm only really just starting my journey in the bewildering world of electronics. Unfortunately the schematics don't give values of components either.
I'm not using the scope at the moment as I'm a bit nervous that if the cap does liberate the magic smoke, it might take something else with it.
I'll post the link to the schematic when I'm back at my computer. Maybe you would be kind enough to take a look?

Sent from my HTC One

[JaMin]

Get 2 20nF 3KV ones, put in series.

Thats a good idea! I had wondered if I could do something like that but wasn't sure if it would work?
Think I read somewhere that caps in series worked like resistors in parallel, but wasn't sure what would happen to the voltage rating.

[JaMin]

https://www.dropbox.com/s/l3992eg1ehmjwbe/Gould%201602%201604%20schematics.pdf

Here's the link for the schematics that I found.

I think it's 'C53' on Fig.6.16a Page 16.

Don't know if this is really usefull without knowing the values of the other components (not that I'd know what to do if the values were there )

[PA0PBZ]

Think I read somewhere that caps in series worked like resistors in parallel, but wasn't sure what would happen to the voltage rating.

Each will get half the voltage across, it's the same with 2 (equal) resistors in series, only then you double the resistance while with capacitors it halves the capacitance.

[Andy Watson]

Page 49 says: "P4   -1700V  trace blanked".  That's the voltage on the other side of R104. Since it is supplying a grid I would expect negligible current flow and therefore negligible voltage across R104. So C53 is working at -1700V plus whatever the blanking signal is (tens of volts probably). Looks like your lucks in - 3kV should cover it.

[HackedFridgeMagnet]

If you go the series caps route than you have to watch your creepage and clearance distances between the three nodes of the two series caps.
You should look up creepage and clearance distances but at a guess about 6mm for 3kV and 9mm for 5kv minimum.

[JaMin]

Guy, thank you all so much for your help
I think using 2 caps in series might give me problems, after doing a bit of reading about clearance I don't think I'll be able to keep them far enough apart from the chassis let alone each other! It's quite tight in there.

Page 49 says: "P4   -1700V  trace blanked".  That's the voltage on the other side of R104. Since it is supplying a grid I would expect negligible current flow and therefore negligible voltage across R104. So C53 is working at -1700V plus whatever the blanking signal is (tens of volts probably). Looks like your lucks in - 3kV should cover it.

On the strength of what you have said, I think I'm gonna leave it be and hope for the best. (if it goes bang I'm holding you entirley responsible )
Andy, I'm very greatful for you trawling through the service manual to help me! Have you got the complete manual or the same one I have that runs out at page 68?

Thanks again

[Andy Watson]

Andy, I'm very greatful for you trawling through the service manual to help me! Have you got the complete manual or the same one I have that runs out at page 68?

It was the first manual that I found on the internet. Loads of scribbling on the front page.

[JaMin]

That sounds like the same one I found too.

Anyway, I had a play with the scope last night (I say play because I don't know what I'm doing) and it was on for over an hour.
No excesive heat, no strange smells, and no magic smoke

All seems to work fine, but I can't get ch2 to display a signal. Probably something I'm doing wrong as the other 3 channels apear to work. Or maybe I disturbed something while I had it in bits....
Thats for another day though, in the mean time I've got LOTS to learn.