10,000 M Ohm resistor !! ??

[MartyMart]

where does HV come from?
voltage on both sides of the 82k towards ground?
raw HV? (probably small blue xfmr...)

Medium size black XFMR ( 3cm X 3cm X 2.5cm )
450 volt 15uf caps to ground, 82k across the + of them, then on to 100k plate resistor.
This "node" should read 300v DC,  it's a nominal 0.81 volts.

MM.

[A.S.P.]

0.81V: there you are... 

[A.S.P.]

check, if the traces from the low voltage side of the xfmr lead to any fuse.
measure the HV diodes back-/forwards resistance (PLUG OUT POWER SUPPLY  BEFORE!).
measure the xfmr windings resistances...

[MartyMart]

No fuse ( only the 240v line slow blow )
Of 11 1N4007 diodes, 10 show forward current of 0.7Xv
1 shows forward current of 0.034v ... !

I'm going to remove the "burnt up" area parts once more ( though when removed measured all OK )
The "pads" got a bit damaged, so I may need to scrape back some copper trace and re-do that section.
This is where the 1N4007 reads 0.034v .... It's a "new" diode though !
There was some "over heating" here, but couldn't find any failed components .
Onwards,  I'm being very slow and carefull ... this isn't 9v's that you can put across your tongue   

thanks,
MM

[A.S.P.]

there`s been a reason, why the diode gave up...
probably not overvoltage, coz they`re 1000V rated.

overcurrent? for the 1A they`re rated, it musta been a heavy short...
(I`ve had the case, that a tiny snipped off wire fell through the ventilation-
openings of my scope...).

maybe the tube had a plate/cathode short inside,
this caused the "10000M" resistor to overheat and play chameleon,
and finally the weakest diode couldn`t take it anymore...
hopefully, it didn`t take the xfmr with it into nirvana!

[MartyMart]

Thanks ASP, you're being VERY helpful here
I think its the transformer, which I don't fully understand :
There's one large Torodial ( 10cm X 10cm ) and this supplies the original
22v line .... its working, this is first thing after 240v in/caps/fuse
This "smaller" block type on the schem say's "EI 15/250 2VA"
This looks like it's 15v in 250v out which is then rectified up to 300v ( which it isnt now )
this 300 hits the large cap/resistor network to the plate ...
When looking at replacements, they all say "input 240v output 2X 15v 2VA"  ?
Can these be reversed for "step up" ? when I look at "step up" transformers they are
huge metal block things, not a small plastic package for 8 bucks, like this !!

Hmm, Confused and not sure what's the scene here ....

MM.

[A.S.P.]

"input 240v output 2X 15v 2VA"  ?

that`s what they say...

but they can do opposite, when you use them backwards!

(many threads in the "search"-function...)

[gez]

when I look at "step up" transformers they are
huge metal block things, not a small plastic package for 8 bucks, like this !!

As mentioned, just use a stepdown in reverse.  If it just powers the plates there shouldn't be a problem with exceeding power ratings.  Plenty of dinky sealed transformers around (even Craplin sell them!).

[MartyMart]

Thanks Chaps .... exactly what i wanted to hear !
think I'll go with an RS (Radio spares) one for £8

MM.

[A.S.P.]

too much for a 2VA...

have you measured winding-resistances?

[Paul Perry (Frostwave)]

I've seen a resistor not too far off this value.. it was a couple of feet long, on glass, immersed in oil, as part of a 150,000V transmission electron microscope. No way to get in in a BB case...

[MartyMart]

too much for a 2VA...

have you measured winding-resistances?

Yeah, it's about £3.00  !
on it's way along with a bunch of new 4w and 1w resistors.
15v secondary side is OK  250v primary is DOA !
The 2w 15ohm r directly connects to the 15v side and that's where
voltage readings end ....
I found a replacement, same spec and same physical size ( to fit the board )
though it's a "15v/230v 2VA" not "15v/250v 2VA", so may have to adjust the plate
resistor value ... ? that's a 100k at the 'mo ... or will that work fine ?

MM.