Telequipment D83 problem

[Martin White]

Hi there,

Apologies if this has been covered, but i could find quite a few questions
on the net, but no answers! Was directed here from another newsgroup.

So, i have one of the above scopes from the 70's and as per quite a few
other people, one day about 3 years ago the trace just didn't come on. Last
night i finally got around to looking at it.

I've established that it at least has problems on the psu board. The
+24, -24 and +31v unreg are spot on (well, +31 is a little off, but hey,
it's unregulated!) but i only get +80v max with very little adjustment on
the +105v outputs. Outputs from the transformer are good and from the
rectifying diodes i get 139vDC so i assume that looks good too. I'm assuming
that the +105v is the driving voltage for the HV, focus etc.

Now then, i have the full manual and schematics for this beast, but i have
to confess that if it isn't digital microprocessor type electronics, i don't
much understand them. My thinking was that there are only 6 transistors in
the 105v section of the psu board and a bunch of resistors / caps so i was
going to just systematically check all the resistors and caps and then
shotgun all the transistors.

Before i do this though, as i hear that some of these transistors are
obsolete now is there anything else that could cause the low voltage output
other than the 105v psu section itself?

Any other suggestions at all - i know others have had this problem, but as i
say, i've not seen any answers?

I am hoping that with a return to 105v the trace will come back as you can
just get a glimpse of it flash on the screen when you switch the scope off.
It's like the beam is working but the deflection and focus are just not
happening...

Any help greatly appreciated.

Many thanks,
Martin.

 

[N Cook]

Hi there,

Apologies if this has been covered, but i could find quite a few questions
on the net, but no answers! Was directed here from another newsgroup.

So, i have one of the above scopes from the 70's and as per quite a few
other people, one day about 3 years ago the trace just didn't come on. Last
night i finally got around to looking at it.

I've established that it at least has problems on the psu board. The
+24, -24 and +31v unreg are spot on (well, +31 is a little off, but hey,
it's unregulated!) but i only get +80v max with very little adjustment on
the +105v outputs. Outputs from the transformer are good and from the
rectifying diodes i get 139vDC so i assume that looks good too. I'm assuming
that the +105v is the driving voltage for the HV, focus etc.

Now then, i have the full manual and schematics for this beast, but i have
to confess that if it isn't digital microprocessor type electronics, i don't
much understand them. My thinking was that there are only 6 transistors in
the 105v section of the psu board and a bunch of resistors / caps so i was
going to just systematically check all the resistors and caps and then
shotgun all the transistors.

Before i do this though, as i hear that some of these transistors are
obsolete now is there anything else that could cause the low voltage output
other than the 105v psu section itself?

Any other suggestions at all - i know others have had this problem, but as i
say, i've not seen any answers?

I am hoping that with a return to 105v the trace will come back as you can
just get a glimpse of it flash on the screen when you switch the scope off.
It's like the beam is working but the deflection and focus are just not
happening...

Any help greatly appreciated.

Many thanks,
Martin.

A number of D83 repair briefs (and D75 muchly the same) on the 3 repair
files below.
Your problem more likely the EHT supply osc. area than main ps.

electronic hints and repair briefs , schematics/manuals list on
http://homepages.tcp.co.uk/~diverse

 

[Martin White]

Thanks for the link.

To be honest, i've given up and it's gone back under the bench until i can
find someone that can fix it! Problem is, i looked for some of the parts and
most were obsolete, so i'm personally wasting my time as i won't have the
parts anyway.

However, i found that with SK401 unplugged, and hence no voltage even going
to the eht area i'm 25v down on the 105v. Plug it back in and that drops
right down to 10v so something in the eht section and probably also
something in the psu section too :O(

Martin.

 

[Charlie+]

On Sat, 28 Aug 2004 14:54:22 +0100, "Martin White"

Well it sounds as though you are not really keen on putting the effort
in! Id start off testing all the transistors in the main psu - a
doddle as all are in plug in holders, then the cans, then the diodes.
If you find a transistor gone I have some more modern numbers for
replacements. PSU is stab so you should be getting your 105V even with
no load. You may find that the psu problem is caused by eht partial
failure as you say. On mine the eht ladder resistors had changed very
substantially with old age!
Charlie+

 

[Charlie+]

On Sat, 28 Aug 2004 14:54:22 +0100, "Martin White"

Well it sounds as though you are not really keen on putting the effort
in! Id start off testing all the transistors in the main psu - a
doddle as all are in plug in holders, then the cans, then the diodes.
If you find a transistor gone I have some more modern numbers for
replacements. PSU is stab so you should be getting your 105V even with
no load. You may find that the psu problem is caused by eht partial
failure as you say. On mine the eht ladder resistors had changed very
substantially with old age!
Charlie+

 

[Martin White]

I'll revisit when i get back from a work trip :O)

The main reason i was probably sounding like i wasn't willing to put in the
effort was because i couldn't find the info on the transistors (probably
down to me not knowing where to look more than anything admittedly!) so it
was a simple case of no point in putting in the effort if i wouldn't be able
to find replacements for any bad parts i found anyway - if you see what i
mean! - made sense to me :O)

SO, many thanks for coming forward. I WILL put in the effort as this was a
birthday present a few years back and would therefore love to get it going
again.

I'm sure those equivalent part number are likely to come in handy.

Thanks,
Martin.