On Sat, 20 Feb 2016 09:36:18 -0500, JC wrote:

The transformer secondaries go (on the left side viewed from the top of
the transformer).

1.5kV 1.0kV Gap/no pin HV Common 60 60 12 12 6
6
0

Well on that basis there may be nothing wrong after all.

As others have pointed out this PSU will not run happy without a load
and I don't know what would be suitable. When I worked on these I always
just left the psu connected to the scope. Lets face it, the scopes been
turned on at some point with the psu connected so its not going to do
much more damage and at least you will know the loading is correct. The
EHT multipliers on these break down internally on these.

It's not possible to test this board with it connected to the scope. On
this model, it slots inside the two main signal boards which make access
under proper, full working conditions impossible. Just *another* obstacle
I've faced with this repair.
The EHT multiplier has been totally disconnected all through my tests
except where explicitly stated otherwise.


In one of your pictures there are a couple of diodes that look messed up
(V1809 and V1811) near the bridge. They are supposed to be BY208-1000
(1000v rectifiers), I can see "40" on one, maybe 1N4007?

I like your thinking! But no, the one nearest the bridge is a BY208-1000
alright, the other one to the side of it is a BY134. They both tested
fine out of circuit.

On 2/21/2016 8:36 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 09:36:18 -0500, JC wrote:


It's not possible to test this board with it connected to the scope. On
this model, it slots inside the two main signal boards which make access
under proper, full working conditions impossible. Just *another* obstacle
I've faced with this repair.
The EHT multiplier has been totally disconnected all through my tests
except where explicitly stated otherwise.

Hi, Its been some time since I worked on these but I'm pretty sure we
ran these with the board out, turned round so you can get the connectors
on and I guess without the HT connected. Alternately put a suitable load
on the PSU.


In one of your pictures there are a couple of diodes that look messed up
(V1809 and V1811) near the bridge. They are supposed to be BY208-1000
(1000v rectifiers), I can see "40" on one, maybe 1N4007?

I like your thinking! But no, the one nearest the bridge is a BY208-1000
alright, the other one to the side of it is a BY134. They both tested
fine out of circuit.


That might be one problem, the sine voltage around T1801 is 800v, your
BY134 is a 600V diode. Also HV diodes can go reverse leaky, try a high
ohmsmeter on it (10-20 meg range). Shouldn't be any reverse leakage.

I guess you saw my next post on this? Try a load on the board before you
do any more work. It will tell you if the PSU runs silent or not under
load. The one I tried was screaming like heck then silent with a load.

On Sun, 21 Feb 2016 09:46:52 -0500, JC wrote:

That might be one problem, the sine voltage around T1801 is 800v, your
BY134 is a 600V diode. Also HV diodes can go reverse leaky, try a high
ohmsmeter on it (10-20 meg range). Shouldn't be any reverse leakage.

Will a DVM suffice or should I do this with my faithful old analogue AVO?

I guess you saw my next post on this? Try a load on the board before you
do any more work. It will tell you if the PSU runs silent or not under
load. The one I tried was screaming like heck then silent with a load.

You're a late-comer to this party, so you will be unaware that even when
tested under full working conditions with all the loads plugged in, this
twitcher/switcher still hisses and the 20 Ohm power resistor R1814 (just
below right from the chopper transistor on the schematic) quickly starts
to burn up.

I take your point on the dummy load, though. I must rig one up before
doing any more live testing.

On Sun, 21 Feb 2016 13:36:49 -0000 (UTC), Cursitor Doom
wrote:

snip
In one of your pictures there are a couple of diodes that look messed up
(V1809 and V1811) near the bridge. They are supposed to be BY208-1000
(1000v rectifiers), I can see "40" on one, maybe 1N4007?

I like your thinking! But no, the one nearest the bridge is a BY208-1000
alright, the other one to the side of it is a BY134. They both tested
fine out of circuit.

The BY134 is a lower frequency part with 2uS recovery time and is
probably unsuited to replacement of BY208-1000 in any of the snubber
or conversion positions indicated on the schematic primary. It should
be soft recovery, medium speed (200-600nS) avalanche-rated part with a
minimum 800Vprv.

I'd avoid the use of anything advertised as 'ultrafast' (ie UF4007),
as this circuit may need a modest recovery time in order to reduce
power loss and EMI, but they could be used temporarily in
troubleshooting.

RL