On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:

Recheck pin function before jumping to conclusions.

Right; now re-checked. DC measurments proved (unsurprisingly) too close
together so I re-tested using 100khz instead. These are the impedances WRT
ground of the output taps of the long winding in the order they actually
come out of the transformer:
GND, 0.17ohms, 0.17ohms, 0.26ohms, 0.28ohms, 3.7ohms, 3.8ohms.
So this doesn't seem to tally up with the schematic. Or does it? I need a
pint of strong coffee to kick-start my head on this one. :-/
Anyway, later...


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

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.

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?

All noted, thank you, gentlemen. I'll have to check those tips out
tomorrow or a divorce will be in the offing.
Until I report back tomorrow then, thanks...

On 2/20/2016 11:52 AM, Cursitor Doom wrote:
All noted, thank you, gentlemen. I'll have to check those tips out
tomorrow or a divorce will be in the offing.
Until I report back tomorrow then, thanks...


I managed to find a PM3264 PSU to try out.

Unloaded it squeals as expected so I tried a makeshift load with what I
had lying around, 6 x 470R 5Watt w/w resistors.

You can pull one of the connectors out of the scope for a connection.
(See photos)

Just for fun (and I'm running this off an isolation transformer), pull
V1812 and scope T2 with T1 as probe ground. you should see a nice drive
waveform for a few seconds and you can check the frequency is 20KHz.

Incidentally the core on L1806 on this board was loose (came apart) and
also caused squealing but of a different note.

If you want me to take any readings let me know, nothing too time
consuming though

Photos of load (It gets hot so take care)

https://www.flickr.com/photos/404665.../shares/H24830
https://www.flickr.com/photos/404665.../shares/J18jga

On Sat, 20 Feb 2016 15:45:34 -0500, JC wrote:

Have you posted this before on another thread related to this? Either you
have or it's deja vu. Sorry if you did, I didn't note it properly.

I managed to find a PM3264 PSU to try out.

Unloaded it squeals as expected so I tried a makeshift load with what I
had lying around, 6 x 470R 5Watt w/w resistors.

You can pull one of the connectors out of the scope for a connection.
(See photos)

Yeah, those Stocko connectors. On another Philips manual I have for a
different scope, they actually publish the proper values for a dummy load
which would be really helpful to have on the current problem I face.

Just for fun (and I'm running this off an isolation transformer), pull
V1812 and scope T2 with T1 as probe ground. you should see a nice drive
waveform for a few seconds and you can check the frequency is 20KHz.

Not sure why you say pull V1812, but here's the waveform I got between
those two points when I did this test a few weeks ago:

https://www.flickr.com/photos/128859...n/photostream/

But then it goes downhill. Here's V1812's C/E junction:

https://www.flickr.com/photos/128859...n/photostream/

And the B/E junction of the main chopper transistor:

https://www.flickr.com/photos/128859...n/photostream/

Not surprising it doesn't work properly with control voltages like that!!!

On 2/21/2016 9:34 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 15:45:34 -0500, JC wrote:

Have you posted this before on another thread related to this? Either you
have or it's deja vu. Sorry if you did, I didn't note it properly.

I managed to find a PM3264 PSU to try out.

Unloaded it squeals as expected so I tried a makeshift load with what I
had lying around, 6 x 470R 5Watt w/w resistors.

You can pull one of the connectors out of the scope for a connection.
(See photos)

Yeah, those Stocko connectors. On another Philips manual I have for a
different scope, they actually publish the proper values for a dummy load
which would be really helpful to have on the current problem I face.

Just for fun (and I'm running this off an isolation transformer), pull
V1812 and scope T2 with T1 as probe ground. you should see a nice drive
waveform for a few seconds and you can check the frequency is 20KHz.

Not sure why you say pull V1812, but here's the waveform I got between
those two points when I did this test a few weeks ago:

https://www.flickr.com/photos/128859...n/photostream/

But then it goes downhill. Here's V1812's C/E junction:

https://www.flickr.com/photos/128859...n/photostream/

And the B/E junction of the main chopper transistor:

https://www.flickr.com/photos/128859...n/photostream/

Not surprising it doesn't work properly with control voltages like that!!!

Pulling V1812 lets you see the clean output from the driver chip without
all the crap feedback from the transformers. Your frequency looks good.

I got the same crap and ringing/distortion on my PSU without a load. Put
a load on it. switchers don't work off load.

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

On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:

Recheck pin function before jumping to conclusions.

Right; now re-checked. DC measurments proved (unsurprisingly) too
close together so I re-tested using 100khz instead. These are the
impedances WRT ground of the output taps of the long winding in the
order they actually come out of the transformer: GND, 0.17ohms,
0.17ohms, 0.26ohms, 0.28ohms, 3.7ohms, 3.8ohms. So this doesn't
seem to tally up with the schematic. Or does it? I need a pint of
strong coffee to kick-start my head on this one. :-/ Anyway,
later...
...
... 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?

Well, if that is true then beware! V1808, V1809 and V1811 are supposed
to be very fast. Any slow (more than a microsecond) diode in these
positions will likely cause symptoms akin to a heavy overload.

Particularly V1811, if replaced with any 1N400x, is likely to render the
energy recovery circuit around L1806 as good as inoperative, thereby
dumping the entire energy from the switcher harmonics into R1814, which
will cause it to overheat fast.

Please recheck L1806 (both windings) for turn-to-turn shorts (with a
signal generator), and if any of the 3 diodes (V1808, V1809, V1811)
looks like it had previously been replaced (possibly improperly
replaced), consider replacing all 3 of them together, using the proper
parts.

Use fast soft-recovery diodes rated for 1kV here. If you can't find any,
use ultrafast ones. They're maybe not optimal from an EMI standpoint
here, but at least they should work well enough for testing.

If you can't find a BY208-1000 replacement, a MUR4100E should work.

Check C1806 for dielectric breakdown. It should be able to withstand at
least 500 V (or something in that ballpark). If it doesn't, replace.

Don't underestimate that L1806 energy recovery circuit. Although it
doesn't by itself transfer any power to the load, this supply heavily
relies on it for proper resonant operation of the main transformer. It
must be working properly before you can test the main transformer
waveform and have any chance of making correct measurements.

Besides, your description of heavy switching noise on V1806 (when you
tried to measure the base drive waveform), up to the point of the
waveform being unrecognizable in the noise, seems to indicate that the
L1806 circuit is shorted at high frequencies. This can be a result of
either a winding short in L1806 or a breakdown in one of its diodes or
some of these diodes being replaced by a generic slow silicon diode.

Dimitrij

On 21.02.2016 18:27, Dimitrij Klingbeil wrote:
On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:

Recheck pin function before jumping to conclusions.

Right; now re-checked. DC measurments proved (unsurprisingly)
too close together so I re-tested using 100khz instead. These are
the impedances WRT ground of the output taps of the long winding
in the order they actually come out of the transformer: GND,
0.17ohms, 0.17ohms, 0.26ohms, 0.28ohms, 3.7ohms, 3.8ohms. So this
doesn't seem to tally up with the schematic. Or does it? I need a
pint of strong coffee to kick-start my head on this one. :-/
Anyway, later...
... 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?

If you can't find a BY208-1000 replacement, a MUR4100E(G) should
work.

Sorry, that may be physically too big to fit. A MUR1100EG or something
similar should work and fit in the available space too.

Dimitrij

On Sun, 21 Feb 2016 20:09:47 +0100, Dimitrij Klingbeil wrote:

On 21.02.2016 18:27, Dimitrij Klingbeil wrote:
On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:

Recheck pin function before jumping to conclusions.

Right; now re-checked. DC measurments proved (unsurprisingly) too
close together so I re-tested using 100khz instead. These are the
impedances WRT ground of the output taps of the long winding in the
order they actually come out of the transformer: GND, 0.17ohms,
0.17ohms, 0.26ohms, 0.28ohms, 3.7ohms, 3.8ohms. So this doesn't seem
to tally up with the schematic. Or does it? I need a pint of strong
coffee to kick-start my head on this one. :-/ Anyway, later...
... 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?

If you can't find a BY208-1000 replacement, a MUR4100E(G) should work.

Sorry, that may be physically too big to fit. A MUR1100EG or something
similar should work and fit in the available space too.

Dimitrij

Thank you. If you've had the chance to read my follow up to JC (I think
it was) then you'll be aware that one of those BY208 diodes was replaced
by a BY134. If the design is that critical of the speed of the diodes it
uses then maybe it won't function properly as you suggest. I can only
imagine the technician who replaced it was unaware of the critical nature
of the part concerned.
I'm kind of unhappy with this design overall, to be honest. It was
critically appraised on s.e.d and found generally unsatisfactory. I'm
strongly tempted to just save the transformers, junk everything else and
start afresh with a modern design. The rest of the scope is mint and
untouched, it's only the psu section that's been butchered around and
shows signs of burning and scorching in places. Maybe the best thing to
do would be to bin it? :-/

On 2/21/2016 3:59 PM, Cursitor Doom wrote:
On Sun, 21 Feb 2016 20:09:47 +0100, Dimitrij Klingbeil wrote:

On 21.02.2016 18:27, Dimitrij Klingbeil wrote:
On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:


Thank you. If you've had the chance to read my follow up to JC (I think
it was) then you'll be aware that one of those BY208 diodes was replaced
by a BY134. If the design is that critical of the speed of the diodes it
uses then maybe it won't function properly as you suggest. I can only
imagine the technician who replaced it was unaware of the critical nature
of the part concerned.
I'm kind of unhappy with this design overall, to be honest. It was
critically appraised on s.e.d and found generally unsatisfactory. I'm
strongly tempted to just save the transformers, junk everything else and
start afresh with a modern design. The rest of the scope is mint and
untouched, it's only the psu section that's been butchered around and
shows signs of burning and scorching in places. Maybe the best thing to
do would be to bin it? :-/

Switching supplies are all designed with the diode (and other component)
parameters in mind, its how they function efficiently. Bunging any old
diode in is asking for trouble. Same as the ESR and temp rating of the
caps used. Seriously you need to sit back and chill. These scopes were
very well designed and I would say exceptionally reliable. I'd like to
see you build a replacement. I've designed switching supplies,
everything needs to be right or it goes wrong fast.

FR107G seems to be the current equivalent for the BY208-1000

On 2/21/2016 4:28 PM, JC wrote:
On 2/21/2016 3:59 PM, Cursitor Doom wrote:
On Sun, 21 Feb 2016 20:09:47 +0100, Dimitrij Klingbeil wrote:

On 21.02.2016 18:27, Dimitrij Klingbeil wrote:
On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:


Thank you. If you've had the chance to read my follow up to JC (I think
it was) then you'll be aware that one of those BY208 diodes was replaced
by a BY134. If the design is that critical of the speed of the diodes it
uses then maybe it won't function properly as you suggest. I can only
imagine the technician who replaced it was unaware of the critical nature
of the part concerned.
I'm kind of unhappy with this design overall, to be honest. It was
critically appraised on s.e.d and found generally unsatisfactory. I'm
strongly tempted to just save the transformers, junk everything else and
start afresh with a modern design. The rest of the scope is mint and
untouched, it's only the psu section that's been butchered around and
shows signs of burning and scorching in places. Maybe the best thing to
do would be to bin it? :-/

Switching supplies are all designed with the diode (and other component)
parameters in mind, its how they function efficiently. Bunging any old
diode in is asking for trouble. Same as the ESR and temp rating of the
caps used. Seriously you need to sit back and chill. These scopes were
very well designed and I would say exceptionally reliable. I'd like to
see you build a replacement. I've designed switching supplies,
everything needs to be right or it goes wrong fast.

FR107G seems to be the current equivalent for the BY208-1000

FR107G Ebay (UK) #390565307743 cheap enough.

On Sun, 21 Feb 2016 17:13:55 -0500, JC wrote:

On 2/21/2016 4:28 PM, JC wrote:
On 2/21/2016 3:59 PM, Cursitor Doom wrote:
On Sun, 21 Feb 2016 20:09:47 +0100, Dimitrij Klingbeil wrote:

On 21.02.2016 18:27, Dimitrij Klingbeil wrote:
On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:


Thank you. If you've had the chance to read my follow up to JC (I
think it was) then you'll be aware that one of those BY208 diodes was
replaced by a BY134. If the design is that critical of the speed of
the diodes it uses then maybe it won't function properly as you
suggest. I can only imagine the technician who replaced it was unaware
of the critical nature of the part concerned.
I'm kind of unhappy with this design overall, to be honest. It was
critically appraised on s.e.d and found generally unsatisfactory. I'm
strongly tempted to just save the transformers, junk everything else
and start afresh with a modern design. The rest of the scope is mint
and untouched, it's only the psu section that's been butchered around
and shows signs of burning and scorching in places. Maybe the best
thing to do would be to bin it? :-/

Switching supplies are all designed with the diode (and other
component)
parameters in mind, its how they function efficiently. Bunging any old
diode in is asking for trouble. Same as the ESR and temp rating of the
caps used. Seriously you need to sit back and chill. These scopes were
very well designed and I would say exceptionally reliable. I'd like to
see you build a replacement. I've designed switching supplies,
everything needs to be right or it goes wrong fast.

FR107G seems to be the current equivalent for the BY208-1000

FR107G Ebay (UK) #390565307743 cheap enough.

Thank you, JC. You and Dimitrij have both given me some subs for suitable
replacements which I'm quite happy to go along with. But I'm not prepared
to spend much more time on this repair, to be honest. I'd relish the
prospect of a comprehensive re-design. Even if it's beyond me at this
stage I'd learn a lot from it.

On Sun, 21 Feb 2016 16:28:50 -0500, JC wrote:

On 2/21/2016 3:59 PM, Cursitor Doom wrote:
On Sun, 21 Feb 2016 20:09:47 +0100, Dimitrij Klingbeil wrote:

On 21.02.2016 18:27, Dimitrij Klingbeil wrote:
On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:


Thank you. If you've had the chance to read my follow up to JC (I think
it was) then you'll be aware that one of those BY208 diodes was
replaced by a BY134. If the design is that critical of the speed of the
diodes it uses then maybe it won't function properly as you suggest. I
can only imagine the technician who replaced it was unaware of the
critical nature of the part concerned.
I'm kind of unhappy with this design overall, to be honest. It was
critically appraised on s.e.d and found generally unsatisfactory. I'm
strongly tempted to just save the transformers, junk everything else
and start afresh with a modern design. The rest of the scope is mint
and untouched, it's only the psu section that's been butchered around
and shows signs of burning and scorching in places. Maybe the best
thing to do would be to bin it? :-/

Switching supplies are all designed with the diode (and other component)
parameters in mind, its how they function efficiently. Bunging any old
diode in is asking for trouble. Same as the ESR and temp rating of the
caps used. Seriously you need to sit back and chill. These scopes were
very well designed and I would say exceptionally reliable. I'd like to
see you build a replacement. I've designed switching supplies,
everything needs to be right or it goes wrong fast.

FR107G seems to be the current equivalent for the BY208-1000

Well, the modern ones may well be super reliable, but this old thing is
very dated and shows many signs of its age and the scars of previous
faults and questionable repairs. I wouldn't attempt another resonant
converter; there must be something simpler with fewer critical
parameters, surely.

On 21.02.2016 21:59, Cursitor Doom wrote:
On Sun, 21 Feb 2016 20:09:47 +0100, Dimitrij Klingbeil wrote:

On 21.02.2016 18:27, Dimitrij Klingbeil wrote:
On 20.02.2016 15:36, JC wrote:
On 2/20/2016 7:55 AM, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:

Recheck pin function before jumping to conclusions.

Right; now re-checked. DC measurments proved (unsurprisingly)
too close together so I re-tested using 100khz instead. These
are the impedances WRT ground of the output taps of the long
winding in the order they actually come out of the
transformer: GND, 0.17ohms, 0.17ohms, 0.26ohms, 0.28ohms,
3.7ohms, 3.8ohms. So this doesn't seem to tally up with the
schematic. Or does it? I need a pint of strong coffee to
kick-start my head on this one. :-/ Anyway, later...
... 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?

If you can't find a BY208-1000 replacement, a MUR4100E(G) should
work.

Sorry, that may be physically too big to fit. A MUR1100EG or
something similar should work and fit in the available space too.

Dimitrij

Thank you. If you've had the chance to read my follow up to JC (I
think it was) then you'll be aware that one of those BY208 diodes was
replaced by a BY134. If the design is that critical of the speed of
the diodes it uses then maybe it won't function properly as you
suggest. I can only imagine the technician who replaced it was
unaware of the critical nature of the part concerned. I'm kind of
unhappy with this design overall, to be honest. It was critically
appraised on s.e.d and found generally unsatisfactory. I'm strongly
tempted to just save the transformers, junk everything else and start
afresh with a modern design. The rest of the scope is mint and
untouched, it's only the psu section that's been butchered around and
shows signs of burning and scorching in places. Maybe the best thing
to do would be to bin it? :-/

Hi

As for the BY134, sorry, I must have overlooked that somehow, or maybe
it did not register in my memory right away. Anyway, it's just as bad a
choice as a 1N4007 and its ilk. It's designed for mains rectification
and doesn't even make an attempt at being fast.

No use in an active snubber or energy recovery circuit whose task it is
to "strip out" the high frequency components from a square wave.

Get rid of it, and while you're at it, consider the condition of the
other two (V1808+9) identical ones. Sometimes a person who does an
improper repair will try swapping nearby components hoping that another
one might be "less critical". So if you see signs of unprofessional
manual soldering on them, take that whole trinity and replace them.

Same with C1806. If it looks suspicious, does not pass a withstand test
at some 105% of its rated voltage or shows high ESR, change it too.

BY208-1000s are hard to come by nowadays, so here is a list of some more
modern candidates: MUR1100E, BYV26E, UF4007. They should fit, and even
though they are faster than the original BY208-1000, they should work.

There are also: RGP30M (slightly large, modest speed), UF5408 (slightly
large), MUR4100E (slightly large), STTH112U (smd), US1M (smd) BYG23M
(smd). They may or may not fit due to size and space constraints, and
the SMD ones would likely need some wire leads soldered on (won't look
professional, but hey, if others are too hard to come by, that's ok).

Once you've fixed that botched repair on the energy recovery circuit,
connect a taillight lamp to the 12.7 V output, test it again and tell
your results here (make sure you put all the proper parts back in,
before you switch it on, this supply may be unforgiving if any parts
are missing and it's powered on).

As for the design being "generally unsatisfactory", let me disagree.
Resonant converters do have a well earned place in the world of power
electronics, but the design of them is, in a way, a black art. They have
lots of pitfalls for the unwary and not so many engineers can actually
design them properly and they tend to use special components (inductive
ones in particular) that would be rather unsuitable for other topologies
too. Yet they do have certain benefits, low noise operation that is
suitable for sensitive measurement instruments, being one of them. They
are not so easy to understand, compared to "simple" flyback topology
supplies - so people go screaming "this is too complex" or "this uses
too many parts". In fact your supply's energy recovery circuit is
actually a little unregulated flyback converter of its own! But so far
(and considering the design's day and age), all the parts that I've seen
in that schematic seem to me to have a good reason for their existence.

Greetings
Dimitrij

On Mon, 22 Feb 2016 00:52:28 +0100, Dimitrij Klingbeil wrote:

As for the BY134,
[...]
Get rid of it, and while you're at it, consider the condition of the
other two (V1808+9) identical ones. Sometimes a person who does an
improper repair will try swapping nearby components hoping that another
one might be "less critical". So if you see signs of unprofessional
manual soldering on them, take that whole trinity and replace them.

Will do. I'm guessing the tech who replaced that diode was solely
concerned with its voltage rating. In all honesty, I'd have been the same
before this speed importance was drawn to my attention in this thread.

Same with C1806. If it looks suspicious, does not pass a withstand test
at some 105% of its rated voltage or shows high ESR, change it too.

That one actually looks fine appearance-wise, but I'll test it
electrically of course.

BY208-1000s are hard to come by nowadays, so here is a list of some more
modern candidates: MUR1100E, BYV26E, UF4007. They should fit, and even
though they are faster than the original BY208-1000, they should work.

Once you've fixed that botched repair on the energy recovery circuit,
connect a taillight lamp to the 12.7 V output, test it again and tell
your results here (make sure you put all the proper parts back in,
before you switch it on, this supply may be unforgiving if any parts are
missing and it's powered on).

Will do. I'll order the parts tomorrow if I can't find any in my spares
bin.

As for the design being "generally unsatisfactory", let me disagree.
Resonant converters do have a well earned place in the world of power
electronics, but the design of them is, in a way, a black art. They have
lots of pitfalls for the unwary and not so many engineers can actually
design them properly and they tend to use special components (inductive
ones in particular) that would be rather unsuitable for other topologies
too. Yet they do have certain benefits, low noise operation that is
suitable for sensitive measurement instruments, being one of them. They
are not so easy to understand, compared to "simple" flyback topology
supplies - so people go screaming "this is too complex" or "this uses
too many parts". In fact your supply's energy recovery circuit is
actually a little unregulated flyback converter of its own! But so far
(and considering the design's day and age), all the parts that I've seen
in that schematic seem to me to have a good reason for their existence.

I read somewhere that resonant converters are poorly understood by
engineers who don't specialise in them and that accurate, detailed
literature on them is not easy to find. So it's very valuable to have
knowledgeable people like yourself and others here who do understand how
they work; otherwise I'd have nowhere to turn for advice on how to
proceed with this!
I'm going to work through the steps you've outlined here and elsewhere
and hope they work. But if the problem remains, I shall definitely be
mothballing it for the foreseeable future. My patience isn't infinite!