Component idenification...

On Fri, 11 Nov 2022 13:34:01 -0500, legg <legg@nospam.magma.ca> wrote:

On Fri, 11 Nov 2022 15:31:20 +0000, Cursitor Doom <cd@notformail.com
wrote:

On Fri, 11 Nov 2022 00:35:03 -0800 (PST), whit3rd <whit3rd@gmail.com
wrote:

On Thursday, November 10, 2022 at 9:57:16 AM UTC-8, Cursitor Doom wrote:

One question, though: they\'ve mounted these transistors without using
a screw through the mounting tab of this TO-220 package. The devices
are just \'leaning\' against some white pad mounted on the heat sink
(see photo). Is there some special grease required to mate the
transistor to the white pad or can I just use the regular silicon
grease for this?

That\'d be \'silicone\' grease; the picture showing studs over the power package indicates
that an L-shaped or Z-shaped pressure arm (maybe three separate ones) was designed
into the system. Why else would there be a threaded rod there?

https://e2e.ti.com/resized-image/__size/1230x0/__key/communityserver-discussions-components-files/196/7888.Mounting-a-TO220.png

https://media.rs-online.com/t_large/RD504059-01.jpg


If that\'s the case, maybe it was removed at some time, shortening the
life of the chopper. I\'ll do something about that regardless. BTW, the
threaded rods are there to retain plastic covers which encase the 3
devices (don\'t ask me why as I haven\'t a clue what purpose they serve
but certainly not to press bear the transistors to the heatsink, that
much is certain).

The \'plastic\' covers may serve the purpose of forced device retension.

Depends on material composition and design.

\'just leaning\' is not what is intended here.
...

Diode test for other shorts.

Check resistance gate to source pads of empty fet location.

Should read diode drop minimum, or gate drive cct is
Click to expand...

\"cct\"? Is that short for circuit?

compromised.

RL
Click to expand...

Thank you, Legg, I shall duly do so. I\'m just hoping the transformer
windings are still okay and that the shorted chopper hasn\'t sucked too
much current through them, \'cos that would probably be terminal for
the scope.
 
Cursitor Doom wrote:
On Fri, 11 Nov 2022 12:41:19 -0500, JC <Chipbee40_SpamNo@yahoo.com
wrote:

Cursitor Doom wrote:
On Sun, 06 Nov 2022 20:48:15 +0000, Cursitor Doom <cd@notformail.com
wrote:

Gentlemen,

Having put together a dim bulb tester for current-limiting purposes
and fired up the faulty \'scope, this component gets hot. I\'m not
really sure what it is, though and there are no markings on it at all
except for two tiny blobs of paint.
Anyone know?


https://disk.yandex.com/i/jmR716xUNBt8OQ


Got \'er!

I\'m not at all familiar with the topology they\'ve used in this PSU,
but I believe it\'s the chopper transistor (I should have guessed)
that\'s gone low-resistance: Q1050. It was releasing quite a bit of
heat even at only 18VAC supply with the current limited via 4x40 bulbs
to boot. I removed it from circuit to check and it was dead. So I\'m
hoping that\'s the only part that\'s been damaged. It\'s an IRF820 and I
have a bunch of IRF840s so will press one of those into service in
place of the burnt-out one. Pin-outs just the same, just a lower Rdson
figure, which is no bad thing.
One question, though: they\'ve mounted these transistors without using
a screw through the mounting tab of this TO-220 package. The devices
are just \'leaning\' against some white pad mounted on the heat sink
(see photo). Is there some special grease required to mate the
transistor to the white pad or can I just use the regular silicon
grease for this? It doesn\'t seem like a very good way of ensuring
thermal conductivity. :-/


https://disk.yandex.com/i/cJDolujNfYucYw

look at the third photo, you will see that you have a lot of parts
missing.

https://bradthx.blogspot.com/2017/05/tektronix-2465b-capacitor-replacement.html

Not sure what you mean by \"a lot of parts missing\" as I do have the
individual plastic transistor covers shown in your photo but not in
the one I posted of mine. Apart from those covers (which were out of
shot in the picture I posted) there\'s nothing else missing AFAIK -
apart from whatever fastening they used to hold the trannies against
the heat sink (assuming they used something at all which they
certainly *should* have, I\'d have thought).

you should also change out the 0.01uf cap, it has tendency to burn up
that area.

Yes, that and the two other RIFAs on that board. They all look end of
life AFAICT.
Click to expand...
The plastic covers hold the transistors against the ceramic heatsink
insulator. They are bolted on as shown in the photo.
 
On Fri, 11 Nov 2022 15:40:30 -0500, JC <Chipbee40_SpamNo@yahoo.com>
wrote:

Cursitor Doom wrote:
On Fri, 11 Nov 2022 12:41:19 -0500, JC <Chipbee40_SpamNo@yahoo.com
wrote:

Cursitor Doom wrote:
On Sun, 06 Nov 2022 20:48:15 +0000, Cursitor Doom <cd@notformail.com
wrote:

Gentlemen,

Having put together a dim bulb tester for current-limiting purposes
and fired up the faulty \'scope, this component gets hot. I\'m not
really sure what it is, though and there are no markings on it at all
except for two tiny blobs of paint.
Anyone know?


https://disk.yandex.com/i/jmR716xUNBt8OQ


Got \'er!

I\'m not at all familiar with the topology they\'ve used in this PSU,
but I believe it\'s the chopper transistor (I should have guessed)
that\'s gone low-resistance: Q1050. It was releasing quite a bit of
heat even at only 18VAC supply with the current limited via 4x40 bulbs
to boot. I removed it from circuit to check and it was dead. So I\'m
hoping that\'s the only part that\'s been damaged. It\'s an IRF820 and I
have a bunch of IRF840s so will press one of those into service in
place of the burnt-out one. Pin-outs just the same, just a lower Rdson
figure, which is no bad thing.
One question, though: they\'ve mounted these transistors without using
a screw through the mounting tab of this TO-220 package. The devices
are just \'leaning\' against some white pad mounted on the heat sink
(see photo). Is there some special grease required to mate the
transistor to the white pad or can I just use the regular silicon
grease for this? It doesn\'t seem like a very good way of ensuring
thermal conductivity. :-/


https://disk.yandex.com/i/cJDolujNfYucYw

look at the third photo, you will see that you have a lot of parts
missing.

https://bradthx.blogspot.com/2017/05/tektronix-2465b-capacitor-replacement.html

Not sure what you mean by \"a lot of parts missing\" as I do have the
individual plastic transistor covers shown in your photo but not in
the one I posted of mine. Apart from those covers (which were out of
shot in the picture I posted) there\'s nothing else missing AFAIK -
apart from whatever fastening they used to hold the trannies against
the heat sink (assuming they used something at all which they
certainly *should* have, I\'d have thought).

you should also change out the 0.01uf cap, it has tendency to burn up
that area.

Yes, that and the two other RIFAs on that board. They all look end of
life AFAICT.

The plastic covers hold the transistors against the ceramic heatsink
insulator. They are bolted on as shown in the photo.
Click to expand...

You are correct. I\'ve taken a closer look now and they do have that
capability; many thanks.
 
On 2022-11-11 16:31, Cursitor Doom wrote:
On Fri, 11 Nov 2022 00:35:03 -0800 (PST), whit3rd <whit3rd@gmail.com
wrote:

On Thursday, November 10, 2022 at 9:57:16 AM UTC-8, Cursitor Doom wrote:

One question, though: they\'ve mounted these transistors without using
a screw through the mounting tab of this TO-220 package. The devices
are just \'leaning\' against some white pad mounted on the heat sink
(see photo). Is there some special grease required to mate the
transistor to the white pad or can I just use the regular silicon
grease for this?

That\'d be \'silicone\' grease; the picture showing studs over the power package indicates
that an L-shaped or Z-shaped pressure arm (maybe three separate ones) was designed
into the system. Why else would there be a threaded rod there?

https://e2e.ti.com/resized-image/__size/1230x0/__key/communityserver-discussions-components-files/196/7888.Mounting-a-TO220.png

https://media.rs-online.com/t_large/RD504059-01.jpg


If that\'s the case, maybe it was removed at some time, shortening the
life of the chopper. I\'ll do something about that regardless. BTW, the
threaded rods are there to retain plastic covers which encase the 3
devices (don\'t ask me why as I haven\'t a clue what purpose they serve
but certainly not to press bear the transistors to the heatsink, that
much is certain).
Click to expand...

I would guess insulation - is that part connected to mains?


--
Cheers, Carlos.
 
On Sun, 13 Nov 2022 23:17:29 +0100, \"Carlos E.R.\"
<robin_listas@es.invalid> wrote:

On 2022-11-11 16:31, Cursitor Doom wrote:
On Fri, 11 Nov 2022 00:35:03 -0800 (PST), whit3rd <whit3rd@gmail.com
wrote:

On Thursday, November 10, 2022 at 9:57:16 AM UTC-8, Cursitor Doom wrote:

One question, though: they\'ve mounted these transistors without using
a screw through the mounting tab of this TO-220 package. The devices
are just \'leaning\' against some white pad mounted on the heat sink
(see photo). Is there some special grease required to mate the
transistor to the white pad or can I just use the regular silicon
grease for this?

That\'d be \'silicone\' grease; the picture showing studs over the power package indicates
that an L-shaped or Z-shaped pressure arm (maybe three separate ones) was designed
into the system. Why else would there be a threaded rod there?

https://e2e.ti.com/resized-image/__size/1230x0/__key/communityserver-discussions-components-files/196/7888.Mounting-a-TO220.png

https://media.rs-online.com/t_large/RD504059-01.jpg


If that\'s the case, maybe it was removed at some time, shortening the
life of the chopper. I\'ll do something about that regardless. BTW, the
threaded rods are there to retain plastic covers which encase the 3
devices (don\'t ask me why as I haven\'t a clue what purpose they serve
but certainly not to press bear the transistors to the heatsink, that
much is certain).

I would guess insulation - is that part connected to mains?
Click to expand...

We eventually got to the bottom of it. The plastic covers are supposed
to bear against the transistors underneath and hold them tight against
the heatsink assembly. But it\'s a really unsatisfactory approach,
because they\'re not secured at the bottom in any way, so the devices
are not pressed evenly against their pads. I\'m looking into whether it
can be improved whilst waiting for the spares to arrive. Perhaps I can
get some of those clips that Whit3rd posted a link to.
 
On 2022-11-13 23:50, Cursitor Doom wrote:
On Sun, 13 Nov 2022 23:17:29 +0100, \"Carlos E.R.\"
robin_listas@es.invalid> wrote:

On 2022-11-11 16:31, Cursitor Doom wrote:
On Fri, 11 Nov 2022 00:35:03 -0800 (PST), whit3rd <whit3rd@gmail.com
wrote:

On Thursday, November 10, 2022 at 9:57:16 AM UTC-8, Cursitor Doom wrote:

One question, though: they\'ve mounted these transistors without using
a screw through the mounting tab of this TO-220 package. The devices
are just \'leaning\' against some white pad mounted on the heat sink
(see photo). Is there some special grease required to mate the
transistor to the white pad or can I just use the regular silicon
grease for this?

That\'d be \'silicone\' grease; the picture showing studs over the power package indicates
that an L-shaped or Z-shaped pressure arm (maybe three separate ones) was designed
into the system. Why else would there be a threaded rod there?

https://e2e.ti.com/resized-image/__size/1230x0/__key/communityserver-discussions-components-files/196/7888.Mounting-a-TO220.png

https://media.rs-online.com/t_large/RD504059-01.jpg


If that\'s the case, maybe it was removed at some time, shortening the
life of the chopper. I\'ll do something about that regardless. BTW, the
threaded rods are there to retain plastic covers which encase the 3
devices (don\'t ask me why as I haven\'t a clue what purpose they serve
but certainly not to press bear the transistors to the heatsink, that
much is certain).

I would guess insulation - is that part connected to mains?

We eventually got to the bottom of it. The plastic covers are supposed
to bear against the transistors underneath and hold them tight against
the heatsink assembly. But it\'s a really unsatisfactory approach,
because they\'re not secured at the bottom in any way, so the devices
are not pressed evenly against their pads. I\'m looking into whether it
can be improved whilst waiting for the spares to arrive. Perhaps I can
get some of those clips that Whit3rd posted a link to.
Click to expand...

maybe drill a hole to put a small bolt?

--
Cheers, Carlos.
 
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