Component idenification

[Cursitor Doom]

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

[legg]

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

That's an NTC inrush limiter. It has roughly constant power
dissipation, when in use and is expected to run warm.

What is the actual current draw? Is your bulb dim?

You should be able to identify parts from board designators
and the reference manual.

RL

[Cursitor Doom]

On Sun, 06 Nov 2022 16:05:00 -0500, legg <le...@nospam.magma.ca> wrote:

>On Sun, 06 Nov 2022 20:48:15 +0000, Cursitor Doom <c...@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
>
>That's an NTC inrush limiter. It has roughly constant power
>dissipation, when in use and is expected to run warm.

That's what I thought too. But there's no such component visible on
the schematic:


https://disk.yandex.com/i/UhxATIy-6Y0LIA

>
>What is the actual current draw? Is your bulb dim?

4x40W bulbs for 160W all together and they're all full-on.

[Phil Allison]

Cursitor Doom wrote:
________________________

> >
> >That's an NTC inrush limiter. It has roughly constant power
> >dissipation, when in use and is expected to run warm.
>
> That's what I thought too. But there's no such component visible on
> the schematic:
>
>
> https://disk.yandex.com/i/UhxATIy-6Y0LIA

** What about " RT1010" ???

Resistor Thermal 7.5 ohms.



..... Phil

[legg]

On Sun, 06 Nov 2022 21:29:37 +0000, Cursitor Doom <c...@notformail.com>

wrote:

>On Sun, 06 Nov 2022 16:05:00 -0500, legg <le...@nospam.magma.ca> wrote:
>
>>On Sun, 06 Nov 2022 20:48:15 +0000, Cursitor Doom <c...@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
>>
>>That's an NTC inrush limiter. It has roughly constant power
>>dissipation, when in use and is expected to run warm.
>
>That's what I thought too. But there's no such component visible on
>the schematic:
>
>
>https://disk.yandex.com/i/UhxATIy-6Y0LIA
>
>>
>>What is the actual current draw? Is your bulb dim?
>
>4x40W bulbs for 160W all together and they're all full-on.
>
>>You should be able to identify parts from board designators
>>and the reference manual.

Then it's time to find the short cct.

RL

[Cursitor Doom]

Sure is. I came across a section of the service manual I hadn't found
before (I find service manuals in PDF format *so* difficult to
navigate) and it's provided a list of parts to check in the event of
this fault arising. The list is quite lenghty, but is at least limited
by virtue of there being a multi-pronged jumper which interconnects
certain sections and by removing that you can drastically reduce the
number of parts to take a closer look at if the fault then clears,
which it has. Fingers crossed, eh?

[Peter W.]

The list is quite lenghty, but is at least limited
> by virtue of there being a multi-pronged jumper which interconnects
> certain sections and by removing that you can drastically reduce the
> number of parts to take a closer look at if the fault then clears,
> which it has. Fingers crossed, eh?

OK - if the fault clears when a jumper is removed, then you know that the fault is in the removed section, correct? Not the other way around?

Too many individuals think it works the other way, sadly.

Peter Wieck
Melrose Park, PA

[Cursitor Doom]

Yes, you're effectively removing the connection to the damaged
section; the section which has some component in it which has gone
low-resistance.

[Cursitor Doom]

This kind of fault would be so much easier to detect if we had IR
glasses to view the suspect areas with. Someone really needs to invent
those.

[Peter W.]

> This kind of fault would be so much easier to detect if we had IR
> glasses to view the suspect areas with. Someone really needs to invent
> those.

https://www.grainger.com/search/test-instruments/temperature-and-environmental-measurement/infrared-temperature-measurement/infrared-cameras?filters=webParentSkuKey&webParentSkuKey=WP8398465 Not cheap. The lenses exist - military versions for the most part. But not particularly fine-pitched.

[legg]

On Mon, 07 Nov 2022 23:22:26 +0000, Cursitor Doom <c...@notformail.com>

You should be able to track a short this close to the
AC input with a multimeter on diode test mode.

brodge rec,
inverter fets
bulk caps

to start.

RL

[three_jeeps]

yes, and actually, tracking down the short on the downstream sections (modules) should not be that difficult either, using the same approach. After all the faulty component is SHORTED which should be easy to find. Yea, it is a PITA to check every single component (sort of the brute force way to check), but judicious studying of the schematic and identifying appropriate points to isolate and subsequently test can prune away a number of branches in the tree fairly quickly so as to speak.

Maybe bounce over to the tek google groups and ask if there are any common component failures that should be targeted first?
Otherwise, enjoy getting intimate with the guts of your scope....

[Cursitor Doom]

Turns out those early items in the power chain are fine, so I'm moving
on down the line to the less obvious items. The service manual has a
list of chief suspects to look at first, so hopefully that should
prove invaluable.

[Cursitor Doom]

Yup. Totally agree. I'm doing just that now.....

[Cursitor Doom]

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

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

[whit3rd]

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>

[Cursitor Doom]

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

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).

[JC]

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

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

[legg]

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

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
compromised.

RL

[Cursitor Doom]

On Fri, 11 Nov 2022 12:41:19 -0500, JC <Chipbee4...@yahoo.com>
wrote:

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.

[Cursitor Doom]

"cct"? Is that short for circuit?

>compromised.
>
>RL

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.

[JC]

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

[Cursitor Doom]

On Fri, 11 Nov 2022 15:40:30 -0500, JC <Chipbee4...@yahoo.com>

You are correct. I've taken a closer look now and they do have that
capability; many thanks.

[Carlos E.R.]

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


--
Cheers, Carlos.

[Cursitor Doom]

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.

[Carlos E.R.]

maybe drill a hole to put a small bolt?

--
Cheers, Carlos.