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Old 06-09-2021, 07:22 AM   #1
izools
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Default This can't be right...

So I'm here repairing an old IBM 5151 monitor, and I've ordered replacements for all the electrolytic capacitors.

I've found like for like replacements for each one except weird one which, according to the circuit diagram sits at a centre tap of the flyback and the collector of the horizontal output transistor?

It's C505.

Non polarized, 18uF, 25v. Apparently.

So the replacement caps arrive, I've had to obtain a 22uF 25v NP cap as there weren't any 18uF 25v available, not sure what the consequences of this might be.

But check out the attached.

The replacement caps are tiny, I ordered two different 22uF 25v NP caps and one is pictured beside the original 18uF 25v NP cap.

Something's afoot. That size difference is alarming.

I feel I've missed or misinterpreted something.

Would really appreciate some input here...
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File Type: jpg DSC_0001.jpg (224.1 KB, 34 views)
File Type: jpg _20210609_141723.JPG (218.2 KB, 22 views)
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Old 06-09-2021, 08:35 AM   #2
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Default Re: This can't be right...

the cap is probably for horizontal yoke drive,
the difference in uf could effect the image width.

it's common to replace them with film caps these days.
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Old 06-09-2021, 09:13 AM   #3
izools
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Default Re: This can't be right...

But how would you explain the physical size difference?

That's what I'm alarmed by, the old cap is a lower capacity same voltage as the replacement capacitor so why is it 20x the size?

I really feel we are missing something here...

None of the other replacement capacitors have shrunk in dimensions by anywhere near that magnitude, the others are maybe 10-20% smaller, not 2,000+% smaller.
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Old 06-09-2021, 09:26 AM   #4
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Default Re: This can't be right...

Yeah, I've zoomed out on the SAMS for this monitor and you can see that C505 is positioned between the flyback and the horizontal deflection coil.

You can also see that it's referenced to the HV output of the flyback, it's not galvanically isolated.

I really feel like we are missing some kind of special characteristic that this capacitor exhibits... Either that or the voltage rating on the old cap is just plain wrong on the label?

Probably not given the collector of that tranny is rated 14.63v on the diagram... But then why is C504 rated 400v?

You see what I'm scratching my head over?

I'm not confident on this, never worked on something like this before...
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Last edited by izools; 06-09-2021 at 09:31 AM..
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Old 06-09-2021, 09:38 AM   #5
Per Hansson
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Default Re: This can't be right...

Take a look at these threads:
https://www.eevblog.com/forum/beginn...apacitor-size/
https://www.eevblog.com/forum/beginn...s-im-confused/
https://www.eevblog.com/forum/chat/r...his-large-cap/

In short I would just leave that capacitor as-is.
Because I am not sure you will find any modern replacement.
The Nichicon UHA series recommended in one of the threads does not seem to be available any longer.
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Old 06-09-2021, 09:41 AM   #6
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Default Re: This can't be right...

The small cap will not handle the current, it will last about 1 minute and blow. Leave the original in place. If it was bad (as in open) you would have no horizontal deflection. You could use a 18f polyester film capacitor.

Last edited by R_J; 06-09-2021 at 09:53 AM..
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Old 06-09-2021, 09:50 AM   #7
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Default Re: This can't be right...

Marvellous, thanks guys, will reinstall the old one!
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Old 06-09-2021, 02:08 PM   #8
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Default Re: This can't be right...

Anyone measure the ESR if these caps? The big ones and the small ones?
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Old 06-09-2021, 03:18 PM   #9
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Default Re: This can't be right...

Yes here, he describes the graphs meaning on the previous page.
But he got different values after reforming so I'm linking that as it is the correct one:
https://www.eevblog.com/forum/beginn...629/#msg487629
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Old 06-09-2021, 04:11 PM   #10
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Default Re: This can't be right...

How do the compare between the different brand/models? Seems to be only measuring fleabay nichicons?

I'd suspect the big ones should have much lower ESR than the tiny ones whether they're fake or not, but wonder if anyone measured.
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Old 06-10-2021, 01:38 AM   #11
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Default Re: This can't be right...

Well I've replaced every electrolytic in the monitor except for the Horizontal drive big boi.

Unfortunately I still get the same symptoms - raster collapsing after two minutes of being powered on from cold (less if warm):

https://youtu.be/jPryi-k_XXY

I think it might be time for me to buy an oscilloscope...

Last edited by izools; 06-10-2021 at 02:05 AM..
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Old 06-10-2021, 02:40 AM   #12
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Default Re: This can't be right...

get some freezer spray or a heat source.
raster collapsing into a horizontal line??
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Old 06-10-2021, 05:11 AM   #13
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Default Re: This can't be right...

The raster seems to shrink from both directions at once, and when power is disconnected a brief vertical line blinks on the screen (which would only happen if HV were still present?).

I've never worked on anthing like this before, I'm not sure what freezer spray or a heat source would show me?

There's a really good SAMS for this monitor thankfully, showing me what voltages and resistances I should expect to see at key probe points and what signal patterns an ossiloscope should show me at certain points too so I guess I'll go down that rabbit hole...
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Old 06-10-2021, 07:00 AM   #14
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Default Re: This can't be right...

the fault only happens when something warms up - you find the part by heating and cooling it.
a vertical line wouldnt normally be possible because the horizontal drive forms part of the high-voltage generator.

i would suspect - in this order,
bad soldering related to the tda1170,
bad tda1170.

incidentally, if you need one then check cricklewood electronics.
https://www.cricklewoodelectronics.c...keep_https=yes

Last edited by stj; 06-10-2021 at 07:03 AM..
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Old 06-10-2021, 06:51 PM   #15
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Default Re: This can't be right...

Is this the original problem with the monitor BEFORE you replaced the capacitors?
That fault looks like it is with the power supply, What is the voltage on TR24 collector?
Upload the service manual/Schematic
Post #4
Quote:
Probably not given the collector of that tranny is rated 14.63v on the diagram... But then why is C504 rated 400v?
The cap has to handle high voltage spikes at the horz. frequency so it is rated 400v

Last edited by R_J; 06-10-2021 at 07:18 PM..
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Old 06-11-2021, 07:36 AM   #16
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Default Re: This can't be right...

Wow, I am really grateful for how helpful you're being. Thank you.

Yes, fault was present before recapping. The recapping was just a stab in the dark. The picture is at least sharper now following the recap so it wasn't in vein.

Will check those transistors as suggested.

SAMS for the monitor is here:

https://tinyurl.com/3npws544
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