Need help buying power supply capacitors

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ral-clan
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Need help buying power supply capacitors

Post by ral-clan »

Hi have an old T912 Tektronix oscilloscope which I was given:

http://www.binatech.se/hem/Vintagesound/912a_e.html
(not my web site, just linking to it so you can see the model)

Actually I was given two of them. I am pretty sure the power supplies on both are bad, as when turned on, the power supply light flickers, which in the service manual is an indication of a bad power supply (undervoltage). I opened it up and although the capacitors aren't obviously bulged or leaky on the power supply. I'm thinking about replacing all the capacitors on the power supply board.

Image
(click to enlarge)

This is what's on it:

2x 16V 100uf axial caps.
2x 16V 2200uf axial caps.
1x 10uf 500V axial cap.
1x 25V 2200uF axial cap. (correction)
1x 200V 100uf axial cap.
1x 160V 3.3uf axial cap.

I've managed to locate replacement capacitors online, they have the correct Voltage and MicroFarads capacitance rating, and are the correct type (electrolytic, axial).

However, I also have read that in power supply capacitors it's important to get the correct RMS current rating.
http://www.eetimes.com/document.asp?doc_id=1273335

I have the service manual for the T912 but cannot find any information on what the RMS rating of the original capacitors was, so not sure how to proceed in purchasing replacement capacitors.

I have the ability swap out components,but I am not an electronics engineer, just a hobbyist, so analogue power supply electronics are a little out of my league. Can anyone advise?

I think the capacitors are going to come to about $20US, then shipping from the US is another $20US, so I'm not sure if it's actually worth it to repair this scope, as there might also be a trigger sync problem.

Thanks.
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Re: Need help buying power supply capacitors

Post by eslapion »

ral-clan wrote:However, I also have read that in power supply capacitors it's important to get the correct RMS current rating.
http://www.eetimes.com/document.asp?doc_id=1273335
RMS current rating in capacitors is really important on high current PSUs and it's almost never mentioned on the schematics of electrical apparatus. I would say age is a far more important factor most of the time.

One example of a power supply where RMS current of capacitors is critical is the lower voltage power source usually built on modern PC motherboards for the CPU, such as those carrying high power processors like the Intel i7. On these you can see a cluster of capacitors located near the CPU socket as currents can be as high as 60 amps. To increase the supported RMS current of this type of power supply, you'll see a large number of smaller capacitors connected in parallel instead of a single large one.

If you are dealing with a power supply where you do not have multiple capacitors connected in parallel to boost supported current then RMS current supported by the capacitors may not be so important.

You also mentioned you found a source online - online being where exactly ? In the case of power supplies (I see some of your capacitors are rated for 200V), always try to get capacitors which haven't been sitting on a shelf collecting dust for years. Get the newest, fresh out of the factory ones in which the dielectric material hasn't had time to age.

Digikey and Mouser usually provide fresh out of the factory capacitors.

Capacitors which haven't been used for years can degrade unpredictably and that's generally what destroys VIC-20s and C64 computers too.
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Re: Need help buying power supply capacitors

Post by ral-clan »

Hi Eslapion,

My source was Digikey in the US. Unfortunately, their shipping to Canada is high, but all other sources I've investigated do not carry all the capacitors I need (Jameco does not carry the 200V one).

From what I have read, only the farad rating must be exact, while the voltage rating can be the same OR HIGHER than what is specified on the original capacitor I am replacing. Correct?
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Re: Need help buying power supply capacitors

Post by Bobbi »

Correct.
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Re: Need help buying power supply capacitors

Post by highinfidelity »

I think that RMS current has to be considered only in high-current, switching-type power supplies. In my opinion your Tektronix has a "classic" regulated power supply, and the instrument shouldn't draw high currents; so you can ignore that parameter.

If you have difficulties in finding the appropriate capacitors, you can consider also a slightly higher or lower capacity. In other words: a capacitor of a slightly higher/lower capacity but correct (or overrated) voltage is better than a capacitor with perfect capacity but incorrect working voltage. Keep in mind that there is a 20% tolerance anyway.

You can also try to check RS Components, they usually have EVERYTHING in store, I think that in Canada they are called Allied Electronics:

http://ca-en.alliedelec.com/
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Re: Need help buying power supply capacitors

Post by eslapion »

ral-clan wrote:Hi Eslapion,

My source was Digikey in the US. Unfortunately, their shipping to Canada is high, but all other sources I've investigated do not carry all the capacitors I need (Jameco does not carry the 200V one).

From what I have read, only the farad rating must be exact, while the voltage rating can be the same OR HIGHER than what is specified on the original capacitor I am replacing. Correct?
Digikey offers free shipping on all orders of 100$CAD or more. For orders of less than 100$, shipping is 8$CAD, that's high to you ???

As for the farad and voltage rating, you are correct.

Added edit:
You indicated in there a capacitor for 100uF rated for 2200V. This one must be huge as the amount of energy stored in a capacitor is calculated as 0.5*C*V^2. I don't think it is present in the posted photo.
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Re: Need help buying power supply capacitors

Post by ral-clan »

I too thought that the shipping price was $8, but when I went to check-out it was $20US on about $25US worth of goods.

I'll double check that I have not got that "large" capacitor value incorrect.
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Re: Need help buying power supply capacitors

Post by eslapion »

I am presently trying to build up a basket with the components you listed above and right now, it is impossible to find a 2200V 100uF capacitor, it is impossible to find a 10uF 500V capacitor in axial format.

The price for the other components turns to 30.66$CAD when choosing the highest quality components. Shipping is 8$CAD.
ralclanbasket.jpg
I don't know the taxes in Ontario, in Quebec you'd have to add 4.60$CAD of taxes.

Added edit:
I think I know why you ended up with 20$ of shipping cost. I suspect you have chosen backordered parts. This means you'll receive multiple shipments if you place the order as it is.

None of the components in the above basket are backordered.
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Re: Need help buying power supply capacitors

Post by MCes »

could you post the image of 2200V 100uF capacitor?

In your pic the fuse F722 (150mA Fast) is missing....
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Re: Need help buying power supply capacitors

Post by MCes »

I found it on the web.....

https://elektrotanya.com/tektronix_912_ ... nload.html

Which capacitor is it (2200V 100uF)? (C782? C742? C743? .....)
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Re: Need help buying power supply capacitors

Post by ral-clan »

Hi, I see now I made a typo in the top message (now corrected).
This is what I get from not being consistent in the order I write values (i.e. sometimes I wrote uF first and voltage second, sometimes the other way around).

Here are the proper capacitor values and their locations on the circuit board:

C758 & C778 - 16V 100uF
C742 & C743 - 16V 2200uF
C712 - 500V 10uf
C782 - 2200uf 25V
C722 - 200V 100uf
C738 - 160V 3.3uf

Also, the fuse has been replaced.

Here is a photo of the power circuit board from a better angle (more component labels visible) - attached image.
HPIM1087.jpg
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Re: Need help buying power supply capacitors

Post by eslapion »

ral-clan wrote:Here are the proper capacitor values and their locations on the circuit board:

C758 & C778 - 16V 100uF
C742 & C743 - 16V 2200uF
C712 - 500V 10uf
C782 - 2200uf 25V
C722 - 200V 100uf
C738 - 160V 3.3uf
I am preparing you a basket at Digikey with all of these except C712.

This one I can only get either in radial format or a ultra high performance Polypropylene film (a plastic brick with short pins on both ends) with a spacing between pins of 32 or 41.5 mm (1.26 or 1.634 inch).

On the photo, it looks like 41.5 mm would be better for you.

Using a radial capacitor would require having one of the leads hang out more than one inch and safety requires that you isolate this exposed lead with at least 2 layers of heat shrink tubing.

What's your preference ?

Present content of the basket is:

Code: Select all

4201PHCT-ND		CAP ALUM 100UF 20% 16V AXIAL	2
4122PHBK-ND		CAP ALUM 2200UF 20% 16V AXIAL	2
4182PHBK-ND		CAP ALUM 100UF 20% 200V AXIAL	1
493-14836-ND		CAP ALUM 3.3UF 20% 160V AXIAL	1
495-3724-ND		CAP ALUM 2200UF 25V AXIAL	1
P16734-ND		CAP FILM 10UF 10% 500VDC RADIAL	1
Subtotal is 44.49$CAD (taxes must be added)
Shipping is 8$CAD, all components are stocked.

I suggest you use the part numbers listed here and check them on Digikey's web site to see if they suit your particular requirements. The last item in particular should be checked for dimensions.
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