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Old 20th Jul 2021, 5:30 pm   #1
pentoad
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Default Bush DAC90A filter capacitors?

The original capacitors are 16uf and 32uf if I replaced both of them with 22uf and 22uf 450v - would that put any undue strain on the rectifier?
I don't think it will but just checking

https://frank.pocnet.net/sheets/046/u/UY41.pdf
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Old 20th Jul 2021, 6:24 pm   #2
snowman_al
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Default Re: Bush DAC90 filter capacitors?

I assume you mean DAC90A? As no one else has responded...

Use 32uF for both. A dual can perhaps? The capacitances are not additive, the first cap on the rectifier cathode needs to be within the manufacturers, which is 50uF for the UY41. The cap after the smoothing resistor can be increased in value from 16uF. So 16, 22 or 32uF is fine.

If you do not use a dual can type, make sure you use a cap with a good 50hz ripple value for the one connected to the cathode. (Because the UY41 is a half wave rectifier and produces 50Hz ripple pulses.)
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Last edited by snowman_al; 20th Jul 2021 at 6:37 pm. Reason: Info and typo.
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Old 20th Jul 2021, 9:57 pm   #3
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Default Re: Bush DAC90 filter capacitors?

Dac90A
Datsheet mentioned the maximum should be limited to 50uf
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Old 20th Jul 2021, 10:47 pm   #4
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Default Re: Bush DAC90 filter capacitors?

There's not usually anything wrong with the original smoothing double can capacitor in these particular sets - I hope you haven't been watching that twit on YouTube!
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Old 23rd Jul 2021, 4:47 pm   #5
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Default Re: Bush DAC90 filter capacitors?

Quote:
Originally Posted by Techman View Post
There's not usually anything wrong with the original smoothing double can capacitor in these particular sets - I hope you haven't been watching that twit on YouTube!
What twit is that!

I usually change smoothing caps on old gear as they are inexpensive and just in case of failure causing a catastrophe
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Old 23rd Jul 2021, 6:27 pm   #6
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Default Re: Bush DAC90 filter capacitors?

Cannot disagree, though only ever needed to change one on a DAC90 I must add.
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Old 24th Jul 2021, 3:04 pm   #7
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Default Re: Bush DAC90 filter capacitors?

Which set are we dealing with here? The later set has rimlock valves.
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Old 24th Jul 2021, 10:19 pm   #8
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Default Re: Bush DAC90 filter capacitors?

dac90A
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Old 25th Jul 2021, 10:39 am   #9
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Default Re: Bush DAC90A filter capacitors?

The filter can on these is usually fine, If replacement is necessary, I would leave the original in place (disconnected) and fit two new caps of the correct value under the chassis.


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Old 25th Jul 2021, 2:01 pm   #10
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Default Re: Bush DAC90A filter capacitors?

At least it's now known what version it is.
The original can would most likely come back to life after spending some time on a reformer, the earlier DAC90 cans tend to have less success, the can itself has to be used as a means to mount replacement capacitors, this involves removing the guts and set new caps in there with mastic.
The later DAC90A can have new caps underneath the chassis without any problems.
Can't remember off the top of my head what the values are, I think it's 32uF for the rectifier & 16uF for smoothing.
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Old 29th Jul 2021, 1:46 pm   #11
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Default Re: Bush DAC90 filter capacitors?

Quote:
Originally Posted by pentoad View Post
What twit is that!

I usually change smoothing caps on old gear as they are inexpensive and just in case of failure causing a catastrophe
I'm not going to mention names and the truth is that I can't remember anyway - and for the record I'm not talking about the chap who used to specialise in repairing them. What I will say is that the chap started off with a working radio which he then did a blanket re-cap on and lastly removed the smoothing can. He had a new replacement sat there which he did a test on for ESR etc. He then proudly went on to demonstrate the same test on the removed original one to show how bad it would be...and it proved to be perfect and as good as the new replacement, but he then said that seeing as it had a date of 1953 on it then it must be dud and would need replacement, or something along those lines. After doing all this the radio didn't work at all! Fair play to him for publishing the video - I think I would have been too embarrassed! I think he said at the beginning he was more used to working on guitar amplifiers which always puts up a red flag to me when they start working on vintage radios!

A while after I found a follow up video where he found he'd made a wiring error or something similar that caused the formally working radio not to work and corrected it, which is what can happen if you do blanket re-caps like he did.

As for changing smoothing capacitors regardless in case of catastrophe, how do we know that modern capacitors will have the same life span as the originals did? Look at all the modern failures caused by electrolytic capacitors, makes you think! How do you know how long a replacement capacitor has been on the shelf in storage before being sold? It's a very small market for the sale of this type of capacitor, so many may not be actually manufactured now and could be old stock from years ago, we don't know.

You can always tell the difference between those who went to college and were taught and did it for a living and those who have been watching others on YouTube who in turn got their information from watching others on YouTube who never went to college to be taught properly, there's a huge difference, not that there's anything wrong with YouTube, I watch it all the time, but you have to be selective.
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Old 4th Aug 2021, 9:14 am   #12
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Default Re: Bush DAC90A filter capacitors?

I believe the chap you may be referring to on YouTube mainly works on guitar amplifiers, and he did say he doesn't do radios, that said he did fault finding and found the error. I have made errors, we all have!
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Old 4th Aug 2021, 8:57 pm   #13
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Default Re: Bush DAC90A filter capacitors?

Hi my 2p worth I have never had to replace a smoothing capacitor on a Bush DAC90A but my DAC90 & AC91 have been replaced.

Kind regards, Bob.
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