Re: Tek 2247A [SN B029000] Recap and diode part list ?


Darren Ting
 

Hi Menahem Yachad

Great info and advice, appreciated.
However, for me to do those in 2247A is a challenge, this PCB board place in-between Main-Board and Processor board, all components lead hide in-between PSU PCB and Main Board making it not accessible to perform live probe as you suggested.
I have to solder wire lead to probe. a bit challenging in this unit


From: "@yachadm [TekScopes]" <TekScopes@...>
To: TekScopes@...
Sent: Sunday, 20 March 2016, 18:04
Subject: Re: [TekScopes] Tek 2247A [SN B029000] Recap and diode part list ?

  When I've had an issue like this, I connect my Fluke DMM to each capacitor lead, and check the voltage - plus or minus, stable or fluctuating.

Then scope the ripple on each lead (if one lead is GND, obviously there won't be any ripple!).


In a couple of seconds, I can determine whether a Regular or BP cap is needed, and what the appropriate voltage rating needs to be.


In older equipment, many times a cap with a voltage rating many times over the actual circuit voltage was required.
Why?
Simply, because an available cap with the correct voltage rating couldn't handle the ripple current.
There are many DISadvantages to using a capacitor with a vastly over-spec'ced voltage rating, but I'm not going to go into that here.








A Let's say Example (simplified for easy understanding):


Capacitor C1234 must dampen ripple of 1086mA, and it has 20.8VDC passing through it.


In the old days, Nichicon's "XYZ" series was the best there was for this circuit, and the 2200uF 25V was capable of only 763mA ripple.
But the XYZ 2200uF 50V could handle up to 1204mA, so that's what the designer specified in the BoM.


Today, Nichicon's HE 2200uF 25V can handle 2860mA, so EVEN THOUGH a 50V capacitor was previously installed (and so specified in the BoM), I am now going to install the 25V capacitor.


This decision would be valid for ONLY Nichicon's HE, because that is what I tested and verified.






To put it simply, you may NOT need a BiPolar in that position, and you may NOT need a 100V or 160V rated capacitor in there, AT ALL.


But you'd have to do some solid measurements to determine what really NEEDS to be in there, and if you'd like to take up that challenge, I would be very interested to know what you discover!


Menahem Yachad
Israel










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