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465B soft start?

treasurer@...
 

Is there much to be gained by powering up my 465B with a variac, lamp dimmer or similar arrangement? It popped (shorted) its 1200 ufd C4429 at power on recently. This cap does not have much head room with its 100 volt rating and 70 volts sitting on it. I'm thinking a soft power up may help prolong the life of those caps. Any thoughts or past experience from the group using this power-on approach?
Bruce

Tom Gardner
 

Switching power supplies operate to keep the output power constant.
Hence if the input voltage is lower, the input current must be higher.

Increased current stresses components such as the rectifiers and
switching transistors.

On 18/11/2019, treasurer@... <treasurer@...> wrote:
Is there much to be gained by powering up my 465B with a variac, lamp dimmer
or similar arrangement? It popped (shorted) its 1200 ufd C4429 at power on
recently. This cap does not have much head room with its 100 volt rating and
70 volts sitting on it. I'm thinking a soft power up may help prolong the
life of those caps. Any thoughts or past experience from the group using
this power-on approach?
Bruce



 

On Mon, Nov 18, 2019 at 10:01 PM, Tom Gardner wrote:


Switching power supplies operate to keep the output power constant.
Hence if the input voltage is lower, the input current must be higher.

Increased current stresses components such as the rectifiers and
switching transistors.
The 465B has a linear power supply.
Low-spec'ed (if so) working voltage hasn't affected those caps for the past 35-odd years. Also, those are (were?) high-quality caps so I wouldn't bother. Epoxy-encapsulated (i.e. dipped) tantalum caps are another matter, especially those with little voltage headroom. The 465B is stuffed with those but slow-on won't help those much I'm afraid. Lots about all that to be found in this group.

Raymond
Power-on current surge puts a strain on the bridge rectifiers. That has been an issue with some 465's.

Tom Gardner
 

On 18/11/19 22:58, Raymond Domp Frank wrote:
On Mon, Nov 18, 2019 at 10:01 PM, Tom Gardner wrote:

Switching power supplies operate to keep the output power constant.
Hence if the input voltage is lower, the input current must be higher.

Increased current stresses components such as the rectifiers and
switching transistors.
The 465B has a linear power supply.
Doh! Too much time spent inside a 2465 :(

Apologies.

treasurer@...
 

Appreciate the thoughts from Tom & Raymond. Guess we should be thankful these 35 + year old scopes are still serviceable. I will need to confirm it is indeed the capacitor and not the bridge rectifier which is shorted, if that has been a past issue.
Cheers

DaveH52
 

I've got a 465M and I've had to replace several caps in the power supply. You will probably have problems getting an exact replacement, After the 30 to 40 years it's been around you can expect the caps to go bad. 70 V on a 100V rated cap is adequately derated. When you go looking for replacements consider the voltage rating, capacitance, size, ( you can probably get more C in the same size) and ripple current rating (higher is better) and ESR (lower is better). Get 105 degree rated parts. They will probably be good for another 30 years.
Just my 2ยข