开云体育

BTW I have a 2 port divider feeding the upper channel of my SD-22s and the
waveforms look to be exactly the same. The pieces of hardline from the
divider to the heads are different lengths and I have not been able to shift
one relative to the other to see how closely they overlay as there is a 340 ps
delay between the two. The divider increases the rise time to 42 ps. It's an
MBC Technology unit. No frequency rating specified.

Switching supplies can be a problem when they fail. I recently had a
Agilent VXI E4808A chassis that the main power supply failed, at least
its 12 volts out did. That power supply is huge with a logic board mounted
about it ( I think it's a power supply monitor board). It's a lot of
physical work just to get it apart to check caps. Anyway, I doubt
HP/Agilent made that power supply themselves. I really don't know, as to
get information out of them is like pulling hen's teeth. They no longer
sell, or support, those 4 slot VXI chassis's, so it would be nice of them
to release the schematics, or service information. I think you can only
hope to buy a used chassis somewhere. I would love to have the schematics
for them.

In other switches that fail to start, I often find what I call the
"start-up" capacitor in the primary is open or leaking or has a high ESR.
I call it the "start-up cap" as I do not fully understand switching
supplies, and often if there are schematics, there is not a theory of
operation with them, but there is often a capacitor in the primary circuit
that looks to be a short to ground and then charge-up to create the initial
switch swing to get it going and then afterwards, the power supply self
sustains.

So a lot of times with a dead switcher, I have been able to fix them by
replacing the small electrolytic in the primary side of the circuit. It is
usually a 4.7uf or 10uf. Other than that, if the rest are not physically
leaking or swelled and the supply is working I don't touch it.

One instrument that I work on has an on-board +5volt switching regulator
circuit. It is very reliable. Two times (since 1999) I have seen the
switch regulator fail where the output drive to the FET shorts to ground
allowing the supply voltage (~+12volts) go through to the output with no
over-voltage protection, not even a +5.6volt zener to short. It blows
every TTL chip on the +5volt rail, and some some regulators that follow it.
Stupid, stupid German design. You have to be very urber careful probing
that +5volt switching regulator circuit. One slip of the scope probe and
you can simulate the switching regulator IC output shorting to ground
turning on the pass FET full time. So, I've learned "If it aint broke,
don't fix it"!

Sorry it that was slightly off topic.

Dave

SMD electrolytic capacitors are a tricky problem in several ways:

First, they are easily damaged by the heat of the reflow oven,
and solvents that may be used in cleaning flux,... though cleaning
flux isn't done much if at all, on consumer grade equipment.

Second, it is almost impossible to find out what was originally
installed. There are no standardized markings to identify the
manufacturer, or what grade capacitor was installed.

Third, it is hard for a re-worker to find a complete spectrum of
ESR values from which to select replacement capacitors.

When you grab an assortment of SMD electrolytic capacitors off
of ebay, you are unlikely to be able to match more than just the
capacitance and voltage of the original. Never the ESR or lifetime
rating.

When you buy from Mouser, or DigiKey, you will find that cute
little 20uf, 16V cap you need to replace, (because its ESR is 3 ohms
while its cohorts are all 0.5 ohm), can only be had with ESR's of
0.7 ohm, 3 ohms, and 12 ohms! Replace the 0.5 ohm ESR caps with a
0.7 ohm cap, and you have taken the expected life of the cap and cut
it in half (or more). Put in a 3 or 12 ohm version, and you will have
improved nothing.

Tricky indeed!

-Chuck Harris

Kevin Oconnor wrote:

I have to agree with Chuck. Most test equipment with imbedded/integrated

switchers are going to be very difficult to qualify parts if working. Their
functionality can be extremely subtle. Blanket replacement of components
will more likely cause additional reliability issues.

My 1978 Tek 485 has had spot repairs, but it’s still working fine. I’d

never replace caps in it without good reason.

I have an HP 500MHz digital scope with one of this 3rd party modular

switchers. (I forget the HP model but 5digits). Switcher went bad. No docs,
no schematics. Really hopeless to diagnose. Best option was an eBay
replacement switcher. $50 and never looked back.

Now if you are talkin about bad caps in an LCD monitor or TV, that’s

where “replace em all” will serve u well! Lots of crap used in those
devices.

K

Sent from kjo iPhone

I have to agree with Chuck. Most test equipment with imbedded/integrated switchers are going to be very difficult to qualify parts if working. Their functionality can be extremely subtle. Blanket replacement of components will more likely cause additional reliability issues.
My 1978 Tek 485 has had spot repairs, but it’s still working fine. I’d never replace caps in it without good reason.
I have an HP 500MHz digital scope with one of this 3rd party modular switchers. (I forget the HP model but 5digits). Switcher went bad. No docs, no schematics. Really hopeless to diagnose. Best option was an eBay replacement switcher. $50 and never looked back.
Now if you are talkin about bad caps in an LCD monitor or TV, that’s where “replace em all” will serve u well! Lots of crap used in those devices.

K

Sent from kjo iPhone

I have a few questions about power supplies and would like some opinions. Are switching powers supplies inherently harder on components than linear power supplies? If you picked up a 70's-80's vintage scope with a SMPS would you replace power supply components before using it as a daily driver? Or do you treat all power supplies the same and visually inspect, check for ripple and call it good? From my reading it seems that when a SMPS melts down the chain reaction damage has the possibility of being bad.


Brendan