开云体育

(of that replacement capacitor). My ESR readings looked *way* too low, so
I re-measured a NOS Sprague 1000 uF cap today, using the GR 1689 LCR
Digibridge.

Today, a good 1000 uF cap measured:

Test
Freq.........Capacitance...........D...................ESR.............
120 Hz..............999.21 uF.........0.0857..........0.11493 ohm...
1 kHz................952.74 uF..........0.4704.........0.07851 ohm...
10 kHz..............844.97 uF..........?????...........0.07212 ohm...

Because my earlier measurements were 9 months ago, I'm not entirely sure
which LCR meter I used for that project.

Anyway, I think today's measurements are more believable. Maybe I had a
"short between the headsets" so to speak 9 months ago during the original
measurement?

John, the GR 1689 LCR Digibridge automatically switches from "Capacitance"
to "Resistance" mode if your cap is shorted. In other words, it's
immediately obvious your cap is shorted. IIRC, the same would be obvious
if you're using a HP LCR meter.

-Greg


----- Original Message -----
From: Pa4tim
To: hp_agilent_equipment@...
Sent: Wednesday, August 31, 2011 11:26 AM
Subject: Re: [hp_agilent_equipment] Favorite bad capacitor debugging
techniques?



What model ESR meter you use ? I guess a HP bridgde. With 6.5 digits this
is not one of the ESR "indicator" kits. How low can you measure.
I use an analog meter, it goes upto 40 ohm ( but I made it non lineair for
better resolution under 1 ohm, and accurate down to 100nF. But I think I
change this and make it switchable between 1 ohm full scale and 40 ohm. (
not that this is important because if the meter corners it is good, the
last cm on the scale is 1 Ohm so about 50 mOhm resolution)

Fred PA4TIM

----- Original Message -----
From: "Pa4tim" <fredschneider@...>
To: <hp_agilent_equipment@...>
Sent: Wednesday, August 31, 2011 10:26 AM
Subject: Re: [hp_agilent_equipment] Favorite bad capacitor
debugging techniques?


What model ESR meter you use ? I guess a HP bridgde. With
6.5 digits this is not one of the ESR "indicator" kits. How
low can you measure.
I use an analog meter, it goes upto 40 ohm ( but I made it
non lineair for better resolution under 1 ohm, and accurate
down to 100nF. But I think I change this and make it
switchable between 1 ohm full scale and 40 ohm. ( not that
this is important because if the meter corners it is good,
the last cm on the scale is 1 Ohm so about 50 mOhm
resolution)

Fred PA4TIM

FWIW, I usually check caps on a GR-1650-A but often
leakage is important and is not measured by the usual
impedance bridge. My usual arrangement is to use a small,
regulated DC power supply and a meter capable of indicating
quite low values of current. My Tektronix DMM goes low
enough. I've found paper caps that test good on the bridge
but have excessive leakage and won't work in some circuits.
Leakage is a primary test for electrolytics. There _are_
capacitance bridges like the GR 1617-A that will also
measure leakage and supply a variable bias internally but I
don't have one. A megohmeter will also do provided you have
one which will run at voltages suitable for the caps.



--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@...

One thing I would never do is put in a bigger fuse.
A switcher might not start up using a lightbulb.
Besides things allready named like following the current and looking for
low voltages i do the following:
If the instrument is still working I use my DVM to check DC levels and
ripple uing the duall display ( but with a good DVM in the AC position is
also possible) a el-cheapo meter will get confused by the mix of AC and DC
especially if it is a switcher. And often to be ure I use a scope to look
at ripple levels.
If it is not working, I desolder caps and test them on DC leakage,
capacitance and ESR. Diodes, resistors and tranistors I test in circuit if
possible. This with a DVM and a tracker. I use a Pace Sucker, they are
really great.
But I'm no oldtimer ;-)


Fred PA4TIM

----- Original Message -----
From: David Speck
To: hp_agilent_equipment@...
Sent: Tuesday, August 30, 2011 7:55 PM
Subject: [hp_agilent_equipment] Favorite bad capacitor debugging
techniques?

Looking over the list, I have noted that the majority of problems with
old equipment is failure of the electrolytic capacitors in the power
supplies over time.

I wonder if the veterans can offer any suggestions on their favorite
techniques for finding bad caps in circuit, especially those that have
not been so considerate to have blown their end caps off or otherwise
developed obvious physical failures.

I have a critical undocumented circuit board of relatively recent
construction with about 25 vertically mounted axial lead caps. It's
begun eating fuses at an increasing rate, and now, even a 2 amp fuse
blows instantly in a slot intended for a half amp fuse. No way to lift
one lead of the cap without pulling it out of the PCB, with the
attendant risk of ruining the irreplaceable board. (well, I could
replace it for another $6-7,000, but that's not in the budget this
week!)

I figured I could put in another 2 amp fuse, and run it on a Variac at
reduced voltage while watching the current consumption, and see what
component heats up, but I really don't want to trash the microcomputer
on the board.

Any one have suggestions for any specific low voltage instruments for
in-circuit cap checking, or home-brew gimmicks that one can work up, to
give better information than the typical DVM and scope?

Thanks in advance,

Dave

Jeanette,

I had heard that as well. Do you remember what year the audio express article was published?

George

On Aug 30, 2011, at 7:26 AM, "Jeanette" <jdwalton@...> wrote:

There was such a modification in June and July issues of AudioXpress (which, I am going to guess, you don't have!). Uses a pair of 1N4742 12V 1W zeners to split the supply.

There's an important utility to running the oscillator off batteries -- you're going to get a much lower noise floor and no pesky 60, 120, 180Hz power supply artifacts. You can rebuild the battery packs with Nicads, or button cells and use the existing charge circuit, or use one of the intelligent charger chips from, for instance Texas Instruments (Unitrode.)

I rebuilt the battery pack of my HP3581 wave analyzer -- and it was worth the investment.

--- In hp_agilent_equipment@..., "willcoele@" <willcoele@> wrote:

I have three 204B oscillators I'm restoring. One had a problem where C2 (the 3 section, 1% capacitor) had a shorted section. I replaced the shorted section with single 1%, .3uF. Close enough for the Government!

My problem is, I'm converting the battery units to AC. It has a transformer a bridge rectifier and some filtering but secondary of the transformer isn't center tapped and it's not referenced to ground. I used a 7812 and a 7915 regulator to provide the +/- 12volts but when I wired it to the transformer the plus side dropped to 5.6 volts and the minus side was 37.6 volts. The plus side was drawing more current than the minus side. To balance it out I used a 15v Zener diode on the minus side and the plus side increased to 25.75 volts.

It appeared to be working but when the frequency was set to 250to 1KHz, there was a small amount ripple on the + 12v. To reduce the ripple, I used a 1000uF electrolytic on the +12v side.

Has anyone done a similar modification? Is there a better way?