HP 871XB VNA CRT module capacitor identification help

[TK]

I am fixing a CRT module on a recently purchased HP 8714B VNA.  The fuse at the +12V (The fuse is rated @ 125V 2A) was blown.  I replaced the fuse and it blows, so I started looking at suspicious components and found Capacitor C732 (50V 3.3uF) bulged.  Removed it and it measures around 4.2uF ESR 6.2 ohms.  I want to replace it but I cannot find a BiPolar capacitor 50V 3.3uF that is the same size.  All the new parts from Nippon Chemi Con seems to be much smaller.  Does anyone know what series or type of capacitor is the part? 



I measured the voltages on the Power Supply and they are all spot on.  The CRT module PCB was super dirty, maybe the cap was already bulged because I did not hear or smell anything wrong when I powered the instrument.

The CRT module is Hitachi CDK-10BA242 (The same one used on the HP 54500A scopes).

[TK]

I found the answer in the yahoo HP test equipment group.  Someone named Brad had the exact same problem:

Brad [hp_agilent_equipment] wrote:
> I've gotten back to this after a bit of a break and it may have helped. To recap,
> the fuse on the video board blows when The board is connected to the CRT. I tested
> a lot of components in circuit but finally noticed that C732 which is part of the
> high voltage circuit is bulging on the top. It looks like in aluminum
> electrolytic, but appears to be unpolarized. It is a 3.3 µF 50 V. capacitor. The
> one on the board, which appears to be original since it was secured with a tie
> wrap that has become very brittle, is a Nippon Chemicon CE04P. There is also a
> longitudinal band of six rectangles with the letters BP running along one side.
> It's marked 85° C. The capacitance tests low, about .49 microfarads and the ESR is
> about 35 ohms (which may not be too high for this value, but I don't have much
> experience with this.) My question is what should I replace it with. Digi key
> doesn't carry this brand, and the ones they have are physically quite a bit
> smaller. This one is about .6 inches in diameter and about 1.6 inches tall. Can
> anyone help? Brad

And someone named Chuck answered:

Your capacitor is toast!

"BP" is the universal marking for bipolar.

Bipolar caps are always used in pretty low current, lightly
loaded places in circuits. Anything BP will be fine.

If you must use Nippon Chemicon, go to Mouser, and use a
United Chemicon... they are the same company.

Smaller is normal. There has been a lot of improvements
in capacitor technology since the original was made. If
the size bothers you, go for higher voltage.

-Chuck

[TK]

The new capacitor is about 10% the size of the original one, and it worked fine... until the other day that I heard an explosion inside the VNA unit, I opened it and I found the remains of the new capacitor, just the two leads.  I am sure the capacitor had the same value and it was rated at 63V (the original cap is 50V), so probably I need to check and replace all the electrolytic capacitors on the display PCB just in case the fault is somewhere else and is causing this capacitor to fail.

I found out that the CRT module is the same one used on the HP 54500 and 54600 oscilloscopes.

[SeanB]

3u3 is well within the ability of a common film capacitor, and you can easily get a 3u3 100V polyester one which will last almost forever there. RS  192-102 is around $1 for example, though you probably could still fit a 250VAC version in there with a little effort. Polyester will have lower ESR than the bipolar one, and will have a better pulse rating, which is what killed the old one.

[TK]

I will try with the polyester 3u3 250VDC capacitor.  I think I found the schematics for the CRT module and I marked the capacitor using a red circle.

[SeanB]

That scan capacitor leads a hard life, 15kHz sawtooth waveform through it at around 10A or so of scan current, and very fast edges on the one side as the line output transistor turns on, so they do fail with time, but the reason that they were always electrolytics was simply that film capacitors at the time were not as capable of handling high current in a small volume. That has changed, but the caps stayed the same electrolytics.

[TheSteve]

Should you need an entire replacement board I have the PCB and CRT out of my 8714C.

[TK]

Should you need an entire replacement board I have the PCB and CRT out of my 8714C.

Thank you TheSteve!  I ordered some caps and will try to fix the broken board.  I will send you a PM if I need the replacement.

[Chris56000]

Hi!

Thanks for posting the CDU–10BA241 Circuit Diagram!

That bulged 3μ3 50 V non–polarised electrolytic capacitor is of a type called "high–frequency Bi–Polar", and is much larger than those tiddly small things they use in audio applications, they were specifically made to withstand the very large peak currents in line scan deflection circuits!

In the old days when CRT monitors were repaired these electrolytics were stocked by suppliers that specialised in Monitor Repair Parts, and I don't think they're available new anymore!

However there is an axial type of polypropylene capacitor called Visaton MKT–A, available in the UK from CPC and Impact Audio, as well as eBay, in a good range of values to replace these special non–polarised electrolytic scan coupling capacitors!

Chris Williams