Repair of the power board from an HP DeskJet, damaged by leakage from capacitors.

Hello again,

Photo here of the power board from an HP DeskJet printer.

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This is the back of the board.

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I removed the three medium sized capacitors in the upper left and, not surprisingly, found residue with the appearance of thinned molasses beneath them.

The photo of the back side shows where this residue has flowed through some of the holes and corroded along traces.

I've never tackled anything this badly damaged. Cleaning exposed surfaces with swabs and alcohol won't help the corrosion in the holes and under the green polymer coating. What are the recommendations? Is repair a realistic possibility? If so, pointers about technique will help.

Thanks, ... Peter E.

Reply to
Peter Easthope
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Looks like a trivial wiring problem to me. Be glad it's not a 10 layer SMT board with tiny feedthrus everywhere. Try a solvent like Simple Green as well as alcohol. They dissolve different stuff. Scrub it several times with each. Then take a wire brush to it. You're gonna rewire the traces anyway. Dry it well...then dry it some more.

Reply to
mike

"Peter Easthope"

** I have repaired many PCBs that were the victims of electrolyte leakage - do the obvious, replace copper tracks that show corrosion with wire links and bits of plastic covered wire.

Wash the area with a good PCB cleaner spray and/or de-natured alcohol - watch out for places where the electrolyte can hide like down component holes and under ICs etc. Dry thoroughly with hot air.

Long as the circuitry is not highly sensitive to PCB leakage, it should be fine.

... Phil

Reply to
Phil Allison

As others have mentioned, just clean up the mess with your favored cleaner. Then tin what's left of the traces with plenty of solder. If there's little copper left, run a bare wire over the traces, and solder the bare wire to the traces. The bare wire should be about #20 AWG to avoid turning it into a fuse. Bigger wire is fine. If the copper tries to detach from the PCB, do the bare wire trick, and then bury it under some acrylic or urethane conformal coating to stick it to the board. Don't worry about what it looks like as you're the only person that will ever see the work.

You might also check the remaining capacitors with an ESR tester. Just because the look good, doesn't mean they are good.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
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Reply to
Jeff Liebermann

Do you always disconnect the cap for measuring ESR? I've seen a mention of checking the ESR of an installed capacitor.

Thanks, ... Peter E.

Reply to
Peter Easthope

Doing in circuit testing only works if you have a tool that operates at low voltage at the test point, one that is below diode drop voltages.

These days, diode drops are in the ~.295 volt area.. So the test device must be presenting voltages below that to get a fair assessment of the cap, while in circuit. Also, it makes sense to make sure any test gear you use does not present a voltage to the circuit above that level, otherwise, you could damage sensitive components, just testing them.

THe last time I looked, 100 mv. was a good area...

When expecting low ESR readings on a cap, it's pretty much straight forward, doing in circuit test. Because the value to be expected would be low to start with. Circuit paths away from that area are more than likely going to register more.

There are times when in circuit testing fails the ESR test with a tester that is designed for low voltage sampling.

If you are testing for dielectric break down (voltage ratings), then yes, you most likely will need to extract the cap.

Jamie

Reply to
Jamie

No. I measure them in the circuit. This works because the ESR of the caps, even when there are a string in parallel, is much less than the conduction resistance of most active devices that are likely to be in parallel with the caps.

I have this ESR tester:

in addition to several home made contraptions. Since I don't need to check batteries with the tester, I have a pair of back to back 1N4002 diodes across the test leads to deal with charged cazapitors.

What's critical is knowing what the value should be. I have a chart that helps (borrowed from Bob Parker's web site):

Since I have a fairly good collection of replacement caps, I also like to compare values with a brand new cap.

Since the major effort involved in recapping is often extracting the board from its overly protective enclosure, I tend to replace first, and test later. If one cap, of a specific type and value is obviously bad, it's highly likely that all the caps of that type and value are also bad. So, I replace all the likely culprits, and not worry about testing.

Please note that it's not just the big caps, which tend to experience high ripple current heating, that die. The smaller electrolytic have their share of problems, especially in switching power supplies. These are never visibly bulging and must be tested with an ESR tester.

There's also running assumption that the bulging capacitors are somehow defective. That was true when various manufactories were shipping electrolytics with bogus electrolyte about 14 years ago. That's no longer the case and those caps are (hopefully) long gone. What replaces them is marginal design and planned self destruction. In order to save a few pennies, many manufacturers reduce capacitor values and voltages to their absolute minimum. If there are three caps in a row, and it will work with two caps, it's shipped with two. The result is that the theoretical capacitor life, which is a function of temperature and ripple current (causing internal heating), moves fairly close to the desired lifetime. If the factory offers a 3 year warranty, designing for a 6 year life is fairly common. The actual failures will occur along a bell shaped curve, but the average will be about 6 years. This is why I like to replace capacitors with the next higher voltage rating. The ESR is actually higher, but the lifetime is much longer.

When I want the board to work forever (such as for my own equipment) I try to use tantalum or polymer caps in place of electrolytics. They're not suitable for every application and not available in every capacitance and voltage rating, so be sure to think about what's important in the circuit.

Then, there's idiot error:

I replaced all the electrolytics in the picture, and couldn't understand why they would blow up after about an hour of operation. When I posted the question to this newsgroup, it was pointed out that they were all installed backwards. (Thanks Phil A.)

Now, I have a question. Is this unspecified model Deskjet worth repairing? Even when working perfectly, the print speed, reliability, quality, and cost per page are all problematic when compared to a laser printer. The only thing good about an inkjet printer is the initial cost, which is less than a laser and the power consumption.

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Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Reply to
Jeff Liebermann

Presumably traces are blackened where the electrolyte has corroded between the board and the trace and between the polymer coat> I have this ESR tester:

Same or similar here; the date label has "7/24/09".

By simple financial calculation, probably not worth saving. But I have three DeskJets. Aside from power boards they are in good shape and I have a lifetime supply of ink. So with my aversion to waste, scrapping them is difficult.

Thanks to everyone for the instructions, ... Peter E.

Reply to
Peter Easthope

Slop cleaner all over the board. Let soak for about a minute. Blow the cleaner THROUGH the holes in the board with compressed air (from an air compressor, not canned air). If desperate, shove a needle or wire through the holes. When you're done, use the compressed air to blow out any liquid that might be hiding under parts.

Ok. You're ecologically correct in using up your ink supply. However, when I have the same problem, I just donate the printer and ink to a worthy recipient. I just hate dealing with clogged nozzles and sloooooooow printing. Laser printers are so much better.

Incidentally, I'm in the process of fixing a Behringer PMH518M portable mixer/amp with a similar problem. The PCB is a mess. The clever owner was running at least four 8 ohm speakers in parallel on the output. That caused considerably more current to flow in the FET output devices. The increased current also increased the ripple current through the 4700uf 50V power supply caps, which got hot and spewed electrolyte all over the PCB. When the electrolyte was finally gone, the FET's finally decided to short out. That caused a dramatic increase in current, which burned and eventually fused open several traces on the PCB. It also cooked a few power resistors. The fuse never blew. So, I have a burned PCB, with blackened electrolyte goo all over everything, and a mess of parts to replace. The mixer is probably only worth about $100, so I don't know why I bother. Photos when I remember which camera has them.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Reply to
Jeff Liebermann

What model is that HP printer? I will be scrapping at least 50 of them before long and might find a good board.

Reply to
Michael A. Terrell

This is after I cleaned off the leaky capacitor goo and removed the fried PCB traces:

The replacement parts haven't arrived yet.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Reply to
Jeff Liebermann

of

From memory, the oldest is a DeskJet 500 but will try to remember to get the numbers this weekend. Thanks, ... Peter E.

Reply to
Peter Easthope

Three of them. DeskJet 500 DeskWriter DeskJet 560C (color)

The three external power adapters are interchangeable. The internal power boards of the DeskJet 500 and the DeskWriter also appear to be interchangeable. I haven't removed the internal board from the

560C.

If you have a board or two to spare, I'm happy to buy it or them.

Thanks, ... Peter E.

Reply to
Peter Easthope

One other detail I'm curious about. The manual mentions "Power-supply PCA", "Head-driver PCA" and "Firmware on the logic PCA". PCA is printed circuit assembly? Something more arcane?

Thanks, ... Peter E.

Reply to
Peter Easthope

"Head-driver PCA" and "Firmware on the logic PCA". PCA is printed circuit assembly? Something more arcane?

Nothing more arcane. That's just HP-speak. Technically "PCB" is just a board. When populated it is an assembly, so HP are just being pedantic to the point of being anal.

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who where

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