Printed-circuit de-crudding?

[KA1ZGC]

Hey Guys,

Got me a Tektronix 466 storage scope that stopped working correctly some time in the recent past, before it got to me. The trace is extremely small unless I press the "Beam Find" button, then I suddenly get a usable trace that covers about 3/4 of the CRT. The horizontal position pot is almost useless.

Suspecting a power supply issue, I pulled its pants down and started measuring voltages. I got as far as measuring the -15V supply when I noticed a small spark in the area of a disc ceramic cap. A closer look at the cap revealed that something leaked on to a small area of the storage amplifier board, and that familiar green crud had formed around the cap, a couple of resisitors and diodes. Whatever leaked also got around the voltage regulator for the +15V supply on the adjacent board (interface board).

This may or may not be the cause of the issue I was troubleshooting, but it needs to be addressed before I go any further with the unit.

I know that my odds of successfully undoing the damage are about 50/50. If I have to lose the storage functionality, I'm fine with that. That's where most of the damage is. I don't mind pulling the gazintas, gazoutas, and power from that section and running without it, but getting it fully functional again would be nice. The +15V supply is a must, however.

Anyone have any luck de-crudding PCBs that have been similarly afflicted? I know the board itself may be compromised, but I'm hoping the crud is merely topical and that I've been lucky enough not to seriously damage anything I can't replace.

Just curious what chemicals/methods folks have used in times like these, and what results they've had with them.

Thanks in advance for any and all advice.

--Thom
Keep Away One Zorching Green Crud

[WA1GFZ]

Check the transistor sockets.

[KA1ZGC]

I don't think you understood the question, Frank. I'm not asking for troubleshooting assistance. I'm asking if anyone has had any luck cleaning boards that have green crud caked around some of the components.

FWIW, there are no transistor sockets.

[W1EUJ]

Try grain alcohol. 100% ethanol if you can get it.

Apply liberally.

[WBear2GCR]

Although it is "wrong" and politically incorrect - a toothbrush (clean) and alcohol is a good first pass for gummy stuff and green stuff. Keep ur face away so you do not breathe the fine particulate matter, since that sort of alcohol is poison.

Beyond that, I have used water washes using Fantastik, 409 and much much stronger degreasers as well, just need a good summer sun dry for 24 hrs or similar. Hair dryers work for crevases. Must be bone dry before refiring up. Never damaged any circuit that way.

I prefer MEAN GREEN degreaser cleaner myself - it is an alkaline cleaner.

cleaner + brush is sometimes useful.

Dishwasher detergent is useful too, it etches stuff slightly.

THere are also "electronic" spray cleaners with very volatile solvents...

            _-_-bear

[Mike/W8BAC]

I have been using a 50/50 mix of original 409 and non sudsing ammonia for almost 20 years. This mix will cut just about anything. Use it in a well ventilated area as it will take your breath away. A toothbrush will help remove the caked on dirt. Rinse with lots of water.

For sensitive circuits I use the purest ethanol I can find (90% plus) to flush and finish the job. Less pure alcohol contains more water. In any case a careful drying will be needed.

If that board can be removed you can limit the cleaning to the effected area. You might have to wash the whole chassis to fix it but as you say,

I know that my odds of successfully undoing the damage are about 50/50.

This type of cleaning takes a leap of faith but let me assure you I have lost very few pieces of equipment in the past 20 years from this kind of cleaning and I have washed ton's of stuff. The biggest problem is not waiting long enough to make sure everything is dry. The best way to dry is in a very low oven  but don't use the kitchen. The smell will last a week.

P.S.

As you might suspect many things are sensitive to detergent and water like silk screened dials and glass plus some lettering on faceplate's.  Another problem area is heat sink compound on transistor and regulator heat sinks. It's a good idea to reapply a new coat of compound after cleaning. A test wash will help figure out what is sensitive. In any case work fast. Squirt, scrub, rinse and get the liquid off as soon as possible.

Mike

[KA1ZGC]

TNX, guys!

Glad you mentioned the heat sink compound. I hadn't considered that, and one of the effected areas is the +15V regulator.

Fortunately, the damage is limited to a very small area (near as I can tell, anyway, I haven't seen the solder side yet), mostly on the storage amp board. I can get the storage amp board out without too much trouble, the interface board may be a different story. That's where the regulator lives.

All told, I'm only dealing with 3-4"².

Thanks again for the tips, guys. It sounds like I can save this thing if I'm thorough and careful about doing it.

--Thom
Killer Agony One Zipper Got Caught

[WA1GFZ]

al the 4XX series scopes I have seen have transistor sockets and they get crudded up after a while. I've seen many of them die this death.
409 is a great cleaner also the green stuff. You can yank the board and run it through the dish washer if Mom isn't looking. I cleaned a used fuel oil tank with dish washing soap and it worked great.

[Rob K2CU]

IN my lab the rule is to use 91% isopropyl alcohol, the type you can buy in the supermarket for years, with one of those useless too stiff toothbrushes you get from the dentist for $85, er I mean free. some baked flux rrequires the use of an exactor knife. Also, and very important, check the area of the observed arc for carbon tracks. you may have to make cross cuts with same exacto knife and then coat with Q-dope if you can't completely remove them. many scopes have deliberate spark gaps built in to limit HV and these can create carbon tracks.

[WQ9E]

Be careful as a lot of the household cleaners leave both a conductive and corrosive residue so you will need to rinse with copious amounts of distilled water after to make sure you flush away all of the residue.  WD-40 is another possible cleaner since it is non-conductive and non-corrosive but you will need to only use it in an area where you can wipe off any of the light oil residue that is left so it will not capture dust.  It is basically a kerosene like product with a light oil added so it doesn't do anything bad unless you spray a lot of it where it does not belong or spray it where it (or its fumes) can ignite.  I have used this to clean the leakage from failed electrolytic caps on circuit boards among other things and it works well, just use a cotton tip or rag instead of spraying it everywhere. Somewhere in my collection of Tektronix manuals and service information is a note from Tektronix where WD-40 is the preferred cleaner for the pots in the old 1 and letter series plug-ins.

I believe the 466 was built in the era where Tektronix was starting to use a lot of tantalum capacitors (most of my Tek scopes are either 500 series prior to tantalum or 76/77/7800 series which are full of tantalum caps).  I have had a number of these fail and they are the only components I have replaced a fair number of in this vintage of Tektronix scopes.  Many of the 7000 series scopes use a switching type supply that is directly line operated and in overcurrent mode the supply ticks on and off until the overload is removed; every time I have had this happen was due to shorted capacitors in one of the plug-ins.

Rodger WQ9E

[w4bfs]

tantalum caps are VERY VERY sensitive to overvoltage spikes ... failure mode is increased leakage ... if you would use a 15 volt electrolytic cap in a 12 volt circuit, use at least a 25 volt tantalum, everything else being equal ... 73...John

[N3DRB The Derb]

there's some spray junk we used at work called Flux Off that would make short work of cap bleedage like that. I believe it's a chemtronics product. We were forbdden to use any non solvent based cleaners on PC boards. Instant fire as in yo ass out the door if you got caught using a water based cleaner on a board.

There was not a PC board in North America that could stay dirty on me. These were 8 layer SMD boards with plated thru holes.

[KB2WIG]

If ur daring, try carb cleaner....

 good for cleaning out firearms tooo......  klc