My tek 2235A just blew up the PSU cap - diyAudio
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Old 14th February 2004, 10:16 AM   #1
pukka is offline pukka  Spain
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Default My tek 2235A just blew up the PSU cap

I was doing some tinkering last night when the big cap on my 2235 scope PSU blew
up: bad smelling smoke, and browny electrolite made its way to the IEC socket _
external side (bottom).

I switched it off as soon as I heared the fatidical (fffffft!) sound and hinted
the smoke.

Apart from that, the CRT side wall of the scope felt medium hot, though I
don't know if that's normal.

Do anybody knows if blown PSU caps take away other components when they
fail? I inspected the surrounding area but didn't see any obviously failed-
looking components. And I saw tons of protection devices on the circuit... It
would be certainly a ton of luck that the problem was just an aging capacitor
(marked 1NE30 dunno what date is that) and solved by its simple replacement...

any opinion?

I bought it used on December (from USA, I live in Spain). I opened and
inspected it when i got it and was marvelled at its shape and clean layout,
there were no signs of repair, and everything looked nice and clean.
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Old 14th February 2004, 12:19 PM   #2
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you can get a service manual from www.w7fg.com -- the cause is probably not just the capacitor itself -- did you have adequate ventillation.
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Old 14th February 2004, 12:29 PM   #3
markp is offline markp  United States
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How old is the scope? Did it die on turn on or after it was on for a while? These caps tend to die of being old and dried out(yes, even if goop come out it can be dried out). Replace the cap and you should be ok if it died on start up.
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Old 14th February 2004, 02:00 PM   #4
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Quote:
Originally posted by markp
How old is the scope? Did it die on turn on or after it was on for a while? These caps tend to die of being old and dried out(yes, even if goop come out it can be dried out). Replace the cap and you should be ok if it died on start up.
highly unlikely that "old age" afflicted a 2235. sometimes the power supplies in the TEK scopes of 2XXX series will go out due to heat stress. that's why I asked if there had been adequate ventillation.

the 22XX series of scopes are still repairable, but they are so inexpensive ($200 - $400) it hardly seems worth the effort unless you are going to do it yourself.
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Old 14th February 2004, 02:23 PM   #5
SY is offline SY  United States
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IME, it's often the other way around- something like a rectifier fails and it manifests by taking the filter cap with it.
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Old 14th February 2004, 02:35 PM   #6
pukka is offline pukka  Spain
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I didn't expect that amount of replies in such a short time! you guys are great

Now the answers:

* It died after an hour of so of work, not on power on.
* It had adecuate ventilation, this is a portable scope, and it was placed at about 30 degrees to the flat surface (table) it was laying on (handle bar below chassis).

However, the PSU side of the chassis was only slightly warm; where temperature was certainly above the rest was on the rear CRT side.

* It had an appropiate (I'd swear still the original) fuse.

I'll try to check the rectifiers and will replace the cap.

Are the tek service manuals worth? Are they simply schematics or do they include troubleshooting procedures, etc?
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Old 14th February 2004, 02:51 PM   #7
mcp is offline mcp  United States
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Quote:
bad smelling smoke, and browny electrolite made its way to the IEC socket
Same thing happened to my 2235A.

In my case, it was the IEC EMI filter that went. Stripped it out, wired directly to mains and it worked. Apparently, the EMI filter is good for 110V but not 240V, eventhough it's supposed to be.
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Old 14th February 2004, 03:09 PM   #8
pukka is offline pukka  Spain
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what!?!? that makes a ton of sense! I couldn't see goop on the PCB, and I wondered how in earth could the goop reptate to the IEC socket and reach out without leaving traces near the cap.

Also, what I was doing was measuring back EMF from electric motors to determine achieve RPM, could that be the thing that aided in frying the IEC?

This scope has "lived" always in the US, and I have used it here in Europe only a dozen times... so what you say makes a lot of sense. I wish!!!

When you say "IEC EMI filter" you mean the whole IEC socket? or is the filter detachable?

I happen to have some spare IEC sockets including filters at hand...
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Old 14th February 2004, 03:23 PM   #9
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The main power supply in the 2235 is a switching type. Those supplys are more likely to blow the switching transistors out long before a capicator drawing alot of current would fail. Parts are difficult to get for any CRT based Tektronix scope as Tektronix discarded all of their spare parts to the surplus makets a number of years back. Buying another scope of the same model number os about the only way to aasure yourself parts availability. There also a few places on the net that specialize in spare parts for test gear and probably the largest is Sphere up in Canada.

http://www.sphere.bc.ca/test/index.html

Mark

P.S. I forgot to mention that in repairing TEK scopes and many others such as H-P and Fluke the main problem is lack of availability of the many custom I'C's that were used in them. These are not re-labeled I'C's but actually custom designed and manufactured by Tektroniz and others. These Chips, some are hybrids, allowed them to meet or exceed the published specifications and manufacture test equipment using normal assembly line production methods. Most TEK scopes far exceed their published specs. My 465B is good to about 150 mhz!
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Old 14th February 2004, 04:04 PM   #10
mcp is offline mcp  United States
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Quote:
When you say "IEC EMI filter" you mean the whole IEC socket? or is the filter detachable?
The IEC socket at the back of the 2235A is actually an EMI filter. I opened the case, unscrew the filter, desoldered the 3 mains wires, threw out the filter, and wired directly to mains. Switched it back on and it worked.

When I bought it used, I was cautioned to use a step down transformer to 110V. I forgot about it and about half an hour or so after switching it on, there was this hissing, smoke and blackish brown mess bubbling out of the IEC socket. I wasn't even measuring anything, just powering it up.

A friend of mine also bought the 2235A the same time. He ran it off a step down transformer (110V) and no problems. Seems the EMI filter doesn't like 240V. Strange.
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