Author Topic: Tek 7904 tick-tick-tick caused by a ceramic capacitor!  (Read 1829 times)

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Offline mmagin

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Tek 7904 tick-tick-tick caused by a ceramic capacitor!
« on: April 18, 2016, 04:13:41 am »
I recently got a Tektronix 7904 "for parts or repair" off ebay because I'd been wanting a faster scope and I have a stubborn love of older things.

True to form, it wasn't working.  Just got the ticking sound.  It was pretty clean and still had calibration seals.  (No label with a date though.)

Fortunately I read through the documentation where it tells you to check the resistances to ground on each of the power supply rails.  And indeed, one was zero ohms to ground -- the 130V supply.

After disconnecting enough of those little connectors, I'd isolated the trouble to somewhere in the high voltage / focusing part of the "high-efficiency supply".  Oh boy.

Well, after some time thinking about it, I dove in and opened it up.  I guess they get high marks for putting almost all the dangerous voltages in places that are hard to reach with just the side covers off the scope.  On the other hand, that power supply is layers and layers of stuff to remove.  I would have even had to desolder 5 transformer wires from the auto-focus board if I had followed the instructions to the letter.

Anyway, it was this damned 0.01 uF cap.  Supposedly a ceramic cap according to the parts listing.  Shorted the 130V supply.  Replaced it with a much higher voltage ceramic cap (only suitable thing I had on hand).

I am also kind of cranky that their darned proprietary .1" header housings seem to break down when in close proximity to -3kV.

But I'm well on my way to a 500 MHz scope (with a 7A19 and 7B92 in addition to my existing sub-100 Mhz plugins)!  Just need to go through all the performance checks and adjustments and clean up a lot of stuff in the plugins.
 

Offline alsetalokin4017

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Re: Tek 7904 tick-tick-tick caused by a ceramic capacitor!
« Reply #1 on: April 18, 2016, 04:41:49 am »
Looks like a pad came off along with the capacitor.
The easiest person to fool is yourself. -- Richard Feynman
 

Offline mmagin

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Re: Tek 7904 tick-tick-tick caused by a ceramic capacitor!
« Reply #2 on: April 18, 2016, 04:55:38 am »
Looks like a pad came off along with the capacitor.

Quite possibly.  I need to get a fancy-pants vacuum desoldering iron.
 

Offline macboy

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Re: Tek 7904 tick-tick-tick caused by a ceramic capacitor!
« Reply #3 on: April 18, 2016, 12:28:57 pm »
Looks like a pad came off along with the capacitor.

Quite possibly.  I need to get a fancy-pants vacuum desoldering iron.
You need to make sure to heat the joint until all the solder - including on the top side - is melted before pulling. With any luck, that blob on the capacitor lead is just solder and the pad is still on the board.
 

Offline MrSlack

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Re: Tek 7904 tick-tick-tick caused by a ceramic capacitor!
« Reply #4 on: April 18, 2016, 12:55:05 pm »
 :palm: My poor eyes! That probably is solder but please be careful.

Use solder wick for this sort of rework and a lower temperature iron.

Empty the joint properly into some solder wick, then carefully twist the duff component leads about 45 degrees to pop the last bit of solder, pull it out, clean the joint up with more solder wick and then put a new component in and re-solder.

Vacuum desoldering will just suck the pads off as well if the iron is too hot. Since I did rework for a bit I have never touched a single vacuum based device. They are destructive and spew tiny bits of solder all over everything.

Source: did 6 months on milspec through hole rework then 6 months inspecting other people's soldering.
 
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Offline mmagin

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Re: Tek 7904 tick-tick-tick caused by a ceramic capacitor!
« Reply #5 on: April 18, 2016, 05:03:58 pm »
:palm: My poor eyes! That probably is solder but please be careful.

Use solder wick for this sort of rework and a lower temperature iron.

Empty the joint properly into some solder wick, then carefully twist the duff component leads about 45 degrees to pop the last bit of solder, pull it out, clean the joint up with more solder wick and then put a new component in and re-solder.

Vacuum desoldering will just suck the pads off as well if the iron is too hot. Since I did rework for a bit I have never touched a single vacuum based device. They are destructive and spew tiny bits of solder all over everything.

Source: did 6 months on milspec through hole rework then 6 months inspecting other people's soldering.

Thanks for the advice.  I don't think I'm using too much heat (I was using a Metcal STTC-036R tip).  Maybe I worry too much about heating for too long rather than about applying vertical forces to the pads.  "Empty the joint properly" indeed.

(Edit: pulled the cap out of the trash can and put a soldering iron to the lead.  The blob melted.  What a relief, wasn't the pad.  I was not eager to disassemble all of that again.)
« Last Edit: April 18, 2016, 05:12:33 pm by mmagin »
 

Offline MrSlack

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Re: Tek 7904 tick-tick-tick caused by a ceramic capacitor!
« Reply #6 on: April 18, 2016, 05:47:25 pm »
I've never used a metcal so I can't comment on them. I use a Weller TCP and recently an Antex C15 which I have slightly fallen in love with (£21 and it's tiny).

Phew at the solder :)

If you're worried about it getting too hot, another technique is to clip the blown component's leads top side flush to the board, heat the joint (it will be easier without the part sinking the heat away) and push the remaining lead through quickly with a wooden tooth pick. Comes right out before the solder goes solid. Then clean with solder wick.
 

Offline mmagin

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Re: Tek 7904 tick-tick-tick caused by a ceramic capacitor!
« Reply #7 on: April 18, 2016, 06:02:22 pm »
The "0" and "6" prefix Metcal tips are the 675°F / 357°C maximum ones "for temperature sensitive applications".  In practice, I use that series for all 63/37 leaded soldering stuff except for one big chisel tip I have in the 775F "1" prefix.  (There is an even cooler 575F series of tips, which I might try getting one of someday.)  Any of them will put out 60 watts to maintain temperature if the work is sucking enough heat away to demand it.   I got this Metcal station used and I am totally hooked.  My old Hakko 936 lives on a hard-to-reach shelf as a backup only :)
 

Offline MrSlack

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Re: Tek 7904 tick-tick-tick caused by a ceramic capacitor!
« Reply #8 on: April 18, 2016, 06:10:28 pm »
Interesting. Thanks for the info. Will do some reading.
 

Offline mmagin

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Re: Tek 7904 tick-tick-tick caused by a ceramic capacitor!
« Reply #9 on: April 18, 2016, 09:49:39 pm »
Thermaltronics makes some cheaper compatible gear, including tips.  I'm not sure they're as good quality, I have a couple I've been meaning to try, but I do so much with a .1" chisel tip that I only end up changing tips for special things.

I think the underlying technology (curie point of a material changing the amount the tip receives energy from the RF supply) is out of patent now and being made by a couple other companies.  The temperature regulation is awesome, but you only get one temperature per tip.

The real win for me is that you hold it closer to the tip, and as someone with a (slight) congenital hand tremor, this is really helpful when dealing with finer pitch components.
 


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