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Everything depends on the +50 supply. Even the regulator op amps get their supply from the +50. So yes, you need to get the 50 volts working first.

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I would find a radial lead cap (wire leads) that is at least higher in the capacitance and voltage ratings?than the original can capacitors. Just jumper the three ground holes together where needed.

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For the 1000 uF try this:

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For the 5500 uF try this:

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HTH,

Tom

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----- Original Message -----

From: fakecrap@...

To: TekScopes@...

Sent: Monday, July 16, 2012 10:50 AM

Subject: Re: [TekScopes] Tektronix 475 with no display

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I did some more reading, and then more testing, over the past week. I also purchased an ESR meter to aid my testing.

What I found was that two of the main caps are definitely bad (one had too high of an ESR, roughly 100 times too high, based on average charts I found, and one appears to be an open circuit based on every test I performed).?

I removed those caps; despite all of the warnings about damaging traces, and being careful, I still damaged one pad/hole in the process of the first removal. ?I mention that just so others reading this will hopefully take more care in the future. Specifically, my damage was not from rough handling, but too much heat over time. I should have cooled things down in between changing the desoldering wick.

Luckily, the damaged part was only on the stand-alone circular pad on the bottom side (opposite the cap). I should be able to make a fair enough repair that won't affect the way it functions too much (if at all noticeable). The second cap came off with no issues as I took even more care removing that one (letting it cool down more in between solder removal, etc).

So this post has two purposes: 1) to say thanks for the info and pointers so far. C1414 (1000uf 75V) is the open cap and C1442 (5500uf 30v) is the bad one. 2) to ask whether it makes sense that my -15V line would be high (roughly -16.4) when the others are low (including -8 at roughly -6). From the schematic, it doesn't seem that -15V is related to either of these bad caps, so that was a bit confusing to me. Perhaps I'm missing something though.?

I'm now researching replacement capacitor options and hopefully that will fix everything. I'll know more once I get the new caps and adjust/test things. Any suggestions for caps would be welcomed. Otherwise, I'm searching the list now and various parts suppliers that I normally use (mouser, digikey, etc).

I assume the main reason people drill out the old ones and place new ones in there is mostly just for looks and partly for the 3 ground pins, correct? Is there any other benefit of going through that work, or should I just fit a new cap in there and jumper the ground pads where applicable?

Thanks again folks, much appreciated.

Josh

On Sat, Jul 7, 2012 at 6:47 PM, Tom Miller <tmiller11147@...> wrote:

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That's a real nice deal. The 475 is a 200 MHz scope and is quite nice.

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The problem sounds like a bad electrolytic capacitor in the 50/105 volt section of the power supply. Check C-1412 and C-1414. Most likely C-1414?has high ESR and needs to be replaced. You might take a look through the files and photos section as well as do a search through the messages for capacitor replacement. The capacitor that most likely is bad is on the main interface board grouped with some other can capacitors. You will want to be very careful removing the bad cap as there is track under the can and it pulls up easily as well as the plated through hole.

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Good luck,

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Tom

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----- Original Message -----

From: i814u2.geo

To: TekScopes@...

Sent: Saturday, July 07, 2012 7:01 PM

Subject: [TekScopes] Tektronix 475 with no display

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I picked up a 475 on craigslist for $10. The owner stated it worked a couple years ago but when recently powered up, he "couldn't get a trace".

For $10, I figured it was worth the effort to fix it. I have experience fixing electronics and while my soldering/component testing/safety skills are pretty decent, my troubleshooting/tracing/etc is beginner level. Any help would be appreciated.

It came with a stand, the manual, two 10x probes (one missing the 10x pin, but otherwise intact), extra parts (probe parts, etc), and the service manual. It also has the front cover and styrofoam intact. So the unit appears to have been kept well over the years.

I've opened the unit up and everything is surprisingly clean there as well. I see a couple of spots where the solder appears slightly burnt (like a semi-poor repair job, but not terrible). I've tested most components in those areas (but not all yet) and they have been testing fine.

Nothing appears terribly loose and nothing appears to be bubbled or blackened.

One major thing I've noticed is that all of the voltage readings are low. I read that most of them reference the +50V line and so I followed the manual's instructions to adjust that first. The best I can get is roughly 42.3V.

To me, that says I need to review the power section before anything else. Am I on the correct track, or not? Also, any common failure point that would cause my 50V line to drop?

Thanks for the help. I can post pictures of those help. Otherwise just let me know what other info would be useful.