B+K Precision 15MHz Oscilloscope Repair

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Jarod
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B+K Precision 15MHz Oscilloscope Repair

Post by Jarod » Sat Apr 07, 2012 12:26 am

So I recently was given an older B+K O'Scope, model number 1477.

It seems to work, has a nice trace, will measure square wave from the calibration port thingy, but sometimes its kinda flaky and goofy and twitches and the trace moves around.

I'm wondering if theres some bad caps or possibly some solder joints that are broken or something, theres a few that I'm wondering about, all the caps look ok from my quick look over it but I'm not sure.


Heres a ton of pictures:
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Smallenge
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Re: B+K Precision 15MHz Oscilloscope Repair

Post by Smallenge » Thu May 03, 2012 1:05 pm

I think you have to check all the wires first. It's a tough job, but most likely the problem is somewhere there.

Tom
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Re: B+K Precision 15MHz Oscilloscope Repair

Post by Tom » Sun May 13, 2012 3:07 am

Iiiittttsssss ALIVE!!!!!!!!!

So the first thing was figuring out what was actually wrong with it. The b channel showed nothing. Just a flat trace. The a channel was sometimes kinda buggy too for no apparent reason.

Turns out there was a resistor (q154 470 ohm on the schematic, under a metal cover on the b side of the vertical board) that was black, completely toasted.
I replaced it and its working fine as of tonight. I cleaned up all the knobs and all the metal and stuff. I've got 2 probes now (found a brand new one in the ladys basement that I'm cleaning out) so I've been having a blast with it.

Still figuring out what I'm going to use it for other than just messing around lol

foxmiles
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Re: B+K Precision 15MHz Oscilloscope Repair

Post by foxmiles » Sun May 13, 2012 11:26 am

That's awesome! You will have a lot of fun with it. You have to do the youscope thing. It looks horrible on my digital scope, I want to see it on your analog. It should look good there. It's a neat test to see if you've solved all the problems too.

One thing to watch, some of those resistors are really tight tolerance (0.01% or less) if you use a 5% your readings may be off. If you have the service manual you may be able to calibrate it against mine.
Digging with a spoon will accomplish more than leaning on a shovel.

Jarod
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Re: B+K Precision 15MHz Oscilloscope Repair

Post by Jarod » Sun May 13, 2012 7:28 pm

Heres a few photos of the problem and where it was found.

I'm not 100% sure if its perfectly accurate or not, I'm probably going to have to check it and possibly get a higher tolerance resistor.

I played with youscope and oscillofun this morning, couldn't quite get them to work, oscillofun displayed some parts but they were incomplete, I'm not sure what to set the v/div or timebase to, and I still have no idea what all the knobs do.
Attachments
IMG_1286.JPG
the whole board. problem was under a metal cover just like on the A channel side
IMG_1280.JPG
the blown resistor
IMG_1268.JPG
Where the problem was first noticed

Jarod
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Re: B+K Precision 15MHz Oscilloscope Repair

Post by Jarod » Mon May 14, 2012 9:48 am

I'm pretty sure its accurate. well its at least as accurate as the A channel.

However I have run into an issue with the oscillofun/youscope... my mp3 player, laptop and cell phone have very noisy audio output so they don't make a very nice image, some seem to be mono only even if I'm using a stereo cord and all that... also none of them play FLAC files so I'm stuck to wavs or mp3s.

I do get an image but its kinda squished because of the lack of true stereo and all the random noise. (up to 3v line noise on some of the devices)

logical solution would be use my desktop right? its got higher quality components and supports flac and has about 700 audio jacks on it....
wrong. for some reason my desktop puts out a triangle wave around 20-40v out of all the audio jacks.
I have no explanation for that other than my computer wants to be irritating. so I'm stuck for the moment.

I'm going to have to build a signal generator, or possibly I'll find one later today when I'm sorting in the basement where i got the oscilloscope.
I may have to burn a cd of oscillofun and play it in a stereo and hook up to the speaker outputs or something to get this working, idk.

Jarod
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Re: B+K Precision 15MHz Oscilloscope Repair

Post by Jarod » Mon May 14, 2012 10:20 pm

I just got a few new toys found buried deep in the ladys basement...

A whole ton of motherboards and cpus and video cards and hard drives. I've got 5 external drives, one 40Gb one 80, a 200 and 2 320Gb units.
A couple legit windows XP discs with the COA intact and valid, a ton of cdrom drives and floppy drives.
truckloads of ancient software that is quite likely not 100% legit and will be destroyed.
A bunch of CD-rs and DVD-rs, a box full of cd cases, and loads more pci netcards, wifi cards, network printer adapter thingy and tons more stuff.

Most of it is a little dated but a lot newer than some of my friends pcs. PCI-E capable and SATA era but my 5ish year old desktop with mid range components obliterates anything I've found so far.

there's a bunch more stuff left, however these are the most interesting finds from the sorting today:

I found an extra probe for the scope, and the things seen below in the pics :)
I'm not sure 100% of what they do or how to use them but its all in nice shape and seems to work, the function generator is missing a fuse and the cap that holds it on, so I'm going to have to find one or scrounge one up somehow.
Attachments
downsized_0514021635.jpg
HP Frequency counter Model 5314A it works, has a battery but I'm not sure if it holds a charge, will probably disassemble to make sure battery isn't leaking.
0514021633.jpg
HP Function Generator Model 3311A
0514021634.jpg
I think its missing the fuse holder and fuse. i haven't tested it yet so I'm not sure

Jarod
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Re: B+K Precision 15MHz Oscilloscope Repair

Post by Jarod » Tue May 15, 2012 12:14 am

These things are confusing. the signal generator does work, however it seems to have a rather nasty filter cap failure somewhere as all the signals it produces seem to wave around at what looks about 60hz. it has this 60hz wave around 5-10v even when not powered up at all.

the frequency counter seems to work but is extremely complicated and cranks around 40v into whatever you're measuring the frequency of... which is rather odd.

possibly more bad caps in there.

anyway, at least its all solid state, I'll have to hunt around for some replacements and see if i can find what caps are bad.

I'm gonna need to build an ESR meter soon huh? lol

Jarod
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Re: B+K Precision 15MHz Oscilloscope Repair

Post by Jarod » Wed May 16, 2012 9:19 am

Been messing with the signal generator more, replaced a couple 5000pf caps (with similarly sized ones, but not exactly the same) that were connected from hot to ground and neutral to ground, I'm assuming those were the filter caps for the 110 line in.

the problem is i still get a huge amount of 60hz noise. I can't see anything visibly damaged or anything, there's a few more 5000pf caps that are darker colored than the usual ceramic cap color, I'm thinking maybe i should buy 6 or 8 5000pf caps and replace all of those ones that are inside of it. there's a couple big electrolytic caps but they look ok, so I'm not sure where to go next.

foxmiles
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Re: B+K Precision 15MHz Oscilloscope Repair

Post by foxmiles » Wed May 16, 2012 12:06 pm

Are you sure you have a good ground? Check your plugs if you haven't already. I plugged my scope into a extension cord with a broken ground prong and got 60hz noise all over the place. Not all 3 prong outlets have 3 wires to the breaker box, some older houses have new plugs, but they aren't grounded. Those caps and any like them in your scope will pass 60hz noise to ground. Without a ground connection to absorb the noise it can go almost anywhere.
Digging with a spoon will accomplish more than leaning on a shovel.

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