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Old 28th March 2015, 01:32 PM   #1
jitter is offline jitter  Netherlands
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Default Oscilloscope woes

Inspired by this thread that went a bit off topic, I decided to continue the part about oscilloscopes here and perhaps shredhead can continue this thread with the search for the cause of a dead channel on his Tek scope.

Here's my contribution to start this thread:
I have a fully functional Philips PM 3208 20 MHz oscilloscope which experienced dropouts in the connected signals. Touching the vertical scale knobs of both channels was enough to send the trace on the screen in all directions.

As suggested by anatech, I applied some contact cleaner to the innards of both rotary switches, but without spraying. Since these switches aren't sealed or shielded, I thought I'd go a step further and clean them with a cotton swab (Q-tip). However, access to the switches was largely restricted by a small pcb hanging over them. Unfortunately, I couldn't access all the screws without having to do a major dismantling job, something I wasn't looking forward to.
Instead, I decided to loosen the so-called "vertical board" (ironically mounted horizontally, it deals with the signals on the vertical axis) and move it back a couple of cm. That meant removing the transformer, desoldering the input AC-GND-DC switches, unplugging a lot of connectors and unscrewing a remarkable amount of ground leads from the chassis.

But in the end I managed to access the vertical scale switches and the black deposit came off rather easily, leaving a brass like colour. It was worth the hassle, the traces now remain steady when I touch the vertical scale rotaries. Job finished.

Below the rotary switches in all their glory.

Click the image to open in full size.

Last edited by jitter; 28th March 2015 at 01:37 PM.
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Old 29th March 2015, 12:00 AM   #2
anatech is offline anatech  Canada
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Hi jitter,
Quote:
However, access to the switches was largely restricted by a small pcb hanging over them. Unfortunately, I couldn't access all the screws without having to do a major dismantling job, something I wasn't looking forward to.
Which is probably why people like to spray switches.

Why not use a spray?
The fluid tends to get into everything. It changes the dielectric constant of the switch wafers and PCB for one. That changes as the fluid then moves or evaporates very .... very ... slowly over time.
This fluid also creeps into trimmer capacitors. May as well replace those now.
The fluid can creep into capacitors and transistors (yes, that can happen). Scrap those and replace.
Finally, the cleaner will often wash away any lubricants off of contacts and even shaft bearings. Ever felt a sloppy radio control? Thank the TV tech who sprayed it years ago.

Controls and switches actually require only a tiny bit of cleaner placed in the correct spot. The carpet bombing approach taken by most only destroys more than it fixes.

Ever want a chance to rebuild an entire front end in a 'scope? Easy, spray it. Yes, it will probably still work, but the calibration is almost surely out the window. Adjust it and you might find that this instrument will never "hold" its calibration.

I learned this a long time ago from an engineer who patiently taught me how to use chemicals. The one thing he stressed was that chemicals are tools. Use the right tool in the right way.

A can of spray cleaner is much like using a hammer to fix everything. Too bad this will work in the short term.

-Chris
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Old 29th March 2015, 12:08 AM   #3
anatech is offline anatech  Canada
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Hi jitter,
That is exactly how I clean contacts. Some times you need to use a strip of business card or paper. Cutting off 50% or so of the Q-Tip makes it easier to clean the contacts without getting the Q-Tip stuck in those contact "fingers". If you do that, carefully rotate the switch the other way while pulling very lightly on the Q-Tip. It should feed out without damaging the contact finger. If not, be careful and patient! Bending these fingers can often spell disaster as that connection may never be reliable again. Take it from someone who was patient one day. I'll remember that incident. Thank goodness it was my equipment (which is probably why I wasn't careful).

I'm really happy this worked for you jitter. Thank you for beginning this thread and posting your work. I almost recognize that 'scope. It is very similar to many others I have worked on.

-Chris
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Old 29th March 2015, 02:55 PM   #4
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by anatech View Post
Hi jitter,
I almost recognize that 'scope. It is very similar to many others I have worked on.

-Chris
Today, I also cleaned the timebase rotary switch, it's as good as new now... well almost...

Here are pictures of top and bottom to jog your memory :

Click the image to open in full size.

Click the image to open in full size.

Last edited by jitter; 29th March 2015 at 02:57 PM.
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Old 30th March 2015, 05:27 AM   #5
anatech is offline anatech  Canada
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Hi jitter,
That is a nice clean interior. It strikes me as a really nice little backup 'scope. I probably worked on a similar model at some point. It does look familiar ...

Now that you have spent a little time (and very little cleaner), this little fella should be back up and running.

-Chris
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Old 30th March 2015, 06:52 AM   #6
Mooly is offline Mooly  United Kingdom
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Good work with the repair

The mechanical layout of that scope is almost identical to my Hung Chang 100MHz one. I pulled some images of the Philips front panel and it looks very similar in layout. These have come out of the same factory methinks
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Old 30th March 2015, 08:00 AM   #7
timpert is offline timpert  Netherlands
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That's a good looking scope on the inside. Most old beasts aren't so clean anymore. The nice thing about the old analog ones, is that they have switches that you can actually clean and get working again. My woes are with a Philips/Fluke PM3384A. It has rotary encoders and push buttons for everything, and relays in the front end for vertical range switching. And you guessed it... It has a bad relay and a bad encoder. The relay may still be obtainable on Ebay, but I am out of luck with the encoder, it seems impossible to find an encoder that fits. Opening the encoder for cleaning, and then successfully re-assembling it is almost impossible. So my only hope is to find someone with a broken 'scope that has the same encoders as mine. Anyone?
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Old 30th March 2015, 11:07 AM   #8
Mooly is offline Mooly  United Kingdom
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You're probably best asking that as a new thread over in 'Parts'. You never know, someone might have something suitable.
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Old 30th March 2015, 01:42 PM   #9
timpert is offline timpert  Netherlands
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True, but there's also another message: hold on to those old beasts, because they are much more repair friendly than the newer ones.
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Old 30th March 2015, 01:47 PM   #10
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by Mooly View Post
Good work with the repair

The mechanical layout of that scope is almost identical to my Hung Chang 100MHz one. I pulled some images of the Philips front panel and it looks very similar in layout. These have come out of the same factory methinks
I thought the same about the layout of Tek's 22xx scopes:

Click the image to open in full size.
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