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Old 20th May 2019, 5:16 pm   #1
ekjdm14
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Default Philips PM3200 X 10MHz 'scope deflection.

Firstly, a big Thank You to Ian (superscope) for taking the time to send this oscilloscope. On first inspection, the reasons for it being described as totally dead were seemingly down to tarnish on the plug pins & fuse holder. With this attended to & after opening up to give the switches and post a dose of Super 10, I've brought it up on the Variac and we have power & a CRT which works for the moment (more on that later).

The problem it has appears to be to do with the vertical deflection circuit, since there is activity from the CRT if powered off & on before it's cooled, a single dot (plenty of brightness) runs from bottom to top and vanishes off the edge of the phosphor with just a faint smear visible down the tube if I turn the intensity and focus up high enough. No adjustment can bring it near centre vertically although I can definitely move it horizontally & it can be seen to "twitch" when changing timebase settings.

The only service info I have found so far is in Dutch, but if anyone has an English version that'd be much appreciated, as would any suggestions where to start with the deflection issue.

It could, of course, be an internal CRT fault since on removing it to have a look the getter didn't look too healthy, being faint and quite milky. Not sure why the tube would be gassy though as although there are signs of damp storage there's really no corrosion to speak of & nothing around the CRT pins. Could a gassy CRT cause deflection issues seeing as it's all internal to the tube?
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Old 20th May 2019, 7:05 pm   #2
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Default Re: Philips PM3200 X 10MHz 'scope deflection.

Ah, looking further at the Dutch manual there are some handy annotations with voltages and waveforms to check so not totally in the dark at least Next order of business is to get photos of how it's put together, dismantle and check for dry joints &/or dirty nicotine shorts and spot check a few components, then power back up and do some voltage checks.

Will try and get a photo of the getter spot and upload that as this is my main concern for now, emission seems good and I couldn't detect any untoward glowing from inside the CRT but it really does look gassy to me even though I don't see how it could've gone to air at all.
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Old 20th May 2019, 8:27 pm   #3
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Default Re: Philips PM3200 X 10MHz 'scope deflection.

Further bit of playing around and I'm pretty sure the Y deflection is in fact working to some degree, I can see a change in the "smear" when I adjust the Y position control & on disconnecting the Y deflection wires I can centre a dot nicely on the screen.

Whether any of the input circuits are awake is another matter again, but for now just onto voltage checking and working out what controls the Y position bias apart from the pot at the front. I think either a preset has gone open due to tarnish or something is just way out of tolerance.
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Old 18th Jul 2019, 10:00 pm   #4
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Default Re: Philips PM3200 X 10MHz 'scope deflection.

A while on and I've got this oscilloscope back on the bench & have made some progress. After spending too much time studying the circuit diagrams and using Google translate to type out and convert the text from the PDF file into English, I've sorted the first fault which was the Y deflection being stuck virtually off the top of the CRT.

This was caused by a BF194 transistor in the vertical amplifier being dead, tested as a diode between B-C. With this replaced, I now have full control over Y position and can deflect it somewhat with an input of a few mV. I found another BF194 in the external X deflection circuit that also thought it was a diode B-C, but I need to keep digging as the timebase generator isn't running and X deflection is skewed to one side (although not as badly as Y was).

I have a feeling this 'scope was damaged by an erroneous input or static at some point, given the faults so far, although I also have another possible theory since the 2 dead transistors have both been BF194's with the same batch/mfr. marking (CED or CEO?).

Since I have zero experience of oscilloscopes, I'm feeling my way a bit here but refuse to be beaten by it! Would appreciate input from those with more experience in these insturments, whether the faults on this are most likely user-created or if there is a history/knowledge of BF194's failing at all?

Thanks in advance,
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Old 18th Jul 2019, 10:09 pm   #5
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Default Re: Philips PM3200 X 10MHz 'scope deflection.

Lockfit transistors are well known for giving trouble. I ended up having to replace all of them in a Pye 697 decoder panel.
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Old 18th Jul 2019, 10:44 pm   #6
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Default Re: Philips PM3200 X 10MHz 'scope deflection.

Ah, that's what a Lockfit is! I've heard/read of them & their less than favourable reputation a fair bit but never had occasion to look up what actually constitutes one. (I have been meaning to look it up for a while just never got around to it).

At least that gives me good reason to act further on my suspicions of one of the little blighters in the X deflection/timebase generator circuit. (seems the internal cal signal isn't running reliably either). Although simple voltage measurements led me to the one in the Y amplifier, I'm fast approaching a similar catch-22 as replacing a soldering element in that I need a working example to repair the faulty one, or at least it'd be advantageous. (I simply don't trust my pocket "20MHz" kit DSO to tell me anything more than if there is a waveform or not.)

Now I know my enemy so to speak though, I can at least look up the specs and try to find a suitable equivalent for reasonable price/in scrap boards (ISTR I did temporarily sub in a BC107 during testing and it didn't seem too unhappy with it).
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Old 18th Jul 2019, 10:50 pm   #7
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Default Re: Philips PM3200 X 10MHz 'scope deflection.

Further to last, I'm now reading up more on Lockfits and their foibles. Makes sense that a Philips instrument would be chock-full of them. The partial failure symptom I just now read about makes sense with some of the other symptoms I notice too so now the decision I need to make is whether to diagnose/replace all faulty ones or "shotgun" them all. My head says I just want it working, though my heart says that's just not the way to do things.
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Old 19th Jul 2019, 5:52 am   #8
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Default Re: Philips PM3200 X 10MHz 'scope deflection.

This bloke did a thorough repair on a Philips scope and though not your model these vids might help -
https://www.youtube.com/watch?v=iPYzzsRXaMY

Andy.
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Old 19th Jul 2019, 1:52 pm   #9
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Default Re: Philips PM3200 X 10MHz 'scope deflection.

Decision made, I've put in an order with Nikkoe for a batch of NOS Telefunken 194's as they were pretty cheap. Buying a good few means I ought to have enough quiet, thermally stable working ones to evict any more sub-par units from the 'scope and have some acceptable spares for future. The others like BC149 and 158 should be easier to find a close substitute in a more reliable package if need be.

Thanks for the link Andy much appreciated, will have a look shortly.

Also one more question (I'll be researching properly but just wondering about experiences), are BF179's known to leak a few uA to their TO39 can? there's 4 in the X deflection and they have up to 200vDC on their can (don't ask how I found out, suffice to say lesson learned and future temperature checking will not involve skin contact!)

Last edited by ekjdm14; 19th Jul 2019 at 1:53 pm. Reason: typo'
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Old 19th Jul 2019, 3:16 pm   #10
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Default Re: Philips PM3200 X 10MHz 'scope deflection.

Quote:
Originally Posted by ekjdm14 View Post
Also one more question (I'll be researching properly but just wondering about experiences), are BF179's known to leak a few uA to their TO39 can? there's 4 in the X deflection and they have up to 200vDC on their can (don't ask how I found out, suffice to say lesson learned and future temperature checking will not involve skin contact!)
I should say so, can's collector.

Lawrence.
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Old 19th Jul 2019, 4:32 pm   #11
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Default Re: Philips PM3200 X 10MHz 'scope deflection.

That'd do it alright!
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Old 19th Jul 2019, 10:00 pm   #12
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Default Re: Philips PM3200 X 10MHz 'scope deflection.

Well I'm pleased to say some more progress on this 'scope before my parts have even arrived. I removed and tested some more of the BF194's and found another 2 faulty (one unidentifiable by my tester, the other thought it was 2 resistors C-B and C-E).

Then I remembered that the box of parts I recently purchased from Rob (Whiskas) contained a lot of Philips/Mullard related parts & had a look through the transistor drawer. Lo & behold, 2 unused TFK BF194's amongst a good selection of other unused Lockfit types. With these fitted I now have a somewhat functional oscilloscope on which I can select V/div and timebase and actually see a change not to mention getting an actual, real, trace with an input.

I have no doubt there are other failed/failing Lockfits in there & will replace the remaining 194's when the TFK ones arrive & probably go forward from there to diagnose/replace any other temperature sensitive or noisy ones since there's some drift, focus movement and still nothing from the cal output. But certainly a major step forward from where this instrument was just 2 days ago and I feel like my learning is starting to advance at a better rate along with it.

Thank you to all who have contributed, including Rob for the parts selection boxes & Ian (Superscope) from whom I got this instrument. It's certainly turned out more challenging than I originally hoped (it was 100% dead originally, I had expected some PSU repairs!) but I'm pleased that it was/is since I've had many hours of enjoyment and learning so far, and may end up with a useful instrument saved from the parts bin at the end of this journey.
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Old 28th Jul 2019, 4:27 pm   #13
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Default Re: Philips PM3200 X 10MHz 'scope deflection.

Been very hectic here lately but finally got a couple of hours to play yesterday. The remaining faulty BF194's are replaced and we're making good progress. The 'scope is now largely working although I'm sure there's some more work to make it 100%, most likely substituting Lockfits for more modern equivalents but for now it's working well enough that I'm now ready to look into the crusty/damaged intensity pot and give all the switches a good cleaning and then check/amend calibration.
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