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I have a Phillips PM3264 100MHz 4 channel analogue oscilloscope, I have not powered it up in around 8 years. I have not been able to find a users manual, nor service manual as of yet, only a few pages of specifications from a brochure [pdf].

I am concerned that due to the time it has spent in storage, and the overall age of the oscilloscope that merely plugging it into mains and turning it on may not be the wisest plan of action. There do appear to be reports that the line filtering circuit can be problematic on these oscilloscopes.

If it is of any importance we have a mains ground circuit breaker (I was honestly shocked to discover that in some areas of the world it is not a legal requirement), and I do not have a variac (even so, I'm not sure that a variac would be the best idea at all). Mains power in the area is 240v at 50Hz, 240v is right at the upper end of the oscilloscopes power supply voltage range. I do not have probes for it at current, and I would prefer to at least get an idea as to whether it is still functional prior to purchasing probes given budget constraints, and location.

So, are there any precautions I can take when powering it up to prevent damage to the oscilloscope?

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  • \$\begingroup\$ I inherited an old bush radio that I powered up a few years ago and it still works but takes about 3 minutes to warm up because of the tubes inside. I thought about getting a fire extinguisher on hand when I powered it but eventually, after about 30 seconds I took the plunge. If it breaks down and smokes when you power the scope just throw it away - it's likely that the custom asics won't work because I've got one of those at work and they failed on me producing an unimaginably shaped signal trace. They can't be bought or stolen for love nor money. \$\endgroup\$
    – Andy aka
    Commented Jul 2, 2015 at 23:57
  • \$\begingroup\$ I normally power stuff up via an extension cord + switch (such as those commonly found on surge protector cables) if I don't want it to blow up in my face \$\endgroup\$
    – user2813274
    Commented Jul 2, 2015 at 23:59
  • \$\begingroup\$ @Andyaka I've got an old valve radio, and amplifier in one kicking around, still works, and I love the sound. It blew 2 fuses before it came to life. I was unaware of the benefits of lowering voltage at the time, though I had the top off to watch if anything started getting out of hand. \$\endgroup\$
    – Phizes
    Commented Jul 3, 2015 at 9:09
  • \$\begingroup\$ @user2813274 I have done that before, (un)fortunately I have yet to witness a large release of magic smoke. My main concern here is the oscilloscope as I will be unable to get a new 'scope of any form for quite some time. \$\endgroup\$
    – Phizes
    Commented Jul 3, 2015 at 9:09

1 Answer 1

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Usually the most problematic parts in old (but not ancient) electronic equipment are electrolytic capacitors with liquid dielectric.

The most common problems are:

  • Some caps dried up (electrolyte dried up).

  • Some of the dielectric oxide reduced.

In the first case the damage is permanent (unless you change those caps) and the result could be reduced capacitance values (even drastically). This could lead to erratic behavior and even short circuits as a consequence. Usually the problem is apparent as soon as you switch on the power: a fuse could blow up immediately or in short time. If the apparatus is not fused, that could be a problem.

The second problem is less severe. If charged at reduced voltages the oxide layer will form back in minutes and everything will be fine. Of course you would need to power the device with a reduced supply.

But you could try a bit riskier operation, which often works: power the equipment up for a short time (5 seconds), if it doesn't break because of the first problem, it usually can bear its full voltage for such a short time. Then power down and leave it alone for half an hour. Then repeat, now keeping the power on for 10s, then power down and let it rest for half an hour again. Repeat again, ever increasing the interval you keep the power on (1min, 2min, 5min, 10min). Always keep an eye on the thing while it is powered up, looking for magic smoke, funny sounds or strange smells (and shut down the thing if it happens). When you reach a point where the device can survive half an hour of continuous operation probably its caps, if not damaged because of the dry-up, will be regenerated.

Note that 8 year storage is not too bad, if the device was stored in a dry, cool place, with no temperature extremes or other atmospheric "danger factors" (salt from seaside air, for example). I expect it to be in good shape since it is professional equipment from a reputable manufacturer, and not some el-cheapo consumer electronic gadget.

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  • \$\begingroup\$ Given that the oscilloscope can take an input of 100 - 240v AC, (50W, 50 - 400Hz) would it be beneficial to drop the voltage with an incandescent bulb in series on the supplied voltage? Given the wide operating voltage, I'm assuming a switched supply, so would a lowered voltage benefit the mains side at all? \$\endgroup\$
    – Phizes
    Commented Jul 3, 2015 at 8:57
  • \$\begingroup\$ @Phizes Mmmh, I don't know if that method would do more harm than good. If the scope has a 100-240V AC mains range it means that it has an internal switching power supply, which could comprise quite complex circuitry. That circuitry is designed to be powered from an almost ideal AC voltage source, i.e. a very stiff source. Putting a bulb in series turns the voltage source adds impedance to the AC source (and a non-linear one, since the resistance of the bulb when it is cold is much less than when it is lit up)... \$\endgroup\$
    – LorenzoDonati4Ukraine-OnStrike
    Commented Jul 3, 2015 at 14:06
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    \$\begingroup\$ @Phizes ... The bulb will act as a current limiter, not a voltage limiter. The SMPS will detect a lower AC voltage and will try to draw more current to compensate (assuming it needs to deliver the same amount of power to the scope circuitry), this will cause more drop, etc. I cannot be sure without analyzing the topology, but what you suggest could make the internal power supply go crazy. To summarize, I don't think it is a good idea. Anyway, since the SMPS will try to adjust to the AC voltage, the internal circuitry will get the same voltage on its DC power rails, defeating your attempt. \$\endgroup\$
    – LorenzoDonati4Ukraine-OnStrike
    Commented Jul 3, 2015 at 14:11
  • \$\begingroup\$ Ah, thanks for that explanation. I was aware the internal components would get the same voltage, but was wondering about the small bit dealing with mains. I hadn't considered the increased current draw in this case, despite knowing that 110V appliances (generally) require higher current than their 240V counterparts. I'll give this a bash later this evening, and report back. \$\endgroup\$
    – Phizes
    Commented Jul 3, 2015 at 14:28
  • \$\begingroup\$ On the whole this seems to have worked, but either some of the power supply, or CRT related caps have died. I took it extremely slowly with the whole process, and I left it over night after hitting the 5 minute mark, and started the following day from 2 minutes. Progress was good until a 20 minute run when there was what I can best describe as a scratchy sound which I'm assuming was capacitors boiling off. (The scent was reminiscent of ceramic capacitors.) The trace now dips off of the screen within a couple of seconds of power. I think I may be able to work from here though. Thanks! \$\endgroup\$
    – Phizes
    Commented Jul 6, 2015 at 12:06

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