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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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25th Nov 2014, 5:26 pm | #1 |
Retired Dormant Member
Join Date: Nov 2014
Location: Hereford, UK.
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Noise on PM3355 traces
Hi All,
I just acquired a Philips PM3355 and it arrived today. It powers up OK, the backlight to the LCD display is not working but I've found that on another post. The big problem is there's a static 40-50mv of noise on each of the traces, whether there's any signal there or not. I've tried the internal cal signal and a cal signal from another scope (my old Hameg HM512) and the result is the same. (my reason for getting another scope or two is a: they were fairly cheap and b: the HM512 is misbehaving a bit lately - intermittent no trace, only 1/2 the screen, trace shape changing when moving vertical position) I'm wondering if the fan in the back might be inducing noise in the scope? I'll try disconnecting that temporarily and see what the result is, has anyone got any other ideas? I've also bought a Gould 4074 and OS-1400 recently and these also seem to suffer from interferance on the traces to varying degrees (which the HM512 doesn't seem to suffer from) - the 4974 seems to be mains hum from somewhere - possibly the fan. Wondering if there's any other general things I should be looking into? Many Thanks Tim |
25th Nov 2014, 5:37 pm | #2 |
Heptode
Join Date: Jun 2010
Location: Zurich, Switzerland
Posts: 538
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Re: Noise on PM3355 traces
Tim,
On PM3355 can you see the same noises with GND-d inputs too_search in a dark room for HT coronas? You can disconnect the fan for a short time and check your noise situation. I think the fans are running on DC(?), not so a sure source for mains hum. K. Last edited by karesz*; 25th Nov 2014 at 5:48 pm. |
25th Nov 2014, 9:49 pm | #3 |
Retired Dormant Member
Join Date: Nov 2014
Location: Hereford, UK.
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Re: Noise on PM3355 traces
Hi,
I've just been out trying a few different things, here's the results: * Grounding the input has no noticable effect on the noise level or frequency. * Removed cover and powered it on then turned off light- No corona arcing was visible. * Fan appears to be DC, disconnecting it only made it audibly quieter, no difference to the signal noise displayed (originally thought it could have been noisy brushes on the motor). * Signal noise is most pronounced when on the '2' ranges (2v, 0.2v, 20mv) and less on other ranges, although below 10mv obviously this magnifies due to ambient noise (as I would expect). * Noise Frequency seems to be around 40kHz, although not pure sine wave. Amplitude seems fairly constant (about 3/5 of one division on the '2' ranges, less on other ranges but doesn't noticably increase until below 10mv) * Noise is present whether using on analogue mode or digital storage mode (measurements on frequency taken on digital mode with the cursors - the frequency and amplitude seem about the same in both modes) I also noticed that the scale illumination bulb has gone as well, another one to replace... (although I may have one to fit - failing that it looks to be a similar size to a 5mm LED - might that be a better replacement) - I might have to look into replacing the LCD backlight with a LED as well... Does anyone know what voltage they're supposed to run at? (the bulbs that is - I know about the LEDs) Tim |
25th Nov 2014, 11:08 pm | #4 |
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Location: Boston, Massachusetts, USA
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Re: Noise on PM3355 traces
You may have some bad electrolytic caps on the power supply . The 40 kHz could be switching power supply noise.
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25th Nov 2014, 11:26 pm | #5 |
Retired Dormant Member
Join Date: Nov 2014
Location: Hereford, UK.
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Re: Noise on PM3355 traces
OK, I'll have a look into that. Next question, has anyone got the schematics? (or a service manual), preferably in English.
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25th Nov 2014, 11:54 pm | #6 |
Nonode
Join Date: Mar 2008
Location: Virginia Water, Surrey, UK.
Posts: 2,876
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Re: Noise on PM3355 traces
I had one of this series of scopes (a 3350?) with noise on the traces, and on the on-screen display.
With the help of Michael Maurice and his ESR meter we found that most of the smaller Mullard/Philips blue electrolytics in the switchmode PSU were high-ESR. I replaced them all - about 22 capacitors - and the noise definitely went away. Oh, and the scale lamp was gone as well. I fettled an LED-based replacement to keep me going until I finally found a replacement from a specialist lamp supplier. Hope this helps.
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26th Nov 2014, 12:39 am | #7 |
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Join Date: Nov 2014
Location: Hereford, UK.
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Re: Noise on PM3355 traces
Sounds like quite a bit of work, but probably worth it, don't suppose you remember the values of the caps so I can order replacements? (I'd be tempted to leave the LED in place - less chance of it going again. Now which colour should I use.)
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26th Nov 2014, 11:18 am | #8 |
Heptode
Join Date: Jun 2010
Location: Zurich, Switzerland
Posts: 538
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Re: Noise on PM3355 traces
Hello Tim,
You can download SM for PM3350_or PM3365 too, may be its helpful for you too? I found yet these topic_according that, the PM3355 & PM3065 are the same(?) scopes... K. |
26th Nov 2014, 4:14 pm | #9 |
Retired Dormant Member
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Re: Noise on PM3355 traces
Hi K,
Thanks for that, manuals now downloaded. Also mailed the guy in the post about the pots (mine has the top Y-pos knob broken off) - see if he has one he could sell me. Now to get on and get the front panel off and see if I can change that bulb. |
26th Nov 2014, 5:19 pm | #10 |
Heptode
Join Date: Jun 2010
Location: Zurich, Switzerland
Posts: 538
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Re: Noise on PM3355 traces
Tim,
I remember yet, Mr Lüders from Hannover has lot of Philips docs and good contacts, mail to him_if you need... greetings, Karl |
26th Nov 2014, 9:25 pm | #11 |
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Join Date: Nov 2014
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Re: Noise on PM3355 traces
OK, progress report. I'm going to spend a few hours tonight studying the manual to try and get the values of all the caps I need to replace the ones in the power supply - I have some already but this is probably a good time to upgrade them to a better spec (good excuse to go offline for an hour or two).
I managed to find a bulb in my stash that fitted and was even almost a perfect match for the scale illumination bulb (28V 80ma, the one I found was 28V 40ma). I also managed to string 3 orange SMD LEDs together along with a 470R resistor to make a backlight for the LCD. It's a bit dim but it's readable (and miraculously I got the polarity right first time), I'll try again when I get some white or yellow SMD LEDs (on order for another project - the one I got the scopes for in the first place see if I can't get the display a bit brighter. All in all a fairly productive day. |
26th Nov 2014, 9:40 pm | #12 |
Nonode
Join Date: Mar 2008
Location: Virginia Water, Surrey, UK.
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Re: Noise on PM3355 traces
By the way, I am pretty sure these are the people from whom I got my 28V bulbs for my Philips 3350 scope.
http://www.cp-lighting.co.uk/
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28th Nov 2014, 1:28 am | #13 |
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Re: Noise on PM3355 traces
No pots available from the other guy, and I can't see any available elsewhere without buying another unit for a LOT of cash and breaking it (even the faulty ones seem to be fetching 3x what I paid for mine).
I think what I'll do is remove the pot from the scale illumination position and swap it with Y-pos 1 - If I cut a slot in it I can adjust the scale illumination (occasionally) with a trim tool until I can get a replacement. As for the caps - I finally got the PSU board out - there's a couple of moulded tabs on the board supports that it seems need to be cut to allow the board to be removed (warranty seals?) - I couldn't find any other way of removing the board so I assume it must be. I think I will replace all of the electrolytic caps with newer ones of a better grade - hopefully it'll mean I never have to do it again. I realise there's a cost involved but looking at how much a replacement is going to cost me it's the cheap option. |