Psion XP power circuit schematic, LZ constant buzz, whine, beep. Re-capping

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MartinP
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Psion XP power circuit schematic, LZ constant buzz, whine, beep. Re-capping

Post by MartinP »

The Organiser is a robust device and many are still going strong at 30+ years old, but some users have ocassionally reported a constant buzz/whine/beep coming from their machine. This seems to particularly effect the LZ model, but replacing the electrolytic capacitors is reported to solve the problem. Perhaps an LZ batch received some poor quality capacitors, or maybe there was a circuit change that made the problem more likely. Either way, electrolytic capacitors don't last forever and can leak which could potentially damage the board, so this could become a problem for all Organisers at some point. In general it's best to replace all of the electrolytics, but there may be one or two which are more likely to fail. There seem to be between 7 to 9 electrolytics depending on which board you have.
I've attached a circuit schematic of the power PCB in an attempt to identify the capacitors. There may be some errors in the schematic, but it's my best guess at the moment and is based upon an old drawing for an Organiser II XP isssue 3 power board. I've put question marks on the names or values of components I'm less confident about.
Psion Organiser II XP power PCB schematic.PNG
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Lostgallifreyan
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Re: Psion XP power circuit schematic, LZ constant buzz, whine, beep. Re-capping

Post by Lostgallifreyan »

Are you wondering which capacitors should be tried first?

Most small electrolytics, especially from that time before the 'bad caps' debacle with the dodgy electrolyte or the counterfeiting, won't be a problem, but C5 is interesting for its location, it looks closely linked to one side of the piezo sounder so if it's not decoupling efficiently on the 22V programming voltage line, noise from the charge pump might get to the sounder and be audible.

The sounds I've sometimes heard on an Organiser can be complex, hard to locate by ear. I once assumed the charge pump coil might be noisy but the sounder might carry AC to air as sound. I can't remember what sort of pitch it was but a lot lower than 32768 Hz! (Also, I doubt the 22V system would be on all the time, even if unloaded, too wasteful..)

That makes me wonder about the LCD contrast changer, which is unusual. It's not adjusting DC, there's a cap there that blocks it according to that schematic, so I assume it's AC (at 1024 Hz marked on the diagram) and used to drive a pump to get a negative voltage, and attenuating the AC drive would have a similar effect as varying the DC, and maybe with better control.

Does modifying the contrast change the sound at all? I doubt enough energy is involved there but it's worth testing, it's the easiest test you could make. If there is any detectable change it might indicate other caps associated with that LCD negative bias system, as likely to need replacing to limit spread of that 1024 Hz.

Another test is to find a new cap of half the value of a cap you suspect of trouble. Adding it in parallel by temporary contact might change the behaviour of the existing capacitor enough to indicate a need to replace, if the sound changes as a result. If the existing cap was good, adding half its capacitance in parallel is unlikely to do anything much, but if it's weak, the change might amount to five times more than it still has on its own, so this test is safe, and fairly easy if you have various caps around to try. Even a short intermittent contact might provoke a useful response if you do it in a quiet room.
MartinP
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Re: Psion XP power circuit schematic, LZ constant buzz, whine, beep. Re-capping

Post by MartinP »

Yes, the aim is to see if one capacitor is usually to blame for this problem. I don't have a faulty Organiser, so I can't do any testing, but I like your suggestion of probing with some good capacitors.
A second aim is to reproduce the circuit schematics of the Organiser, they were becoming lost, I think, but should have been included with an original copy of the Technical Reference Manual, does anyone have a printed copy of it? I've started a GitHub page to redraw the schematics: https://github.com/martinprest/Psion-Or ... Schematics
When looking for a likely capacitor for the fault, I also suspected the contrast circuit, it is fed by roughly 1 kHz, so that may match the sound quite well. However, I found a post in the Facebook Psion group which says that they replaced only C3, and this fixed the problem. This capacitor has the lowest voltage rating 6.3 V, which doesn't give much margin on the regulated 5 V. This capacitor is on the main supply to the Organiser Vcc1, and is powered all the time, although it switches from a low current supply in standby to the high current supply of the op-amp when the Organiser powers up. The power board circuit is itself quite interesting and a nice example of using op-amps for power regulation. It's good to see that the Organiser supply uses standard components and shows that it should be possible to keep Organisers running for at least another 30 years!
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MartinReid
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Technical Reference Manual

Post by MartinReid »

Martin
I'm not sure if you have it, but during lock down I republished the Technical Reference Manual (here) . The contents of which were salvaged from the Archived PSION2.ORG website created by Dave Woolnough and maintained by Jaap Scherphuis and Boris Cornet (RIP).

Let me know if it is of help, then I know my time wasn't wasted.

Always sincere
Martin
MartinP
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Re: Psion XP power circuit schematic, LZ constant buzz, whine, beep. Re-capping

Post by MartinP »

I used your version of the Technical Reference Manual when I made the circuit schematic. The power circuit is described in the section beginning on page 42, components are referred to by reference numbers which I have matched to the schematic. The schematic is missing from your manual, but the manual is written as if it should have been there originally. There is a poor image quality schematic at the archive of Psion2.org. That schematic was difficult to read and had some bits missing, that's why I created the new one, you could include it with the manual if you like.
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MartinReid
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Publishing the schematic..

Post by MartinReid »

Martin

That's a great idea to include your schematic in the original place in the republished manual. I've got the 'master' in Word so will need either a PNG, DXF, or DWG and you better let me know exactly where is belongs. You better PM me your reference / credit details along with a disclaimer. I see the copy above is on A3. With the manual having A5 pages I could put a PNG in the correct location then reference an appendix D where I could publish it across two pages (A4 Landscape)

In good faith
Martin
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MartinReid
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More information on repairing 'buzzing' LZ's

Post by MartinReid »

Others with 'buzzing' LZ's

James has a post (here) regarding his LZ64 suffering from a persistent and quite irritating whistle, which he believed came from an ageing capacitor.

Regards
Martin
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