Useful inspections (capacitors a. o. things) before using a QL? Update 24.03. - changed the membrane

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YuT666
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Useful inspections (capacitors a. o. things) before using a QL? Update 24.03. - changed the membrane

Postby YuT666 » Thu Feb 22, 2018 6:44 pm

Bought a working QL and my question is ...

Are there any special capacitors and other hardware related things, i should inspect or repair BEFORE seriously using the QL?

Thanks in advance & cheers
Last edited by YuT666 on Sat Mar 24, 2018 3:29 pm, edited 2 times in total.


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1024MAK
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Re: Useful inspections (capacitors a. o. things) before using a QL?

Postby 1024MAK » Thu Feb 22, 2018 7:11 pm

Depends...

If the membrane is thought to be intact, it best not to open it up.

If the membrane is known, or suspected to be broken, then just a visual inspection of the board and its components is a good thing to do.

If however, it is known or suspected that someone has done work inside, then a closer inspection is warranted.

Things to look out for include, but is not limited to:-
  • The 7805 voltage regulator being firmly fixed to it's heatsink,
  • The electrolytic capacitors in the power supply part of the board look okay (the tops are flat, not domed and no leakage can be seen),
  • Check that the chips are secure in their sockets and the pins are all inserted correctly (with the correct chips in the relevant sockets, and they are the correct way round),
  • That the ROM/EPROM selection links are correct for the ROMs/EPROMs that are fitted,
  • If relevant, what Sinclair official modifications have been done or not done.
Can't think of anything else.

Unlike the ZX Spectrum, threre is no auto-destruct circuitry :geek:

Mark


QL, Falcon, Atari 520STFM, Atari 1040STE, more PC's than I care to count and an assortment of 8 bit micros (Sinclair and Acorn)(nearly forgot the Psion's)
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YuT666
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Re: Useful inspections (capacitors a. o. things) before using a QL?

Postby YuT666 » Thu Feb 22, 2018 7:30 pm

Well, here we meet again ... ;)

Thanks for the hints ...


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Re: Useful inspections (capacitors a. o. things) before using a QL?

Postby 1024MAK » Thu Feb 22, 2018 8:51 pm

If you intend to use the microdrives, then these need checking and maybe cleaning.

The expansion connector should be checked for any bent or broken pins.

Mark


QL, Falcon, Atari 520STFM, Atari 1040STE, more PC's than I care to count and an assortment of 8 bit micros (Sinclair and Acorn)(nearly forgot the Psion's)
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tofro
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Re: Useful inspections (capacitors a. o. things) before using a QL?

Postby tofro » Thu Feb 22, 2018 9:39 pm

I have actually never had a failed capacitor ever in a QL (now that I said that, I'll probably have one tomorrow...)

The keyboard membrane is very likely to be toast if it has never been replaced before.

Beyond that, Microdrive cleaning, re-seating chips in sockets, checking the expansion connector for bent pins, and, if you want to use some cartridges, felt replacement, might be in order.

Actually, the last item is the most likely to fail.

Tobias


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Re: Useful inspections (capacitors a. o. things) before using a QL?

Postby vanpeebles » Fri Feb 23, 2018 7:40 am

Did early 80s caps use a different material/liquid? It's funny how on a lot later japanese hardware, caps are prone to fail, but I've never once changed one on my Sinclairs etc.


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Pr0f
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Re: Useful inspections (capacitors a. o. things) before using a QL?

Postby Pr0f » Fri Feb 23, 2018 8:40 am

Capacitor life depends a lot on the circuit they are in, the conditions they are in and the material in the capacitor.

Consider the old valve radios with waxed paper capacitors - some of those are still working even now!

High voltages, large ripple currents, high ambient heat, very low temperature, humidty - both too high and too low can all affect capacitor longevity.

Electroylitc caps have the lowest life expectancy - particularly those in high ripple high temperature environments such as a switch mode PSU. It's not so easy to tell with axial electrolytic caps (wires out of each end), but the radial caps have the 'pressure vent' head pattern and any bulging is a good sign that replacement is due :-)

As for the QL - I've seen a couple of boards recently - and the caps look in pretty good nick. I've unsoldered a couple of the larger ones and tested them - still within tolerance.


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Re: Useful inspections (capacitors a. o. things) before using a QL?

Postby tofro » Fri Feb 23, 2018 10:01 am

Actually, Sinclair Spectrums are quite well-known for developing problems caused by bad caps in a critical area, the +/- 12V supply of the RAM. Due to a kind of adventurous but money-saving design of that circuitry, failing caps (or other failing components) have a high probability of completely destroying the lower 16k of RAM there...

QLs seem to be constructed in a much more traditional way.

Tobias


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Re: Useful inspections (capacitors a. o. things) before using a QL?

Postby Pr0f » Fri Feb 23, 2018 10:46 am

The QL power supply is 9V from the power brick, and 15V AC which is just halfwave rectified into a couple of smallish caps, that drive a couple of small 12V regulators for +/- 12V. They +12v is used for video encoder chip, and RS232 serial drivers, and the -12v only for the RS232 serial drivers (actually not strictly true - I think it's also used via resistors as a bias voltage for Microdrive circuitry).

The 9v feeds the Microdrive boards, and the expansion connector, and also the main regulator that produces 5V for all the memory and logic in the QL. The standard regulators are fairly robust devices, and tolerate a lot of abuse, and even failing supply caps to a limited degree, provided they don't fail shorted. A lot of the decoupling capacitors are either tantalum or ceramic, so these have very good longevity. Although the design is done on the side of cheapness, some thought has gone into it.



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