ZX81 rebuild

[Lardo Boffin]

I guess at £2.15 with p&p I will take my chances but would rather keep things simple with a proven supplier! Is it possible to test them in any way before fitting them?

What I would really like to do is prove they work before fitting them to the rebuild (to make the trouble shooting easier when I finally build it, switch it on and nothing happens...). I have a slightly trashed test bed zeddy I can use. Would it be possible to desolder one of this zeddys transistors, push one of these into place, bend the legs out to ensure contact and stick them down on the solder side using insulating tape? Would this give a good enough contact for a brief test?

Lardo

[1024MAK]

To test all the characteristics of a transistor requires some expensive test gear. However, a lot of low cost multimeters now have a simple transistor test socket, the meter will provide base and collector bias, and display the DC gain (HFE) on the display. If the transistor is healthy, the gain should be in the range indicated on the datasheet.

There are also dedicated testers like those from Peak.

If you don't have a meter that has a transistor tester, then you could always put a simple circuit on a plug board. You just need a couple of resistors and a meter that can read mV as well as volts. Example test resistor values for a NPN transistor are 20k or 22k from +5V to the base. 100 ohm (this resistor may get hot) from +5V to the collector. The emitter goes to 0V. If you test and record the voltage across each resistor (from one lead to the other), you can then use ohms law to work out the currents. Then divide and you get the DC gain at that bias (the base current and collector current).

Mark

[gozzo]

oh, what were these eeproms that were bad? and from what supplier? i've bought a fair few but not got round to using them yet...

[gozzo]

and i think that supplier above is where i got some mps2369s from..havent tried them yet..something about the type number printing doesnt look quite right, though..... maybe something else relabelled?!!

[gozzo]

these will almost certainly be genuine, very good supplier.. http://www.ebay.co.uk/itm/MPS2369A-MOTO ... 0653829622 the type number 'print' is totally different, seems to be 'etched' ...

[1024MAK]

oh, what were these eeproms that were bad? and from what supplier? i've bought a fair few but not got round to using them yet...

Read through this thread over on StarDot.

Mark

[Lardo Boffin]

I have ordered all the bits on the list. Fingers crossed it is all the right stuff!
When it arrives I can start the rebuild...

[Lardo Boffin]

Some time later...

Some bits! All the bags neatly marked up. Its amazing what you will do to delay having to actually start building this. The sooner I start the sooner I switch it on and nothing happens...

Other bits are lurking in various places.

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PCB prepared for the 16k mod as i don’t have a 1k chip.

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Lardo

[Lardo Boffin]

Resistors fitted. Who knows I may have even got the resistor packs the right way round!

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[Lardo Boffin]

Capacitors

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