Hiyas Scooby,
There is a guide to spectrum common diagnostics and repair, I shall have a hunt for it but I am sure I found the link on WoS (
www.worldofspectrum.org/forums/).
In truth, the best tools to have are
1. Good Quality Multimeter, spend a good £40 in this area and you can even get one that reads
frequency and
duty cycle*
2. Oscilloscope (25Mhz)
3. Plan of Attack
1. Good Quality Multimeter
I can recommend the Vichy 97 / 99 and the Uni-T UT61D, they have a lot of features and a great price point to boot. Both can even check clocks (timers), oscillators and resonators up to 30MHz as well... this can reduce the need for a full oscilloscope - although to be quite honest when you are diagnosing nothing beats having a scope to hand.
2. Oscilloscope
In regards to scope, you should consider sample time and bandwidth. Most people are throwing the old analogue display scopes out - you can get one for very little investment, if any at all. to work with most retrolicous kit, look at something in the 25MHz bandwidth range with a resolution of 4.5ns or lower. Here is a link to know what you need to buy (
click me)
3. Plan of Attack
Now this will depend on your target devices, you need to know where to start and what to isolate to test. There is a wealth of info in google, but at times it can be a pain to data-mine - sometimes a pick-axe would be faster. The best thing to do is immerse yourself with the community that a given system is, like with the Spectrum, the best place to learn would be WoS, as with an Atari it would be Atari-Age.
A common rule of thumb - if there is one is to get to know the components of the device, start with the power input and work your way towards the logic. check passives and wet-ware caps first. check transistors second, check coils and windings third.... after that its becomes specific to the machine in question.
Another thing to do is ALWAYS test a system before you put ANY work into it. You do not know what state it is in before you begin and once you add additional variables with your own work it can be a tad of a nightmare to debug / diagnose what you have done. If there is nothing obvious that needs attention you should always power-cycle before beginning any diagnosis, repair or component replacement.
), but only because of the sentimental value. I've done the same for quite a few others too though. It's nice to have a fully socketed Spectrum too 
.
if there are still problems on the board)
