Bought my first Master 128 off eBay. Description said "spares - not working" so I knew I was in for a challenging time.
The guy also said it switches on ok but needs a PSU to which I decided he mean't -"you need to replace the blown 30 odd year old capacitors". I'd already encountered (and solved) this problem with my loft based BBC Model B so I felt quite confident. Sure enough one of the 'usual suspect' capacitors was a mushroom shape showing its somewhat crisped innards.
I replaced all the power supply caps (using RetroClinic's full kit), tested the outputs for +5v and -5v, cleaned out the dust filled keyboard, swept the motherboard and discovered that the MOS ROM was missing. Replaced that with another RetroClinic product (4-way MOS ROM + switch) and checked 5v and ground connection to the ICs.
All looked good so hooked up a TV via RGB and switched it on. The left keyboard LED lit but no beeps and no display. CAPS LOCK and SHIFT LOCK lights didn't come on when pressing those keys.
Now I've hooked up a BitScope Micro oscilloscope and am testing the clock signals but I'm getting odd results. Hooking up pin 2 on IC43 to the 'scope did not show a waveform so that might be the main cause of my problem (or I don't know how to measure clock signals). In order to check the latter I hooked up pin 8 on IC42 (VIDPROC) to the scope. When I did this the power relay clicked on and off a couple of times and the speaker started emitting a buzzing noise.
Can anyone suggest why the speaker should be emitting noise?
I presume I'll have to replace IC43 and probably the RAM chips too
Thanks in advance
