"Black screen fix" biscuit
5 May 2021
TI99 / 4A composite mod
31 May 2021
Power supply that smokes ?!
One of the problems that afflicts this computer is on the power supply, there are a couple of capacitors on the 230V input that are just right and over time they deteriorate until they burn and make that famous smoke .. but sometimes it's not just that!
I took this model to see what goes into when we have this failure, and also because the price for a well-made BBC is much more expensive, so let's see the restoration!
In the beginning it was the dirty one
Watch the video repair 🙂
The first thing I did was clean all the plastics, the board and the keyboard ..
After a good wash it is watchable again!
There will be some retrobright to do but otherwise I'm not complaining!
Now we come to the power supply.
All smoke and no capacity !.
Obviously I replaced the 2 capacitors on the 230V input, one was really open (C2 100nF 250V) while the other had some cracks (C1 10nF 250V).
But that's not enough, besides these capacitors I saw that a couple on the final leveling stage (the electrolytic ones) were damaged, they had not swelled on top but they had lost the electrolyte on the pins. On this occasion I changed them all.
It is not enough yet, when I turned on the power supply I had no output voltage and so I started looking for the fault, seeing how far I had voltage.
Here comes the scheme of the power supply that is not found on the network if not this done at best without values.
It is still a start !, I had the voltage up to the ends of C6-C8 and I checked the diodes and transformer to see that there were the windings.
At this point the problem was localized on the oscillator stage composed of Q1, Q2 and C9. Control C9 and in fact it was to be thrown away, but it still did not work, I change Q1 with one of luck that I had at home a 2n2222 quite similar even if it has a slightly lower voltage level.
Well, it started working again !! all voltages present: + 5v -5v + 12v.
Now we can see the real computer, I had initially seen a lot of dirt on the pcb but with a good clean with isopropyl alcohol it went away. There was a 74ls00 that had blackened pins and in fact they came off just looking at them, so off with the socket and… all new!
I plug everything in and try! Well a little luck now, in fact everything worked at the first start
