Friday, March 27, 2020

Amiga 500 PSU repair

I have an Amiga 500 PSU which is a bit unstable. Sometimes my Amiga 500's don't start when using this PSU.

I measured the voltages of the PSU (when connected to the Amiga) and the 5V rail was a bit low, and slowly rising. When the voltage was under 4.8, the Amiga wouldn't start. So in order to troubleshoot, I started with drawing a schematic of the PSU. The schematic is a work in progress, I change it as I find errors, so the best version is at GitHub: .

So, in accordance to the schematic references, I measured the parts that are involved in the 5V regulators feedback: 5V reference was 5.008V, excellent in other words.
5V R2 = 1.500k
R4 = 4.702k
Internal reference R1 = 1.498k
R5 = 4.700k
These resistors show signs of getting hot, so I measured them swell.
R6 = 179.8
R8 = 179.9

The first idea I had was to replace the output transistor. It was difficult to find the exact replacement , so I ordered a D45H11G (instead of the original D45H2). This did not change the faulty behaviour.

Finally I tried swapping the regulator IC, SG3524,  to a new one. I didn't think this would work, but actually it did. Voltage in a running Amiga 500 was measured to 4.93V and this was enough for stable activity.

PSU with replaced regulator IC (SG3524)

1 comment:

  1. Grazie per la pubblicazione , potrĂ  servire ad altri appassionati!