Pinballs

So, what happened to the Sinbad? Well, it had its share of love, but unfortunately I couldn't go very far:

I first ran the preliminary checks, as outlined in the ever useful Gottlieb sys1 pinrepair guide. All the coils were ok, though there was two blown fuses ("playboard" in the cabinet, and "bank setup" under playfield). That was the least of my problems, see below ;-)

I then moved on testing the power supply. It's an interesting, yet somewhat complex piece of circuitry: it provides no less that 6 different voltages! The schematics can be found here. It appeared quite immediately that ground level wasn't "even". Between the input and the output of the board, it shifted a few volts (both on the high and low voltage rails). Changes suggested by pinrepair helped a little bit, but were not enough. Pinrepair only "fixes" low voltage GND, by connecting the negative pin of C1 to the ground pane. This leaves high voltage GND floating. I'd thus recommend to also connect the negative pin of C6 to the ground pane, to bring both grounds to the same level.

I then moved on to testing the CPU board, unfortunately it quickly appeared that it would have to be brought to the workbench to investigate it further. I have trouble with (at least) the clock circuit. Stay tuned for more info!

Now, a note on rectification

Pinrepair states (in section 2b) that low voltage is only half-wave rectified because there are only two diodes used for rectification, namely CR1 and CR2. This is incorrect. This particular diode configuration is typical of a center-tapped back-to-back rectifier (for the records, half-wave rectification only requires one diode). Both 11.5VAC and 14VAC inputs are full-wave rectified.

In fact, the small transformer providing low input voltages is center-tapped (which is why you can get the 11.5VAC and 14VAC mentioned on the schematics when comparing pins 1-2 and 4-5 with GND pin 3). If you wonder how back-to-back center-tapped rectification works, take a look at Wikipedia ;-)

Thus, CR1/CR2 provide full-wave rectification for 11.5V and CR3/CR4 for 14V. The 69VAC section of the transformer is not center-tapped (you have the peak-to-peak 69VAC voltage swing between both lugs, and only half the swing if comparing with GND), this is why this section of the power supply uses a full diode bridge to rectify the input.