Is there any way to search for a transistor that will substitute for one that is no longer available.
The transistor was mounted underneath batteries which also took out the MPU and the NE555. This CPU board no longer boots and replacement boards are not available.
Here are links to the Data Sheet and the Schematics in which the transistor is used.
That’s not a transistor, despite the 3-pin TOx package. It’s a power-on reset generator IC. What it does, basically, is detect when the power supply rail has reached a certain voltage level at power-up and drives the reset pin of the MPU to the appropriate logic level so that the logic can start cleanly. I’ll see what I can find by way of a suitable replacement. If I can’t find anything online, I can make use of the engineering cross reference resources at work.
Thanks Q.E.D.. If no substitute can be found I’ll have to build a daughter board with some kind of similar circuit, something cribbed from another manufacturer.
On preview - good find!
On further investigation, I see that the PST518 was available in two trip voltages: The A version @ 4.2 V and the B at 4.4 V. See if you can tell which one you have. The MCP-120 is available in 7 trip voltages. The one that most closely matches the original PST518 A and B is the MCP120-450 @ 4.5 V. If you have the B version, this will most likely work fine, but if it’s an A, this may prove unstable if your +5 V supply rail is operating low or experiences large voltage drops during heavy loading periods. The next lowest trip voltage for the MCP120 series is 3.15 V, which may be too low to provide stable operation for the 6808. Try the 4.5 V device and see if that works properly. If not, we can add a voltage divider to “trick” the MCP120 into tripping at a slightly lower voltage level.
The PST518A is used.
It take it the numbers following the MCP120 are the voltage levels.
The +5V power supply was is a little low so I replaced the 2N6057 regulator. It’s still a little low for my taste but there is no adjustment. The filter cap is a hard to find value (18000uf @ 25V axial) but I’ll look some more and even if I can’t find it cheaply enough I can make do with four 4700uf @25 radials I have in stock. (I have them in stock because I didn’t realize that the power supply board was an early production model without the through holes for the four radials. Sigh, may be time to break out the tracing repair kit.)
If the Williams (another manufacturer’s) CPU boards are any indication I’ll need that voltage divider. On those pins, if the +5 bridge rectifier was failing in the least then the machine reset when you used the flippers. At least I’m pretty sure it was the rectifier…if you’ve got the board out you might as well shotgun the regulator and cap at the same time.
So, if it’s not asking too much, you’ve been a lifesaver so far Q.E.D., what do you suggest for a voltage divider?
I’d need to know what the 5 V supply is running at. In the meantime, try the MCP120-450 by itself. If you look at the data sheet, the actual trip voltage varies between 4.25 and 4.50 V, with the typical unit tripping at 4.35 V. If you pick up, say, a half dozen of them you’re bound to find one operating at the low end of the range, which falls right in line with specs on the PST518A. Odds are good you won’t have to rely on any resistive trickery to achieve stable operation. If you have any difficulties, my email is in my profile.
Cool. What pinball machine are you repairing?
4.87 with my uncalibrated fancy-dancy Fluke but that was at the power supply board and not across the CPU board chips.
Will do. I’ll also take my supply of resistors and a small breadboard, pencil and paper and a copy of the voltage divider equation.
Well, first I’ll tie the Reset line high (after reseting after power up) until I can get the Test Eprom to pulse the PIAs. Then if I have to I can order any other needed components at the same time.
Thanks again.
By the way, how did you find the component. Sometimes I spend a small fortune talking to the engineering departments of large manufactures to find such a substitue.
Just to entice you to tell me how: