Jennings J400 replacement boards

[Op-Bell]

Available again soon. I ran out of PCBs and parts to make these last year and have been reluctantly turning people away when they asked for one, but now I have five inquiries I'm going to order a new batch of boards and parts. So if you need one, drop me a line. Remember, I need a pic of the pay table to choose the right program.

[erbs]

Thanks Pete for your service. The Board I purchased from you works flawlessly. Much better than the original. 

[Badbaud]

I have one that was a redesign conceived by IGT. I guess IGT bought the concept and designed their own version.
The MPU board is the same size as the original Jennings board.
The problem is the MPU board is totally designed differently then the original concept and there are no schematics to aid in getting it up.
I'll be hitting you up for a board for the second one I have, it has been sitting in the shop for 6 months waiting for repair.

pm me with your contact info please.

[mrcrazy]

I would be interested in one!! Please send me your info.

Greg

[Op-Bell]

petehand@gmail.com

12 left.

[Op-Bell]

A question that's puzzled me for years - Why do Jennings 400 boards always fail?

I know it's something to do with the battery, but it didn't seem to make much difference how corroded it was. However, I think I worked it out now. Here's a snippet from the J400 schematic, showing the battery charging circuit. Note the zener diode, circled in red.

Now here's a picture of that circuit on an actual board. Note the zener diode, circled in red. Now note that there's no position for it laid out on the board. It's an afterthought.

Referring to the schematic, where it says "+8V to +12V" it should actually say, "+15V to +18V" because that's what the J400 power supply delivers. In a board with a healthy battery, the battery will hold the voltage down to 4.5V. But if the battery is dead, leaking, or missing, it will pass the 15 to 18V through. Now if that zener diode is installed, it will hold down the voltage to 5V. But if it isn't, and remember it's an afterthought, the CMOS chips get 15 to 18 volts. Being CMOS, they may shrug that off, but all the chips they connect to are TTL, for which 15V is sudden death. Bye bye board.

[Badbaud]

I posted a sticky on how to bypass the battery circuit and make all chips use the 5 Volt supply and it seems to work fine across the 4 board repairs I have done.
Have you tried it? According to my scope everything seems to be working fine unless I have missed something. Please let me know.

[Op-Bell]

It would certainly work so long as you disconnected the 17V charging circuit, and so long as the chips weren't blown (or you replaced them). The battery is only there to comply with the regulations.