09-05-2022, 01:51 PM
(09-05-2022, 12:34 PM)Snuffytime Wrote: Hi All,G'day Snuffytime.
Just got a Premium 128k which the seller said had been working until recently when it made a pop and released the magic smoke.
When I plugged it in I got a "fizz" and some smoke from c80 and the top of it even started glowing! I replaced with a cap borrowed from another 'Bee but I'm getting nothing. Core board works when tested on a similar machine (although I don't have a boot disk). Any idea what else might go at the same time as c80 before I start checking everything (eg. should I replace c79 as well)? I saw on another post that Someone recommends replacing the tantalums with low induction electrolytics
Also, where can I get the circuit diagram for the Premium base board?
PS. Just read a post where it was pointed out that the 2xFDD base unit uses a switching power supply (the MB base board has a smaller filter cap - as mine does) and that using a standard Microbee PSU brick can give problems - such as blowing the tantalums.
Cheers.
The plug pak power supplies can produce up to 16 or 17v unloaded, which can be a problem for the premium baseboard.
The input tantalum caps at the 5v regulators should really have been specified at 25v working voltage, but a lot of the time
16v tantalums were used - fine if used with a normal drive box supplying the power, and some plug paks, but too close for comfort
with some other plug paks.
The premium baseboard schematics are available in the repository.
Check this post for details on repository access.
You may well have another tantalum that has gone short circuit (check C12 - there are 2x 5v regs under the keyboard frame that supply the baseboard)
so with the plug pak disconnected, check for shorts / low ohms on the 12v input to the regulators. C12 (left side of PCB) and C80 (right side)
should be replaced with 2.2uF or 4.7uF 25v tantalums.