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+--- Thread: Power Problem with Premium Base Board? (/thread-485.html)
Power Problem with Premium Base Board? - Snuffytime - 09-05-2022
Hi All,
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.
RE: Power Problem with Premium Base Board? - MbeeTech - 09-05-2022
(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.
RE: Power Problem with Premium Base Board? - someone - 09-05-2022
Irrespective of the Tag Tantalum capacitor's rated VW (working voltage), they can become dangerous Roman Candles with age launching a red hot fireball.
Old capacitors of many varieties fail with age.
Watch this incident.
Some old electrolytic capacitors can be reformed.
Capacitors can fail on both sides of the voltage regulators.
Modern low ESR and low impedance ceramic and electrolytic capacitors can be used as replacements.
Whether they suffer the same issues in 20-40 years time has yet to be determined.
Did you know that with the correct PCB design most bypass/decoupling capacitors can be removed?
The first XBOX 360 prototypes were designed with no bypass/decoupling capacitors & worked perfectly well.
Bypass/decoupling capacitors were added as decoration & serve no functional purpose to placate those who didn't understand Power Distribution Network design.
Here's another!