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IIe power hooked up backwards HELP

Mattsal

New Member
Joined
Jan 22, 2022
Messages
2
I have a makeshift power connector that has worked for a long time, I ended up plugging it in backwards and turned it on for about 2 seconds. When I corrected it and turned it back on, C72 smoked.
I have replaced the cap but now get no boot.
What I know
All voltages are there
power LED comes on
Video signal but nothing displayed at all.
no beep
Tested a few locations on the board and there seems to be a clock signal
80 column board got quite hot, removed it. but didn't help
no other chips get hotter than they should.

Any ideas on where to start?

Thanks in advance
 

bladamson

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Nov 11, 2018
Messages
732
Location
Appalachia
If you plugged it in 180 degrees off, you applied 12v to the 5v rail and both -5v and -12v to the ground rail. My supposition is that you probably fried several ICs, and it's probably time to start pulling chips and testing them. If it were me I'd start with the CPU and move outwards from there.
 

daver2

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Jun 19, 2012
Messages
6,890
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UK - Worcester
The dynamic RAM could be quite susceptible to a power supply ‘glitch’...

Unfortunately, a ‘snafoo’ of this type could have killed anything (semiconductors or polarised power capacitors) on the board.

First of all, replace the connector with a none-reversible type (as it should have been). Next, grab the schematics and work through the logic in a systematic way. I seem to remember a good manual (containing oscilloscope traces) for this machine.

If you have an oscilloscope, probe the points identified looking for the identified traces and see which ones align and which ones don’t.

Work from the clocks and reset circuitry, through the CPU and buffers, memory decoding etc. You may have to also develop some bespoke tools or test firmware. For example, on the PET we have a NOP generator and I wrote a diagnostic chip. Others have developed RAM/ROM replacement boards that include the diagnostic tools etc.

As per the post above, if everything is in an IC socket, you may be advised to get a TTL chip tester and work through each chip at a time.

Where are you based out of interest? Oh, and welcome to VCFED (as this is your first post). Sorry it is under these circumstances though.

Dave
 

bladamson

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Nov 11, 2018
Messages
732
Location
Appalachia
I know it's fun to fix stuff, so I am not trying to poo on your fun or anything, but.... At this point I'd be considering just replacing the board, if it's just a working machine that you're after. It's not like they are rare/expensive yet. I would wager that it's going to be a *whole* lot of aggravation to troubleshoot and fix that thing, and may end up costing more than just buying a board in the long run.

It's possible that your disk controller and drives are damaged in some way, too. They all run off the same 5v supply that got hardcore-overvoltaged.
 

daver2

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Joined
Jun 19, 2012
Messages
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Location
UK - Worcester
It might be worth a 'look see' - you never know, something could have gone short circuit to protect the rest. I have known both scenarios:

1. The 5V voltage regulator failed on a quadraphonic decoder (this is going back now) and put the best part of 30 Volts on the circuitry. After replacing the voltage regulator - everything still worked! It is still in use 20 years after replacing it and the owner is still very, very happy...

2. A computer had every single chip (apart from 1) blow... Fortunately, all of the ICs were in sockets and (at the time) were easily replaceable. Not so now of course...

Might be worth setting yourself some targets (money, time etc.) and then just look for a replacement as suggested after that.

Dave
 

NeXT

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Joined
Oct 22, 2008
Messages
6,754
Location
Kamloops, BC, Canada
I don't even know where to start. Voltage reversals are always a nightmare to troubleshoot because now you have multiple component failures, PLUS you're going to have other IC's now either running weak or intermittent.
It's a IIe. Start by verifying the ROMs are still good, then test the 6502 in something else. If you have not already, pull every card you have installed, because as you saw with the 80 column card, they will likely be compromised as well.
 
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