I ended up with a non-working 755CE and a load of parts from previously dismantling a 755CX and decided to see if I could use the doubles of a lot of the parts to test everything and see if I could repair either machine.
I knew from past experiences that the 755 model range went through a vast evolution of parts. Each machine is quite modular, but as previously covered the the 755C is extremely similar to the 750C and there is a vast change between it and the 755CX, which is closer in evolution to the 760 than the 750 - essentially a 3 generation gap across the first and last 755 models.
This is for several reasons - the 755 firmly crossed a huge change in personal computers, for at least 3 reasons; the Pentium CPU was heavily marketed as the powerful machine you must have. Multimedia was also being marketed - which in real terms meant CD-ROM and either a powerful accompanying hardware chipset to compress/decompress video and audio on the fly, OR a powerful CPU to do the same in software. And finally Windows 95 hugely increased hardware requirements along with the expectation you could access CD-ROM and the multimedia surrounding it. So the 755C was a 486 with floppy drive and not a lot of custom hardware, whereas the 755CD and CX are fatter machines, packed with additional custom hardware.
Anyway.
Here are notes of things I discovered along the repair journey:
Some Dismantling Notes:
When dismantling either machine, this run of the LCD's power cable (to the inverter) and the lid's microphone cable are shown here, to be tucked out of the way to get the LED bezel back on:
Here is the 755CE board removed from the base. What you're looking at is the motherboard buried underneath almost everything you can see. At the top, with the speaker and the green switch (which detects when the keyboard has been lifted), is the audio board with volume control which is not screwed down. It latches on the left onto the push on/off switch. The top audio board is also wired to the main battery connector which sits underneath the board underneath this board. If that's pulled up and out, you will see the DC power board, which is actually two boards joined in the centre with a push connector. It's got one screw holding it down, just above that blue bit on the left (which is a push-out connector for the optional modem).
The whole massive velvety black section bottom-right is the heatsink for the CPU daughterboard, which also just push-pull unclips from the bottom.
So remove the motherboard from it's metal frame, you need to remove all the connector bolts on the back and two M2.5 fat screws above and below the PCMCIA bay.
Also on top, just below the rear connectors is the interposer which is a small interposing board, which connects the motherboard to the LCD, the LED indicator board and all 3 keyboard connectors.
On the CE only you'll also see this small black plastic spacer back-left which sits underneath the metal plate covering the top and goes ontop of all the boards:
Some Daugherboard Notes:
Here is the top audio board...which is exactly the same on the CE and CX.
These are the two original capacitors on this DC power board. They don't seem to leak but I always replace them just because a lot of this era do.
And this is the left side of the DC board with 5 surface-mounted electrolytic daughterboard - the bit with the power connector and volume control on it. I believe these are the same on both the CE and CX but I can't be sure as I no longer have the one for the CX:
Here we have two images showing the bottom heatsink detachable CPU boards. As you can see these are extremely different but actually interchangable between the CE and CX and usable, but I don't know if maybe the CE motherboard is then missing components needed for full CX compatibility. The CE board is plaintly a 486 DX board, extremely simple with a few bolts connecting the above and below layers of the heatsink (removed in this picture):
The 486 CPU and heatsink have this part code on them:
And here we have the 755CX board which has the Pentium 75 CPU and a bunch of accompanying chips which might mean that everything CX/Pentium required is on this daughterboard and not the CE/CX motherboard. But this I don't know for sure:
Note that both boards have that copper covered tape around them which help transfer heat onto the shell of the motherboard.
Memory Woes
The next problem I faced is that I know the later 755 machines suffer from memory issues with the 'on board' off-board memory. These later 755 machines do not have RAM on board and instead have them on a small double-connector daughterboard, in the least-accessible location, underneath the CPU board and inside the deepest part of the motherboard and it's metal chassis. So not only do you have to dismantle the entire machine to get to it, you almost have to rebuilt the entire base to test it - and keep going backwards and forwards as you test and check.
Here is the memory daughterboard with the 755's motherboard upside down and it's metal cage removed:
Secondly the CX uses a memory board which has the same dimensions and connectors as the other later 755 motherboards, and the machine will boot, but display similar memory errors that would occur if the machine had a faulty board. This is the board for non-755CX:
And these are the boards, I think are only for 755CX machines:
These boards connectors are keyed exactly the same, and the machine will boot to Easy Setup, but won't go any further if you get them the wrong way around. On the CX you'll get something like this (ignore the system board error, it's just that I don't have the front/rear IR connected):
If you manage to get a good RAM board, which matches the type required for your 755 variant and you get it all back together with both IR boards connected, you'll be rewarded with this: