I purchased an Apple II+ a few years ago as it was cheap at the time, I never powered it on at the time and only recently uncovered it, so I decided to try and see if it worked.
Unfortunately although the light came on, there was no-one at home. I checked the PSU voltages and although they were not perfect on the negative rails it should not have stopped the computer booting: -4.85v / -11.13v | +5.18v / +12.43v
I took a closer look at the mainboard and noticed a lot of discoloured IC legs, I was hoping they were just tarnished, so I removed the worst looking ones, 6502 CPU and all the ROM’s, I tried cleaning them up but the legs were quite badly corroded especially on the ROM’s and one in particular lost 3 legs during the process.
I initially thought this was not too much of a problem, I’ll just write a compatible EPROM and replace them, however the original 9316B ROM’s were not that standard so I can not just use a 2716 EPROM, the issue is that the 9316B had 3 Chip Select lines, while the 2716 had Output Enable and Chip Enable lines as well as a Program line that needs to be kept high during reads, which looks somthign like this:
Pin 9316B 2716
18 CS Active High CE Active Low
20 CS Active Low OE Active Low
21 CS Active Low Vpp Read High
On the Apple 2 Pins 20 and 21 are shorted, Pin 18 is used to disable all on board ROM’s when an upgrade card is installed.
So to replace the 9316B with 2716’s we need to invert both Pins 18 and 21. I decided to do this with a dual hex inverter 2704 SMD packaged device. I will create a PCB to do this for all the ROM’s but will test with a birds nest type method first.
One of the first things you will notice on most of these units is they are loud, thing is these were purchased for heavy use, lets be honest you don’t pay upwardly of $10k and never turn it on.
The good thing is the noise can be cured, it is built from 2 castings and is on supreme quality. The unit is held together with a cir-clip and uses a top and bottom sealed ball bearing (604ZZ) this is a metric bearing of 4x12x4mm in size and over time simply wears out, luckily it is used in many modern RC cars so are easy to find.
I use a high quality bearing that cost around £10GBP each to replace the old noisy ones.
It is also important to fit a new air filter and nearly all the ones I have serviced had them missing completely.
Once the fan has been overhauled the 29B becomes much easier to live with and that’s why when I have one in for servicing it’s the first thing I do.
Below in pictures are the steps I take to refurbish the fan side of things in these units:
This is the fan removed from the 29B chassis.
Removing the plastic cap from the top of the fan.
Circlip removed from shaft.
Rotors removed from body.
Top Bearing removed, I push the rotor back in to remove this.
Bearings, spring, Shim, circlip and topcap removed.
I apply a light covering of electrical grease on all the shafts and bearings before reassembly, with this work done, it will result in a lot quieter fan and many years of continued use.
Welcome to 1byte; my name is Clint and this site is dedicated to my adventures in vintage 8 bit computing. I am a trained component level engineer and spent many hours fixing 90’s computers and video monitors and satellite systems for a living. This board pictured here is from my current repair that’s on the bench, exciting; 30 years dust to clear and you have to keep it close to period parts as possible; A whole 1Byte point for telling me that its out of, before i post the build blog.