Early this year, a friend of mine asked me to take a look at their 1990s Apple Macintosh LC475 computer & matching 12″ RGB CRT display – these worked until a couple of years ago, but the LC475 was no longer booting & the CRT was no longer displaying an image.
The first step was to disassemble the units and check over everything inside.
Macintosh LC475 Repair
The Macintosh LC475 suffers from two major problems: they have a lithium battery in a holder on the mainboard to power the battery-backed PRAM and RTC, and these have a habit of physically leaking corrosive battery alkali (see “battery bomb“) and causing damage to the mainboard and chassis; both the SMD electrolytic capacitors on the mainboard and the through-hole electrolytic capacitors in the power supply have a habit of failing and physically leaking corrosive electrolyte, which can not only prevent the system from working, but can also cause damage to the circuit boards and the components on them.
I must therefore emphasise: if you have an Apple Macintosh LC475 or similar pizza-box style Macintosh in original condition, it needs to be serviced! The original battery needs to be removed, and the original electrolytic capacitors need to be replaced. These systems are dying, day by day.
Pizza-box Macintoshes like the LC475 are nice and easy to disassemble – the top cover pops off and you’re straight in, and the internal parts are just held in with clips.
With the mainboard removed and stripped for easier inspection, it seemed that the SMD electrolytic capacitors had started to leak, although not too badly.
Aluminium electrolytic capacitors are commonly used for filtering, smoothing, and decoupling in both high- and low-voltage electronics. They are quite cheap in comparison to their solid-electrolyte counterparts (such as tantalum and polymer electrolytics), so are very common in consumer electronics.
Their useful lifetime is highly dependent on the specific application that they are used in (i.e. frequency, ripple current) and temperature, as well as the manufacturer and series of the specific component. They typically comprise aluminium windings which are coated with a liquid electrolyte, which can dry out over time (negatively affecting the performance of the capacitor, often causing them to fail dead-short), or even leak out and cause corrosion to the PCB and surrounding components.
SMD aluminium electrolytic capacitors are common, well-documented failure points in other pieces of equipment from the 1990s (i.e. Amiga 600, Amiga 1200, Apple PowerBooks, Apple Macintosh Classic / Classic II / SE / SE/30, etc).
The LC475 mainboard is fairly easy to recap as it only has eleven SMD electrolytic capacitors, as follows:
- 9 x 47uF 16V SMD
- 2 x 100uF 6.3V SMD
You can usually buy capacitor packs for these machines from sellers such as Console5, but I just made up my own by noting the specifications of all of the electrolytic capacitors on the board, and ordering a set of high-quality known-brand parts.
I decided to use tantalum electrolytic capacitors, an equivalent to aluminium electrolytic capacitors – these use a solid electrolyte, meaning that they will not physically leak.
When substituting electrolytic capacitors, the capacitance needs to be the same, and the voltage rating can be the same or higher (within reason) – when you’re going through all this effort to recap something, be sure to use high-quality replacements.
For SMD capacitors, I usually remove all of them at once using a hot air rework station with kapton tape and aluminium foil to protect the surrounding areas, or by carefully twisting them off using needle-nose pliers (this technique may not be suitable if the pads are damaged, as they could delaminate from the board – and push downwards, don’t pull upwards!). The pads can then be cleaned up using new solder and either desoldering braid or a desoldering station. The board should then be thoroughly cleaned to remove any leaked electrolyte and leftover flux, using isopropyl alcohol and an ESD-safe brush.
When fitting new electrolytic capacitors, you must take care to ensure that the value, voltage rating, and orientation of the new capacitor are correct – electrolytic capacitors are polarised, so must be installed the correct way around, else they’ll get hot when powered on (and probably explode). The polarity is marked on the case: for aluminium electrolytic capacitors, the negative side is usually shown by a white stripe (for through-hole) or a black bar (for SMD); for tantalum capacitors, the positive side is usually shown by an orange or white bar (for SMD). This catches a lot of people out!
You can’t always trust the orientation markings on the PCB silkscreen (if it even has them, not all boards do), as sometimes mistakes were made in the design from the factory (take the PCB layout of the audio circuit on the Commodore CD32, for example), so care must be taken to match the orientation of the new capacitor with the original. Make sure to take lots of “before” pictures for reference, and double-check throughout.
I pulled and dismantled the power supply to inspect it – there wasn’t any obvious signs of leakage from the outside, but sure enough, there was underneath some of the output capacitors – relatively minor, but enough to cause problems, and it would only get worse.
The Macintosh LC, LCII, LCIII, LC475, and Quadra 605 all use the same internal power supply, the Sony/TDK 614-0003 – this is fairly easy to recap as it only has ten through-hole electrolytic capacitors, as follows:
- 1 x 180uF 385WV snap-in bulk capacitor, typically reliable so I usually leave it.
- 1 x 8.2uF 50V radial.
- 1 x 47uF 25V radial.
- 1 x 56uF 25V radial.
- 1 x 270uF 10V radial.
- 2 x 270uF 25V radial.
- 3 x 1000uF 10V radial.
Again, you can usually buy capacitor packs for these power supplies from sellers such as Console5, but I just made up my own by noting the specifications of all of the electrolytic capacitors on the board, and ordering a set of high-quality known-brand parts.
For through-hole capacitors, I usually remove each one-by-one using my desoldering station, then immediately install its replacement part – this minimises the likelihood of getting it wrong. However, in cases of leakage like this, I usually remove all of the capacitors at the same time to aid cleaning – the board should be thoroughly cleaned before fitting new parts, then again afterwards to remove any leaked electrolyte and leftover flux, using 99% isopropyl alcohol, cotton buds, and an ESD-safe brush.
The machine originally had a SCSI2SD v6 SD card adapter installed, but the owner wanted to swap this out for a BlueSCSI, for various reasons: they’re easy to find, cheap to build/buy, are easy to set up and use, and because I’d already made some for my Mac Plus, and had spare parts in stock.
The BlueSCSI fits into a 3D-printed 3.5″ drive adapter– this is available to buy from the creator, which I didn’t realise at the time, so I used the open-source design files and had several parts made by PCBWay, and bought some of the appropriate PCB mounting screws (M2.5 x 5mm).
The 3.5″ adapter can then be installed into the drive caddy using the original screws.
With the mainboard and PSU recapped, the LC475 now seemed to boot OK.
M1296Z CRT Display Repair
The CRT display had apparently started to suffer from vertical collapse and then died completely, which is a common problem on the M1296Z and is apparently caused by electrolytic capacitor leakage.
I dismantled the unit and removed the board caddy for further inspection.
It became obvious quite quickly that the front of the display board had suffered from some serious corrosion, probably due to electrolyte leakage from an electrolytic capacitor.
The part which had leaked was the Rubycon 2200uF 6.3V radial aluminium electrolytic capacitor at C418 – I removed this part and cleaned around and underneath it using an anti-static brush and 99% IPA, and gave this area of the board a run through my ultrasonic bath to remove as much of the leaked electrolyte as possible.
C418 is a very common failure point on the M1296Z, so if you own one of these displays, you should get it serviced as soon as possible to minimise damage – it may be that the particular Rubycon part used in this position is prone to seal failure, that it is under-rated for its application in voltage or frequency, that the nearby power resistors bake it and reduce its useful operating life, or a combination of all three.
Given how far the leakage had spread on the underside of the board, I removed some of the larger components around C418 to clean under those too.
I fitted a high-quality 105C-rated 2200uF 10V part at C418 – I tested the parts that had been affected by the leakage, and the 1R power resistor at R534 measured open-circuit. I replaced this and the similar power resistor at R525 with 1R 2W parts.
Depending on which direction the leakage creeps, other power resistors in the area such as R416, R417, and R514 can also be susceptible to damage.
I also cleaned off the damaged, discoloured, and bubbling solder mask on the underside of the board using a gentle polishing tool, taking care not to damage the copper underneath – as all of the traces had good continuity, I then covered this bare section of the board with clear nail polish to help protect and insulate it.
I also took this opportunity to reflow the solder joints on the power switch, which were badly cracked, and the other connectors and transformers across the display board, all using fresh solder – in particular the heavier parts such as the flyback.
I also took this opportunity to clean the brightness and contrast control potentiometers with contact cleaner, and I reflowed the solder joints on the control PCBs.
With the display reassembled, I powered it up – the power LED lit up and I could hear high-voltage kick in, so the display seemed to be working now.
Reassembly and Testing
After their respective repairs, the LC475 and its matching 12″ RGB CRT display looked great and worked perfectly! The owner was very happy with the outcome.
