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Why X68000's caps are almost broken

In 1980s, aluminum electrolytic capacitors with quaternary salt were used. It was said high performance and long life. Many companies made them. But about 5 years goes, they started leaking solutions that was high pH (having strong alkalinity). It was found that quaternary salt became alkaline belong long time use. That problems was not found in condensed test in labs. As a result, almost high performance caps in 1980 leaks in about 10 years. High pH caps not only leaks but makes short circuit. Then one day, PSU fires.

To prevent break down,we must need to renew all caps. But now, it is too late. Almost PSU is broken,isn't it?

We must replace not only caps but zener-diodes, photo couplers, some transistors, resistors, and so on. Ideally, all pars to be replaced but some parts was discontinued.

I researched succeeding parts, but it is imperfect. Any advices is welcome.

p.s. Original X68000 PSU seems not to use quaternary salt. But they are too old, so they should be replaced.

Original model

ACE (SH2 Unit)

It has the same design as the PSU of Expert but features lower rated capacitors. It is recommended to use use the capacitor ratings which are mentioned in the Expert section.


Almost all info taken from:

All X68000 power supplies fail because of the capacitors. Here are a few common symptoms and solutions:

  1. Main power is connected, front LED is glowing red but the system does not react to front soft-power switch. Check if ZD31 or anything around it is burnt. Replace ZD31 and all electrolytic capacitors. There are some cases when Q32 also needs to be replaced. In some rare cases PC31, PC51, PC52 also need to be replaced.
  2. R35 rupture is caused when C31 capacity has decreased to the point that it is no longer able to supply constant voltage and current. In this case it is advisable to replace Q31, Q32, R35, ZD31.
  3. The fuse gets blown if C9 is faulty. Replace all electrolytic capacitors, 4A fuse and check if there are no signs of damage to the ZD31 or near it.
  4. Most parts though discontinued have generic replacements available. The only custom-made part is the transformer but it is very reliable and does not break easily.


Part number Part description Notes
F1 125V 4A fuse (MZ) small type
ZD31 RD6.8EB 6.8V 1/2W Zener diode
ZD32 RD4.3EB 4.3V 1/2W Zener diode
ZD51 RD15EB 15V 1/2W Zener diode
PC31, PC51 PC8170, TLP521-1 Photocoupler
PC52 S12MD1, TLP541G Photothyristor
Q31 2SC2333 / 2SC2335 Transistor
Q32, Q33, Q53 2SC1815Y Transistor
C9 200V 470μF Original: 180V 470μF. Height must not exceed 41mm and diameter 25.8mm.
C24 25V 1000μF
C25 16V 1000μF Original: 16V 680μF (25V can also be used)
C27 25V 220μF Original: 25V 120μF
C28 16V 100μF
C31 16V 100μF Original: 10V 100μF
C33 16V 220μF Optional: 25V can also be used
C34 10V 100μF Original: 6.3V 100μF
C35 50V 10μF Original: 25V 10μF
C38 50V 3.3μF
C40, C41 16V 6800μF Original: 10V 5600μF
C42 10V 2200μF Original: 6.3V 2200μF
R33, R34 100Ω 1/6W
R35 3Ω 1/4W Original: 3.3Ω
R40 2KΩ 1/6W
R98 2.2KΩ 1/4W Original: 1/6W


Compact XVI,030 Compact

These are more tolerant. Their capacitors leak but usually do not kill anything and a simple recap is all that is needed.


preface/why_x68000_s_caps_are_almost_broken.txt · Last modified: 2019/08/27 20:45 (external edit)