H7821 power supply in MicroVAX 3100, SCSI disk enclosures and others

Joshua Rice Rice43 at btinternet.com
Fri Jan 31 13:48:49 CST 2020

The main failure point of all old electronics are the capacitors. Over time, electrolytic capacitors dry out, and can cause a wide array of different faults. From ones that are leaky, and or way out of spec, they can exhibit all sorts of “fun” errors in hardware. 

The main one i’ve come across is “RIFA” brand rectangular thin-film filter capacitors. These can be quite violent when they fail, and they fail often. Many power supplies can be brought back (for a time) by just replacing these capacitors.

Electrolytics, whilst far more reliable (and less explosive) than the RIFA capacitors, also dry out and go bad over time. However, uness your machine was built during the “capacitor plague”, these can still be surprisingly reliable. However, if you’re planning on using the electronics regularly, they are still recommended to be replaced.The SMD electrolytics from the late 80’s and onwards  often need replacing,. These can leak and rot out the leads, and being surface mount, they can be quite tricky to replace.

Outside of electrolytics, tantalum capacitors can also fail. Whilst they don’t “dry out” as such, they still degrade over time. These can fail quite spectacularly, resulting in smoke and flames if you are not careful. They are generally more reliable, and probably not worth replacing across board, but when they do fail, it can be quite impressive.

Other electrical components, such as PCB’s, resistors, and silicon chips, have no discernable lifetime outside of corrosion and physical damage. Provided they are looked after, these components can carry on working indefinitely. 

Obviously, mechanical components (fans, relays, rotary switches), vacuum tubes, and much older cardboard tube components (resistors in vacuum tube TV’s/radios) stiff suffer from degredation from wear and old age. Often replacements can be as scarce as the equipment itself, so extra care should be taken when powering that sort of stuff on.

> On Jan 31, 2020, at 7:22 PM, P Gebhardt via cctalk <cctalk at classiccmp.org> wrote:
> Yep, that's what I call preventive electronics maintenance: I power up for a little time at least once a year all electronics that were in operational order up to a year ago, no matter what it is, to make sure they don't turn defective from "non-running", which is mainly affected by caps indeed. 
> Best regards,
> Pierre

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