Reversed Polarity on factory-installed LC3 caps at C22 ?

[Mr. Fahrenheit]

I just stumbled upon a thread over at 68k mla regarding an error in the silkscreening AND installation of factory caps by Apple on the LC3 logic boards, at C22.

The thread mentions that testing of the circuit shows the cap should be orientated backwards from factory installation, and that it's a -5v circuit. Testing factory caps which are installed with positive towards the floppy drive shows improper readings, and testing replacement caps orientated wrong show correct -5v.

@max1zzz Thoughts on this?

 

[max1zzz]

I just stumbled upon a thread over at 68k mla regarding an error in the silkscreening AND installation of factory caps by Apple on the LC3 logic boards, at C22.

The thread mentions that testing of the circuit shows the cap should be orientated backwards from factory installation, and that it's a -5v circuit. Testing factory caps which are installed with positive towards the floppy drive shows improper readings, and testing replacement caps orientated wrong show correct -5v.

@max1zzz Thoughts on this?

I'm mixed on this, Given they have been installed 'the wrong way round' from factory for 30 years and it hasn't caused a issue I'm kind of tempted to say leave them the way apple installed them, but on the other hand I do remember seeing a well known member of 68kmla saying he had seen tants explode on LC III boards when installed 'the apple way'

When I assembled my replica LCIII's I left them the same way apple installed them but I was using electrolytic or aluminum polymer caps which are a little more resilient to reverse voltage than tants

I seem to remember hearing @Branchus saying he installed thants the apple way round, any thoughts?

 

[YMK]

Clearly the correct solution is to install a capacitor with reversed markings.

 

[Mr. Fahrenheit]

Clearly the correct solution is to install a capacitor with reversed markings.

If we install a non-polarized ceramic cap then we solve the problem, no?

 

[max1zzz]

If we install a non-polarized ceramic cap then we solve the problem, no?

That's one option, In fact one of the 3 repro boards I built up had a ceramic's installed for all the 47uf's and it works just fine

 

[YMK]

If we install a non-polarized ceramic cap then we solve the problem, no?

It should be fine if the ceramic has a comparable ESR.

Can't be any worse than a reversed electrolytic.

 

[Mr. Fahrenheit]

That's one option, In fact one of the 3 repro boards I built up had a ceramic's installed for all the 47uf's and it works just fine

A local guy has a LC475 that was recapped prior to him getting it. I examined it, and it was recapped completely with ceramic non-polarized caps, for everything, and it works just fine, strangely enough.