BBC Micro Fixers Wanted

[AlanBeckett]

Anybody near Bletchley?

http://www.bbc.co.uk/news/technology-33291036

Alan

[ed731pdh]

Horrible little sods to fix, so old now that the motherboards are often brittle and the slightest flexing kills the tracks stone dead. Hard to get a lot of the chip as well, the WD 1771 disk controller IC is virtually impossible to source and the 8255s are going the same way.

Bit of a poison chalice that request.

[zx80nut]

The main issue I've had with them is the suppression cap fizzing and popping. (makes a horrible smell and looks quite alarming). This has happend on 3 BBC micros recently (two are mine - a BBC B and a Master) and also on my friends BBC B. New caps bought from Farnell (although you could just remove them and risk a bit of interference). Also happened on my Sharp MZ700 as well, AND an Apple II... I've bought a pack of them ready to replace any others in my collection.

So, just a warning... if you dig out your old Beeb from the attic, stay with it for 30 mins after switch-on otherwise you may fill the room with smoke!

Grant

[ppppenguin]

There's a lot of trouble like that with RIFA brand class X and Y caps. Other makes are generally OK unless somebody knows otherwise. I've also had a Schaffner filtered main inlet fail unpleasantly in a Tektronix 1755A vectorscope. I suspect it had RIFA caps inside.

The RIFA failures I've had on my own kit or kit under repair in my lab were in a Quad 33 pre-amp and a Gould/Advance switchmode PSU.

[zx80nut]

Would you recommend that the class X caps are replaced with class Y ones? I've used new class X ones as replacements at the moment but I wonder if they are any more reliable now than they used to be. From what I understand, class Y are "guaranteed" not to short on failure (so should not smoke) - is that correct?

Any help in maintaing a "smoke free zone" is most appreciated, especially considering the size on my computer collection...

http://searle.hostei.com/grant/my_machin...hines.html

Regards.

Grant

[ppppenguin]

Generally class Y caps aren't available in the values you would often use for class X applications. Also they are physically bigger for a given value.

As you say class Y caps mustn't fail short circuit because they are used between live and earth and could cause shock hazard if they short. Their maximum value is quite low to limit the normal reactive current that will flow between live and earth. Class X caps are for use between live and neutral (or phase to phase) must merely fail safe. Failing short and blowing a fuse is allowed AFAIK.

There are subclassifications of X and Y (X1, X2 etc) but X1, Y1, X2, Y2 are all suitable for the ordinary single phase 230V stuff that we usually deal with. You rarely see X3, Y3, Y4 which may not be adequate for our use.

You may also find dual rated (eg X1 and Y2) caps. These meet both specifications.

[zx80nut]

Thanks for the info. I'll stick with the X caps that I have then

[Cardigan]

Easy way of remembering what cap goes where is that class X go 'a cross' the mains.

Some thirty years ago I repaired one of these when an electrolytic failed in the power-on reset circuit: from memory, a monostable based on the 555 timer.