Different outcomes of similar situations... Mac Plus

[solidpro]

Hey everyone

I'm still trying to work out why one of my analogue board is giving out low voltage. That saga continues. I bought a second working machine in order to restore and then use as a swap machine to troubleshoot with. I tested all the voltages and it boots up with the chime and disk icon. So I disassembled, recapped the analogue board - triple checking all my capacitor values and replacing like for like.... and then booted it up on the bench with no CRT and the original mainboard - BONG so I check all the voltages on a multimeter post-cap on the bench - all dead on what they should be. I think "OK all good"

I re-assemble it and use a new spare clean "known good" Plus mainboard and I get.... HORIZONTAL LINES!! I can't believe this - the same lines I don't see anyone else reporting but on a different unit now...

So I get a second mainboard out. This time chequerboard.

So I get a third mainboard out. Horizontal lines.

So I get the original mainboard out which tested fine. Nothing on the screen.

So I disconnect the CRT cables to the neck. Nothing.

So I disconnect the yoke and FDD. Chime.

So I reconnect the yoke. Chime

So I reconnect the CRT cables. Chime

So I reconnect the FDD. Chime.

So what changed? Was it something to do with a bit of liquid flux or IPA burning off in some corner of the analog board? 

Does anyone have a definitive meaning of chequerboard or horizontal zebra stripes??!

Finally - C1 - the bi-polar capacitor which is impossible to source. What do I buy instead (ideally from Mouser or RS Components)?

Thanks

 

[IlikeTech]

I can't answer most of the questions, but you might have a cracked solder joint somewhere that's making bad contact.  I think it's the yoke connector on the Plus AB that normally has these problems.  Also, I'm not sure running the AB without a CRT connected is a good idea.  you have an exposed HV lead that will climb in voltage without the load of the CRT tube, and might damage the output circuitry responsible for producing the HV.  You might have broken something doing that.  Not sure though.