HP15C displays gibberish. Is it broken?

[BillBee]

(10-06-2020 05:56 PM)GreyUser Wrote:  No clock in a voyager results in blank display. Every time.

<snip>
The display pattern shown on the failed unit of the OP is a processor/r2d2 comms failure, any number of causes. If a reset doesn’t fix it, it’s most likely a bad r2d2.

It is not bad LCD contact. Due to the way the display is multiplexed, regardless of the differences between the original 3:1 and the later 4:1 mux schemes, you could never create that exact display with a loss of connections, it’s physically impossible as the missing segments are common with segments that are in fact displayed.

Thank-you for that explanation.

-Bill

[sebi]

(10-06-2020 04:38 PM)lyuka Wrote:  It was not the axial lead inductor that was hot, but the dip tantalum capacitor at the power input.

Can you show me the dip tantalum capacitor you are talking of on the picture: is it the blue capacitor-looking component on the left of the green axial lead inductor?

(10-06-2020 04:38 PM)lyuka Wrote:  After that, it is presumed that the short-circuited state became open and the operation was restored because the fuse in the tantalum capacitor was blown or the solid tantalum was completely burned down.

Sorry but I didn't get what you explained: A capacitor is mounted in parallel whereas a fuse is mounted in series, right? Didn't the tantalum capacitor acted more like a diode in parallel, thus creating a short-circuit when being reversed-powered and protecting the processor? Blowing a fuse in that tantalum capacitor would have resulted in no more short-circuit thus no more protection for the processor, right?

[sebi]

(10-06-2020 05:56 PM)GreyUser Wrote:  ... if you have Tony Duell’s schematic...

Could you point me to the schematic you are talking of please?

(10-06-2020 05:56 PM)GreyUser Wrote:  The display pattern shown on the failed unit of the OP is a processor/r2d2 comms failure...

As a newbie, can you help me with the jargon: OP? r2d2? comms stands for communications, right?

(10-06-2020 05:56 PM)GreyUser Wrote:  Due to the way the display is multiplexed ... you could never create that exact display with a loss of connections

Very interesting~~