HP 48S IR Loop-Back Test fail

[Jonathan Busby]

(03-12-2021 11:10 PM)calculon Wrote:  Thanks for the helpful tips. All is not lost, as it sends and receives by wire when connected to a PC.

If the problem is indeed in the IR receive circuit on the main PCB, then you can "easily" repair the circuit, assuming that you can open up your 48 with no cosmetic damage using the excellent guide by Han Duong here . The schematic for the circuit can be found in the "HP 48 I/O Technical Interfacing Guide" here ( on page 8 ). To test whether the Clarke 1LT8 SoC's IR receiver is working, you'll first need to determine if the IR receive circuit on the main PCB is damaged, and, if so, fix it. Having done that, if you still can't receive from another HP48S/SX via IR, then I'd check the trace(s) on the main PCB to see if there's a connection to the IR receive pin. If that checks out and you still can't receive via IR from another S/SX, then it's probably a hardware fault in the SoC. All is still not lost though : If you're good with soldering then I'd recommend using a Hakko, an SMD attachment for a soldering iron and some solder paste to de-solder the Clarke SoC and then replace it with a known functioning one from an "as is" or "for parts" HP48S/SX which has, say, a broken display or keyboard etc. but a functioning Clarke 1LT8 SoC. It might be a PITA, but you could definitely fix it

Regards,

Jonathan