Replaced LH1056 solid state relay
TL;DR: replaced LH1056 solid state relay / optocoupler at U3
One of the USR Courier HST modems I purchased recently had a peculiar problem that if I plugged the phone line into the “wall” jack of the modem it returned NO DIALTONE when I tried to dial something, yet I was able to dial something if I plugged it into the “phone” jack. After a while I realized from looking at the ATA that when plugged into the “phone” jack it was immediately taking the line off-hook, even if the power was turned off!
This sounded like a relay was stuck open or something was commanding a relay open somewhere so I went hunting for it. I had no idea what a relay looked like on a PCB so I hooked a multimeter to one of the phone line terminals and went poking around the board around the phone jacks to see what was connected to it. This eventually lead me to a pair of chips labeled LH1056, which the datasheet told me was a solid state relay. The datasheet told me pins 1 and 2, 4 and 6 were connected so I checked these on the board. On one I noticed between 4 and 6 I had nothing, but on the other between 4 and 6 I had continuity. The datasheet said these were normally open relays, so that sounded like my culprit.
LH1056 were apparently long obsolete, I didn’t put any effort into trying to find a replacement and just ordered a few off eBay. They finally arrived today so I got to desoldering the old relay and putting on a new one. Just like that, problem solved! The modem now dialed, taking the line off-hook and on-hook as expected.
Now I have enough spare relays to fix several more modems, so if you encounter this problem I guess hit me up!
LH1056 spares
