HP3478A repair attempt, A:D LINK FAIL error - Page 1
Forum section: electronics / repair

author: polymood
author_rank: Newbie
timestamp: August 26, 2025, 05:20:47 pm
content: Hey all,I’ve been trying to bring back to life an HP 3478A that I picked up and I’ve hit an interesting snag. Maybe this will help someone else down the line too.The instrument boots up, but the display just sits in a loop: most of the time it shows A:D LINK FAIL, then briefly flips over to mVDC, and then right back to the error.Over and over.First thing I did was check whether the two processors were actually alive. The chassis side (U501, an 8039 with external EPROM) looks fine: the crystal is running, ALE is a clean square wave at about a third of the clock, PSEN is active, so it’s fetching code as it should.On the floating side (U462), things were more suspicious. The crystal there was actually dead when I first got the unit — it’s supposed to be 10.98 MHz, but I didn’t have one in stock so I dropped in an 11.000 MHz replacement. That’s close enough, and on the scope it’s a nice clean 11 MHz sine, about 1.9 Vpp, no signs of dropping out. So the oscillator is solid.But the reset pin on U462 wasn’t happy. Instead of staying low (inactive), it kept pulsing high periodically, which explains the front panel behaviour: when reset is low the CPU runs for a moment and the display changes, then reset spikes high and the CPU gets kicked back into reset, and the whole thing repeats.Tracing through the schematic, the reset comes from a watchdog chain: U466 (a CD4020 counter) divides down the ALE signal, two Q taps go into a NOR gate (U467C, a 74LS02), and the NOR drives /RESET. On the scope the 4020 clock was stable, the Q taps were counting, but the output of the NOR was still producing periodic reset pulses. The 4020 reset input itself wasn’t being toggled, so the RC power-on network isn’t the one to blame. Looks very much like glitches/hazards at the NOR inputs, or just a weak/leaky LS02 gate.So far so good… except there’s a bigger red flag. The chip sitting in the U462 socket is marked P8039AHL. That’s a ROM-less microcontroller. On the floating board there is no external EPROM to feed it. Which means even if the watchdog/reset circuit was perfect, this CPU has nothing to execute. According to the HP parts list, U462 should be HP 1820-2726. That’s HP’s part number for a mask-programmed MCS-48, essentially an 8049/80C49-class device with internal ROM. That matches the schematic too: the only connections across the isolation barrier are TX and RX through little transformers, not a parallel bus. The floating processor is supposed to run its firmware internally and talk serially to the chassis CPU.That would explain everything I’m seeing. With a ROM-less 8039 in there, the poor thing has no code. It just sits there being reset by the watchdog, while U501 can’t establish the A/D link and complains.So my next step is twofold:Replace the watchdog chips (U466 → CD4020BE, U467C → SN74HC02 or HCT02) to clean up the spurious resets.More importantly, find the right floating CPU. Either an original HP 1820-2726 from a donor unit, or possibly an 8749 programmed with the 3478A floating firmware image if anyone has dumped it.Has anyone here got experience with swapping U462 on these? Or even better, does anyone have a spare 1820-2726 or a programmed 8749 they’d be willing to part with?Or any info on HP part 1820-2726.I will post follow ups and try to document everything here.

author: polymood
author_rank: Newbie
timestamp: August 26, 2025, 05:37:50 pm
content: Quick update after doing some more research on the floating CPU (U462) in the 3478A.The service manual lists it as HP part 1820-2726, and for a while I wasn’t sure what that actually mapped to. After cross-referencing the manual and some Intel docs, it’s now pretty clear that this is an Intel MCS-48 family device – specifically an 8049-class microcontroller with 128 bytes of internal RAM and 2KB of internal mask ROM. The manual even says U462 “has 128B of RAM and 2K of ROM” which matches the 8049 spec perfectly.That explains why there’s no EPROM sitting next to U462 on the floating board: unlike the chassis CPU (U501, which is an 8039 that runs from an external 8K EPROM), the floating CPU has its firmware baked into its own mask ROM. HP simply ordered a custom mask version and gave it their own part number (1820-2726).So if someone stuck an 8039 in that socket (as in my unit), it will never work – the 8039 has no internal ROM and there’s no external ROM to fetch code from. That would explain the endless resets and the A:D LINK FAIL I’m seeing.As for replacements:The “real” part is an HP-masked 8049 (1820-2726).Pin-compatible options would be an 8749 (EPROM version of the 8049) or a CMOS variant like the 80C49, MSM80C49, etc. But they’d only be useful if programmed with the HP firmware image for the floating CPU.Unfortunately I haven’t found a public dump of that ROM yet. The only ROM image floating around online is for the chassis EPROM (03478-65501).So at this point the real fix is to source an original 1820-2726 from a donor 3478A, or to find someone who’s already dumped and burned the floating CPU firmware into an 8749.Hope this helps clarify what that mysterious HP part number actually is.

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 27, 2025, 02:20:06 pm
content: I got a parts unit I can strip the chip from, or just send you the entire motherboard pending postage costs? The motherboard works IIRC, it doesn't have the input hybrid though.

author: fenugrec
author_rank: Frequent Contributor
timestamp: August 27, 2025, 11:04:25 pm
content: there is a publically available 8049 dump, look on ko4bb it should be clearly identified.I was just skimming your post, are you saying someone plonked a plain ROMless 8039 instead of the mask-ROM 8049 in there?

author: polymood
author_rank: Newbie
timestamp: August 29, 2025, 11:44:47 am
content: Yes, if you are willing to send either the part or the the entire board I would be more than willing to pay for postage costs !How can we exchange info ?

author: polymood
author_rank: Newbie
timestamp: August 29, 2025, 11:46:20 am
content: Thank you I will look into ko4bb !Yes there is an 8039AHL on U462 at the moment, when I checked the pins for activity and all seemed strange, reseting constantly, no sort of activty on the data bus, etc...

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 29, 2025, 01:15:56 pm
content: Can you send me a private message?You might need to make more posts on the forum to be allowed to send PM's...