Family BASIC v2: UI loads but crashes entering BASIC — ROMs verified, logic checked

[AdamT117]

Hi NESDev — first-time poster here.

I've been reading the forums while working on Famicom hardware repairs and have already learned a great deal. I'm currently working on a Family BASIC v2 cartridge and have run into an issue I can't quite resolve.

Problem:
The cartridge boots to the menu/UI without any issues.
When attempting to enter BASIC mode, the system crashes or freezes.
No BASIC prompt appears—just a blank screen or a complete lockup.

Steps Taken:

Removed and socketed both PRG ROMs (IC2 and IC3: HVC-FBI-1 A and B). Dumped both ROMs and verified the mask ROMS are readable.
Reprogrammed new 28c256 ROMs with a split online image dump—same behavior.
Replaced the 7805 regulator in the Famicom with a 2A version—thinking it may be a power issue.
Confirmed the 74HC20 (IC4) is known good.
Ran the cartridge’s built-in memory test by pressing "T" at boot—displays "RAM OK", suggesting the MB8416A-15 SRAM (IC1) is functional.
Inspected front and back of PCB for broken traces or damaged pads—no obvious defect.

Current Observations:

UI loads properly, which confirms PRG A and CHR ROM are functioning.
Despite working ROMs, proper power, working logic, and confirmed RAM, the BASIC mode consistently fails to start.

Looking For:

Suggestions on any known failure modes or quirks specific to Family BASIC v2
Ideas for deeper testing—especially related to logic or timing during PRG B access
Advice on anything that might be getting overlooked during the switch from UI to BASIC

Happy to provide board photos, ROM dumps, or signal traces if helpful. Thanks in advance, and I really appreciate the depth of knowledge shared on this forum.

[Pokun]

If it stays on the RAM test screen when booting wile holding "T" key it's a v1 ROM, not v2. The v2 ROM boots into BASIC mode when booting with "T". It sometimes quickly displays "RAM OK" when doing this but I don't know if it actually does the RAM test on v2.

[Ben Boldt]

I am wondering if there is a button stuck in the keyboard or one of the controllers. We've seen some games can't start when there is a button stuck (i.e. Super Mario Bros.). Is this a real Famicom or is it a clone?

[AdamT117]

Thanks for the pointers. As an update to this it looks like I made a mistake in comparing and preparing the image dumps. I must have been comparing a defect to the defect!

The B ROM was defective. Looks like fairly subtle changes to the defective ROM that threw me a bit.

As a useful helper I used Mesen to troubleshoot, this allows export and import of the ROM space as well as a trace function. From this functionality can be checked before committing to the real hardware.

The BASIC was v2 so have replaced with the latest revision. Again useful to know that 27c or 28c can be used.

Incidentally it's a pain that the cartridge cases cannot fit sockets so a lot of careful desoldering needed.

[TakuikaNinja]

That or you had a mismatch between inidividual PRG ROMs for 1.0 (PlayBox), 2.0A, 2.1A, or 3.0.

[Pokun]

There's also Family BASIC v1.0 (has the same version number of BASIC as Playbox but it replaced the Biorythm Board with fortune telling and the Message Board) and the Playbox BASIC prototype (which uses BASIC version number v0.0).

Holding "T" for the RAM test works on Playbox BASIC v0.0, Playbox BASIC v1.0 and Family BASIC v1.0.