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I can't find this anywhere on the internet so here's as good a place as any. Search engines will surely find this page.
Most retail gameboy cartridges, seemingly those with battery-backed RAM saves, have four test pads on the rear side of the PCB, labelled TP1, TP2, TP3 and TP4.
Connections:
TP1: RAM supply voltage/VCC
TP2: Battery voltage (after 1kOhm resistor R1)
TP3: Battery voltage direct
TP4: Ground/negative for the whole board
Useful tests:
Battery voltage: Voltage between TP3 and TP4. Could be more convenient than measuring across the battery terminals. Low voltage here indicates a dead battery or short circuit on input side of R1 (i.e. close to the battery).
Battery drain current: Voltage between TP2 and TP3. The reading in volts equals the current in milliamps, or the reading in millivolts equals the current in microamps. Large voltage (near battery voltage) here indicates a short circuit on output side of R1 (i.e. not necessarily close to the battery, probably around U4) or faulty R1.
RAM supply voltage: Voltage between TP1 and TP4. Zero/low voltage here with a good battery indicates a broken trace, or faulty U4 (may be caused by short circuit near U3 or U4)
Components:
All these components have the same numbers on all official boards as far as I can tell.
R1: 1k ohm resistor between the battery and U4. Acts as current sense (for manual testing) and short circuit protection. Tiny black rectangle or large brownish blob mounted in a hole, near battery or U4.
U3: Save RAM chip. Part number varies between games. Large chip usually near the bottom-left of the board.
U4: Power management chip. Also known as MM1134 or 134A. A small 8-pin chip usually near the middle-top of the board.
Most retail gameboy cartridges, seemingly those with battery-backed RAM saves, have four test pads on the rear side of the PCB, labelled TP1, TP2, TP3 and TP4.
Connections:
TP1: RAM supply voltage/VCC
TP2: Battery voltage (after 1kOhm resistor R1)
TP3: Battery voltage direct
TP4: Ground/negative for the whole board
Useful tests:
Battery voltage: Voltage between TP3 and TP4. Could be more convenient than measuring across the battery terminals. Low voltage here indicates a dead battery or short circuit on input side of R1 (i.e. close to the battery).
Battery drain current: Voltage between TP2 and TP3. The reading in volts equals the current in milliamps, or the reading in millivolts equals the current in microamps. Large voltage (near battery voltage) here indicates a short circuit on output side of R1 (i.e. not necessarily close to the battery, probably around U4) or faulty R1.
RAM supply voltage: Voltage between TP1 and TP4. Zero/low voltage here with a good battery indicates a broken trace, or faulty U4 (may be caused by short circuit near U3 or U4)
Components:
All these components have the same numbers on all official boards as far as I can tell.
R1: 1k ohm resistor between the battery and U4. Acts as current sense (for manual testing) and short circuit protection. Tiny black rectangle or large brownish blob mounted in a hole, near battery or U4.
U3: Save RAM chip. Part number varies between games. Large chip usually near the bottom-left of the board.
U4: Power management chip. Also known as MM1134 or 134A. A small 8-pin chip usually near the middle-top of the board.
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