Texas Instruments
Interactive Flowchart Texas Instruments TI-99/4A
TI-99/4A Mainboard & Internal LPS-2 Power Supply Board — 16-Bit TMS9900 + TMS9918A VDP + triple-rail LPS-2 regulator
Complete Diagnostic Flowchart: Capacitive Load, ESR & Power Rail Ripple Diagnostic Tree (Universal Retro Mainboards (C64, Amiga, ZX Spectrum, Game Gear, Consoles))
STEP 1: Measure 100Hz/120Hz Peak-to-Peak AC Ripple on the +5V / +12V Bulk Filter Cap
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Set your oscilloscope to AC Coupling, 200 mV/div, and 2.5 ms/div (20 MHz bandwidth limit ON). Probe directly across the main bulk filter capacitor (e.g. C19 2200µF / C88 1000µF on C64, or C303/C304 on Amiga) under full system load. Measure the peak-to-peak sawtooth ripple amplitude (mVpp).
Expected Reading: < 50 mVpp clean regulated rail (post-regulator) or < 350 mVpp on raw unregulated bulk DC before the 7805/7812.
STEP 2: Check Linear Regulator (7805 / 7812) Input-to-Output Dropout Margin
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Switch oscilloscope or multimeter to DC Volts and measure Pin 1 (Input) of the 7805 (+5V) or 7812 (+12V) regulator at the bottom of the ripple trough. Verify that Input Voltage remains at least 2.0V above the regulated output (e.g. ≥7.2V into a 7805, ≥14.2V into a 7812).
Expected Reading: 7805 Input ≥ 7.2V DC; 7812 Input ≥ 14.2V DC. Below this dropout threshold, ripple punches straight through to the logic rail.
STEP 3: Check Local IC Decoupling & Audio/Video AC Coupling Capacitors
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Probe audio output coupling caps (e.g. C91 100µF on C64 SID Pin 27, or C324/C334 22µF SMD on Amiga Paula) — compare upstream vs. downstream signal amplitude. Inspect SMD electrolytic pads (Amiga 600/1200, Game Gear, DMG-01) under magnification for fishy odor, dull grey solder corrosion, or lifted copper traces.
Expected Reading: Full audio/video swing across the coupling capacitor with zero DC leakage on the output side and clean, shiny solder pads.
[ISOLATED FAULT: RES-BULK-CAP] Fault Isolated: High-ESR Bulk Filter Capacitor (>100 mVpp Rail Ripple)
VERDICT
Excessive 100/120Hz sawtooth ripple on the DC rail confirms that the primary reservoir/filter electrolytics have dried out and lost effective capacitance.
[ISOLATED FAULT: RES-REG-DROPOUT] Fault Isolated: Linear Regulator Dropout / Bridge Rectifier Diode Open
VERDICT
The pre-regulator DC rail is sagging below the minimum 2V dropout headroom required by the 7805/7812 regulator.
[ISOLATED FAULT: RES-COUPLING-CAP] Fault Isolated: Open/Leaky Audio or Video Coupling Capacitor & Trace Corrosion
VERDICT
Signal attenuation, muffled audio, or DC offset hum across the coupling stage indicates dried-out or leaking coupling capacitors.
[ISOLATED FAULT: RES-RAILS-HEALTHY] Verification Passed: Power Rails, Ripple & Coupling Capacitors Within Spec
VERDICT
All measured DC supply rails, AC ripple levels (<50 mVpp), and coupling stages pass bench specification. If performing a preventive recap, use the verified board BOM below.
Summary
The TI-99/4A takes dual AC voltages (18V AC and 8.5V AC) from its external step-down transformer brick and rectifies/regulates +5V, +12V, and -5V on a long internal power supply board along the right side of the console. Both the TMS9900 16-bit CPU and the eight 4116 VRAM chips depend on all three rails.
Interactive Bench Oscilloscope — Verify Pre- & Post-Recap Waveforms
Open Full Signal Visualizer →Click any test point below to compare a healthy recapped rail or clock against high-ESR electrolytic capacitor ripple and droop:
Safety & Warnings
Both the TMS9900 CPU and 4116 VRAM require +5V, +12V and -5V!
Always verify all three DC voltages on the 4-pin harness from the internal power supply board before connecting the mainboard.
Is This Your Board?
- The internal power regulator PCB sits under the right-hand cartridge/switch bezel and connects to the mainboard via a 4-wire harness (+5V, +12V, -5V, GND).
Verified
Check
Verified rows are agreed by two or more independent sources. Check rows are revision-dependent or single-sourced — confirm the value against your own board before ordering.
Internal Power Supply Regulator PCB (LPS-2)
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C1 |
4700 uF | 16 V | Axial / Radial | +5V raw DC primary reservoir | Verified | |
C2 |
1000 uF | 35 V | Axial / Radial | +12V raw DC reservoir | Verified | |
C3 |
220 uF | 25 V | Radial | -5V raw DC reservoir | Verified | |
C4 |
100 uF | 16 V | Radial | +5V regulated output filter | Verified | |
C5 |
47 uF | 25 V | Radial | +12V regulated output filter | Verified | |
C6 |
10 uF | 25 V | Radial | -5V regulated output filter | Verified |
Internal Power Supply Regulator PCB (LPS-2) — shopping list
- 1 x 4700 uF 16 V Axial / Radial
- 1 x 1000 uF 35 V Axial / Radial
- 1 x 220 uF 25 V Radial
- 1 x 100 uF 16 V Radial
- 1 x 47 uF 25 V Radial
- 1 x 10 uF 25 V Radial
TI-99/4A Main Logic Board
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C101 |
100 uF | 16 V | Axial | +5V mainboard rail filter | Verified | |
C102 |
22 uF | 25 V | Axial | +12V TMS9900 / VDP rail filter | Verified | |
C103 |
10 uF | 25 V | Axial | -5V substrate bias filter | Verified | |
C205 |
10 uF | 25 V | Radial | TMS9919 sound chip audio coupling | Verified |
TI-99/4A Main Logic Board — shopping list
- 1 x 100 uF 16 V Axial
- 1 x 22 uF 25 V Axial
- 1 x 10 uF 25 V Axial
- 1 x 10 uF 25 V Radial
Whole-Machine Shopping List
- 1 x 4700 uF 16 V Axial / Radial
- 1 x 1000 uF 35 V Axial / Radial
- 1 x 220 uF 25 V Radial
- 1 x 100 uF 16 V Radial
- 1 x 47 uF 25 V Radial
- 2 x 10 uF 25 V Radial
- 1 x 100 uF 16 V Axial
- 1 x 22 uF 25 V Axial
- 1 x 10 uF 25 V Axial
Recap Tips
- Clean the felt dust strip inside the cartridge port - lint and oxidation on the 36-pin cartridge edge connector cause frequent lockups.
Related Symptoms
- Garbled characters in specific vertical columns on title screen Faulty 4116 VRAM chip feeding the TMS9918A VDP.
Sources & References
- Console5 TechWiki - TI-99/4A Internal power board and mainboard capacitor tables.