Atari 16/32-bit
Interactive Flowchart Atari ST (520ST / 1040STFM / 1040STE)
C070789 / C103414 Mainboard & Mitsumi SR98 / ASP Internal PSU — MC68000 + Shifter + GLUE + MMU 16/32-bit workstation
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
[node-1]
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
[node-3]
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
On the Atari 520STFM, 1040STFM, and 1040STE, the internal switched-mode PSU (Mitsumi SR98, Phihong, or ASP) sits directly above the rear of the board under a metal shield. Drying secondary filter capacitors on the +5V and +12V rails cause floppy read errors, wavy RGB video, and 68000 exception bombs.
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
Lethal 340V DC primary reservoir capacitor on the open-frame Atari ST PSU!
The primary heatsink inside the Mitsumi SR98 PSU can be live at mains potential! Always unplug the mains cord and verify 0V DC across the large 200V/400V primary capacitor before touching the PSU.
Is This Your Board?
- Lift the metal shield over the internal PSU to read the model label (e.g. "Mitsumi SR98" or "ASP34-2").
- Mainboard part number (e.g. C070789 on STFM, C300598 on STE) is silkscreened along the front edge under the keyboard.
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.
Mitsumi SR98 Internal Switched-Mode PSU
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C5 (Primary) |
100 uF | 400 V | Snap-In Radial | Mains HV DC primary reservoir | Verified | |
C8 |
47 uF | 50 V | Radial (105 C) | PWM oscillator startup | Drying C8 prevents PSU from starting when cold. | Verified |
C14 |
2200 uF | 16 V | Radial (Low-ESR) | +5V DC primary output filter | Verified | |
C15 |
1000 uF | 16 V | Radial (Low-ESR) | +5V DC secondary pi-filter | Verified | |
C17 |
1000 uF | 25 V | Radial (Low-ESR) | +12V DC floppy/audio output filter | Verified | |
C18 |
470 uF | 25 V | Radial (Low-ESR) | +12V DC secondary filter | Verified |
Mitsumi SR98 Internal Switched-Mode PSU — shopping list
- 1 x 100 uF 400 V Snap-In Radial
- 1 x 47 uF 50 V Radial (105 C)
- 1 x 2200 uF 16 V Radial (Low-ESR)
- 1 x 1000 uF 16 V Radial (Low-ESR)
- 1 x 1000 uF 25 V Radial (Low-ESR)
- 1 x 470 uF 25 V Radial (Low-ESR)
Atari 1040STFM / 1040STE Mainboard
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C1 |
470 uF | 16 V | Axial | +5V mainboard entry filter | Verified | |
C2 |
100 uF | 25 V | Axial | +12V mainboard entry filter | Verified | |
C11 |
10 uF | 25 V | Radial | 555 /RESET & /HALT power-on delay | Bad C11 causes failure to boot on cold power-up until reset is pressed. | Verified |
C28 |
22 uF | 25 V | Radial | YM2149F / DMA audio coupling | Verified | |
C34 |
47 uF | 16 V | Radial | Shifter / video RGB DAC decoupling | Verified |
Atari 1040STFM / 1040STE Mainboard — shopping list
- 1 x 470 uF 16 V Axial
- 1 x 100 uF 25 V Axial
- 1 x 10 uF 25 V Radial
- 1 x 22 uF 25 V Radial
- 1 x 47 uF 16 V Radial
Whole-Machine Shopping List
- 1 x 100 uF 400 V Snap-In Radial
- 1 x 47 uF 50 V Radial (105 C)
- 1 x 2200 uF 16 V Radial (Low-ESR)
- 1 x 1000 uF 16 V Radial (Low-ESR)
- 1 x 1000 uF 25 V Radial (Low-ESR)
- 1 x 470 uF 25 V Radial (Low-ESR)
- 1 x 470 uF 16 V Axial
- 1 x 100 uF 25 V Axial
- 1 x 10 uF 25 V Radial
- 1 x 22 uF 25 V Radial
- 1 x 47 uF 16 V Radial
Recap Tips
- Clean and inward-tension the PLCC-68 sockets for the GLUE and MMU chips while the shield is off.
Related Symptoms
- Needs warm-up or Reset button press to boot Dried-out C8/C14 in the Mitsumi PSU or C11 in the 555 reset circuit.
Sources & References
- Console5 TechWiki - Atari ST & Mitsumi PSU Mitsumi SR98 and STFM/STE mainboard capacitor lists.