Tangerine / Oric
Interactive Flowchart Oric-1 & Oric Atmos
Issue 3 & Issue 4 Mainboard — 6502A + HCS10017 ULA + 7905 negative-regulator mainboard
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
[node-2]
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
The Oric-1 and Oric Atmos use an unusual power supply architecture where the positive +9V DC input is tied to the +5V rail and a 7905 negative regulator regulates the ground/return rail! Drying reservoir and LM386 audio capacitors cause hum and cassette loading failures.
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
Oric uses a 7905 Negative Regulator (+9V DC Centre-Positive Input)
Do not confuse the 7905 regulator in the rear corner with a 7805 - its pinout is Pin 1 GND, Pin 2 Input, Pin 3 Output!
Is This Your Board?
- Look along the front-right PCB edge for "ORIC ISSUE 3" or "ISSUE 4".
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.
Oric-1 / Oric Atmos Mainboard (Issue 3 / 4)
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C1 |
2200 uF | 16 V | Axial / Radial | 9V DC input primary reservoir | Verified | |
C2 |
100 uF | 16 V | Radial | 7905 regulated +5V rail filter | Verified | |
C5 |
10 uF | 25 V | Radial | 6502A /RESET timing | Verified | |
C11 |
220 uF | 10 V | Radial | LM386 internal speaker coupling | Verified | |
C12 |
10 uF | 25 V | Radial | AY-3-8912 audio input coupling | Verified | |
C21 |
47 uF | 16 V | Radial | HCS10017 ULA & RAM rail decoupling | Verified |
Oric-1 / Oric Atmos Mainboard (Issue 3 / 4) — shopping list
- 1 x 2200 uF 16 V Axial / Radial
- 1 x 100 uF 16 V Radial
- 2 x 10 uF 25 V Radial
- 1 x 220 uF 10 V Radial
- 1 x 47 uF 16 V Radial
Recap Tips
- Replacing the hot 7905 linear regulator with a negative-rail switching regulator (or modding to a 7805 positive regulator) eliminates heat buildup under the keyboard.
Related Symptoms
- Black/white vertical bars on boot Check the eight 4164 DRAM chips and HCS10017 ULA socket.
Sources & References
- Oric-1 & Atmos Service Manual Official Tangerine Computer Systems parts list.