Tandy / RadioShack
Interactive Flowchart TRS-80 & Tandy CoCo (1 / 2 / 3)
Tandy CoCo 1/2/3 & TRS-80 Model I/III Mainboards — MC6809E Color Computer & Z80 TRS-80 mainboards
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
Tandy Color Computers (CoCo 1, 2, and 3) house an internal linear power supply transformer and regulator section on the right side of the mainboard. Drying 4700 uF and 1000 uF filter cans cause 60Hz/50Hz video hum bars and GIME/SAM resets.
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
Unplug mains power cord before opening CoCo 1/2/3 or TRS-80 Model III
The power switch and transformer primary wiring inside the CoCo and Model III carry live 120V/240V AC mains voltage.
Is This Your Board?
- CoCo 1/2/3 boards have the power transformer and large filter capacitors mounted inside the rear-right corner of the case.
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.
Tandy Color Computer 2 / 3 Mainboard & Power Stage
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C1 |
4700 uF | 16 V | Radial / Axial | +5V raw DC primary bridge reservoir | Verified | |
C2 |
1000 uF | 25 V | Radial | +12V regulator reservoir | Verified | |
C3 |
470 uF | 25 V | Radial | -12V / RS-232 bias reservoir | Verified | |
C4 |
100 uF | 16 V | Radial | 7805 +5V output filter | Verified | |
C12 |
10 uF | 25 V | Radial | MC6809E /RESET delay timing | Verified | |
C24 |
22 uF | 25 V | Radial | 6-bit DAC audio coupling | Verified | |
C31 |
47 uF | 16 V | Radial | MC6847 VDG / GIME video decoupling | Verified |
Tandy Color Computer 2 / 3 Mainboard & Power Stage — shopping list
- 1 x 4700 uF 16 V Radial / Axial
- 1 x 1000 uF 25 V Radial
- 1 x 470 uF 25 V Radial
- 1 x 100 uF 16 V Radial
- 1 x 10 uF 25 V Radial
- 1 x 22 uF 25 V Radial
- 1 x 47 uF 16 V Radial
Recap Tips
- On TRS-80 Model III and Model 4 units with Astec switched-mode PSUs, replace the RIFA X2 mains filter capacitors before applying power.
Related Symptoms
- Dark rolling hum bar on TV / Composite monitor Dried-out 4700 uF main filter capacitor C1.
Sources & References
- Console5 TechWiki - Tandy Color Computer CoCo 1, 2 and 3 capacitor schedules.