Commodore 8-bit
Interactive Flowchart Commodore PET Series
PET 2001N (320351) & 4032 / 8032 (8032080) — Dynamic RAM 40-column & 80-column Universal CRTC 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
Commodore PET mainboards carry four linear voltage regulators (two 7805s, one 7812, and one 7905 for the 4116 DRAM rails) fed by a large chassis-mounted computer-grade filter capacitor (23,000 uF) and local axial tantalum/electrolytic decoupling cans.
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
Discharge the 9-inch / 12-inch CRT monitor anode
The integrated monochrome CRT monitor operates at 10-12 kV. Discharge the anode cap to the CRT chassis frame before servicing the monitor PCB.
Check -5V regulator VR2 (7905) before powering 4116 DRAM
Just like on the ZX Spectrum, missing -5V on Pin 1 of the sixteen 4116 DRAM chips while +12V is present will destroy the RAM bank.
Is This Your Board?
- ASSY 320351 is the non-CRTC 2001N / 3032 dynamic RAM board using sixteen 4116 DRAM chips.
- ASSY 8032080 is the Universal 4032 / 8032 board with the 6845 CRTC chip and piezo chirp speaker.
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.
PET 2001N / 4032 / 8032 Mainboard
| Ref | Value | Voltage | Type | Function | Notes | Confidence |
|---|---|---|---|---|---|---|
C1 (Chassis) |
22000 uF | 16 V | Screw-Terminal / Axial | Primary +8V raw DC reservoir | Original is 23,000 uF 15V Mallory screw-terminal can mounted by the transformer. | Verified |
C61 |
470 uF | 35 V | Axial | +12V regulator VR1 input | Verified | |
C62 |
10 uF | 25 V | Axial | +12V VR1 (7812) output | Verified | |
C63 |
470 uF | 25 V | Axial | -5V regulator VR2 (7905) input | Verified | |
C64 |
10 uF | 25 V | Axial | -5V VR2 (7905) output | Verified | |
C65 |
22 uF | 16 V | Axial | +5V VR3 (7805) output | Verified | |
C66 |
22 uF | 16 V | Axial | +5V VR4 (7805) output | Verified | |
C67 |
1 uF | 35 V | Radial | 555 power-on /RESET timer | Verified |
PET 2001N / 4032 / 8032 Mainboard — shopping list
- 1 x 22000 uF 16 V Screw-Terminal / Axial
- 1 x 470 uF 35 V Axial
- 2 x 10 uF 25 V Axial
- 1 x 470 uF 25 V Axial
- 2 x 22 uF 16 V Axial
- 1 x 1 uF 35 V Radial
Recap Tips
- Many PET boards use dipped tantalum beads at C62, C64, C65, C66 and C67 that fail dead-short and pull down the -5V or +12V rails - replace them with 105 C radial electrolytics.
Related Symptoms
- Random characters (Garbage Screen) on boot Check the +5V/+12V/-5V regulators, 2114 Video SRAM (UF7/UF8), and 6520 PIA.
Sources & References
- Zimmers.net - Commodore PET Schematics (320351 & 8032080) Official Commodore PET parts lists and schematics.