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Amiga 500 Video Output Repair

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Amiga 500 video output circuit (Commodore schematic 312511-03 sheet 5)
The video section of the Rev. 6A/7 board on Commodore's schematic 312511-03, sheet 5: Denise (U4) at the left, the buffers U40 and U41, the video hybrid HY1, and the RGB connector CN9 and composite jack CN10 at the right.

The Amiga 500 builds its picture in Denise (U4), which puts out digital red, green, blue and intensity lines. Two 74HC245 buffers, U40 and U41, pass them to the video hybrid HY1, which converts them to analogue RGB for the 23-pin RGB port CN9 and to a composite signal for the RCA jack CN10. Agnus (U2) supplies the horizontal and vertical sync, which reach CN9 through the resistor pack RP403.[1][2] Commodore lists the composite output as a monochrome monitor output.[3] Commodore's service manual gives a check for each stage, so a missing picture, a missing colour or missing sync can be traced to the part that has failed.

Symptoms

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Commodore's troubleshooting splits video faults into six cases: no or wrong digital RGBI picture, no or wrong analogue RGB picture, no composite picture, no RGB vertical sync, no RGB horizontal sync, and no composite sync. It asks for analogue RGB, digital RGBI and composite all to be checked, with a known good monitor and cable.[2] Ian Perry's Amiga Troubleshooter chart gives these suspects:[4]

Video faults and suspects (Perry)
Symptom Suspects
No video (black screen) R405 and R406 (4.7 Ω); Denise U4; the video hybrid; U40, U41
Incorrect colours (red, green or blue missing) Denise U4; U40, U41; the video hybrid
Scrambled picture, loss of composite sync Agnus U2; U41; RP403
Picture breaks up after 15 to 30 minutes Agnus U2; Denise U4; the U2 socket

A black screen on every output can also mean the machine is not starting. Commodore's basic preliminary checks of the supply rails, reset and clocks must all pass before the system can produce a picture; if they fail, start with the Amiga 500 Troubleshooting Guide.[5] A picture on the RGB port but none on the composite jack is a separate fault; see composite output below.

Tools and parts

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  • An oscilloscope. Commodore takes every troubleshooting reading with one, to within ±10 %.[5]
  • A known good monitor and cable for each output being tested.[2]
  • A Torx T10 screwdriver for the case and top shield.[6]
  • Replacement parts as needed. Commodore's parts lists give U40 and U41 as 74HC245 (part 310003-01), R409 as 150 Ω, and C401 and C402 as 3300 µF 10 V radial electrolytics. The service parts diagram gives the video hybrid HY1 as part 390229-01.[7][8] Amigatronic's AMYVIDHYBRID+ is an open-hardware recreation of the Commodore 390229-03 hybrid built from current parts.[9]

Procedure

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Open the machine

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Remove the six T10 Torx screws along the front and rear edges of the bottom case and lift off the top case. Unplug the keyboard from CN13 and its ground lead from the drive, and lift the keyboard out. Remove the four T10 screws of the top shield, straighten its four tabs and lift it off.[6] The parts and their order are shown in Commodore's service parts reference diagram, File:Amiga 500 service parts reference diagram (Commodore 314981-04 p. 4-1).png.[8]

Find the parts

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Amiga 500 Rev 5 board layout, video area (Commodore 312510)
The video area of Commodore's Rev. 5 layout, marked "VIDEO OUTPUT": the hybrid HY1 along the rear edge, U40 and U41 below it, and Denise below them.

On Commodore's Rev. 5 layout the video section is at the left rear of the board, marked "VIDEO OUTPUT". HY1 is the long single-in-line module behind U40 and U41, with Denise (U4) below them and CN9 and CN10 on the rear edge. R405 sits at the right-hand end of HY1.[10]

Amiga 500 rear panel ports (Charles Cros collection, BnF)
The rear panel of an Amiga 500: from left, the two joystick ports, the right and left audio jacks, disk drive, serial and parallel ports, the power socket, the RGB video port and the composite jack marked "MONO".

Check the video supply

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  1. Measure +5 V on both sides of R405 and R406. On the Rev. 6A/7 schematic both are 4.7 Ω resistors fed from VCC. R406 feeds +VXX, the supply for U40, U41 and pin 2 of HY1, decoupled by C402; R405 feeds pin 21 of HY1, decoupled by C401. C401 and C402 are both 3300 µF, and a note says HY1 pins 2 and 21 are connected inside the hybrid. Denise takes its own +5 V through E406.[11]
  2. Perry lists R405 and R406 first for a black screen.[4] Commodore's parts lists give both as 5.1 Ω, where the schematic and Perry give 4.7 Ω.[7][11][4]
Amiga 500 video supply resistors R405 and R406 (Commodore schematic 312511-03 sheet 5)
The video supply on sheet 5: R406 feeds +VXX to U40, U41 and HY1 pin 2 with C402; R405 feeds HY1 pin 21 with C401. The note says HY1 pins 2 and 21 are connected internally.

Trace a missing or wrong picture

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Commodore's checks, working back from the output. "B" pins are the buffer outputs, "A" pins its inputs.[2]

Commodore's video checks (Service Manual A500, section 4)
Fault Measure Good result means Bad result means
No or wrong digital RGBI U40 outputs B0-B7, U41 outputs B0-B3 Fault is in RP402 or CN9 Measure the inputs next
U40 inputs A0-A7, U41 inputs A0-A3 U40, U41 or HY1 U4, U40 or U41
No or wrong analogue RGB HY1 outputs AR, AG, AB Fault is in EMI431-EMI433 or CN9 Measure U40/U41 outputs
U40/U41 outputs (as above) HY1 Measure U40/U41 inputs
U40/U41 inputs U40, U41 U4, U40 or U41
No composite picture HY1 composite output Fault is in EMI435 or CN10 Video missing: measure U40/U41 as above. Sync missing: composite sync checks
No composite sync U41 output B7 HY1 Measure U41 input A7: good means U41, bad means U2, U4 or U41
No RGB vertical sync RP403 pin 5 RP403 or CN9 U2 or RP403
No RGB horizontal sync RP403 pin 3 RP403 or CN9 U2 or RP403

Sams Computerfacts adds pin numbers and a level for the analogue stage:[12]

  • For analogue RGB, look for pulses at pins 2 to 9 of U40 and pins 2 to 5 and 7 of U41 (missing: Denise U4), then at pins 11 to 18 of U40 (missing: U40) and pins 13 and 15 to 18 of U41 (missing: U41). Then look for pulses of about 1.7 V peak to peak at pins 7, 12 and 17 of HY1 (missing: HY1). If they are present, check the joints on CN9 pins 3, 4, 5 and 14.
  • For digital RGBI, look for pulses at pins 2 and 6 of U40 and pin 2 of U41 (missing: U4), then pins 14 and 18 of U40 (missing: U40) and pin 18 of U41 (missing: U41), then check CN9 pins 6, 7 and 8.
  • For RGB sync, look for horizontal sync at pin 81 and vertical sync at pin 79 of Agnus (U2). If either is missing, suspect U2; if both are present, check CN9 pins 11 and 12.

Commodore's RGB connector pin-out puts analogue red, green and blue on pins 3, 4 and 5, digital intensity, blue, green and red on pins 6 to 9, composite sync on pin 10, horizontal sync on pin 11 and vertical sync on pin 12.[13] Commodore's troubleshooting text uses the Rev. 5 names EMI431-EMI435 for the filters that the Rev. 6A/7 schematic calls E431-E435.[2][1]

Composite output

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If an RGB monitor on CN9 works but there is no picture on the composite jack, Sams points at the hybrid HY1 and the resistor R409.[12] On the Rev. 6A/7 schematic R409 is 150 Ω from the composite output to ground, with the note "R409 controls composite output level for 390229-01 hybrid"; E435 sits between HY1 and the jack.[1] For missing composite sync, Sams checks for pulses at pin 8 of U41 (missing: Agnus U2), then pin 11 of U41 (missing: U41), then HY1 and R409.[12]

Repair

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  • Replace an open R405 or R406 with a resistor of the same value and power rating as the board's original.[4][7]
  • Replace a failed U40 or U41 with a 74HC245.[7]
  • A failed hybrid can be replaced with a good 390229 hybrid or with Amigatronic's recreation.[9]
  • Agnus sits in a PLCC socket, which Perry lists for a picture that breaks up when warm. Reseat it with a PLCC extraction tool before replacing it.[4][14]

Reassemble

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Reverse Sams' removal steps: refit the top shield and its four T10 screws, the keyboard and its ground lead, and the top case with six T10 screws.[6]

Testing

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Test analogue RGB, digital RGBI and composite in turn with known good monitors.[2] Commodore's A500/A2000 System Test disk includes SCREENTEST, which draws colour bars and grey scales, and SHOW BALLOON, a HAM picture that shows up a bad Agnus or Denise.[15] Leave the machine running for at least half an hour; Perry's break-up fault appears after 15 to 30 minutes.[4]

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References

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  1. ↑ 1.0 1.1 1.2 Service Manual A500, schematic 312511-03, Rev. 6A/7, sheet 5 of 8, printed p. 5-14 (PDF p. 70): U4, U40 and U41 (74HC245), HY1, RP402, RP403, E431-E435, CN9, CN10, R409 and its note.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Commodore Business Machines, Service Manual A500, PN-314981-04, October 1990, section 3, troubleshooting section 4, "Troubleshooting the A500 system video", sections 4.1-4.6, printed pp. 3-8 and 3-9 (PDF pp. 39-40). Hosted at Amiga 500 Service Manual (314981-04).
  3. ↑ Service Manual A500, section 1, specifications, printed p. 1-1 (PDF p. 4): "Supported monitors: Analog RGB, digital RGB, monochrome (composite video), and standard televisions".
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Ian Perry, Amiga Troubleshooter (1993), "Video problems", reproduced at Commodore Amiga problems—link(accessed 2026-10-25)
  5. ↑ 5.0 5.1 Service Manual A500, basic preliminary checks, printed p. 3-3 (PDF p. 34): supply rails, reset and clocks before any further checks; all readings with an oscilloscope, ±10 %.
  6. ↑ 6.0 6.1 6.2 Sams Computerfacts CSCS26, disassembly instructions, printed p. 18 (PDF p. 22).
  7. ↑ 7.0 7.1 7.2 7.3 Service Manual A500, component parts lists, PCB assembly 312510 Rev. 5 (PDF pp. 50-51) and Rev. 6A/7 (PDF p. 52): U40, U41 74HC245, 310003-01; R409 150 Ω; R405 and R406 5.1 Ω; C401, C402 3300 µF 10 V radial, 900100-56.
  8. ↑ 8.0 8.1 Service Manual A500, service parts reference diagram, printed p. 4-1 (PDF p. 48): HY1 video hybrid, 390229-01.
  9. ↑ 9.0 9.1 Amigatronic, AMYVIDHYBRID+ Modern Replacement for Amiga Vidiot—link(accessed 2026-10-25) An open-hardware recreation of the Commodore 390229-03 video hybrid, listed for the A500, A500+, A1000, A2000 and A3000.
  10. ↑ Service Manual A500, A500 board layout #312510, Rev. 5, printed p. 4-7 (PDF p. 54).
  11. ↑ 11.0 11.1 Service Manual A500, schematic 312511-03, sheet 5 of 8, power section at the lower left (PDF p. 70): R405 and R406 marked 4.7 Ω, C401 and C402 3300 µF.
  12. ↑ 12.0 12.1 12.2 Howard W. Sams & Co., Computerfacts CSCS26, Commodore Amiga A500 (1988), troubleshooting, "Composite video", "RGB analog video", "RGB digital video" and "Video sync", printed p. 26 (PDF p. 30). Hosted at Sams Computerfacts CSCS26 Amiga 500.
  13. ↑ Service Manual A500, input/output connectors, "RGB Monitor Connector", PDF p. 78.
  14. ↑ Reseating Agnus—link(accessed 2026-10-01)
  15. ↑ Service Manual A500, A500/A2000 system test diskette commands, printed pp. 3-1 and 3-2 (PDF pp. 32-33).
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