Jump to content

TRS-80 Model II Capacitor Replacement Guide

From RetroTechCollection
TRS-80 Model II AA11080 power supply schematic (Technical Reference Manual Figure 5)
The AA11080 power supply schematic (manual p. 124). The polarised capacitors are marked with a "+"; the manual gives no values for them.

Electrolytic and tantalum capacitors in the TRS-80 Model II display console and keyboard. Radio Shack's TRS-80 Model II Technical Reference Manual (catalogue number 26-4921) has a parts list for every logic board, the motherboard, both versions of the video monitor and the keyboard, and the tables below are taken from them.[1] The one gap is the AA11080 power supply: its schematic shows where the capacitors are, but the manual gives no values for them.

Before working inside the console, read the safety notes on TRS-80 Model II General Maintenance: the power supply's input capacitors and the monitor's C6 and anode lead must be discharged first.[2]

CPU board

[edit | edit source]
CPU board (PCB 8709040) electrolytics (parts list, pp. 25-26)
Ref Value Type Manufacturer's part no.
C2, C15, C24, C27 33 µF 50 V Radial 8326331
C32, C34, C36, C41 33 µF 50 V Axial 8316334
C38 100 µF 16 V Axial 8317101
C43 33 µF 50 V Radial 8326331

The parts list describes C43 as "33µF, 50V, Monlithic, Radial", but gives it the same manufacturer's and Radio Shack part numbers as the radial electrolytics C2, C15, C24 and C27. Check the part on your board before ordering. The other CPU board capacitors are 0.1 µF monolithic and small ceramic disc parts.[3]

Floppy disk controller board

[edit | edit source]
FDC board (PCB 8709063) electrolytics and tantalums (parts list, p. 50)
Ref Value Type Manufacturer's part no.
C18, C19, C30, C34 33 µF 50 V Aluminium electrolytic, axial 8316334
C17 0.47 µF 35 V, 20 % Tantalum 8324471
C27 68 µF 35 V Tantalum 8334683

The list gives C35 as "33µF, 50V, Ceramic Disc", but its part numbers (8300334, ACC330QJCP) are those of the 33 pF disc C9, so C35 is a 33 pF ceramic.[4]

Video/keyboard interface board

[edit | edit source]
Video/keyboard board (PCB 8709048) electrolytics (parts list, p. 69)
Ref Value Type Manufacturer's part no.
C26, C27 33 µF 50 V Axial 8316334

The rest are a 0.01 µF disc, 0.1 µF monolithics and small ceramic discs.[5]

Memory board

[edit | edit source]
Memory board, 32K and 64K (PCB 8709053), electrolytics (parts list, p. 81)
Ref Value Type, as listed Manufacturer's part no.
C13, C39 4.7 µF 25 V Electrolytic, PC 8325472
C25, C57 4.7 µF 25 V Electrolytic 8325472
C38, C68 33 µF 25 V Electrolytic, axial 8316332
C48, C56, C66 33 µF 25 V Electrolytic 8316332
C67 10 µF 16 V Electrolytic, PC 8326101

Every other capacitor on the memory board is a 0.1 µF monolithic decoupler.[6]

Motherboard

[edit | edit source]
Card cage motherboard electrolytics (parts list, p. 127)
Ref Value Type Manufacturer's part no.
C1 470 µF 16 V Radial 8327471
C2, C3 470 µF 16 V Axial 8317471

C4 to C6 are 0.1 µF monolithic parts.[7] The parts list does not say which rail each 470 µF capacitor sits on; trace it on your board before fitting a replacement, and use at least the original 16 V rating.

Video monitor

[edit | edit source]

The manual covers two monitor boards: a Motorola M3970-series board and an RCA board (chassis VDC-1). Identify yours from the designators before ordering parts; the two numbering schemes do not overlap.[1] The monitor runs from the +12 V rail.[8]

Motorola monitor board electrolytics and tantalums (parts list, p. 101)
Ref Value Type Motorola part no. Radio Shack part no.
C6 2000 µF 35 V Electrolytic 23S10255A11 ACC208QGAP
C7, C12 22 µF 160 V Electrolytic 23S10255A74 ACC226QNAP
C10 470 µF 16 V Electrolytic 23S102255A29 ACC477QDAP
C19 100 µF 16 V Electrolytic 23S10255A06 ACC107QDAP
C2, C8, C9 5 µF 15 V Tantalum 23S10218A31 ACC505QPTP
C3 0.68 µF 35 V Tantalum 23R29976A01 ACC684QGTP

The remaining Motorola capacitors are disc, polyester and film types, some rated 400 V and 500 V.[9] C6 is the one Radio Shack tells you to discharge before working on the board.[2]

RCA monitor board (VDC-1) electrolytics (parts list, p. 108)
Ref Value RCA stock no.
C506 5.6 µF 20 V 141028
C517, C518 1 µF 150 V 146113
C519 1 µF 350 V 148889
C521 56 µF 50 V, 10 % 141285
C523 470 µF 25 V 141432
C524 330 µF 25 V 141163
C528 4.7 µF 50 V 142776
C605, C606 220 µF 15 V 139285
C608 47 µF 6 V 137331
C610 0.68 µF 80 V 145307

C511 and C514 to C516 are marked as safety-critical parts, for which RCA says to use only its recommended replacements; the list prints no value for them.[10] One owner recapping an RCA board replaced C521 with a 68 µF 100 V part and the others with like values at higher voltage ratings.[11]

Keyboard

[edit | edit source]

The keyboard has no aluminium electrolytics. C1 and C3 are 10 µF 10 V tantalums (Radio Shack ACC106QCTA and ACC106QTCA); C2 and C4 to C6 are 0.033 µF ceramic discs.[12]

AA11080 power supply

[edit | edit source]

The manual's schematic for the AA11080 gives designators only, and section IX has no parts list.[13] Read your own board's parts and record them before removing anything. From the schematic:[13]

Polarised capacitors on the AA11080 schematic (values not published)
Circuit Designators
Mains input capacitors C5, C6
+24 V output C11, C12, C22
+12 V output C13, C14, C15, C23
+5 V output C16, C17, C18, C19, C24
−12 V output, before and after the regulator IC1 C21, C26
Control module supply and sensing C27, C28, C32

Radio Shack's no-output procedure has you check the input capacitors C5 and C6, and check the secondary rectifiers and capacitors for shorts with the loads disconnected. Its B+ check, made with the probe's ground on the negative terminal of the large input capacitor nearest Q1, expects 280-300 V DC with 95 V AC in.[13] The service notes describe two large grey high-voltage capacitors on the supply and say to discharge them across their own terminals with an insulated, grounded tool before working in the console.[2] The unpolarised C1 to C4 at the mains input are the line filter capacitors.[13]

Some owners have replaced the whole supply with a modern four-output unit instead of rebuilding it.[14]

Replacement notes

[edit | edit source]
  • Use the listed value and at least the listed voltage rating.
  • Check the polarity of every replacement against the board before switching on. A VCF forum member has seen a monitor board whose printed polarity markings were wrong, so compare with photographs taken before removal.[15]
  • After recapping, check the AA11080 outputs under load against the limits on TRS-80 Model II Troubleshooting Guide (+5 V 4.90-5.10 V, +12 V 11.40-12.60 V, +24 V 21.20-26.40 V, −12 V −11.40 to −12.60 V).[13]
[edit | edit source]

References

[edit | edit source]
  1. ↑ 1.0 1.1 Radio Shack, TRS-80 Model II Technical Reference Manual, catalogue number 26-4921, 1980. Hosted here as File:Radio Shack TRS-80 Model II Technical Reference Manual (26-4921, 1980).pdf. Page numbers on this page are the manual's printed page numbers.
  2. ↑ 2.0 2.1 2.2 Technical Reference Manual, p. 91, "Service Notes": discharging the two large grey high-voltage capacitors on the Astec power supply, capacitor C6 on the Motorola monitor board and the second-anode lead.
  3. ↑ Technical Reference Manual, pp. 25-26, "CPU Module Parts List".
  4. ↑ Technical Reference Manual, p. 50, "Floppy Disk Controller Parts List".
  5. ↑ Technical Reference Manual, p. 69, "Video/Keyboard Interface Board Parts List".
  6. ↑ Technical Reference Manual, p. 81, "Memory Board (32K & 64K) Parts List".
  7. ↑ Technical Reference Manual, p. 127, "Card Cage and Motherboard Parts List".
  8. ↑ Technical Reference Manual, p. 89 (Motorola: +12 V at 1.2 A typical) and p. 105 (RCA: +12 V at 1 A nominal).
  9. ↑ Technical Reference Manual, p. 101, "Video Monitor (Motorola) Parts List".
  10. ↑ Technical Reference Manual, p. 108, "Replacement Parts" (RCA video monitor kit, chassis assembly VDC-1), including the product safety note.
  11. ↑ VCF Forums, "TRS-80 Model II repair log", May 2024 (retrieved 2026-10-21). Recapping the RCA monitor board. Community source.
  12. ↑ Technical Reference Manual, p. 137, "TRS-80 Model II Keyboard Parts List".
  13. ↑ 13.0 13.1 13.2 13.3 13.4 Technical Reference Manual, pp. 119-124, section IX "AA11080 Power Supply": no-output checks naming the input capacitors C5 and C6, and Figure 5, the schematic.
  14. ↑ VCF Forums, "Model II power supply advice", May-June 2015 (retrieved 2026-10-21). Community source.
  15. ↑ VCF Forums, "TRS-80 Model 2 - first startup after recapping but CRT shows only disappearing horizontal line", September 2024 (retrieved 2026-10-21). Checking replacement values, ratings and polarity after a recap. Community source.


Retrieved from "https://retrotechwiki.com/index.php?title=TRS-80_Model_II_Capacitor_Replacement_Guide&oldid=28773"