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TRS-80 Model I Capacitor Replacement Guide

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Radio Shack TRS-80 Model I logic board
TRS-80 Model I logic board (Binarysequence, CC BY-SA 4.0). The blue and black axial capacitors near the top edge are C8 (2,200 µF 35 V) and C9 (10,000 µF 16 V), the reservoirs for the +12 V and +5 V regulators.

The logic board of the TRS-80 Model I (PCB 1700069) has eleven aluminium electrolytic capacitors. Radio Shack's parts list in the TRS-80 Micro Computer Technical Reference Handbook gives every value, rating and Radio Shack part number, and the table below is taken from it. The keyboard PCB (1700070) has only two 0.1 µF ceramic disc capacitors, and the Level II ROM adapter board (1700081) has none.[1][2]

The Expansion Interface, the Video Display and the AC adapter are separate units and are not covered here.

Electrolytic capacitors on the logic board

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Model I logic board electrolytics (parts list, pp. 94-95)
Ref Value Type Radio Shack part no. Circuit, as described in the handbook
C1 200 µF 16 V Axial 1500059 Filters the negative output of CR8 (about −11 V) feeding the −5 V supply[3]
C2 10 µF 16 V Radial 1500012 With C7 and R30, the filter on Q1's collector in the video output stage[4]
C4 10 µF 16 V Radial 1500012 −5 V output filter, with ceramic C3[3]
C5 10 µF 16 V Radial 1500012 Cassette input: on the bias line of the LM3900 amplifier Z4 (schematic)[5]
C6 100 µF 16 V Radial 1500014 Supply decoupling for Z4, at pin 14 after R31 (schematic)[5]
C8 2,200 µF 35 V Axial 1500064 +12 V supply reservoir, about 20 V, feeding Z2 and pass transistor Q6[3]
C9 10,000 µF 16 V Axial 1500058 +5 V supply reservoir, about 7 V from bridge rectifier CR8[3]
C10 10 µF 16 V Radial 1500012 +5 V output filter, with ceramic C14[3]
C11 10 µF 16 V Radial 1500012 +12 V output filter, with ceramic C15[3]
C42 22 µF 16 V Radial 1500055 Power-up reset timing for the CPU[6]
C57 10 µF 16 V Radial 1500012 Reset switch S2 timing[6]

Every other capacitor on the logic board is a ceramic disc or a polyester film part: 0.1 µF and 0.01 µF decoupling discs, the 470 pF regulator compensation discs C12 and C13, the 47 pF oscillator disc C43, and the polyester C26 (0.047 µF 100 V) and C27 (0.022 µF 100 V) in the sync generator.[2][3]

The working voltages leave margin on the supply parts: C8 is a 35 V part on about 20 V, C9 a 16 V part on about 7 V, and C1 a 16 V part on about 11 V.[3] Keep the same voltage rating or higher on every replacement.

Which capacitors matter

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  • C8 and C9 are the reservoirs for the +12 V and +5 V regulators, and C1 for the −5 V zener supply.[3] Check the rails at the test points on TRS-80 Model I General Maintenance before and after replacing them.
  • C42 sets the power-up reset. Radio Shack's section isolation chart sends a unit that shows garbage at the first switch-on but starts after a ten-second power cycle to S2 and C42.[7] On the Model III and 4, community repairers report the equivalent reset electrolytic drifting out of specification with age; the Model I handbook does not discuss ageing of C42.[8]
  • The handbook warns that the polyester capacitors C21 and C27 in the sync generator can short after an impact. Its parts list gives C21 as a 750 pF ceramic disc and C26 and C27 as the polyester parts, so check both C21 and C27 if the sync generator has failed.[9][2]
  • Display interference caused by oscillation on the +5 V bus points to the regulator compensation capacitors C12 and C13, which are ceramic discs, not electrolytics.[10]

Replacement procedure

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  1. Remove the logic board from the case as described on TRS-80 Model I General Maintenance. Keep the interconnect cable to the keyboard free of strain.[11]
  2. Before touching the RAMs, ground yourself with a strap or by touching the right-hand side of C9. The RAMs are MOS parts.[12]
  3. Note the polarity and position of each capacitor before removing it. C1, C8 and C9 are axial; the rest are radial.[2]
  4. Remove each capacitor and clean the pads. Use a rubber eraser, not steel wool or a sharp tool, on solder-coated tracks; scraping raises slivers of solder that short neighbouring tracks.[13]
  5. Fit the new capacitors the same way round as the originals, with the same capacitance and at least the same voltage rating.
  6. Check for solder splashes and solder balls under the board before refitting it. These are the commonest shorts Radio Shack found.[13]
  7. Power up with the boards on the bench and check the rails.

After the recap

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Rail checks after the recap (handbook, p. 60)
Rail Measure at Limits Adjust with
+12 V Top end of R18 11.4-12.6 V R10
+5 V Left-hand end of R4 4.75-5.25 V R5, after +12 V
−5 V Anode of CR2 −5 V ± 5 % None

Measure with a digital voltmeter whose common lead is on the right-hand side of C9.[14] The expected voltages on every pin of the two regulators are tabulated on TRS-80 Model I Troubleshooting Guide. Finish with Radio Shack's burn-in for intermittent faults: a memory test run for several hours with the unit in its case.[15]

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References

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  1. ↑ Radio Shack, TRS-80 Micro Computer Technical Reference Handbook, second edition, second printing, 1982. Hosted here as File:Radio Shack TRS-80 Micro Computer Technical Reference Handbook (2nd edition, 1982).pdf (see Radio Shack TRS-80 Micro Computer Technical Reference Handbook). Page numbers on this page are the printed ones; the PDF page number is one higher.
  2. ↑ 2.0 2.1 2.2 2.3 Technical Reference Handbook, parts lists, pp. 94-95 (logic board, PCB 1700069, Level I) and p. 98 (keyboard PCB 1700070 and Level II kit).
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 Technical Reference Handbook, pp. 52-54, "System Power Supply": C8 filters about 20 V for the +12 V regulator; C11 and C15 are the +12 V output filter; C9 filters about 7 V for the +5 V regulator; C10 and C14 are the +5 V output filter; C1 filters about −11 V for the −5 V zener supply; C4 and C3 filter the −5 V output.
  4. ↑ Technical Reference Handbook, p. 42, "Video Mixing".
  5. ↑ 5.0 5.1 Technical Reference Handbook, TRS-80 schematic, sheet 2 (fold-out at the back of the handbook): C5 10 µF 16 V and C6 100 µF 16 V in the cassette input circuit around Z4 (LM3900), C6 on Z4 pin 14 through R31 (10 Ω).
  6. ↑ 6.0 6.1 Technical Reference Handbook, pp. 14-15, power-up and reset: C42 holds the CPU's reset input low for a few milliseconds after switch-on; reset switch S2 discharges C57, which is smaller than C42.
  7. ↑ Technical Reference Handbook, pp. 62-63, "Section Isolation Flowchart" and Figure 16, block 7.
  8. ↑ Ira Goldklang, "TRS-80 Troubleshooting Guide and Repair Q&A", TRS-80 Revived (retrieved 2026-10-21). Model III/4 answer on garbage at boot cleared by the reset button: replace the power-on reset timing electrolytic (C54 on the Model III). Community source.
  9. ↑ Technical Reference Handbook, p. 71, "Sync Generator": R20 and R21 fail with heat; C20 and C26 are usually dependable; C21 or C27 may short under impact stress.
  10. ↑ Technical Reference Handbook, p. 71, list of display faults: "Severe Display Interference".
  11. ↑ Technical Reference Handbook, p. 58, "Disassembly".
  12. ↑ Technical Reference Handbook, p. 66, "RAM" caution.
  13. ↑ 13.0 13.1 Technical Reference Handbook, p. 79, "The solder hair": cleaning only with a rubber eraser and light pressure; never steel wool or a sharp tool.
  14. ↑ Technical Reference Handbook, p. 60, "Power Supply Checks And Adjustments", and p. 75, Figure 19.
  15. ↑ Technical Reference Handbook, p. 76, "It Doesn't Work Sometimes".


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