TRS-80 Model 4D Capacitor Replacement Guide
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The electrolytic capacitors in the TRS-80 Model 4D are on the gate array main board, in the power supply and monitor, and on the two TEC drives. Tandy's 4D addendum changes only the drives, keyboard and two small ceramic capacitors on the main board, so the power supply is that of the gate array TRS-80 Model 4.[1] See Capacitor Failure Symptoms for the signs of a failing part.
 | The power supply's reservoir capacitors sit on the rectified mains. Unplug the machine, let the supply stand, and measure across them before touching the board. |
Main board
[edit | edit source]The gate array board's electrolytics, from the schematic in Howard W. Sams' Computerfacts CS13 for the gate array Model 4:[2]
| Ref | Capacitance | Position |
|---|---|---|
| C3 | 10 µF | Supply to the phase-locked clock U8 |
| C4 | 22 µF | +5 V rail |
| C5, C7 | 22 µF | −12 V rail |
| C6, C10 | 22 µF | +12 V rail |
| C25 | 22 µF | Power-on reset timing |
| C126, C128, C129, C130, C131, C132 | 10 µF | +5 V rail |
Sams gives capacitance only, except for C3, which its parts list gives as 10 µF 20 V; read the voltage from the part fitted.[2] The addendum's two changes for the 4D are ceramic: C2 is 47 pF 50 V NPO (Tandy 8300473) and C20 is 33 pF 50 V NPO (8300335), the load capacitors on the master clock crystal.[1][2]
Disk drives
[edit | edit source]The manufacturer's parts list for the TEC drive assembly (Tandy 8790133), bound into the addendum, names five electrolytics:[3]
| Ref | Capacitance | Rating | TEC part no. |
|---|---|---|---|
| C14, C16 | 10 µF | 16 V | EBB00-53800 |
| C17 | 1 µF | 50 V | EBB00-34700 |
| C26 | 47 µF | 16 V | EBB00-34800 |
| C28 | 100 µF | 6.3 V | EBB00-34900 |
The same list gives the drive's ceramic capacitors and its one trimmer potentiometer, VR1.[3] After work on a drive, check the motor speed and read-data adjustments in the General Maintenance guide.
Power supply and monitor
[edit | edit source]These are as in the gate array Model 4.[1] Sams lists Tandy's 65 W supply as the gate array Model 4's supply, with the Astec 38 W and 65 W supplies as alternatives.[2] Check which is fitted, then use:
- Tandy 65 W supply 8790049, in the Model 4P guide
- Astec supplies, in the Model 4 guide
The mains filter capacitors are the parts the trs-80.com notes report failing: C1, C2 and C13 on Astec supplies, C32 and C33 on Tandy's.[4]
None of the documents found lists the parts in the 4D's green-screen monitor. The monitor lists in the Model 4 guide are for the RCA and TCE monitors of the original Model 4.
Related pages
[edit | edit source]- TRS-80 Model 4D
- TRS-80 Model 4D General Maintenance
- TRS-80 Model 4D Troubleshooting Guide
- TRS-80 Model 4 Capacitor Replacement Guide
- Capacitor Failure Symptoms
References
[edit | edit source]- ↑ 1.0 1.1 1.2 Tandy Corporation, Addendum to the Model 4 and Model 4 Gate Array Service Manual (Cat. No. 26-1067/8/9) for Model 4D (Cat. No. 26-1070), 1985, hosted as File:Tandy TRS-80 Model 4D Service Manual Addendum (26-1070, 1985).pdf. Introduction and parts list differences, PDF pages 5-6.
- ↑ 2.0 2.1 2.2 2.3 Howard W. Sams & Co., Computerfacts CS13, Radio Shack TRS-80 Model 4 Gate Array, 1986, archive.org scan. System board schematic, pp. 2-7 and 50-57; replacement parts list.
- ↑ 3.0 3.1 A&A Japan Ltd., Whole Parts List for catalogue no. 8790133 (manufacturer TEC), 5 August 1985, in the Addendum ... for Model 4D, pages 11-12 of 13 (PDF pages 121-122).
- ↑ Ira Goldklang, "TRS-80 Troubleshooting Guide and Repair Q&A", Ira Goldklang's TRS-80 Revived Site (retrieved 2026-10-21). "Model 4P" section.