Tandy 1000 Capacitor Replacement Guide

The 54 W supply board 8790070. The large electrolytics C11 to C19 stand in two rows on the secondary side; C4 is the primary reservoir at the left.

Every capacitor value in the original Tandy 1000 (catalogue number 25-1000) is published. Radio Shack's 1985 service manual has parts lists for the main logic board and the 54 W power supply, and its appendix reprints TEAC's parts list for the FD-54 floppy drive control board. The power supply holds most of the electrolytics and is the place to start. See Capacitor Failure Symptoms for the signs of a failing part.

Power supply 8790070

The table lists the electrolytics in Tandy's parts list with their place on Tandy's schematic.

54 W supply electrolytics (parts list p. 100)

Ref	Capacitance	Rating	Tandy part no.	Position on the schematic
C4	47 µF	200 V	CC476MPAP	Primary reservoir after bridge D1
C11, C12	10000 µF as printed (see note)	35 V	CC108MGAP	+12 V, ahead of regulator IC1
C13	1000 µF	16 V	CC108MDAP	+12 V output, after IC1
C14	470 µF	35 V	CC477MGAP	-12 V, ahead of regulator IC2
C15	470 µF	25 V	CC477MEAP	-12 V output, after IC2
C16, C17, C18	4700 µF	10 V	CC478MCAP	+5 V, ahead of choke L2
C19	2200 µF	10 V or 16 V	CC228MDAP	+5 V output, after L2
C20	see note	50 V	CC106MJAP	+5 V sensing circuit at IC3 and VR1

Two lines in the list need care. C11 and C12 are described as 10000 µF, but their part number differs from C13's (1000 µF 16 V) only in the voltage letter. Throughout this list Tandy's part numbers give the value in the same way: CC476 is 47 µF, CC477 470 µF, CC478 4700 µF, CC228 2200 µF, and the letters C, D, E, G, J and P match 10, 16, 25, 35, 50 and 200 V. By that pattern CC108MGAP is 1000 µF 35 V. The description of C20 reads only "Electrolytic 10WV 50WV"; its part number CC106MJAP reads as 10 µF 50 V by the same pattern. The part number for C19 (D, 16 V) agrees with the 16 V option. Read the markings on your own board before ordering for C11, C12 and C20.

The other capacitors on the supply are film and ceramic parts: C1 and C2 (0.1 µF 250 V AC film, across the mains), C3 (10000 pF 400 V AC ceramic), C5 (0.22 µF film), C6 (2200 pF 400 V AC), C7 and C21 (10000 pF 1 kV), C8 (1500 pF 2 kV), C9 (0.047 µF film) and C10 (0.01 µF film). C1 to C3 are the mains filter, with L1. If C1 or C2 is replaced, use an X-class safety capacitor rated for the mains.

Main logic board

Main logic board layout. C13 is near the ROM at the top left; C101 to C108 are below the centre.

The main board has eleven polarised capacitors. The rest are ceramic decoupling and timing parts and one 0.022 µF film capacitor (C6).

Main logic board electrolytic and tantalum capacitors (parts list pp. 94-96)

Ref	Capacitance	Rating	Type as listed	Tandy part no.
C13	22 µF	16 V	Axial electrolytic	8316221
C26	10 µF	16 V	Axial electrolytic	8316101
C51	22 µF	16 V	Axial electrolytic	8316221
C82	22 µF	16 V	Axial electrolytic	8316221
C85A	10 µF	16 V	Axial electrolytic	8316101
C101	2.2 µF	16 V	Axial electrolytic	8315221
C103	2.2 µF	16 V	Axial electrolytic	8315221
C104	22 µF	16 V	Axial electrolytic	8316221
C107	10 µF	16 V	Axial electrolytic	8316101
C108	10 µF	16 V	Radial tantalum	8336101
C124	100 µF	16 V	Radial tantalum	8337100

C13 appears twice in the printed list, both times as 22 µF 16 V. C26 is the reset capacitor: the RES* input of the 8284A clock generator comes from C26, which the reset button or diode CR6 discharges. The board's axial electrolytics lie flat, so mark the polarity on the board before removing each one.

Keyboard

The manual's keyboard parts list names only assemblies. Its Figure 23 shows C1, 4.7 µF, across the +5 V supply at the keyboard connector, with 0.033 µF capacitors C2, C4, C5 and C6.

Floppy drive

The service manual's appendix is TEAC's FD-54B specification, Revision C, with the parts list for the drive's control board, PCBA MFD Control (C).

TEAC FD-54 control board polarised capacitors (Table 504)

Ref	Capacitance	Rating	Type (TEAC code)
C2	100 µF	10 V	Aluminium electrolytic (CE)
C24	47 µF	25 V	Aluminium electrolytic (CE)
C6, C8	22 µF	16 V	Aluminium electrolytic (CE)
C7, C9	15 µF	20 V	Tantalum (CS)
C33, C34	47 µF	16 V	Tantalum (CS); not fitted on some versions
C36	2.2 µF	16 V	Tantalum (CS)
C17	1 µF	16 V	Tantalum (CS)
C5	0.33 µF	16 V	Tantalum (CS)
C13, C14	0.22 µF	35 V	Tantalum (CS)
C11, C12	0.15 µF	35 V	Tantalum (CS)

TEAC notes that parts marked with a double asterisk, such as C33 and C34, are not used on some versions of the board. Check a drive of another make against its own documentation.

Replacement procedure

1. Unplug the machine and remove the supply as described in General Maintenance.
2. Discharge and measure C4 before touching the board.
3. Photograph the board and note the polarity of every electrolytic.
4. Replace the capacitors one position at a time, matching capacitance and at least the original voltage rating. Use 105 °C low-ESR parts for C11 to C19 on the secondary side.
5. Check the solder joints on the output and drive power connectors while the board is out; a cracked joint on the +12 V drive pin has been reported on this supply.
6. Test the supply on a load before refitting it. Tandy's test calls for a minimum load of 2.0 A on +5 V and 0.2 A on +12 V, and warns that the supply must never run without a load.
7. At full load and 120 V AC, adjust VR1 for 5.05 V on the +5 V output, then check all three outputs against the limits in General Maintenance.

Related pages

• Tandy 1000
• Tandy 1000 General Maintenance
• Tandy 1000 Troubleshooting Guide
• Tandy 1000 Documentation
• Capacitor Failure Symptoms
• Floppy Drive Repair