Tandy 1000 TX Capacitor Replacement Guide

Schematic sheet 12 of the TX main board. The filter capacitors at the power connector J8 are at the bottom left, and the expansion bus decoupling capacitors C191, C194 and C263 below the slot connectors.

Tandy's Tandy 1000 TX Technical Reference Manual (catalogue no. 25-1514) has no parts lists, so the capacitor values for the Tandy 1000 TX main board below are read from the revision C schematic in the manual. The schematic gives values for every electrolytic but a voltage rating for only three of them. Read the rating printed on each part before ordering. See Capacitor Failure Symptoms for the signs of a failing part.

Power supply

The TX manual describes two 67 W supplies, single input and dual input, with the same operating characteristics, block diagram and circuit references as the supplies in the SX manual, but gives no parts list. The SX manual lists every capacitor for its supplies 8790085 and 8790084; the tables are on Tandy 1000 SX Capacitor Replacement Guide. Check the part number on your supply and the values printed on the capacitors before using those lists for a TX.

Main logic board

Electrolytic capacitors on the revision C schematic. Ratings are given only where the schematic prints one.

TX main board electrolytics (schematic D-8000288 revision C)

Ref	Capacitance	Rating	Position	Sheet
C100, C208	22 µF	not shown	+5 V filter at power connector J8	12
C210	10 µF	not shown	+5 V filter at J8	12
C202	22 µF	not shown	+12 V filter at J8	12
C205	22 µF	not shown	-12 V filter at J8	12
C220	22 µF	not shown	-5 V, output of regulator VR2 (79M05)	12
C191, C194	10 µF	16 V	Supply decoupling at the expansion connectors	12
C263	22 µF	16 V	Supply decoupling at the expansion connectors	12
C214	22 µF	not shown	+5 V at the parallel port pull-up network RP15	11
C135	10 µF	not shown	Power-on reset timing with R24	3
C177A	10 µF	not shown	Second reset timing with R47A (called C171A in the manual's text)	3
C258	10 µF	not shown	Output of regulator VR1 (78L05), composite video	6
C106	470 µF	not shown	+5 V filter for the sound chip U7, with choke L1	8
C102	10 µF	not shown	LM386 (U2) bypass, pin 7	8
C104	10 µF	not shown	Audio coupling to the LM386	8
C105	100 µF	not shown	LM386 output to the speaker	8
C217	10 µF	not shown	Audio coupling into the LM358 (U76)	8
C259	100 µF	not shown	LM358 audio output	8

For parts with no printed rating, match the marking on the old capacitor. The LM358 runs from +12 V and -12 V, so C217 and C259 should be rated well above 12 V; the same applies to C202 and C205 on the ±12 V rails.

C211 polarity

On one owner's TX, the 22 µF 16 V capacitor marked C211 at the rear right of the main board was fitted the opposite way to its silkscreen. He traced the positive side shown on the silkscreen to the ground plane, concluded that the silkscreen was wrong, and fitted the replacement the same way as the original. Another forum member cited Tandy 1000 Tech Bulletin No. 36, which puts this down to a silkscreen misprint on revision A and C boards. The revision C schematic shows C211 as a 0.1 µF part beside C210, so the board's markings and the schematic do not agree here. Before removing C211, note which way the old part is fitted and check with a meter which pad goes to ground; the negative lead goes to that pad.

Replacement procedure

1. Unplug the machine and remove the main board or the supply as needed.
2. Photograph the board and note the polarity of every electrolytic, paying particular attention to C211.
3. Replace the capacitors one at a time, matching capacitance and at least the original voltage rating.
4. Check the solder joints at J8 while the board is out.
5. Before refitting the supply, test it on a load, as described on Tandy 1000 TX Troubleshooting Guide.
6. Run the machine and check +5 V, +12 V and -12 V against the limits in General Maintenance.

Related pages

• Tandy 1000 TX
• Tandy 1000 TX General Maintenance
• Tandy 1000 TX Troubleshooting Guide
• Tandy 1000 SX Capacitor Replacement Guide
• Capacitor Failure Symptoms