Tandy 1000 RL Capacitor Replacement Guide

A Tandy 1000 RL (photo: Plasticboob, CC BY-SA 3.0).

Capacitor work on the Tandy 1000 RL and Tandy 1000 RL-HD centres on two things: the surface-mount tantalum capacitors on the main board, which can fail short and take the machine down, and the electrolytics in the small fanless power supply. Tandy's service manual for the RL (25-1450) could not be obtained. The values below come from Tandy National Parts' 1995 bill of materials, which merges every assembly into one list, so check each value against the marking on the part before ordering. The same procedure applies to the Tandy 1000 RLX; its values are on Tandy 1000 RLX Capacitor Replacement Guide.

Shorted tantalum capacitors

Symptoms

One owner's RL gave a wailing tone that died away, followed by a smell of burning, the first time he switched it on. The cause was a surface-mount tantalum capacitor, C98, that had failed short and burnt. daver2 explains that a tantalum shorted across a supply rail pulls that rail down, and that vintage tantalums are prone to this failure.

Finding the short

keenerb's checklist, from his working RL:

1. Unplug the supply from the main board. Check the main board's +12 V and +5 V pins for a short to ground; the black wires are ground.
2. With the supply unplugged from the main board, check its output voltages.
3. Before reconnecting, remove the riser board that carries the volume control and joystick connectors, since it can fail on its own.
4. Disconnect the floppy drive. A non-Tandy drive on the RL's floppy cable shorts +12 V to ground, because the cable carries power.
5. Check fuse F1, near the back of the board, for continuity.
6. Check each surface-mount tantalum for a short. There are about a dozen black rectangular parts; measure across each one. A good capacitor reads low at first and then climbs towards open circuit; one that stays low, or reads open from the start, is faulty.

For comparison, keenerb measures 67 ohms between +5 V and +12 V on the drive power connector of his working RL's supply.

Replacing a tantalum

• Read the value from the part or the board. The burnt C98 in the forum case was marked "10 16v", 10 µF 16 V; the board shows the polarity. Tandy's parts list gives C98A as 22 µF, so the marking on the board is the better guide.
• keenerb suggests removing a shorted tantalum and trying the machine without it, which he does not expect to cause harm in the short term.
• Fit a replacement of the same capacitance and at least the same voltage, with the positive end where the board marks it.

Tantalums in the parts list

Tandy's parts list gives the capacitors below the type code TNP. C98, the capacitor that failed in the forum case, is listed among them (as C98A), and owners identified it as a tantalum.

Type TNP capacitors in Tandy's RL parts list

Value	References
10 µF 16 V	C96, C97, C109, C173, C174, C177, C188
22 µF 16 V	C8, C9, C10, C58A, C80A, C98A, C107A, C108, C111
3.3 µF 16 V	C168, C170, C172
1 µF 16 V	C103A

Source: Tandy bill of materials 250-1450. The list merges assemblies, so a reference may belong to the main board, the riser board or the floppy drive.

Power supply

The RL's supply is rated at 25 W and has no fan. The only aluminium electrolytic in the RL parts list is C7, 150 µF 200 V (type code ALP); the supply's other capacitors are not listed, so read their values from the board.

Power conversion parts in Tandy's RL parts list

Reference	Part
C7	150 µF 200 V
T1	Converter transformer TB0337
Q1	MJF13007 transistor
D5	MBR745 rectifier
D7-D10	1N5395 rectifiers
SC1	2N5060 SCR
IC1	TL431C shunt regulator
CP1	4N35 optocoupler
R6	3.3 ohm 2 W fusible resistor
F1	1 A 125 V fast-blow fuse

Source: Tandy bill of materials 250-1450. The RLX and RLX/HD lists give the same parts, which points to a shared supply design. An owner has also used an RL supply as a drop-in replacement in a Tandy 1000 RSX.

Output connector

The supply feeds the main board through a single-row six-pin connector. An RLX/HD owner reads the label inside his supply as:

Pin	Output	Rated current
1	-12 V	0.11 A
2	+12 V	0.6 A
3, 4	0 V	-
5, 6	+5 V	3.3 A

Another owner, working from the wire colours on an RL or RLX, reads the pins from the red wire to the white as +5 V, +5 V, ground, ground, +12 V and a white wire he took to be -12 V. Plasma identifies the main board connector as Molex 41790 series, with mating housing 09-50-3061. Confirm each pin with a meter before connecting a replacement supply.

Replacing supply capacitors

1. Unplug the machine and remove the supply.
2. Photograph the board and note each capacitor's value, voltage and polarity before removing anything.
3. Replace electrolytics with parts of the same capacitance and at least the same voltage, keeping the polarity.
4. Check C7 (150 µF 200 V) for bulging or leakage.
5. Refit the supply and measure the outputs against the table above with the main board connected.

Related pages

• Tandy 1000 RL
• Tandy 1000 RL-HD
• Tandy 1000 RL General Maintenance
• Tandy 1000 RL Troubleshooting Guide
• Tandy 1000 RLX Capacitor Replacement Guide