TRS-80 Model 4P Capacitor Replacement Guide

Component layout of the 4P's 65 W power supply, Tandy 8790049 (board 8709365 Rev C), from page 163 of the service manual. The two 220 µF 200 V reservoir capacitors are C26 at the lower left and C29 at the bottom centre; the mains filter capacitors C30-C33 are to the right of C29, near the line choke T2.

The tables below list the electrolytic capacitors in the TRS-80 Model 4P: on the 65 W power supply, on both versions of the main logic board and on both versions of the 9-inch display. Every value comes from the parts lists in Tandy's TRS-80 Model 4P/4P Gate Array Service Manual (catalogue 26-1080, 1984). Tandy's part numbers code the type: numbers beginning 832 are radial electrolytics in these lists, and 8374104 is the 0.1 µF monolithic decoupling capacitor used throughout the logic boards.

Power supply 8790049

The 4P's supply is Tandy's own 65 W flyback design, on the underside of the top cover.

Mains-side capacitors

In Tandy's input diagram, C30-C33 and the choke T2 form the EMI filter ahead of the bridge rectifier: C30 and C31 run from the line to earth, C32 is across the line, and C33 is a transient bypass across the line that also terminates T2. C28 links the negative side of the rectified supply to earth.

Mains-side capacitors (Tandy parts list, p. 165)

Ref	Value	Rating	Type as listed	Tandy part no.
C28	4700 pF	125 V AC	Ceramic disc	8303475
C30	4700 pF	125 V AC	Ceramic disc	8303475
C31	4700 pF	125 V AC	Ceramic disc	8303475
C32	0.22 µF	125 V AC	Ceramic disc	8393432
C33	0.01 µF	250 V AC	Metal paper	8393106

The trs-80.com repair notes report that a 4P which pops and gives off smoke at switch-on has usually lost one of these; on Tandy's supply they name C32 and C33, and recommend X2-class 275 V AC film capacitors as replacements; capacitors rated for AC mains carry that rating, not a DC voltage. On a 4P fitted with an Astec supply instead, the notes name C1, C2 and C13.

Electrolytics

Power supply 8790049 electrolytics (Tandy parts list, p. 165)

Ref	Capacitance	Rating	Tandy part no.
C1	10 µF	35 V	8326103
C2	4.7 µF	50 V	8325474
C6	1 µF	50 V	8325014
C8	47 µF	25 V	8326472
C9	1 µF	50 V	8325014
C10, C11	2200 µF	10 V	8328224
C12	2200 µF	6.3 V	8328220
C14	100 µF	35 V	8327103
C16	2200 µF	16 V	8328221
C18	3300 µF	16 V	8328331
C19	100 µF	35 V	8327103
C20	100 µF	25 V	8327102
C23	470 µF	16 V	8327461
C25	2200 µF	16 V	8328221
C26	220 µF	200 V	8327226
C29	220 µF	200 V	8327226

All are listed as radial electrolytics. Tandy's troubleshooting chart names C10, C11, C12, C16, C18, C19, C20, C23 and C25 as the output filters to check when the supply cycles in current limit, C14 as the auxiliary supply capacitor, and C26 and C29 on the high-voltage side. C14 charges to about 31 V during start-up and runs at about 15 V; it is one of the parts Tandy's chart checks when the supply will not run at all.

Main logic board

Both main boards use 0.1 µF monolithic capacitors (8374104) for decoupling and a small number of radial electrolytics. The trs-80.com notes say the 4P's main board capacitors very rarely need replacing and advise leaving them alone unless one has an obvious fault.

Electrolytics on the original main board, PCB 8709427 (parts list pp. 79-80)

Ref	Capacitance	Rating	Tandy part no.
C131-C138 (eight)	22 µF	16 V	8326221
C157	33 µF	16 V	8326331
C161, C162	10 µF	16 V	8326101
C201	22 µF	16 V	8326221
C202, C204	10 µF	16 V	8326101

Electrolytics on the gate array main board (parts list pp. 137-138)

Ref	Capacitance	Rating	Tandy part no.
C201-C211 (eleven)	22 µF	16 V	8326221
C215	10 µF	16 V	8326101

The original board's parts list stops at C205. Its theory section names a power-on reset capacitor, C218, which does not appear in the list; on the gate array board the reset capacitor is C214, listed as a 0.022 µF ceramic disc. The trs-80.com notes describe the 4P's reset capacitor as usually a 22 µF 16 V electrolytic. Read the part on your board before ordering.

Display

The 9-inch display was built with a black and white tube (8790612) or a green tube (8790613). The two lists differ in one electrolytic, C212.

Electrolytics on the 9-inch display boards (parts lists pp. 181-188)

Ref	8790612 (black and white)	8790613 (green)	Tolerance
C101	4.7 µF 50 V	4.7 µF 50 V	20 %
C104	22 µF 100 V	22 µF 100 V	20 %
C207, C208	33 µF 16 V	33 µF 16 V	20 %
C209	330 µF 35 V	330 µF 35 V	20 %
C211	220 µF 16 V	220 µF 16 V	20 %
C212	100 µF 16 V	4.7 µF 50 V	20 %
C302	0.47 µF 50 V	0.47 µF 50 V	10 %
C307	4.7 µF 50 V	4.7 µF 50 V	20 %
C309	220 µF 16 V	220 µF 16 V	20 %
C313	1000 µF 25 V	1000 µF 25 V	20 %
C316	1 µF 250 V	1 µF 250 V	20 %
C322	10 µF 25 V	10 µF 25 V	20 %
C401	220 µF 16 V	220 µF 16 V	20 %

Both boards also carry three tantalum capacitors: C204, 0.33 µF 16 V, and C205 and C206, 4.7 µF 16 V. Discharge the tube before removing the display board; see CRT Discharge Procedure.

Procedure

1. Unplug the 4P and remove the case, rear mounting plate and top cover as described in the TRS-80 Model 4P General Maintenance guide. The power supply comes off the top cover with four screws once its three connectors are unplugged.
2. Check that C29 has discharged before handling the supply board.
3. Record the value, voltage and polarity of each part before removing it, and compare it with the tables; the reset capacitor in particular varies between sources.
4. Fit parts of the same capacitance and at least the listed voltage.
5. Refit the mylar insulator between the supply and the top cover.
6. Power up with the computer connected as the load and check the outputs against the table below. Set V1 to +5.05 V with R15.

Checks after the recap

Supply 8790049 outputs and ripple (Tandy specification)

Output	Nominal	Tolerance	Ripple (max.)
V1	+5.05 V	±3 %	50 mV p-p
V2, CRT	+12 V	±5 %	150 mV p-p
V3, disk	+12 V	±5 %	150 mV p-p
V4	−12 V	+25 %, −8.3 %	150 mV p-p

The supply should start within two to four seconds at 120 V AC. Ripple is a mix of 100 or 120 Hz from the mains and switching-frequency components; Tandy measures it through a 50 Ω coaxial cable terminated at the scope end.

Related pages

• TRS-80 Model 4P
• TRS-80 Model 4P General Maintenance
• TRS-80 Model 4P Troubleshooting Guide
• TRS-80 Model 4 Capacitor Replacement Guide
• Radio Shack TRS-80 Model 4P Service Manual
• Capacitor Failure Symptoms