Amiga 500 Audio Output Repair

The audio output stages on the Rev. 6A/7 board, from Commodore's schematic 312511-03, sheet 4. Right channel above, left below; Q301 and U38 at the lower left switch the filter.

The Amiga 500 produces stereo sound from Paula (U3): the right channel leaves pin 30 and the left channel pin 31. Each channel then passes through two sections of U14, a quad op-amp listed as an LF347 or TL084, a switchable low-pass filter, a 22 µF coupling capacitor, a 1 kΩ resistor and an EMI filter, and reaches the right RCA jack CN3 or the left jack CN4. Commodore's service manual gives an expected reading at each stage, so a dead or distorted channel can be traced to the part that has failed. The manual covers the Rev. 5 and Rev. 6A/7 boards; the differences are listed under board revisions.

Symptoms

• One channel silent, both silent, or one channel quiet or distorted. Commodore's audio checks start from either symptom and treat the two channels separately.
• Sound that is not smoothed by the filter. Sams Computerfacts describes the waveforms at the jacks looking "ragged" when the filter is not switching in, and Sams' preliminary service checks point at CIA U7, which drives the filter switch.
• Ian Perry's Amiga Troubleshooter chart lists, for problems on the left, right or both channels: the op-amp U14, the filter FETs Q331 and Q321, cold solder joints on the jacks CN3 and CN4, and the output filters EMI1303 and EMI1302.

If the machine does not reach the Workbench prompt at all, this is not an audio fault; start with the Amiga 500 Troubleshooting Guide.

How the circuit works

U14 runs from +12 V and −12 V through R301 and R302 (10 Ω). Paula takes +5 V through R309 (1 Ω), and R303 and R304 (1 kΩ each) divide it to make the reference Vref.

Commodore's schematic shows these stages for each channel (right channel designators first, left in brackets):

1. Paula's output passes through E305 (E306) to the inverting input of the first U14 section, pin 9 (pin 13). The non-inverting input, pin 10 (pin 12), sits at the reference voltage Vref. R331 (R321), 360 Ω, and C331 (C321), 0.1 µF, form the feedback network. The output is pin 8 (pin 14).
2. R332 and R333 (R322 and R323), 10 kΩ each, with C332 (C322), 6800 pF, and C333 (C323), 3900 pF, make a low-pass filter round the second section, input pin 5 (pin 3), output pin 7 (pin 1).
3. The JFET Q331 (Q321), an MPF102, switches C333 (C323) in or out. Its gate is driven from Q301, a 2N3906, which is driven by U38 (an MC1488) from the _LED signal. The schematic notes: "LED off, Filters bypassed".
4. The output passes through C334 (C324), 22 µF, in parallel with C335 (C325), 0.33 µF, then R334 (R324), 1 kΩ, and E303 (E302) to the jack CN3 (CN4). R335 (R325), 390 Ω, loads the output to audio ground.

U14 is powered from +12 V through R301 and −12 V through R302, both 10 Ω, at pins 4 and 11. Vref comes from R303 and R304, two 1 kΩ resistors across Paula's +5 V supply, which itself comes through R309, 1 Ω. Because the same U38 and the same −12 V rail serve the serial port, a missing −12 V affects both.

Tools and parts

• An oscilloscope. Commodore takes all its troubleshooting readings with one, to within ±10 %, and recommends a dual-trace scope if both channels have failed.
• A test signal. Commodore uses the Music icon on the Extras disk. Sams uses an AmigaBASIC program that plays a 1000 Hz tone on both channels (see Testing).
• A Torx T10 screwdriver for the case and top shield, and a Phillips screwdriver for the drive.
• Replacement parts as needed. Commodore's parts lists give U14 as an LF347 or TL084 (part 390086-01), Q321 and Q331 as MPF102 or PN4302 JFETs (390254-01), Q301 as a 2N3906 and U38 as an MC1488. The coupling capacitors C324 and C334 are 22 µF; the Rev. 6A/7 list rates them at 35 V (part 390101-04).
• A desoldering tool for U14 and the jacks. See Recommended Tools.

Procedure

Open the machine

Commodore's service parts reference diagram: the top case, keyboard, drive, top shield, main board and bottom shield in the order they come apart.

Sams' disassembly instructions:

1. Unplug the power supply and every cable.
2. Turn the machine over and remove the six T10 Torx screws along the front and rear edges of the bottom case. Turn it back and lift the top case off.
3. Disconnect the keyboard connector CN13 from the main board and the keyboard's ground lead from the disk drive. Lift the keyboard out.
4. Remove the four T10 Torx screws holding the top shield, straighten its four tabs and lift the shield off. U14 and the audio filter parts are now accessible.
5. To reach the solder side of the jacks, remove the drive (three Phillips screws from the bottom of the case, then the drive data and power connectors), release the latch at the front of the main board, lift the front of the board and slide it forward. Remove the capture nuts on the rear ports to separate the bottom shield.

Find the parts

The audio area of Commodore's Rev. 5 layout: U14 with Q321 and Q331 at the bottom, C324 and C334 below the audio jacks at the top.

On Commodore's Rev. 5 layout the audio section sits at the right of the board, marked "AUDIO FILTER". U14 stands between Q321 and Q331, with the resistors and capacitors of the filters in rows either side, and C324 and C334 sit just below the two audio jacks at the rear edge.

Trace the faulty channel

Play sound continuously and follow the channel with the scope. Commodore's steps and expected readings, all approximate:

Commodore's audio checks (Service Manual A500, section 8)

Step	Left channel	Right channel	Expect	If wrong
1	U3 pin 31	U3 pin 30	About 100 mV audio	U3, U14, E306 (E305)
2	U14 pin 13 (−)	U14 pin 9 (−)	About 100 mV	E306 (E305), U14 or the parts round it
3	U14 pin 14	U14 pin 8	About 1-2 V	U14 or the parts round it
4	U14 pin 3 (+)	U14 pin 5 (+)	About 1-2 V	R322, R323 (R332, R333)
5	U14 pin 1	U14 pin 7	About 1-2 V	U14 or the parts round it
6	EMI302 pin 1	EMI303 pin 1	About 100 mV	C324, C325, R324, R325, EMI302 (C334, C335, R334, R335, EMI303)
7	EMI302 pin 2	EMI303 pin 2	About 200 mV	EMI302 (EMI303), jack

Commodore's text names CN3 at step 7 of the left-channel checks; its schematic puts the left channel on CN4 and the right on CN3, so check the jack the channel actually goes to.

Sams uses its own test program and gives different levels: 0.8 V peak to peak at the jacks, and 3 V peak to peak at Paula pins 30 and 31 and at U14 pins 1 and 7. If Paula's outputs are missing, it points at U3; if they are present but U14's outputs are not, at U14 and the parts on pins 1 to 14, including Q321 and Q331. Use whichever set of figures matches your test signal.

Check the supplies and the filter switch

1. Measure U14 pin 4 (+AV) and pin 11 (−AV). They come from +12 V and −12 V through R301 and R302, so an open resistor or a missing rail kills both channels. Commodore's preliminary checks measure −12 V at pin 1 of U38.
2. Measure Vref at U14 pins 10 and 12, the junction of R303 and R304. With two equal 1 kΩ resistors across Paula's supply it should sit at about half of that supply. On an Amiga 600, whose LF347 has the same job, Pure Amiga normally measures this reference at about 2.3 to 2.5 V, and found it at 4.2 V when the op-amp had failed internally.
3. If the sound is ragged or unfiltered, check the switch chain while sound plays. Sams checks for pulses at pin 3 of CIA U7, then at pin 3 of U38, and then the parts round Q301, Q321 and Q331. On the schematic U7's PA1 output, _LED, also drives the power LED, so the state of the power LED shows whether the filter should be in.

Repair

• Reflow cracked joints on CN3 and CN4, which Perry lists among the causes of channel faults.
• Replace a failed coupling capacitor, C324 or C334, with a 22 µF part of at least the original 35 V rating, positive end towards U14 as drawn on the schematic. The values for every position are in the Amiga 500 Capacitor Replacement Guide.
• Replace U14 with an LF347 or TL084.
• Replace Q321 or Q331 if the filter does not switch on one channel only; replace Q301 or U38 if it fails on both.

Reassemble

Reverse Sams' removal steps: refit the bottom shield and capture nuts, slide the board back under its front latch, refit the drive and its three screws, the top shield with its four screws and tabs, the keyboard and its ground lead, and the top case with six T10 screws.

Testing

Sams' test program, typed into AmigaBASIC, plays a 1000 Hz tone on both channels:

10 POKE 12575233,3
20 POKE 12574721,2
30 POKE 12574721,0
40 SOUND 1000,10,255,0
50 SOUND 1000,10,255,1
60 GOTO 20

Expect about 0.8 V peak to peak at both jacks, with the filter on while the program runs. With Commodore's Music icon test, expect the readings in the table above. Listen to each channel separately through an amplifier.

Board revisions

Rev. 5 and Rev. 6A/7 differences in the audio section

Part	Rev. 5 (schematic 312511-02)	Rev. 6A/7 (schematic 312511-03)
Input filters	EMI305, EMI306, 100 pF	E305, E306, 270 pF
Output filters	EMI303, EMI302, 100 pF	E303, E302, 6800 pF
C325, C335	0.22 µF	0.33 µF
Paula +5 V feed	EMI301, drawn as a 1 Ω resistor	R309, 1 Ω
C307 (Paula supply)	100 µF	470 µF

U14, Q321, Q331, Q301, the 22 µF coupling capacitors and the resistor values are the same on both.

Related pages

• Amiga 500
• Amiga 500 Troubleshooting Guide
• Amiga 500 Maintenance Guide
• Amiga 500 Capacitor Replacement Guide
• Amiga 500 Service Manual (314981-04)
• Sams Computerfacts CSCS26 Amiga 500