Onkyo TX-SR508 Defective Channel Repair

ID: 218690

Description: [youtubevideo|Aj_1SKS06Yc]Use this guide to...

Steps:

1. Unplug the receiver and disconnect all speakers, sources, and antennas before opening the case.
2. WARNING: The power supply carries lethal mains voltage, and large capacitors can hold charge after the cord is unplugged.
3. Do live testing and bias adjustment only if you're trained to work on powered electronics.
4. Use an isolation transformer for powered tests, but don't treat it as full protection from shock.
5. Wear safety glasses while soldering and clipping leads.
6. Work in a ventilated area, and wash hands after handling leaded solder.
7. Use a Phillips screwdriver to remove the four side screws and three rear screws securing the top cover.
8. Lift the top cover off the receiver.
9. Keep the screws organized for reassembly.
10. Similar Onkyo models may have minor screw or board differences.
11. Set a multimeter to ohms and measure between the outer pins of every heatsink-mounted output transistor.
12. A normal output transistor reads in the megaohm range; good transistors on this unit read about 20 megaohms depending on probe polarity.
13. Don't let the probes touch the heatsink, or you'll measure the heatsink instead of the transistor.
14. Find any pair that reads very low or shorted.
15. The adjacent pair of shorted output transistors marks the defective channel.
16. The same channel often fails on TX-SR507 and TX-SR508 receivers, but test every channel instead of assuming the location.
17. Replace all parts in the defective channel repair kit, not only the shorted output transistors.
18. A shorted output pair stresses the driver transistors, nearby transistors, load resistor, four small resistors, and electrolytic capacitor.
19. A typical channel kit includes six transistors, two 0.2 ohm load resistors, four small resistors, and a 47 uF 50 V capacitor.
20. Some stressed driver transistors can pass basic hFE tests but fail at higher voltage, causing poor sound or only one half-wave.
21. Use the exact parts for the channel and model, because similar Onkyo models may use different values.
22. Have a right angle Phillips screwdriver ready because some output transistor screws are blocked by the chassis and heatsink.
23. Have thermal paste ready for the power transistors and the small thermally coupled transistors.
24. Use a bias measurement plug with two built-in resistors for the idle current adjustment socket on each channel.
25. Connect the multimeter to the plug leads during bias adjustment.
26. Use a Phillips screwdriver to remove the screws from the rear panel so the rear panel can come off.
27. Unplug the cable that runs to the rear panel area.
28. Keep all screws, and separate machine screws that thread into metal from self-tapping screws that cut into plastic.
29. During reassembly, install machine screws into metal threads and self-tapping screws into plastic posts.
30. Cut or release the cable ties that block the main board.
31. Pull the flat ribbon cable straight out while holding the board firmly.
32. The ribbon cable is only press-fit into a spring connector and isn't latched.
33. Caution: Don't yank the ribbon cable at an angle or tear it.
34. Reinstall the ribbon cable by pressing on its stiff backing until it seats in the connector.
35. Remove the two screws from the cable clamp or bracket near the ribbon cable.
36. One screw is above the bracket, and one screw is below it.
37. Remove the remaining screws securing the main board and nearby brackets.
38. Check for hidden screws if the board still feels attached.
39. Use needle nose pliers or sturdy tweezers to pinch the ears on each plastic standoff while lifting the board or chassis bridge.
40. Release both hidden standoffs in the tight rear area.
41. These two clips are the hardest ones to reach.
42. If a cable section prevents the board from lifting, remove the two screws holding that section and move it aside.
43. Use long nose pliers to release the lower hidden standoff under that section.
44. Work slowly so the plastic standoffs don't snap.
45. Lift the main board assembly out enough to access the solder side.
46. Set the board down on an ESD mat with support under the board.
47. Caution: Don't rest the board on tall components, connectors, or transistor leads.
48. The main goal is underside access to the affected channel parts.
49. Use a soldering iron and a desoldering pump, desoldering braid, or desoldering station to remove the affected channel parts.
50. Desolder the leads for the two power transistors, the small transistors, the load resistor area, the four small resistors, and the 47 uF 50 V capacitor.
51. Cut stubborn transistor legs with side cutters before desoldering.
52. Pull the remaining leg pieces out one at a time with sturdy tweezers while heating each joint.
53. Caution: Avoid overheating the board. Lifted traces in this area can usually be repaired because the traces are visible.
54. Use a right angle Phillips screwdriver to loosen the screws holding the two power transistors to the heatsink.
55. Remove the screws and lift the power transistors away.
56. Leave the clear mica insulators stuck to the heatsink if they stay there.
57. If a mica insulator sticks to a transistor, peel it off and reinstall it on the heatsink.
58. Apply thermal paste on both sides of any mica insulator that was removed.
59. Caution: Replace cracked or damaged mica insulators before powering the receiver.
60. Use a multimeter to check the removed resistors if you want to confirm the failure.
61. A good 0.2 ohm load resistor section reads about 0.2 ohm plus multimeter lead resistance, and an open section is bad.
62. A failed 2.2 ohm resistor may read around 1 megaohm, and a failed 100 ohm resistor may read around 180 kiloohms.
63. A 47 uF 50 V capacitor may still test near 45.7 uF, but replace it because the short can overstress it above its rating.
64. Don't trust a basic component tester alone for driver transistors.
65. An oscilloscope component tester can reveal reverse breakdown that a simple hFE test misses.
66. Bend one lead on each small replacement resistor so larger modern resistors can stand upright in the original footprint.
67. Build the shared load resistor from two separate 0.2 ohm resistors if the kit uses individual resistors.
68. Bend one lead from each 0.2 ohm resistor to meet as the shared center connection.
69. Route the two remaining leads down into the board holes, solder the shared joint at the top, and trim the extra lead length.
70. Apply a pea-sized amount of thermal paste to each heatsink and mica area where a power transistor sits.
71. Spread a thin layer over the contact area, including the upper tip of the insulator if needed.
72. Seat the new power transistors and reinstall their screws.
73. Tighten the screws light hand-tight only.
74. Check the other channel transistor screws while the receiver is open.
75. Caution: Don't overtighten the screws, because the heatsink is aluminum and the threads can strip.
76. Insert the replacement resistors, capacitor, and small transistors into the affected channel.
77. Bend each lead outward after inserting a part so it stays in place when the board is turned over.
78. Match the electrolytic capacitor polarity to the board markings.
79. The stripe on the capacitor marks negative, and the board marks the negative pad with a white ring.
80. On this TX-SR508 channel, the capacitor negative side faces the heatsink.
81. Verify capacitor polarity against the board markings and nearby capacitors before soldering.
82. Follow the transistor outline printed on the board for each small transistor.
83. Align the broad flat side with the thicker flat mark on the silkscreen.
84. The side with writing is the broad flat side.
85. On this channel, one transistor's writing faces left.
86. Don't swap the two driver transistors; install each labeled part in its matching position.
87. Leave the transistor that's difficult to access from above out until the board is turned over.
88. Turn the board over and install the remaining transistor from the solder side.
89. Orient its writing toward the heatsink, so the unmarked side faces you while you place it.
90. Trim its legs short before soldering so it sits neatly in the tight space.
91. Fill the gap with normal thermal paste before seating the transistor.
92. Solder one leg to lock its position, adjust the body, and solder the remaining legs.
93. Add solder from the top side if needed.
94. Solder every remaining lead and trim excess lead length.
95. Add thermal paste between the thermally coupled small transistors so the center transistor can sense heat from the neighboring transistor.
96. Use enough normal thermal paste to fill the gap.
97. Adhesive thermal compound isn't required if the transistor rests in place.
98. Inspect the power transistor joints, shared 0.2 ohm resistor joint, all small resistors, all small transistors, and the capacitor.
99. Fix any solder bridges or dull joints before reassembly.
100. Reinstall the main board, standoffs, brackets, flat ribbon cable, and rear panel.
101. Replace any cable ties that were removed if they won't block testing.
102. Don't reinstall the top cover until bias adjustment is complete.
103. Caution: Don't power the receiver with loose boards lying on each other.
104. A loose HDMI board can sit on the mains power board and create a dangerous short.
105. Install at least the rear panel screws for the HDMI sockets and RCA jacks before testing.
106. The rear panel provides the ground connection between the HDMI board, intermediate board, and lower board.
107. Without that ground, the receiver can return to protect mode even if the channel repair is correct.
108. Cover the exposed mains input poles and nearby live area with insulating tape before plugging in the receiver.
109. WARNING: Even with an isolation transformer, touching the mains poles or underside power supply can be fatal.
110. Keep hands away from the underside of the board and all power supply areas during powered tests.
111. Plug the receiver into the isolation transformer or a protected outlet only after the hazardous area is covered.
112. Turn the receiver on and confirm it stays on, the relays click, and the standby LED no longer blinks.
113. If it returns to protect mode, unplug it and look for missing grounds, soldering errors, or additional faults.
114. With the receiver plugged in but switched off, insert the bias measurement plug into the socket for the repaired channel.
115. Connect the multimeter to the plug leads and set it to the millivolt range.
116. The plug contains two resistors, so the multimeter reads voltage across the resistors while you adjust bias current.
117. Keep speakers disconnected and leave the channel with no signal during the adjustment.
118. Place the multimeter where it can be read without reaching over the power supply.
119. Turn the receiver on and wait for the speaker relays to click.
120. If the meter reads 0 mV, the bias trimmer may be turned fully down.
121. Turn the channel bias trimmer slowly until the reading begins to rise.
122. Start around 3 mV to 4 mV, let the receiver warm for about 5 minutes, and watch the reading.
123. For this TX-SR508 surround channel, adjust the warmed reading to 6 mV.
124. Other channel groups can use 9 mV, so check the correct service information or compare with a known-good channel before setting a different channel.
125. Caution: Keep the reading from climbing too high; excessive bias can destroy the amplifier again.
126. Let the bias reading stabilize and make a small correction if needed.
127. Unplug the receiver before removing the measurement plug or changing internal connections.
128. Connect a signal source and a speaker.
129. Play audio through each channel and confirm the repaired channel and all other channels sound clean.
130. Install the remaining rear panel screws and top cover after testing.