Onkyo TX-SR508 Defective Channel Repair

Unplug the receiver and disconnect all speakers, sources, and antennas before opening the case.

Wear safety glasses while soldering and clipping leads.

Work in a ventilated area, and wash hands after handling leaded solder.

Use a Phillips screwdriver to remove the four side screws and three rear screws securing the top cover.

Lift the top cover off the receiver.

Set a multimeter to ohms and measure between the outer pins of every heatsink-mounted output transistor.

Don't let the probes touch the heatsink, or you'll measure the heatsink instead of the transistor.

Find any pair that reads very low or shorted.

The adjacent pair of shorted output transistors marks the defective channel.

Replace all parts in the defective channel repair kit, not only the shorted output transistors.

Have a right angle Phillips screwdriver ready because some output transistor screws are blocked by the chassis and heatsink.

Have thermal paste ready for the power transistors and the small thermally coupled transistors.

Use a bias measurement plug with two built-in resistors for the idle current adjustment socket on each channel.

Connect the multimeter to the plug leads during bias adjustment.

Use a Phillips screwdriver to remove the screws from the rear panel so the rear panel can come off.

Unplug the cable that runs to the rear panel area.

Cut or release the cable ties that block the main board.

Pull the flat ribbon cable straight out while holding the board firmly.

Remove the two screws from the cable clamp or bracket near the ribbon cable.

Remove the remaining screws securing the main board and nearby brackets.

Use needle nose pliers or sturdy tweezers to pinch the ears on each plastic standoff while lifting the board or chassis bridge.

Release both hidden standoffs in the tight rear area.

If a cable section prevents the board from lifting, remove the two screws holding that section and move it aside.

Use long nose pliers to release the lower hidden standoff under that section.

Work slowly so the plastic standoffs don't snap.

Lift the main board assembly out enough to access the solder side.

Set the board down on an ESD mat with support under the board.

Use a soldering iron and a desoldering pump, desoldering braid, or desoldering station to remove the affected channel parts.

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.

Cut stubborn transistor legs with side cutters before desoldering.

Pull the remaining leg pieces out one at a time with sturdy tweezers while heating each joint.

Use a right angle Phillips screwdriver to loosen the screws holding the two power transistors to the heatsink.

Remove the screws and lift the power transistors away.

Leave the clear mica insulators stuck to the heatsink if they stay there.

If a mica insulator sticks to a transistor, peel it off and reinstall it on the heatsink.

Apply thermal paste on both sides of any mica insulator that was removed.

Use a multimeter to check the removed resistors if you want to confirm the failure.

Bend one lead on each small replacement resistor so larger modern resistors can stand upright in the original footprint.

Build the shared load resistor from two separate 0.2 ohm resistors if the kit uses individual resistors.

Bend one lead from each 0.2 ohm resistor to meet as the shared center connection.

Route the two remaining leads down into the board holes, solder the shared joint at the top, and trim the extra lead length.

Apply a pea-sized amount of thermal paste to each heatsink and mica area where a power transistor sits.

Spread a thin layer over the contact area, including the upper tip of the insulator if needed.

Seat the new power transistors and reinstall their screws.

Tighten the screws light hand-tight only.

Insert the replacement resistors, capacitor, and small transistors into the affected channel.

Bend each lead outward after inserting a part so it stays in place when the board is turned over.

Match the electrolytic capacitor polarity to the board markings.

Follow the transistor outline printed on the board for each small transistor.

Align the broad flat side with the thicker flat mark on the silkscreen.

Turn the board over and install the remaining transistor from the solder side.

Orient its writing toward the heatsink, so the unmarked side faces you while you place it.

Trim its legs short before soldering so it sits neatly in the tight space.

Fill the gap with normal thermal paste before seating the transistor.

Solder one leg to lock its position, adjust the body, and solder the remaining legs.

Add solder from the top side if needed.

Solder every remaining lead and trim excess lead length.

Add thermal paste between the thermally coupled small transistors so the center transistor can sense heat from the neighboring transistor.

Use enough normal thermal paste to fill the gap.

Inspect the power transistor joints, shared 0.2 ohm resistor joint, all small resistors, all small transistors, and the capacitor.

Reinstall the main board, standoffs, brackets, flat ribbon cable, and rear panel.

Replace any cable ties that were removed if they won't block testing.

Cover the exposed mains input poles and nearby live area with insulating tape before plugging in the receiver.

Plug the receiver into the isolation transformer or a protected outlet only after the hazardous area is covered.

Turn the receiver on and confirm it stays on, the relays click, and the standby LED no longer blinks.

With the receiver plugged in but switched off, insert the bias measurement plug into the socket for the repaired channel.

Connect the multimeter to the plug leads and set it to the millivolt range.

Turn the receiver on and wait for the speaker relays to click.

Turn the channel bias trimmer slowly until the reading begins to rise.

Start around 3 mV to 4 mV, let the receiver warm for about 5 minutes, and watch the reading.

For this TX-SR508 surround channel, adjust the warmed reading to 6 mV.

Let the bias reading stabilize and make a small correction if needed.

Connect a signal source and a speaker.

Play audio through each channel and confirm the repaired channel and all other channels sound clean.

Install the remaining rear panel screws and top cover after testing.