One channel crackles on a vintage stereo: how to find the bad part

Find the one bad part. When one channel crackles on every source and cleaning has not helped, the fault is a single part in that channel's amplifier, usually a noisy transistor. Do not pull parts at random, and do not order a full set of replacement transistors.

This guide follows the right channel of a Sony HP-510A, an early-1970s all-in-one with tuner, amp, and turntable. The first round of diagnosis fixed the left channel and the phono input. The right channel was still crackling on every source. Each test below is listed with what it showed on this unit, so you can see what each result rules out.

The repair is still on the bench. This page will be updated with the part that turns out to be at fault.

Step 1: Prove it is the channel, not the speaker

Move the speaker that works onto the other channel's terminals. If the crackle stays on one side whichever speaker is plugged into it, the speaker is fine and the fault is in that channel. On this unit, the speaker played clean on the left and crackled on the right.

Then try both outputs on the bad side, main and remote. If both fail, the problem is upstream of where the signal splits to the two jacks, not one bad jack or the speaker selector.

A broken speaker plug needs care while you test. Running a solid-state amp with one channel unplugged is fine. Running it with a broken plug's two wires touching each other or the chassis is not, because a shorted output can burn out the output transistors. Tape the loose ends apart until the plug is replaced.

Step 2: Turn mono off, then use it as a test

Mono feeds both channels the same mixed signal. That makes it a test, and a trap.

• The trap. Leave mono on and the bad channel's crackle plays through both speakers. On this unit, mono was left on by accident, and the "fault" seemed to spread to the good channel. Set mono back to stereo before any other test.
• The test. With mono on, the bad channel receives the same signal that plays clean on the good one. If the bad channel still crackles, the fault is after the mono switch. That rules out the tuner, the phono stage, and the function selector.

Step 3: Clean first

Contact cleaner fixes more old-stereo faults than any part does, so clean before you condemn anything. Unplug the stereo, give each switch and control a short burst of DeoxIT D5, and work it back and forth 20 to 30 times.

On this unit, cleaning brought the left channel back to full, clean volume and made the phono input play again. The right channel was unchanged. When cleaning fixes the good channel and does nothing for the bad one, the bad channel's crackle is not dirt.

Step 4: The volume knob test

With a speaker on the bad channel, play the radio and turn the volume all the way down.

• The crackle fades or changes: it starts before the volume control, in the preamp or tone stages.
• The crackle stays exactly the same: it starts after the volume control, in that channel's power amp.

On this unit, the volume knob had no effect. That narrows the search to the right power amp: the output transistors on the heatsink and the small transistors that drive them.

Step 5: Check the output for DC

A fault in a power amp can put DC voltage on the speaker terminals, and DC can burn a speaker's voice coil. With the speaker disconnected and the stereo on, set a meter to DC volts and measure across the bad channel's speaker terminals. Near zero, under about 0.1 V, is safe. A volt or more means keep speakers off that channel until it is fixed.

Step 6: Find the bad channel's parts

On a crowded 1970s board, the wire from the speaker jack is often hidden. Three landmarks get you there without tracing it by eye.

• The speaker fuses. Each channel's output runs through its own fuse. Unplugged, with the meter on continuity, touch one probe to the bad channel's speaker jack and the other to each fuse clip. The one that beeps is that channel's fuse. Continue from its other clip to the parts near the heatsink: a large output capacitor or resistor, and the transistors beside it.
• The part numbers. Sony boards often number matching parts in pairs, one set for each channel, such as a C501 on one side and a C551 on the other. Matching numbers tell you which part on the bad channel corresponds to a part on the good one.
• The heatsink layout. This HP-510A has five identical transistors bolted to the heatsink, all marked "SONY D291." Each channel's power amp normally uses a pair, so the fifth is most likely the power supply's regulator. The supply is shared, and the left channel works, so that one is not the suspect.

Japanese part numbers usually drop the "2S" printed on the body, so "D291" is a 2SD291 and "A706" is a 2SA706. Transistors are the three-legged parts. They come in three sizes:

• Output transistors are bolted to the heatsink. On this unit they are 2SD291s, a Sony NPN power transistor in a TO-66 metal case (about 40 V, 3 A).
• Driver transistors feed the outputs. They are small square bodies with a metal tab sticking out of the top to shed heat. This unit has four: 2SA706 and 2SC1124 parts, most likely one pair per channel.
• Small-signal transistors are tiny black "D"-shaped bodies or small metal cans on three thin legs, marked with numbers like C632, C458, or A733. They can be tucked under shields or on another board.

Look at the legs in good light. Shiny silver is normal. Black or dark gray legs are the oxidation a former Sony service technician describes as a common cause of crackle on Sony sets from this period.

If a part has a slot or a small dial you could turn with a screwdriver, it is a trimmer, not a transistor. In a power amp, trimmers usually set the output transistors' idle current. Do not turn them.

Step 7: Tap, then freeze

Tap test. With the stereo playing at low volume, tap around the bad channel's parts with a wooden chopstick. A cracked solder joint usually makes the crackle jump or cut out. On this unit, tapping everything changed nothing, which makes a bad joint less likely and a noisy transistor the main suspect.

Freeze spray. A noisy transistor often changes its behavior when its temperature changes. Play the stereo at low volume with a speaker on the bad channel. Give one transistor a short burst through the straw, listen for a second or two, and wait about 30 seconds before the next part. Start with the bad channel's driver pair, then its output pair, then any small-signal transistors near them. The part that makes the crackle change, stop, or get worse is the one to replace.

What not to do

Do not pull transistors one at a time to see which one stops the crackle. In a power amp, a missing transistor can send the output to full DC or let the output transistors overheat. That can take out the output stage, which is the hardest part to replace, and the speaker with it. Old boards also lose copper pads each time a part is desoldered. Freeze spray answers the same question with nothing removed.

Keep your hands off the mains wiring, the power switch, the transformer, and the large filter capacitors whenever the unit is open. Those capacitors can hold a charge after it is unplugged.

Which of those parts is which, on this board, is the HP-510A transistor guide. The five cans on the heatsink are the outputs. The small gray parts are the drivers.

Replacing Sony driver and small-signal transistors

The 2SA706 and 2SC1124 drivers are on Audiokarma's long-running list of failure-prone transistors, flagged because their gain deteriorates with age. The same list flags Sony's small 2SC632 and 2SC458 for noise and static. The originals are out of production, but restorers have settled on current-production substitutes.

The older pick for the drivers was the KSA1220 and KSC2690. The KSA1220 is now obsolete, which is why restorers moved to the Toshiba pair.

Buy them from an authorized distributor. Mouser and DigiKey stock genuine Toshiba parts for about a dollar each. Anonymous marketplace listings for the same part numbers sell for many times that, and counterfeit transistors are a known problem in audio repair. A fake in a driver position can fail and take the output transistors with it. These are not affiliate links.

Before you solder anything in:

1. Replace the pair. If one driver in a channel is bad, replace both drivers in that channel. Many restorers do all four so both channels match.
2. Check the pinout. The 2SA706 and 2SC1124 have their pins in the order emitter, base, collector, with the base in the middle. None of the modern substitutes put the base in the middle, so two legs have to cross. Mark the emitter, base, and collector holes on the board from the original part, check the new part's datasheet, and put insulating sleeving on the crossed legs.
3. Plan for heat. The originals shed heat through their metal tab. The Toshiba parts are a different shape, so the original clip will not fit. A small bolt-on heatsink is a good idea.
4. Power up through a dim bulb tester. Then check for DC on the speaker output, and set the output transistors' idle current the way the service manual describes. This is the moment a mistake shows up, and the bulb is what protects the output transistors.

Once you find it

• A part reacts to freeze spray: write down its number, find the substitute above or in the Audiokarma list, and replace that part and its partner.
• Nothing reacts: compare DC voltages at matching transistors on the good and bad channels with the schematic. For the HP-510A, that is SAMS Photofact MHF-44. The first point where the two channels disagree is the fault.
• You would rather not solder: a vintage audio repair shop can usually find one noisy transistor quickly. Expect a bench fee of roughly $75 to $150.