Turn mono off before you condemn a second channel. On a 1970s stereo, mono mixes the two channels into one. A crackle that lived in one channel then comes out of both speakers. Wait on a recap, and wait on opening the case, until mono is off and the two channels have been listened to separately.
This happened on a Sony HP-510A. The right channel crackled on AM and FM. While the switches were being worked, both sides started crackling. Mono had been left on. With mono off, the left speaker played FM at full volume, on main and on remote. The right side still crackled on both of its outputs. The "worse" stereo was one switch.
The rest of the first pass, stylus, tape monitor, and speaker leads, is the crackle and no-music guide.
What mono did on this HP-510A
| Switch position | Left | Right |
|---|---|---|
| Mono left on | Crackle, copied from the right | Crackle |
| Mono off, FM | Clean, full volume | Crackle on every source |
| Main and remote, mono off | Both outputs work | Both outputs crackle |
Put the panel back to normal
Do this with the volume low.
- Mono off. This is the one that lied. One bad channel was playing through both speakers.
- Tape monitor on source. "Tape," with nothing connected, mutes the radio and the turntable together.
- Balance centered. A balance knob parked on the dead side makes a good speaker sound weak when it is the only one plugged in.
- Loudness off. Turn it back on later if you want it. It is not part of this test.
- Function selector in a detent. Halfway between AM and FM is not a station.
One speaker plug on this set was broken, so only one speaker was connected at a time. Move that speaker from the left terminals to the right. The crackle that stays with the channel, and leaves with the speaker, is in the amplifier. Tape the loose wires on a broken plug so they cannot touch each other or the chassis. A shorted output can take out the output transistors.
If one side is still bad
Mono off, and one channel still crackles on AM and FM, means the radio is not the fault. DeoxIT on the switches and controls is the next cheap check. On this unit it cleared the left channel and the phono input, and it changed nothing on the right. A crackle that ignores the volume knob is downstream of the volume control, in that channel's power amp. That diagnosis is the one-channel guide.
Both outputs dead on the bad side means the jacks and the speaker selector are not the open circuit. Those two jacks share one channel's output. The fault is further back. Getting to that board on this model is the case-opening guide.