Thought I'd post my success in fixing the sound issue I've been having on a Quadra 700 that I picked up recently, in case it helps someone else.
The symptom was that the Quadra would produce a normal startup chime after being powered off for a while. However, if it was restarted there would be no chime and no system sound. If it was powered off and back on again after only a short time, the startup chime would be badly distorted.
The first step was a visual inspection. The logic board was dirty with spider webs and had some strange residue patterns, so I gave it a thorough clean. I also recapped the PSU since it was probably due anyway; there was capacitor leakage and a few capacitors measured out of spec. Unfortunately, neither of those fixed the problem.
At that point it seemed like a good idea to start working through the available schematic and tracing the audio circuitry.
The first thing I did was set up a repeatable test and make probing easier. I discovered I could repeatedly press the reset button just after power on to replay the startup chime over and over until it faded away. The longer the machine had been powered off, the longer the chime would continue before failing.
Probing the logic board while it was installed in the case wasn't practical, so I made a power harness and powered the board directly from my three channel bench power supply. Only +5 V, +12 V and -12 V were required. This gave me plenty of room to probe the board and made it easy to repeatedly power cycle it. (Photo Attached)
After some initial investigation, checking for shorts and measuring resistances, I noticed that the "Sporty" sound chip (U9) and resistor R3 would heat up to over 70 °C, too hot to touch.
Using an oscilloscope, I could see that audio was still present on AINL and AINR, even when there was no sound from either the speaker or headphone jack. That suggested everything upstream, including the TDA1543 DAC, was still functioning correctly.
Probing the outputs HPOL and HPOR, I could see the audio waveform initially, but after a short time it disappeared and the outputs would sit at approximately +5 V DC. At first glance this made it look like the Sporty chip itself had failed, which wasn't an encouraging thought given how difficult those chips are to replace.
After probing further, I found that the DC offset was also appearing on AINL, while OFFADJ remained stable at around 2.2 V. Since OFFADJ provides the reference point for the waveform, AINL drifting up towards +5 V effectively drove the input hard against the positive rail. That also explained why the Sporty chip and R3 were getting so hot.
At that point I had two theories: either the TL064 opamp (U8) had developed a fault and was driving the signal incorrectly, or the Sporty chip itself was somehow pulling the line high. Since a TL064 costs only a few cents from DigiKey, I replaced that first.
Replacing U8 fixed the problem.
This also demonstrated just how resilient the Sporty chip is. Despite running extremely hot due to the fault, it survived without damage.
The hard drive had also failed, so I replaced it with a SCSIknife. The floppy drive was seized solid and full of dust, but after cleaning and lubrication, it now works too.
So the Quadra 700 is now fully functional again.
The symptom was that the Quadra would produce a normal startup chime after being powered off for a while. However, if it was restarted there would be no chime and no system sound. If it was powered off and back on again after only a short time, the startup chime would be badly distorted.
The first step was a visual inspection. The logic board was dirty with spider webs and had some strange residue patterns, so I gave it a thorough clean. I also recapped the PSU since it was probably due anyway; there was capacitor leakage and a few capacitors measured out of spec. Unfortunately, neither of those fixed the problem.
At that point it seemed like a good idea to start working through the available schematic and tracing the audio circuitry.
The first thing I did was set up a repeatable test and make probing easier. I discovered I could repeatedly press the reset button just after power on to replay the startup chime over and over until it faded away. The longer the machine had been powered off, the longer the chime would continue before failing.
Probing the logic board while it was installed in the case wasn't practical, so I made a power harness and powered the board directly from my three channel bench power supply. Only +5 V, +12 V and -12 V were required. This gave me plenty of room to probe the board and made it easy to repeatedly power cycle it. (Photo Attached)
After some initial investigation, checking for shorts and measuring resistances, I noticed that the "Sporty" sound chip (U9) and resistor R3 would heat up to over 70 °C, too hot to touch.
Using an oscilloscope, I could see that audio was still present on AINL and AINR, even when there was no sound from either the speaker or headphone jack. That suggested everything upstream, including the TDA1543 DAC, was still functioning correctly.
Probing the outputs HPOL and HPOR, I could see the audio waveform initially, but after a short time it disappeared and the outputs would sit at approximately +5 V DC. At first glance this made it look like the Sporty chip itself had failed, which wasn't an encouraging thought given how difficult those chips are to replace.
After probing further, I found that the DC offset was also appearing on AINL, while OFFADJ remained stable at around 2.2 V. Since OFFADJ provides the reference point for the waveform, AINL drifting up towards +5 V effectively drove the input hard against the positive rail. That also explained why the Sporty chip and R3 were getting so hot.
At that point I had two theories: either the TL064 opamp (U8) had developed a fault and was driving the signal incorrectly, or the Sporty chip itself was somehow pulling the line high. Since a TL064 costs only a few cents from DigiKey, I replaced that first.
Replacing U8 fixed the problem.
This also demonstrated just how resilient the Sporty chip is. Despite running extremely hot due to the fault, it survived without damage.
The hard drive had also failed, so I replaced it with a SCSIknife. The floppy drive was seized solid and full of dust, but after cleaning and lubrication, it now works too.
So the Quadra 700 is now fully functional again.
