Hi there,
Upon cleaning the controls and switches on this amp, the volume control became scratchy on the left channel. Testing shows 3.7V DC on the volume pot and the balance control (volume all the way down and balance centered).
What I'm describing hereon applies to the left channel; right channel is OK and the amp plays both channels fine (except for the volume scratchiness) and works fine. DC offset is acceptable (30 and 40 mV) and the bias is to spec. The service manual is too large to attach here. It's at hifiengine - dot -com/manual_library/sony/ta-f550es.shtml . The amp in questions is a Canadian model.
It seems the corroded selector switches (sound was cutting off and contact cleaner wasn't enough; I had to take them apart to shine them) were impeding DC.
Measurements and notes:
- Measurements done with no source connected.
- The 3.7V DC at the pot is also present at the line input selected. Shorting the selected input reduces it to 0.3V.
- The DC seems to originate on the Selector board as disconnecting it totally from the other boards (except for bus EH3 to power it from the Main board) does not remove the DC; to the contrary, this puts 35V DC almost everywhere on the Selector board.
- DC voltages at EH3 read (from 1 to 5): 0V - 1.1V - 0V - 0.8V - 51.3V.
- Turning both Direct-in and Mute on reduces DC on the pot to 0.7V.
- Turning both Direct-in and Subsonic on reduces DC on the pot to 0.0V BUT puts 12.8V on the balance control.
- All voltages measure from chassis ground with DMM.
- The Selector board is capacitor coupled to the EQ board and there is no DC present at R419 (or R469 on the R channel). So DC cannot be coming from the Phono stage on EQ board.
- DC at the negative of C302 is -0.4V (close to the specified -0.3V on pin 15 of IC301) so this cap is not leaking. C352 on R channel is similar.
- There is rail coming into the Selector board to power the LEDs and their drivers; however, the schematic does not show any way for this to get onto the volume, balance or tone circuits.
- I also removed the Muting board completely in case it's infiltrating DC, but that didn't resolve anything.
I can't figure out where the DC at the volume, balance and tone section of the Selector board is coming from.
Your thoughts and suggestions would be appreciated.
Regards,
Bill
Upon cleaning the controls and switches on this amp, the volume control became scratchy on the left channel. Testing shows 3.7V DC on the volume pot and the balance control (volume all the way down and balance centered).
What I'm describing hereon applies to the left channel; right channel is OK and the amp plays both channels fine (except for the volume scratchiness) and works fine. DC offset is acceptable (30 and 40 mV) and the bias is to spec. The service manual is too large to attach here. It's at hifiengine - dot -com/manual_library/sony/ta-f550es.shtml . The amp in questions is a Canadian model.
It seems the corroded selector switches (sound was cutting off and contact cleaner wasn't enough; I had to take them apart to shine them) were impeding DC.
Measurements and notes:
- Measurements done with no source connected.
- The 3.7V DC at the pot is also present at the line input selected. Shorting the selected input reduces it to 0.3V.
- The DC seems to originate on the Selector board as disconnecting it totally from the other boards (except for bus EH3 to power it from the Main board) does not remove the DC; to the contrary, this puts 35V DC almost everywhere on the Selector board.
- DC voltages at EH3 read (from 1 to 5): 0V - 1.1V - 0V - 0.8V - 51.3V.
- Turning both Direct-in and Mute on reduces DC on the pot to 0.7V.
- Turning both Direct-in and Subsonic on reduces DC on the pot to 0.0V BUT puts 12.8V on the balance control.
- All voltages measure from chassis ground with DMM.
- The Selector board is capacitor coupled to the EQ board and there is no DC present at R419 (or R469 on the R channel). So DC cannot be coming from the Phono stage on EQ board.
- DC at the negative of C302 is -0.4V (close to the specified -0.3V on pin 15 of IC301) so this cap is not leaking. C352 on R channel is similar.
- There is rail coming into the Selector board to power the LEDs and their drivers; however, the schematic does not show any way for this to get onto the volume, balance or tone circuits.
- I also removed the Muting board completely in case it's infiltrating DC, but that didn't resolve anything.
I can't figure out where the DC at the volume, balance and tone section of the Selector board is coming from.
Your thoughts and suggestions would be appreciated.
Regards,
Bill
Turned out to be a crack in the volume pot that broke the path to ground for the coupling capacitor connected to the wiper. I was probably reading the capacitor's charge. What remains a mystery, though, are:
1. Why I was getting different dc readings on the selector board
2. Why wasn't the reading counting down as the meter load discharged the cap
Anyway, the dc is gone after replacing the pot.
1. Why I was getting different dc readings on the selector board
2. Why wasn't the reading counting down as the meter load discharged the cap
Anyway, the dc is gone after replacing the pot.