My Superphon Revelation Dual Mono has developed a loud buzz in one channel, audible both line level inputs and louder on phono input.
Some background: I was testing some speakers and moved the speaker wires and interconnects during testing (preamp was muted at the time)
After this, the preamp immediately had a loud buzz in the right channel.
Signal is still present on the channel with the buzz.
This preamp is dual mono all the way to the (single) transformer. I've replaced the electrolytics last year (exept for the 4 largest ones in the power supply) and it's been working very well. I've checked for cold solder joints, but see none, the preamp was not moved/dropped etc.
I have an EE degree but have done little circuit level debugging since graduation 12 years ago (more of a software-related job). I do have a scope, DMM, L/C and ESR meters, etc.
So far I have tried isolating any bad components by tapping with a wooden stick but can detect no change in the sound when doing so. The parts count is very low in this unit, so I pretty much tried the tap test on everything. The large 1000 uF electrolytics in the PS are 25 years old but show no sign of leakage.
Can anybody think of anything that I can check? Where would you look first?
The schematic for the preamp is at http://www.diyaudio.com/forums/gallery/uploads/78563/Superphon_Rev_schematic.jpg.
I appreciate any suggestions you might have.
Some background: I was testing some speakers and moved the speaker wires and interconnects during testing (preamp was muted at the time)
After this, the preamp immediately had a loud buzz in the right channel.
Signal is still present on the channel with the buzz.
This preamp is dual mono all the way to the (single) transformer. I've replaced the electrolytics last year (exept for the 4 largest ones in the power supply) and it's been working very well. I've checked for cold solder joints, but see none, the preamp was not moved/dropped etc.
I have an EE degree but have done little circuit level debugging since graduation 12 years ago (more of a software-related job). I do have a scope, DMM, L/C and ESR meters, etc.
So far I have tried isolating any bad components by tapping with a wooden stick but can detect no change in the sound when doing so. The parts count is very low in this unit, so I pretty much tried the tap test on everything. The large 1000 uF electrolytics in the PS are 25 years old but show no sign of leakage.
Can anybody think of anything that I can check? Where would you look first?
The schematic for the preamp is at http://www.diyaudio.com/forums/gallery/uploads/78563/Superphon_Rev_schematic.jpg.
I appreciate any suggestions you might have.
Measure the DC offset at the output between the diodes and before the output caps - what is it for each amp?
Measure the bias as the voltage across R6 and R7 - again for each of the amps.
Are you sure it is the pre amp, have you swapped channels to test or listened through headphones to rule out the power amp?
Measure the bias as the voltage across R6 and R7 - again for each of the amps.
Are you sure it is the pre amp, have you swapped channels to test or listened through headphones to rule out the power amp?
OK, I don't know why I didn't think of this before, but I have 2 of these preamps. So I set them up side-by-side and started comparing voltages. Straight away I found a large voltage drop across one of the 1000 uF electrolytics in the PS, and I can confirm that this cap has failed. (reading 2uF). The power supply in not shown on the schematic.
I checked the voltages across R6 and R7, they are .41 volts across R6 and R7 for both units.
At this point I assume that the problem is the 1000uF cap. Seem reasonable?
I checked the voltages across R6 and R7, they are .41 volts across R6 and R7 for both units.
At this point I assume that the problem is the 1000uF cap. Seem reasonable?
The Basic has at least three models I know of: 1) Two separate power supply boards stacked on top of each other- this could be the original model? 2) A single PSU board with two independent power supplies 3) A single PSU board, but with a single volume control (as opposed tot the two controls for 1and 2 above) and a balance control. There are also different minor mod's to the main boards- I dont think there was much version control happening. The main 1000uf power supply caps do fail, and should be replaced, I have used 2200uf Nichicon VX with good results
Single Volume Knob Basic hum in both channels in phono
All capacitors replaced, still hums. Line source has no hum - ok, just extremely faintly. Did the caps in phases. Two big ones in the ps, then two small in ps. After replacement of the smaller ones, the hum was only in the right channel for a bit, until I replaced the rest of the small caps, now it is back in both channels.
The transformer definitely has a hum, and though I don't have perfect pitch, the frequency of the transformer hum closely matches that coming through the phono section.
New at this - auto mechanic - so please forgive any mistakes in nomenclature!!
Hafler XL-280 is dead quiet with my NAD preamp.
Thoughts??
Thanks!!!!
All capacitors replaced, still hums. Line source has no hum - ok, just extremely faintly. Did the caps in phases. Two big ones in the ps, then two small in ps. After replacement of the smaller ones, the hum was only in the right channel for a bit, until I replaced the rest of the small caps, now it is back in both channels.
The transformer definitely has a hum, and though I don't have perfect pitch, the frequency of the transformer hum closely matches that coming through the phono section.
New at this - auto mechanic - so please forgive any mistakes in nomenclature!!
Hafler XL-280 is dead quiet with my NAD preamp.
Thoughts??
Thanks!!!!
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