My Krell KRC-2 preamp started having a problem of reduced volume and buzzing noise in one channel.
Thinking that it could be an old-capacitor problem, I recapped all the electrolytic capacitors, and now I made the problem worse. It makes the buzzing, static noise even when it's muted, and also the volume has been reduced further.
Just an extra information:
- Switching input source selection works well.
- All the remote controller related operations function OK.
- Left and right volume adjustments works - I turn on one side only to compare how low the volume is in one channel.
I wonder how I should go about troubleshooting this problem. How do people go about repairing solid state preamps with buzzing channel and reduced volume?
Many thanks in advance.
Thinking that it could be an old-capacitor problem, I recapped all the electrolytic capacitors, and now I made the problem worse. It makes the buzzing, static noise even when it's muted, and also the volume has been reduced further.
Just an extra information:
- Switching input source selection works well.
- All the remote controller related operations function OK.
- Left and right volume adjustments works - I turn on one side only to compare how low the volume is in one channel.
I wonder how I should go about troubleshooting this problem. How do people go about repairing solid state preamps with buzzing channel and reduced volume?
Many thanks in advance.
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One of the bridge rectifier was getting very hot after turning it on for a minute. It does not have any heat sinks, and I was wondering if it's supposed to do that.
Just as a side note, I touched every transistor with a heat sink, and found them to be just warm, not hot. I found that only the red marked bridge rectifier was getting really hot.
When I replaced the capacitors, the original capacitors around the red marked bridge rectifiers (that are not shown on the photos) were discolored, and looked burned.
An externally hosted image should be here but it was not working when we last tested it.
Just as a side note, I touched every transistor with a heat sink, and found them to be just warm, not hot. I found that only the red marked bridge rectifier was getting really hot.
When I replaced the capacitors, the original capacitors around the red marked bridge rectifiers (that are not shown on the photos) were discolored, and looked burned.
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I would not think a preamp would pull enough power to need a heat sink on the bridge, but maybe you have a problem there. Probably not the source of your noise though.
One channel is a hint. Do you have a schematic? Are the power supplies independent for R & L? I ask because that is something Krell might do. If not, then it would not be the supply. Move forward comparing the channels. Most likely a bad solder joint, plug, ground, pot etc.
One channel is a hint. Do you have a schematic? Are the power supplies independent for R & L? I ask because that is something Krell might do. If not, then it would not be the supply. Move forward comparing the channels. Most likely a bad solder joint, plug, ground, pot etc.
try changing the diode bridge.
make sure the caps near warm heatsinks and the diode bridge are 105 C rated
make sure the caps near warm heatsinks and the diode bridge are 105 C rated
Thank you, tvrgeek.
I increased the capacitance value of the capacitors by 50% to 100% since they fit into the holes perfectly. Also, the voltage ratings or temperature ratings remained the same or slightly higher. Would this cause a problem?
I don't have a schematic from the web search. I'll call Krell and request it - hopefully, I can purchase or get more pointers.
I would make an educated guess that the power supplies would be independent for L&R channels because there are lots of symmetric parts in the PCB. I'm adding the image of the main PCB.
I'll try to go through all the solder joints as you suggested.
One extra experiment - after taking the AC plug out of the preamp, the amp kept making the buzzing sound in less and less volume. The noise settled to a lower volume after three seconds or so, and kept going. After thirty seconds, I turned the amp off. I can try to measure how long the noise will persist if it helps any trouble shooting.
I increased the capacitance value of the capacitors by 50% to 100% since they fit into the holes perfectly. Also, the voltage ratings or temperature ratings remained the same or slightly higher. Would this cause a problem?
I don't have a schematic from the web search. I'll call Krell and request it - hopefully, I can purchase or get more pointers.
I would make an educated guess that the power supplies would be independent for L&R channels because there are lots of symmetric parts in the PCB. I'm adding the image of the main PCB.
An externally hosted image should be here but it was not working when we last tested it.
I'll try to go through all the solder joints as you suggested.
One extra experiment - after taking the AC plug out of the preamp, the amp kept making the buzzing sound in less and less volume. The noise settled to a lower volume after three seconds or so, and kept going. After thirty seconds, I turned the amp off. I can try to measure how long the noise will persist if it helps any trouble shooting.
First I'd look for anything that looks burned or is too hot. It sounds like you did that. Then I'd signal trace to whatever extent is possible (no schematic?). Check the power supply voltages anywhere you can. If you changed the caps, I'd look at any opamps or transistors closely, then resistors and whatever else there is (relays?). It can be next to impossible without a schematic.
Thank you, ticknpop. Yes, the capacitors are 105 C rated - Nichicon KA.
For replacing the diode bridge, should I look for the same model numbers? I don't know the evolution of the diode bridge last twenty years. I wonder if there will be better substitutes with better lower heats. Which parameters should I look for?
For replacing the diode bridge, should I look for the same model numbers? I don't know the evolution of the diode bridge last twenty years. I wonder if there will be better substitutes with better lower heats. Which parameters should I look for?
Thank you, Bob Richards. I'll check the power supply voltages wherever I can.
Here is the pic of the bad cap compared to the other ones. The discolored capacitor used to be right next to the bridge rectifier.
Here is the pic of the bad cap compared to the other ones. The discolored capacitor used to be right next to the bridge rectifier.
An externally hosted image should be here but it was not working when we last tested it.
discoloring could mean the cap is bad, but it could also mean that the cap was from a different batch after they changed the color of it. If it got hot then it's probably bad. If an ohmeter says it's good then it probably is. The ohmeter should show that it's DC resistance climbs to over 100Kohms or so. Check the others to see what they do. They should all be about the same in that way. the bridge may also be OK. It would possibly get hot if there was a shorted part out in the amplifier circuit.
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