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Hi,
I repaired a marshall 2205 (jcm800) amp for a customer. He complained of the amp 'losing grunt'. Also the clean channel wasn't working.
Clean channel was a dodgy heater connection - the socket was damaged so I replaced it. I also noticed that one of the output valves was arcing internally so I replaced the pair of valves and re-biased to 70%.
This was January some time. I cheekily used the amp for a days recording with one of my bands and ran it up in front of the customer at pickup so I know it was fine. I don't think he powered it up for a while - he's got a few amps!
He's now brought the amp back saying that it went quiet for him after a few minutes of playing. It powered up fine and ran fine for a while so I left it playing pre-recorded music from my phone at a medium volume. After an hour or so I heard a pop and then a low level hum appeared.
After powering down, no hum and the amp seemed fine. I then opened up and checked all the usual suspects. I left VM monitoring the HT voltage and set it playing music again. The hum appeared again after a couple of hours. Flicked to standby and back on and it disappeared.
I then moved the VM probe to the other filter cap (other side of the choke) and suddenly the amp sound changed, becoming tinny and the hum appeared.
Any suggestions? I'm thinking replace the reservoir caps. But I'm not sure why moving the probe between them should cause the problem to become apparent. The input impedance of the VM is 10meg and the caps are 50u so I wouldn't have thought this would discharge the caps very quickly.
Thank you very much.
I repaired a marshall 2205 (jcm800) amp for a customer. He complained of the amp 'losing grunt'. Also the clean channel wasn't working.
Clean channel was a dodgy heater connection - the socket was damaged so I replaced it. I also noticed that one of the output valves was arcing internally so I replaced the pair of valves and re-biased to 70%.
This was January some time. I cheekily used the amp for a days recording with one of my bands and ran it up in front of the customer at pickup so I know it was fine. I don't think he powered it up for a while - he's got a few amps!
He's now brought the amp back saying that it went quiet for him after a few minutes of playing. It powered up fine and ran fine for a while so I left it playing pre-recorded music from my phone at a medium volume. After an hour or so I heard a pop and then a low level hum appeared.
After powering down, no hum and the amp seemed fine. I then opened up and checked all the usual suspects. I left VM monitoring the HT voltage and set it playing music again. The hum appeared again after a couple of hours. Flicked to standby and back on and it disappeared.
I then moved the VM probe to the other filter cap (other side of the choke) and suddenly the amp sound changed, becoming tinny and the hum appeared.
Any suggestions? I'm thinking replace the reservoir caps. But I'm not sure why moving the probe between them should cause the problem to become apparent. The input impedance of the VM is 10meg and the caps are 50u so I wouldn't have thought this would discharge the caps very quickly.
Thank you very much.
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