Mcintosh C28 Hum

Rudes

Member
2009-09-08 6:56 pm
I just picked up a Mcintosh C28 preamp. The line section works fine. In the Aux and Tuner mode there is no hum. When I turn the selector knob to phono, mic or tape head, there is a hum present. So there is a problem in the low level section. I checked the electrolytics on board 044-309 and they are good. Would appreciate any advice!

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I have not hooked a turntable up to this but the owner said that he did and with the ground wire connected. He said there was a hum.

Maybe he has a bad ground on the table?

The problem may be your turntable wiring or ground wire. This is common.

Does the turntable ground terminal on the preamp ground to chassis? Many recievers/amps/preamps are configured like this. It is infortunate.

If so, configure the turntable ground lug so it is connected to the noise ground of the RIAA preamp circuit, or to the star ground if there is one.
 

Rudes

Member
2009-09-08 6:56 pm
I replaced all PS caps. Does anyone know if these Mcintosh preamps are just like this? There is always a slight hum. Louder with the phono section. It is very noticeable without an audio source playing through it and increases with volume. Can not be heard with music playing. My turntable works fine with my Audio Research unit so I know it is not that and also the fact that the hum is there even in the Aux mode, but much lower. Could a faulty transformer be the cause?
 
Noise + Hum for the phono stage is -78dB. Maybe that's high enough for you to hear? If not, then you'd have to do some diagnosis in order to figure out what's going on. First check would be the amount of ripple on the DC rails. You say you replaced the power supply caps, so you got those special cans from where?
 
If the hum level is the same with the new capacitors, then the old ones were not bad. You might as well just undo what you just did. You have to do an investigation to determine the cause of the hum. I would suspect bad regulators if the caps are good, but you have to use some equipment (like a simple AC voltmeter) to figure it out. Your job is to figure out where the hum is coming from. It could be the power supply, or it could be some bad part in the circuit itself.