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In Reply to: RE: Hum Trackdown posted by Chip647 on July 05, 2016 at 19:34:37
Hi Chip. When the inputs are shorted the amp is dead silent. I can hear the hum and it's more than should be there and I'm pretty sure that it's a 60hz hum. I've tried different sets of tubes but it could be anything else.
To measure the ac on the filter do I do that at each stage of the power supply?
Thanks
Follow Ups:
It is not your tubes. When it is quiet with the inputs shorted, your amp is quiet electrically. However, you have a grounding issue. Really check how the RCA input grounds are routed inside the amp compared to how they were before the mod. Check grounds of any shielded wire in the amp. Try taking a piece of wire and touch the amp chassis to another component. Tracking down this type of ground issue is tricky but you will find it.
Thanks Chip. It seems like I've tried everything over the years besides touching the amp to a source so I'll give that a shot. Btw, I've tried countless sources. I'm sure it's a mistake that I keep on overlooking.
This, plus the fact it's 60 Hz and the amp is quiet when the inputs are shorted, strongly implies a shielding issue inside the amplifier ahead of the first amplifier stage. I'm not familiar with the Cit V at this level, but has the signal input wiring been changed from factory? Was it originally twisted pair or shielded wire, now plain single wires? Are the input resistors still in place (1M to ground and 10K in series, according to my schematic)? Have any of the NFB loops been modified or removed?
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Buy Chinese. Bury freedom.
Edits: 07/10/16
hmm. yeah it was changed but I tried a few different things even routing double shielded twisted pair wire the long way to avoid it going by the power supply.
The NFB is left in tact. Same with the input resistors. I'll double check though to see if it wired it wrong.
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