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Hi, I have a McShaned Citation V that has some hum, I'm sure that it's my fault, at the outputs that I dig out and try to figure out every great while without success so far.
Anyway, I finally acquired an oscilloscope, a Tektronicx 2225, and was wondering if you guys had any tips on how to use it to figure out the area most likely responsible for the noise.
I'm guessing that I install a dummy load on the outputs, ground the probe to the amp's ground bus and trace my way backwards starting at the speaker output. What pattern on the scope am I looking for and what should it be set to?
Thanks.
Follow Ups:
A multi meter is the place to start. Measure the AC on the B+ after the filter. If it is more than 0.1 volt you probably have a bad filter cap. However, your hum is most likely a ground loop or a wire layout issue. Your oscilloscope will tell you that you have a noise. Your ears will as well. If it is not a bad cap, you go down the ol' list: wire routing/dressing, grounding points, bad tube, system ground loop, monsters, etc.
I usually start by shorting the signal at the output tubes and listen, short the signal at the phase inverter and listen, short the signal at the next stage, etc. Hum tracing is the spookiest part of the science.
A 120 hz hum indicates a hum with something that has been rectified. A 60 hz hum is something that has not been rectified.
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
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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