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60Hz triangle wave hum Marantz 2270

S

Shikadi

New Member
I just acquired a 2270, and did a full re-cap. There's a 60Hz triangle wave hum I can measure with a scope (which may or may not have been there before) that gets louder with increased volume, independent of source selection. I can also still here it when I disconnect the pre-amp, but it no longer follows volume, leading me to believe it's a power supply issue that's being amplified in the pre-amp. I also have this strange issue where touching the volume knob (with the faceplate off) makes a quiet popping sound similar to when you touch a floating input. In fact, that's sort of what the entire thing sounds like, a floating input. Despite all this, rough audio measurements on my scope look/sound pretty good. I'm also in the process of attempting to fix the bias current on one of my modules, but this noise is on both channels.

Has anyone seen this before?
Thanks =D
 
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Triangle or sawtooth? Sawtooth would be power supply. Using the scope can you determine it's frequency? 1/t is the frequency.

Craig
 
I just went through one of these this last summer & had a similar problem after the recap. Found it to be a broken wire that was being held in place by the insulation only. I wasn't surprised as the unit was filthy when I got it & needed extensive cleaning.
 
Shikadi said:
There's a 60Hz triangle wave hum I can measure with a scope
Click to expand...
Where did you measure this?

Hum in both channels?

Post a pic of the waveform, could it be just the "AC ripple" also what is the peak to peak voltage...

Hum that increases with volume suggests it is introduced before the vol pot.
 
I'm at work at the moment, but I measured at the outputs, 60hz frequency. I'll go through the wires after work if I have time and take a picture of the scope/get peak to peak. I don't think it's just ripple, unless my ripple is really bad for some reason, it's too loud to be ripple. I'm suspicious of EMI, because the sound changes when things change on my TV screen, and gets a little quieter if I turn off the TV
 
Here's my scope capture, looks to be about 300mV peak to peak if you ignore the overshoot. I checked a bunch of the boards for resistance to case ground and everything was close enough to zero, but I didn't check the AM/FM boards that are under shields/other boards. I still agree that a grounding issue sounds possible, especially since nearby devices have a large effect (plugged in transformers, the refrigerator being on, etc). I'm just not sure how exactly to diagnose the grounding issue when all the boards seem to be happily grounded
IMG_20191209_195802.jpg
 
Shikadi said:
I'm at work at the moment, but I measured at the outputs, 60hz frequency. I'll go through the wires after work if I have time and take a picture of the scope/get peak to peak. I don't think it's just ripple, unless my ripple is really bad for some reason, it's too loud to be ripple. I'm suspicious of EMI, because the sound changes when things change on my TV screen, and gets a little quieter if I turn off the TV
Click to expand...
Cell phone? WiFi? Does it stop with all the covers on?
 
Shikadi said:
Here's my scope capture, looks to be about 300mV peak to peak if you ignore the overshoot. I checked a bunch of the boards for resistance to case ground and everything was close enough to zero, but I didn't check the AM/FM boards that are under shields/other boards. I still agree that a grounding issue sounds possible, especially since nearby devices have a large effect (plugged in transformers, the refrigerator being on, etc). I'm just not sure how exactly to diagnose the grounding issue when all the boards seem to be happily grounded
View attachment 1695602
Click to expand...
It's very cyclic. Could be a diode getting noisy.
 
You certainly have a grounding issue. 2 things -
1. The hum is being amplified by the power amp sections - variable with volume change.
2. The popping noise when you touch earth you self.

You have to go hunting.
 
Powertech said:
You certainly have a grounding issue. 2 things -
1. The hum is being amplified by the power amp sections - variable with volume change.
2. The popping noise when you touch earth you self.

You have to go hunting.
Click to expand...
Ahhh yes, I missed the slight popping noise. That certainly points to a grounding issue. Could even be a ceramic snubber cap to ground that has come loose.
 
So this morning I probed around a bit on the preamp board and didn't see the noise anywhere... I may not have looked in the right places, but I would have expected to see it there given its response to the volume knob (or is the volume knob post pre-amp?) I'm a little confused about the schematic, I can't seem to figure out what's the input and what's the output of the pre-amp, and working my way backwards from the pre-amp output jacks leads me to some resistors in the hi filter and being tied together at J407 and J420.

An interesting thing to note is there's a lot of static when connecting chassis to earth ground, and once the connection is solid the buzzing is significantly reduced. I measure 120VAC on the chassis with my probe's ground floating, and 80VAC on a multimeter between chassis and earth ground. I'm guessing that's normal since there's a 2.2M resistor from one of the AC lines straight to chassis, but it's puzzling to me what a difference it makes to ground the chassis to earth. I could try replacing the power switch/snubber capacitor, I keep seeing people talking about doing that, why exactly is it done?

I probed around on the power board too, and didn't see anything noisy or out of the ordinary. My probing today was relatively random though, just picking all the easy to access spots since I didn't have a lot of time.

Tried putting both covers on, noise was the same (if not worse)

Shot in the dark, if R009 and R010 are significantly uneven values, could that cause this problem? I measured them in circuit (which is relatively meaningless given that they're across the giant capacitors) but the readings were pretty far away from each other
 
Try flipping the AC cord plug around in the receptacle. You may have the "hot" in the neutral side of the receptacle. There is a line (or two) embossed the length of the power cord. That "line" is supposed to be the neutral conductor. Check to see which conductor of the power cord goes to your power switch to verify. At least that way you will know if your "hot" is going to the hot in the receptacle to eliminate it as a possible source.
 
Just before heading out to work, I flipped the power cord and the noise was the same, but the phantom voltage is 60v instead of 120v, and the noise gets worse when I connect chassis to ground instead of better
 
I found a lead! I got frustrated probing random things, so I decided to touch random ground points with earth ground and see if any of them sounded different. Along my travels, I hovered near the "Record" inputs (not phono), and the hum got way louder. Out of stupidity, I let the contacts touch just to see what would happen, and POP I forgot my volume was all the way up O.o Anyways, grounding the record inputs makes the hum go away! Now I just need to figure out why... Also, what those even do...

I think I came across this in another thread, but I read "Record" as "Phono" and decided it wasn't my problem...
 
That might have been a dead end... Record is an output, also tied to dubbing out. It's a signal that exists before the tone amp, and on any input, which means I was probably just shorting the input of the tone amp to ground, i.e. setting the volume to zero. But it might still be a clue how sensitive those ports are
 
...Ready for some stupid s***? Decided to ground the inputs to the AUX channel, noise goes away. Dumbstruck, I plugged in my laptop with 3.5->RCA, and no noise. So uhh, sorry about that. Basically none of my measurements were worth anything. The buzzing apparently only happens connected to my TV. I have a 2216b I've been using with the same TV that doesn't have any buzz though, so it's still an issue =/
 
Shikadi said:
That might have been a dead end... Record is an output, also tied to dubbing out. It's a signal that exists before the tone amp, and on any input, which means I was probably just shorting the input of the tone amp to ground, i.e. setting the volume to zero. But it might still be a clue how sensitive those ports are
Click to expand...
In many units, it's also a direct output used for tuner alignment. So there may be something to it there. It's worth looking into to at least eliminate it from the list of possibilities.
 
So things are even stupider than my previous post. The 2216b is probably 5-6 times less powerful than the 2270, not to mention it's in a dying state right now and doesn't fully function. I hooked it back up again, turned the volume to max, and realized I can hear the noise there too, it's just way quieter. The TV just has crappy RCA outputs. The show I was watching at the time I noticed turns out to be super low quality audio with low volume, so I ended up amping the signal way more than needed, so there was probably never a problem... Unless noise from floating inputs isn't supposed to happen. So I guess I'll ask three final questions to wrap this thread up, hopefully someone can answer them for me and future viewers of this thread who get here by searching:

1. Should there be noise on floating inputs/nothing connected? If not, is there some sort of circuitry that senses whether or not an input is present?

2. My noise floor is kind of bad in terms of hissing, not unbearable but it seems worse than my 2260. I've heard it can be old resistors or old diodes, but is there a specific place I can target to bring the noise down a bit?

3. Why do people change out the power switch and snubber?

Thanks everyone for the help in this thread, sorry it turned out to be nothing, but I really appreciate it
 
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