Loud hum through speakers - Yamaha CA-1000

[rjsilva]

I have recently been working on my Yamaha CA-1000. I've opened the unit to look at the function board many times over past weeks and have continued to listen to the unit with no problems.

Just today I finally worked out the test points I needed on the function board and made small adjustments ( I also adjusted the current, mid-point, etc on the amp boards). When I put the unit back together there was a loud hum coming through the speakers. I opened the unit, quadruple-checked everything that could have been affected, fiddled with the adjustments to see if that would do anything, checked again, and again. Nothing looks wrong.

The hum is audible with the coupler switch off, but considerably louder with it on. The mute switch makes it quieter, and the loudness dial and tone controls affect the sound of it somewhat. The balance dial does nothing, nor do any of the input/tape/stereo switches. I disconnected things one by one to isolate, but no change. Audio appears to play fine.

Please, any ideas?

 

[glen65]

Generally hum is going to be a filter or some sort connection issue.
Since what you have is an integrated amp, you will need to find out
which section is generating it. That unit has "pre outs" and "main in" jacks.
I would want to use them to see if you have a hum coming from just
the preamp section. Or is it just in the main amp section. If its in both
then you would need to start focusing on the power supply section.

 

[rjsilva]

Thanks! I'm guessing it's not starting in the preamp since there is still hum with the preamp coupler switch off (though the hum is much quieter). It seems like the preamp amplifies the hum a lot.

Is this a reasonable assumption?

The power supply section is underneath where I doubt anything happened when I was taking the unit apart. But I'll have a look. What should I be looking for? (I did recap the unit a few years ago.)

 

[petehall347]

check for broken grounds .

 

[avionic]

Thanks! I'm guessing it's not starting in the preamp since there is still hum with the preamp coupler switch off (though the hum is much quieter). It seems like the preamp amplifies the hum a lot.

Is this a reasonable assumption?

The power supply section is underneath where I doubt anything happened when I was taking the unit apart. But I'll have a look. What should I be looking for? (I did recap the unit a few years ago.)

What should I be looking for?

Cracked solder connection or broken wiring.

 

[glen65]

Thanks! I'm guessing it's not starting in the preamp since there is still hum with the preamp coupler switch off (though the hum is much quieter). It seems like the preamp amplifies the hum a lot.

Is this a reasonable assumption?

The power supply section is underneath where I doubt anything happened when I was taking the unit apart. But I'll have a look. What should I be looking for? (I did recap the unit a few years ago.)

At this point dont make any assumptions, speculate or take any possibilities off the table.
If your amp doesn't use external jumpers to connect the pre out and main in jacks, just plug a set of RCA cables into the pre out jacks. Do you still get a hum at the output?

 

[rjsilva]

Cracked solder connection or broken wiring.

I'll examine the power supply board later tonight. Maybe I did snag something on the underside when taking the amp out of the case.

If your amp doesn't use external jumpers to connect the pre out and main in jacks, just plug a set of RCA cables into the pre out jacks. Do you still get a hum at the output?

To clarify, are you saying to plug in a set of RCA cables into the pre out by itself? Is this with the coupler switch on or off? And when you say a hum at the output do you mean from the RCA cables or the speakers? Sorry if the answers should be obvious, unfortunately they aren't to me

 

[glen65]

I'll examine the power supply board later tonight. Maybe I did snag something on the underside when taking the amp out of the case.



To clarify, are you saying to plug in a set of RCA cables into the pre out by itself? Is this with the coupler switch on or off? And when you say a hum at the output do you mean from the RCA cables or the speakers? Sorry if the answers should be obvious, unfortunately they aren't to me

OK, I wasn't aware of the switch. Forget the cable and turn the switch off, so that the preamp
is not connected to the main amplifier. Do you still get a hum at the output? I'm talking about the speaker output of the amp.

 

[rjsilva]

Yes, there is hum, just quieter.

 

[glen65]

Yes, there is hum, just quieter.

You shouldn't be getting hum at the output.
It should be fairly quiet, especially now since its not getting an input.
There may be more than one issue needing addressed. But its likley
you have one singular fault thats effecting both the amp and preamp.
For now I would leave the preamp disconnected and focus your efforts
on quieting down the main amp. Start by focusing on the power supply.

There are DC voltages coming from the supply that are Unregulated
and regulated. Its important that the DC voltage is clean,
"meaning no AC mixed in". Below is a schematic with the non regulated
and regulated section marked. With your meter set on "AC" Volts measure
across the main caps C471 and 472 in the unregulated section.
Then measure for AC on the regulated section connecting between E and B+
then E and B-. Does your meter read any AC voltage at any of these spots? If so how much?
It shouldn't be much, in the Millivolt range.

 

[rjsilva]

Ok, I didn't notice anything on the (regulated) power supply board. The solder connections aren't obviously loose or cracked, and nothing seemed out of place or broken.

I didn't have time tonight to take out the board with the big caps (C472/471) but I did attempt to measure E to B+/-. Unfortunately my multimeter is substandard and only measures AC volts to 1/10th of a volt. With that said, I didn't measure an increase with B-. However, on B+ it reported 1/infinity. Not sure what that means here.

Additionally, I could hear the hum coming from the power supply board, and there was also a pronounced buzz coming from what I think was the area with the four caps grouped together (C705-708?). I couldn't verify with certainty where it was coming from though. Either of these sounds might be normal, I don't remember what it sounded like when I recapped it.

Does any of this reveal anything?

 

[glen65]

Ok, I didn't notice anything on the (regulated) power supply board. The solder connections aren't obviously loose or cracked, and nothing seemed out of place or broken.

Many times bad solder joints aren't really visible. Provided the DC line
voltage coming from the supplies is clean, that would be the next thing to focus on.

I didn't have time tonight to take out the board with the big caps (C472/471) but I did attempt to measure E to B+/-. Unfortunately my multimeter is substandard and only measures AC volts to 1/10th of a volt. With that said, I didn't measure an increase with B-. However, on B+ it reported 1/infinity. Not sure what that means here.

If your meter isn't up to the task, the only thing I can recomend here
is get one that is. Get one that has a Millivolt range on both DC and AC.
The Millivolt range will usually be rated anywhere from 200-400 mv.
Check the specs before getting one. Don't buy on price alone, if you
cant afford something like a Fluke, then consider something along the lines of a B&K, maybe a 2705b.

Additionally, I could hear the hum coming from the power supply board, and there was also a pronounced buzz coming from what I think was the area with the four caps grouped together (C705-708?). I couldn't verify with certainty where it was coming from though. Either of these sounds might be normal, I don't remember what it sounded like when I recapped it.

Does any of this reveal anything?

Since youre not certain as to where the sound is actually emitting from,
it would be hard for me to say. Although a small audible hum from the
power transformer is not unusual.

I think I better ease up with my posting, I'm beginning to get in the others way, later.

 

[rjsilva]

I took a look and there is nothing visually wrong with the the "unregulated" board with the big caps.

As far as the hum and buzz, it appears to be coming from both the power transformer (which I know isn't strange) and the "regulated" power supply board. They are separated so I fairly certain I'm not hearing the transformer when listening to the power supply board. The pitch of the hum is the same as is coming through the speakers.

I took a look on eBay for better multimeters. As a temporary solution I might snag a cheap one I saw online at Walmart which looks like it has a much better AC range.

 

[rjsilva]

I'm going to order a better multimeter so it'll be a few days before I get it. I'll post once I can take some measurements. In the meantime if anyone has any further thoughts I'd very much appreciate it.

 

[rjsilva]

Some new info:

I was testing the pre out and observed something interesting. When RCA cables are plugged into the pre out and the coupler switch is on, there is no hum in the speakers. If I unplug one RCA cable, the corresponding speaker starts to hum. (If the coupler switch to off, the hum is there as I described previously regardless of the state of the RCA cables.)

Also, I wasn't getting a signal on the pre out into my portable recorder. Not sure if that's relevant.

Is any of this useful in diagnosing the problem?

 

[rjsilva]

Ok, some more info which I think is useful (hopefully someone is still following this thread!):

I accidentally discovered that when the function board is not connected to the tone board, the hum coming from the speakers is different. So, I tested the speaker terminals and noticed these things:

- With the function board connected to the tone board, the frequency of the hum is 120hz. When it's disconnected, the frequency is 60hz. There is some AC which changes as well, from 63mV to 80mV.

Does this sound like a grounding issue?

 

[avionic]

Ok, some more info which I think is useful (hopefully someone is still following this thread!):

I accidentally discovered that when the function board is not connected to the tone board, the hum coming from the speakers is different. So, I tested the speaker terminals and noticed these things:

- With the function board connected to the tone board, the frequency of the hum is 120hz. When it's disconnected, the frequency is 60hz. There is some AC which changes as well, from 63mV to 80mV.

Does this sound like a grounding issue?

Sure does..Pinched, chaffed or broken wire or cold solder connection.

 

[ev13wt]

Ok, random stupid idea popped in my head: Is it in the same location in your room and on the same wall jack it previously was? A very slight chance its coming through the wall?

 

[avionic]

A very slight chance its coming through the wall?

Termites that don't know the words . Call Orkin!

 

[rjsilva]

I have no experience in spotting what I'm trying to look for and therefore didn't see anything. I was thinking it'd be useful to give a condensed list of everything that's going on, then perhaps someone could more easily comment?

1. The hum is normally at 120hz, very audible, equally in both speakers.
2. Touching the pre-out/main-in plugs with my finger causes the hum to reduce a little bit.
3. Disconnecting the grounding wire at the rear panel circuit board and chassis causes the hum to increase in volume by about 50%.
4. Decoupling the preamp/amp causes the hum to reduce in volume a lot (maybe 70% quieter?).
5. Unplugging the function board from the preamp changes the hum to 60hz.
6. Using different outlets and reversing the plug doesn't change anything.

I was thinking that with the function board disconnected and the preamp/amp decoupled, it must be coming from either the main amp boards or the power supply boards. And since it is coming equally in both channels, I'm guessing it's not the amp boards. But, I know nothing about how a grounding issue will manifest itself - it could be coming from my nose for all I know.

Any help narrowing this down would be greatly appreciated!