CR-2020 Intermittent Hum

aeyb701

Member
I have a Yamaha CR-2020 I found at a waste station last year, and since then I've performed the service bulletin(s) and between me and a technician whom I trust hav replaced many capacitors in the power supply, hoping to sort a 120Hz (I think, maybe it's 60) hum. The hum is not there until, on another part of the room circuit, I turn on one of those electric liquid-filled heaters, the ones that look like upright wall radiators.
It also comes when I have BOTH leads of a 300-ohm FM antenna hooked up, but not if it's just one lead.
It is there regardless of input, and even with no inputs connected.
It is in both channels.
It increases with the volume control, and is not there when set at zero.
Oddly it also increases when the low-filter is flicked up to the 70Hz position.
With the case off and using the end of a fine paint brush to press around the hum may vary with the position of groups of wires ( I do not know which as they're so many running in bunches).
Everything else is perfect.
Even replaced the filter caps with new Nichicons since the power supply board was lifted for other cap replacement and it's a bear to disassemble.
Given the above behaviour, especially with the way an antenna is connected, I'm starting to think it's a ground issues more than any cap.
Anyone have any ideas?
Thanks,
Jon Archibald
Indian River, Ontario, Canada
 
Did you try reversing the AC mains plug 180° ?
Thanks-you for chiming in Avionic.
Yes. Same thing with AC plug reversed, but the fm antenna fully connected didn't induce the hum this time.
Oddly someone had marked the polarity of the plug for some reason, perhaps ground loops encountered with other components.
Maybe a cold solder joint. Pulling all the Pcb's would be a job, but I could try grounding the grounds of each board via alligator wire to the chassis.
I've no experience with an oscilloscope but do have LCR and digital multi meters.
 
I am having a similar problem with CR-2020 after a full recap and doing all of the service bulletins I still have a 60mhz hum in the FM. All pother inputs work great with no hum. The hum is just like a hum I have encountered on SX-1250 with wrong diode on Stabizer board, so I tried changing diodes in PS one at a time, but NTE replacements don't even work well enough for it to come out of protection. Put old diodes back in. I even recapped the FM board which I usually don't do. I have rebuilt nearly 100 receivers and have never had this kind of issue. If I solve problem I will post again!
 
I still have a 60mhz hum in the FM. All pother inputs work great with no hum.
Have you eliminated external noise possibilities? Like florescent lights or light dimmer switches?
 
I would not suspect anything like that, I have rebuilt nearly 100 receivers in this environment. I tried changing the Zeners on the unusually designed Power Supply one at a time, but the NTE substitutes did not work and unit would stay in protection mode. Someone had replaced toasted transistors with unknown parts, etc. Replaced all caps on FM board, and used MLCC caps for the .68 and .22uf caps. Still hums. This unit was in such bad shape when I got it, burned PS board, still had the weak 22 and 33 ohm resistors on the Filter Cap board that had unsoldered themselves, etc, that my client has accepted the unit back with the FM hum. It is such a great sounding amplifier, I do really like the Yanmaha sound. Unit is on it's way back to the client.
 
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