CR-620 Buzz/hum

[Gary_James]

I just bought for $70 a cosmetically pristine CR-620. All functions work, but there a loud buzz or hum for about the first two minutes after it powers on, after which the buzz fades away. The only thing on the front panel (other than volume) that affects the amplitude of the buzz is turning the Bass knob. Rotating the knob from the min position, the buzz is loud, softer at the "flat" position, then loud again at max. I've done what I can to DeOx the Bass adjust pot, although it's tough to reach. Any thoughts? Do I need to pull the tone board and look for bad caps? Any testing I can do short of pulling the tone board? I'd love to have it recapped but the quotes I've received were more than the unit sold for new in 1979, and I'm not sure that I trust myself not to screw it up.
Thanks!

 

[avionic]

Hey Gary
Likely a bad cap or transistor in the power supply if the buzz/hum is in both channels.

I'd love to have it recapped but the quotes I've received were more than the unit sold for new in 1979,

Did you factor in the 2018 dollar value compared to the 1979 dollar value.

 

[Gary_James]

Would a bad p.s. cap or transistor create a buzz that fades away about 2 minutes after powering up? I've been given to believe the buzz they'd create would likely not fade away.

 

[desertmirage]

Would a bad p.s. cap or transistor create a buzz that fades away about 2 minutes after powering up? I've been given to believe the buzz they'd create would likely not fade away.

Hi, I have the same problem! Please let me know if you find the problem, and I’ll keep an eye on this thread and let you know if I find the problem.

 

[ConradH]

I'm guessing the cap(s) are so bad that they only filter after reforming for a couple minutes. Could be wrong, but if you read the other threads on the CR-620, you'll see my comments about the cap problem.

 

[Gary_James]

Hi, I have the same problem! Please let me know if you find the problem, and I’ll keep an eye on this thread and let you know if I find the problem.

So far, I haven't found the cause. I've done all sorts of things to see if its a bad solder joint (flexing the boards, heating the boards with a heat gun, etc.). No luck. I'm planning to replace the capacitors in the power supply after the holidays to see if that solves the problem. However, this receiver will be my first patient for this sort of treatment; hoping I don't kill the patient entirely.

 

[ConradH]

Just buy good parts from an authorized distributor, never eBay, go slowly and pay attention to polarity. Don't overheat the traces. You should be fine. I seem to remember there are some small caps near one edge that are hard to change out. Do those last if at all. The caps on the power board will be the bad ones. The tuner/low level board doesn't seem to fail so quickly, though upgrading the coupling caps as I've described in the past will improve things a bit. If you have irregular noise there are two transistors on the tuner/low level board that are likely culprits.