cr-620 HUM HUM HUM

[rwj0j0]

i just picked up a 620 in a small $35 lot of stuf (also a bsr eq and a kenwood kx-1030)

well she runs but the output hums. i replaced the biggest filter caps and it didn't solve the problem.

any advice?

 

[merrylander]

There are two supplies, there are four caps near the big filter caps, you will see four diodes behind them, could be the culprits.

 

[rwj0j0]

thanks! I'm on it.
Do you know what part I need to replace those diodes with.

 

[merrylander]

The diodes are usually ok but I suspect an 1N4002 would probably work

 

[rwj0j0]

so the other caps in the power supply are more likely to be the problem?

 

[merrylander]

They are located just to the left of the main rectifier diodes, 2 - 100mfd @ 50V and 2 - 100mfd @ 63V. These I would suspect if you get a steady hum unaffected by any controls. I did have a CR-620 pass through here where the problem was on the tone control board - can't recall which cap but the hum was affected by the Bass control.

 

[rwj0j0]

Thanks - i'll see if i can grab those caps at ratshack and throw them in tonight.

 

[rwj0j0]

it does relate to the bass control.

 

[rwj0j0]

and it also relates to the FM muting button. When depressed, the bass control causes hum - when not it does not.

I don't remember it being this way prior to replacing the 100uf caps in the PS - i only replaced the ones with the lower voltage rating - couldn't get the bigger ones - maybe tomorrow i can though.

 

[Sir.Byrd]

Did you install the caps with the right direction and value? Also did you install correct types, as in no polars where bi-polar caps are called for?

 

[rwj0j0]

yes. the caps i replaced are the right value and orientated correctly.
The hum existed prior to changing the caps - i just don't remember if I tried switching the FM muting button or not.

 

[rwj0j0]

so i recapped all the electrolytics on the tone board and i still have hum.

get this though - if i run it for a while it goes away except for if the fm muting button is depressed and then it returns.

i'm a little stumped - I guess it has to be a capacitor somewhere - that starts working after it heats up? does that make any sense?

 

[Sir.Byrd]

Have you checked for cracked or cold solder joints? Use a magnifying lens to examine each joint. Remove the solder from any joint that is questionable and resolder it with some new solder.

 

[rwj0j0]

if I don't allow power to the whole tuner board - there is no hum at all...
i haven't replaced the 1000uf 50v cap and I think it might be that.
(i do get a big speaker pop now when i power it up... not sure what's going on there)