My Copland CTA401 hums , help needed please

[LuvValve]

Recently I noticed my Copland CTA-401 hums. I can hear the hum from about 3 meters away at night. Both channels pruduce the same level of hum. I have had this amp for more than 15 years and really enjoyed the sound.

I have replaced the 4XEL34 and adjusted the bias , same problem.

I have tested the filter caps. there are 2X 220U/500V and 1X100U/500V main caps. No obvious leakage found by my multi-meter ( measured when the caps are disconnected on board). Both 220U show around 190U in value and the 100U shows 95U.

I also tested other two 33U/450V bypass caps, no leakage reading 30U and 29U in value.

What do you think the problem might be? Maybe one of the caps or more are leaking only when applying high voltage? I do not have spare caps to test but will order a few. Before ordering I'd like to hear you experts's view about my situation. Attached is the diagram for this amp. Thanks in advance.

[aardvarkash10]

Originally Posted by LuvValve

Recently I noticed my Copland CTA-401 hums.

Get the sheet music and accompany it on a keyboard...

[whitelabrat]

Originally Posted by LuvValve

...more than 15 years

I would put my money on power supply cap failure. Have you done a sniff test? I've got an old Yamaha C-6 that I've been recapping. It worked fine until the other day. It's humming a nice 60hz and has electrolytic stink about it. Another cap bit the dust.

[kevinkr]

Have you checked the amplifier with all sources disconnected to make sure this is not caused by an external groundloop - the result of some recently added hifi component?

Next I would check the pre-amplifier tubes and make sure that one of them is not the culprit, this can be done by simple substitution.

Finally you need to measure the ESR of your power supply capacitors, not just their capacitance value. After all of these years of use it would not be surprising if some of them were going bad despite the seemingly ok measurements.

Interestingly most new commercial caps tend to test well above their nominal capacitance values in most cases (IME) and the fact that yours all test well BELOW the nominal makes me believe that they are far to the right along the service life curve and might be on the very verge of falling off! It would probably be a wise investment for the future to replace them now before they fail.

[LuvValve]

Thanks for reply Whitelabrat and Kevin, I went to my local electronic store and bought 2X 220UF/500, 100UF/500V ( all Jamicon ), tested last night but the apm still hums no change. Next I want to try is to replace 4X 0.047uf/600v coupling caps, ( been thinking mondorf caps for an upgrade ).

Recently I did add a new DAC to my system. I will disconnect it tonight and see if it makee any difference.

[Salas]

If it does not, tap with the back of a screwdriver all its relays.

[LuvValve]

Originally Posted by kevinkr

Have you checked the amplifier with all sources disconnected to make sure this is not caused by an external groundloop - the result of some recently added hifi component?

Amazing, when I disconnected all the RAC's at back, the hum disappeared ! the amp is almost dead quiet. So, the hum was caused by groundloop. But now what I need to do?

[Andy Bartha]

Plug the rca connectors back in one pair at a time. When you get the hum back you have found the component that is causing the hum. You may have to lift the ground on that component.

[thehappytooter]

Originally Posted by LuvValve

Recently I noticed my Copland CTA-401 hums. I can hear the hum from about 3 meters away at night. Both channels pruduce the same level of hum. I have had this amp for more than 15 years and really enjoyed the sound.

I have replaced the 4XEL34 and adjusted the bias , same problem.

I have tested the filter caps. there are 2X 220U/500V and 1X100U/500V main caps. No obvious leakage found by my multi-meter ( measured when the caps are disconnected on board). Both 220U show around 190U in value and the 100U shows 95U.

I also tested other two 33U/450V bypass caps, no leakage reading 30U and 29U in value.

What do you think the problem might be? Maybe one of the caps or more are leaking only when applying high voltage? I do not have spare caps to test but will order a few. Before ordering I'd like to hear you experts's view about my situation. Attached is the diagram for this amp. Thanks in advance.

Please could you send me a copy of the diagram by email. I am desperately trying to get one.
Regards Michael furmgame@aol.com