Cyrus II Hum problem

[daveshah]

I recently bought a Cyrus 2 amplifier and PSX power supply (early model, button style switch), and noticed mains hum, particularly noticeable with no audio playing.

At first I thought this might be related to a ground loop with my computer, and thus built a simple differential amplifier which removed most of the hum, but a small amount still remains.

This occurs even with no inputs connected, and the input selector set to 'O' (off/mute). It is not affected by the setting of the volume control, and occurs using both speakers and headphones.

Before I attempt to replace all or most of the capacitors, I wanted to make sure this isn't expected behaviour, or whether there is some other cause that is more likely.

 

[JonSnell Electronic]

If you lift C33/34 and the hum is still there it is a poor ground connection. If it clears look for poor ground connections in the pre-amp side if the amplifier. If it is 50HZ mains hum, capacitors will not cure it!

 

[jaycee]

I seem to recall there are some capacitors in the phono stage and the feedback loop which can go bad and cause a problem, even if the PSX is being used. The ones that spring to mind are a red colour and have "ROE" written on them.

 

[daveshah]

Thank you for your suggestions, I will try these over the week-end. On a second listen, I think it could be described as more of a buzz than a hum.

 

[daveshah]

Can anyone suggest a replacement for the big non-polarised 470uF/6V bipolar capacitors? Bipolar 470uF capacitors seem to be hard to come by.

 

[jaycee]

They don't really have to be bipolar. A 470uF 16v electrolytic of good quality will work just fine.

 

[daveshah]

I replaced those capacitors and the capacitors around the voltage regulators and it's working now. Sounds absolutely brilliant, no noise whatsoever on mute.

Thanks everyone for the help

 

[jaycee]

Good to hear you solved it

 

[amptech]

was these caps the old gold colour one's? as these do dry out sitting near the reg's..

 

[daveshah]

Yes, both of those were dead, and visibly leaking

 

[amptech]

Them voltage reg's get fair hot and the best way to solve this is to bolt them to the main chassis and run small wires back to the pcb that way your replacement pre-amp cap's will not dry out again, I recall doing this on the cyrus 2 may years ago with the same problem.

 

[Tombeur]

Cyrus 2 buzz

Got an old Cyrus 2 + psx that has been unused for 15 years... plugged it in and seems to still work but when volume is turned all the way down I get random noise clicks, buzzes... it sounds a little like a digital glitch, but like when u have poor signal on a mobile phone... it’s not constant just comes in randomly. I noticed that when I fiddle with the phono ground I also get glitches, like a bad connection .
Any idea ? I’m thinking of selling the amp with the psx so would like to repair it.
Thanks !