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Old 30th August 2015, 09:22 PM   #1
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Default Refurbished Cyrus II with 'hum'

Evening all

Emboldened by reading widely on this site, and with a soldering skill acquired during work experience at a surface mount electronics factory when I was at school (in 1990!), i thought i'd give this a go!

I purchased two Cyrus II amps (the 07 variant) with a pair of Argonaut speakers. The amps have been configured for mono operation, and they've clearly been worked hard for a living! I've recapped them with Panasonic/Nichicon capacitors, undertaken a few repairs to the pcb (the bypass resistor feeding the power-on LED, and the phono stage voltage regulators had both scorched the PCB nicely). I moved the ribbon cable to underneath the PCB to aid airflow around the power amp section, and fitted some VanDamme interconnect cable from the CD input direct to the pins of the input selector, with the earth shield fixed at one end to the small-signal ground point. Something else I did was fit a small daughter board with a latching relay to enable the fragile on/off switch to operate with a simple push-to-make button behind it (rather than the cranked plastic lever thing which had broken). How's it sounding so far...?

The good news is that they sound absolutely spectacular! The bad news is i've got a hum :-(

It changes in level with the interconnects disconnected, and it gets louder the more the volume is turned up (but its always there). I've cleaned the earth points and made sure the parts of the PCB which need to contact the chassis do so. But i'm stumped? What can I do next to get rid of the dreaded hum?? I have the main driver and power transistors in stock - is it worth changing them for example?
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Old 31st August 2015, 07:22 AM   #2
Mooly is offline Mooly  United Kingdom
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The hum wont be caused by any of the semiconductors. Hum and noise issues are some of the most frequent to appear on the forums....

Have a read at this, totally different amp but the procedures are the same for diagnosing.
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Old 31st August 2015, 09:09 AM   #3
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Which input are you switched to? Does it make any difference? If you short the input phono with a linked phono plug, does the hum increase, (if it does there is an earth loop issue) if it decreases there is a signal path issue and screening/re-routing will be required. Keep thinking outside of the box and the issue will become obvious.
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Old 1st September 2015, 07:20 PM   #4
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Thanks for the tips! The hum is on all inputs, but worse when the pre amp is connected. Disconnecting the phone input and the hum quietens down a lot (but it's still there). Shorting the input makes no change. Still hums with mains lead disconnected and the amp freewheeling down on the PSU caps...what ya reckon?
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Old Yesterday, 07:04 AM   #5
Mooly is offline Mooly  United Kingdom
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If you have hum with the inputs shorted and the amps running on just the charge in the reservoir caps (mains lead pulled from socket) then I can only think it must be stray pickup from neighbouring equipment (the radiated field from other transformers).
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Old Yesterday, 07:13 AM   #6
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Can you provide more details of your VanDamme cable mod. I suspect that is an issue if not the full cause of your hum problem.
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Old Yesterday, 07:45 PM   #7
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Quote:
Originally Posted by richie00boy View Post
Can you provide more details of your VanDamme cable mod. I suspect that is an issue if not the full cause of your hum problem.
You might be right! Basically, i wired the nice VanDamme cable to the phono sockets (the solder pads that take the signal feed from the centre pins), then connected the other ends directly to the solder pins on the input selector. The screening is connected at one end to the small signal ground point. This and the ribbon cable have been flipped over so they run underneath the PCB instead of over the top...i thought this would help with heat dissipation and it looks neat, which pleases me immensely!

...i'm wondering if this is why Cyrus put the ribbon cable where they did :-(
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Old Today, 07:10 AM   #8
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Well you have just made a good antenna with that Van Damme cable. As there is no issue with the original PCB routing I suggest you remove the cable.

The ribbon cable you may be able to leave on the bottom. It is placed on the component side at manufacture because the PCB undergoes an automated solder process, if the cable was underneath it would get melted.
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