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Thread: Amp Hospital: Cyrus One

  1. #1
    Join Date: Dec 2011

    Location: South downs

    Posts: 3,477
    I'm James.

    Default Amp Hospital: Cyrus One

    I recently purchased a Cyrus One amplifier partly out of curiosity & partly because I felt sorry for it.

    It was in a sorry state as you can see with nextel paint flaking off all over the place, all of the controls only worked in certain positions, 50hz hum over the speaker outputs & the phono stage sounded like aliens were trying to make contact !



    I disassembled it & found a very nice Holden&Fisher transformer, LM317 regulators, Alps controls, soft recovery diodes & slit foil psu caps all with a very high quality circuit board & layout... I like !



    Started work on it today, now I dont normally bother with fancy capacitors but decided to give this one "the works" so replaced all of the electrolytics with Nichicon Muse (pretty colours!) The power supply caps got changed for Kendeil 10000uf/50v (originals were 7000uf/40v) both bypassed with MKP films... Also replaced the power amplifiers input caps with MKP films. I've just finished cleaning the volume control/input selectors with Servisol which seems to have helped them.



    Currently giving it a little test run & it sounds very promising indeed ! Just waiting for some more capacitors to arrive + DIP8 sockets for the opamps

    I'll be stripping & respraying the cast aluminium casework at the weekend.... Keep you posted

  2. #2
    Join Date: Jul 2010

    Location: North Cambs UK, Earth, Sol, Orion - Cygnus arm of galaxy

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    I'm MadeOfDeadGiantStarsThatExplodedEonsAgo.

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    I had a Cyrus amp here a while ago for some reason or other, it went back working at any rate

    Consider yourself lucky that your amp has a real heatsink The Cyrus that I was working on only had a shallow U shaped 4mm thick alloy heatsink - a bloody joke

    Think of the outer two fins & flat bottom of yours that the transistors are attached to... It wasn't even black anodised
    Bests, Mark



    "We must believe in free will. We have no choice" Isaac Bashevis Singer

  3. #3
    Join Date: Dec 2011

    Location: South downs

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    I'm James.

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    Quote Originally Posted by Reid Malenfant View Post
    I had a Cyrus amp here a while ago for some reason or other, it went back working at any rate

    Consider yourself lucky that your amp has a real heatsink The Cyrus that I was working on only had a shallow U shaped 4mm thick alloy heatsink - a bloody joke

    Think of the outer two fins & flat bottom of yours that the transistors are attached to... It wasn't even black anodised
    Ah yes, the MK1 ! AKA V6. Mines the V7... They removed the headphone socket, cast a heatsink into the bottom panel & fitted a rocker power switch otherwise the same though.

  4. #4
    Join Date: May 2008

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    I'm David.

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    The main cause of hum on output used to be the supply caps in the phono stage failing (yours had the later ones fitted thoughm the failing ones were the encapsulated Roe's originally fitted). The basic circuit is almost bomb-proof I reckon and is basically a power amp with a volume control on the front. If you have means of measuring, make sure there's no ringing at higher power levels, as it's slight (almost) oscillation that could make the sound "exciting" back then....
    Tear down these walls; Cut the ties that held me
    Crying out at the top of my voice; Tell me now if you can hear me

  5. #5
    Join Date: Dec 2011

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    Quote Originally Posted by DSJR View Post
    The main cause of hum on output used to be the supply caps in the phono stage failing (yours had the later ones fitted though the failing ones were the encapsulated Roe's originally fitted). The basic circuit is almost bomb-proof I reckon and is basically a power amp with a volume control on the front. If you have means of measuring, make sure there's no ringing at higher power levels, as it's slight (almost) oscillation that could make the sound "exciting" back then....
    Your quite right Dave, the phono fault was caused by the 470uf caps in the power supply. I measured them after desoldering & they read 29uf for about 1 second before shorting, Amazing the thing kept working at all !

    I dont have a scope but IME if oscillation is occurring certain parts will get hot. Its now been running 4 hours & everythings cold

  6. #6
    Join Date: Jul 2010

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    Quote Originally Posted by istari_knight View Post
    I dont have a scope but IME if oscillation is occurring certain parts will get hot. Its now been running 4 hours & everythings cold
    Nice to know the heatsink works in that one

    You are pretty much on the money, ultrasonics will heat things pretty quickly



    Enjoy
    Bests, Mark



    "We must believe in free will. We have no choice" Isaac Bashevis Singer

  7. #7
    Join Date: Dec 2011

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    The V7 Cyrus One & Cyrus Two both share the same casework so the heatsink is rated for 50wpc outputs. This being the One outputs 25wpc so it runs stone cold.

    I've seen pictures of the earlier example & the heatsink is woefully inadequate.

  8. #8
    Join Date: May 2008

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    Apologies. When I said "oscillation," I meant slight oscillations on transients, which is something done back then to get an impressive sound and an easy quick dem. Arcams and Marantz's did this as a matter of course at certain times and on certain models to keep their 5 star ratings. You don't always notice this as an issue until you hear a bigger and better amp that doesn't do it...

    Anyway, glad you're getting good sounds from the Cyrus One. These later ones did feel quite solid and phono supply caps apart, they were very reliable. Once upon a time, an owner's girlfriend accidentally turned the record and play switches to the same source with the volume turned up. The squeal took out the tweeters very quickly, but the amp continued like this (inaudibly) for a good while. the board and most components were well cooked, but the amp was repairable, and I don't think a new board was fitted either

    All I'd ask is to use efficient speakers with it and don't push it flat out, since it isn't a soft-clipper
    Tear down these walls; Cut the ties that held me
    Crying out at the top of my voice; Tell me now if you can hear me

  9. #9
    Join Date: Sep 2011

    Location: Bridgwater UK.

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    I'm alan.

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    A great amp,for the money,i used a V7 for a few years on my PC system with a v-dac into an old pair of alesis 4 ohm studio monitors.Sometimes sounded better than my main system,ie late at night with a large glass of malt
    Just sold it for more than i paid for it, because my Mrs seemed to think that my computer room was her dinning room,and why can,t i just use a laptop like everybody else )It never got hot and put up with everything i did to it...
    Look forward to seeing the photo's of the finished restoration.

  10. #10
    Join Date: Dec 2011

    Location: South downs

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    I'm James.

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    Quote Originally Posted by DSJR View Post
    Apologies. When I said "oscillation," I meant slight oscillations on transients, which is something done back then to get an impressive sound and an easy quick dem. Arcams and Marantz's did this as a matter of course at certain times and on certain models to keep their 5 star ratings. You don't always notice this as an issue until you hear a bigger and better amp that doesn't do it...

    Anyway, glad you're getting good sounds from the Cyrus One. These later ones did feel quite solid and phono supply caps apart, they were very reliable. Once upon a time, an owner's girlfriend accidentally turned the record and play switches to the same source with the volume turned up. The squeal took out the tweeters very quickly, but the amp continued like this (inaudibly) for a good while. the board and most components were well cooked, but the amp was repairable, and I don't think a new board was fitted either

    All I'd ask is to use efficient speakers with it and don't push it flat out, since it isn't a soft-clipper
    Its currently driving a pair of Mordaunt-Short Classic 20 standmounts I got for 99p ! apparently they were £400 new & their quality would support this.

    I normally listen at around 75db according to my meter which into 90dbw speakers = what ? 1/2watt output or something silly! I dont think I use more than 5watts ever to be honest so no chance of me getting ringing on transients... Not that I've noticed anyway. I did push it harder than I ever normally would earlier and there was a detectable change in character as you describe I can imagine complimenting alot of 70's speakers.

    If used at a low level with 90db+ speakers theres little need for anything more it would seem.
    Last edited by istari_knight; 30-08-2012 at 17:48.

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