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Old 26th April 2012, 03:55 PM   #1
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Default Cyrus One Bias Issue

Hi,
I have a Cyrus One issue 07 TOG. I checked the bias recently and I have a channel measuring 5/6 mV. The other channel is ok measuring 8mV.
I have the service manual, and so I read there that if the value is low you must put a 220ohm resistor across the R81/R82 (This resistor should be 130ohm if I remember right).
Well, the strange thing is that on my unit the R81/R82 are those kind of jumper/resistors...here is a pix...
http://img857.imageshack.us/img857/5513/img0563vf.jpg
...if I check these resistors with my DVM i read 0....
If I put the resistor across the pins...nothing happens to the bias...
Anybody has the same issue as me?
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Old 26th April 2012, 05:59 PM   #2
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Referring to the schematic on Geoff Moss's site below, you have R81 replaced with that link so you cant read across the test point anyway. Is this mod. the same in both channels? (R82) If so, someone has tried to change the bias before you and been unsuccessful.
The schematic suggests you replace the link resistor, not just parallel a short circuit. It could be worthwhile returning the values to orignal, unless the parts seem original manufacturers modifications.
http://www.tcaas.btinternet.co.uk/cyrus1pwr.gif
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Old 26th April 2012, 06:29 PM   #3
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hi Ian,
I bought this amp new in 1996, and I am the only owner. I think that this is 100% original. This is the schematic for my version
http://img688.imageshack.us/img688/6...rvice2flat.jpg
Here is an overview of my amp:
http://img84.imageshack.us/img84/8545/img7505b.jpg
...as you can see the R81 and R82 are the same type of the X21/X22, etc.....
I was thinking about changing the link with a 130ohm resistor...but I'm afraid to blow everything up!



Quote:
Originally Posted by Ian Finch View Post
Referring to the schematic on Geoff Moss's site below, you have R81 replaced with that link so you cant read across the test point anyway. Is this mod. the same in both channels? (R82) If so, someone has tried to change the bias before you and been unsuccessful.
The schematic suggests you replace the link resistor, not just parallel a short circuit. It could be worthwhile returning the values to orignal, unless the parts seem original manufacturers modifications.
http://www.tcaas.btinternet.co.uk/cyrus1pwr.gif
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Old 26th April 2012, 07:48 PM   #4
jitter is offline jitter  Netherlands
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Looking at the picture and comparing them to the schematic reveals that your amp has different resistors.

R81/R82 carry only a black ring as far as I can see in the picture. This means they're 0R resistors. That would explain why parallelling another resistor to them with has no effect on the bias current, you cannot lower 0R any further
R83/R84 seem to carry white-red-black-black, or 920R, not 680R as per the schematic.
R85/R86 seem to carry red-blue-black-brown, or 2k6, not the 3k6 from the schematic.

Since you're the first owner, I expect it left the factory that way.
If the amp sounds OK, I wouldn't touch it. Since this amp hasn't got potmeters here, the slighly lower bias current in one channel might have been like that from new.
"If it ain't broke, don't fix it."

Last edited by jitter; 26th April 2012 at 07:55 PM.
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Old 26th April 2012, 08:02 PM   #5
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Hi jitter,
actually, I recapped the ampli...the pix I posted is my unit before the recapping. Actually the ampli works fine, with a nice and warm sound. I was just curious how I could adjust the bias on this unit...from what you wrote, it looks like this ampli has fixed bias...am I right?


QUOTE=jitter;3002197]Looking at the picture and comparing them to the schematic reveals that your amp has different resistors.

R81/R82 carry only a black ring as far as I can see in the picture. This means they're 0R resistors. That would explain why parallelling another resistor to them with has no effect on the bias current, you cannot lower 0R any further
R83/R84 seem to carry white-red-black-black, or 920R, not 680R as per the schematic.
R85/R86 seem to carry red-blue-black-brown, or 2k6, not the 3k6 from the schematic.

Since you're the first owner, I expect it left the factory that way.
If the amp sounds OK, I wouldn't touch it. Since this amp hasn't got potmeters here, the slighly lower bias current in one channel might have been like that from new.
"If it ain't broke, don't fix it."[/QUOTE]
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Old 26th April 2012, 08:08 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

I think R81/82 have been replaced by "mystery" components that are in
fact dead shorts, and allow a component stuffing machine to insert PCB
links in the same way as real resistors.

To compare real bias voltages between channels measure the DC voltage
across C51 and /or the voltage across the 0.22R output resisistors.

FWIW replacing a short with 130R in the "Vbe muptiplier" will reduce bias.

rgds, sreten.
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Old 26th April 2012, 08:22 PM   #7
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by sreten View Post
Hi,

I think R81/82 have been replaced by "mystery" components that are in
fact dead shorts, and allow a component stuffing machine to insert PCB
links in the same way as real resistors.
I doubt such a machine would have been used on low volume production.
But who knows, all links seem to be named X.. and close by is another 0R resistor in the position named X30/X31.

Quote:
To compare real bias voltages between channels measure the DC voltage
across C51 and /or the voltage across the 0.22R output resisistors.

FWIW replacing a short with 130R in the "Vbe muptiplier" will reduce bias.
Why would Cyrus have chosen to deviate from the schematics? Or is it that the schematics from the One issue 7 from Geoff Moss aren't actually from an issue 7?

Last edited by jitter; 26th April 2012 at 08:25 PM.
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Old 26th April 2012, 08:26 PM   #8
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I posted an Issue 07 and TOG schematics (same as amp)...not Issue 1....
ah...ok...sorry, you edited your post....
The question why cyrus deviate from the schematics is the key question...what I could add is that my unit actually was assembled in Taiwan...so maybe they messed up the things...but why?

Quote:
Originally Posted by jitter View Post
The schematics posted by pampalini are from an issue 1 (wouldn't we nowadays call that "revision"?).

Last edited by pampalini; 26th April 2012 at 08:28 PM.
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Old 26th April 2012, 08:34 PM   #9
jitter is offline jitter  Netherlands
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[QUOTE=pampalini;3002215]Hi jitter,
actually, I recapped the ampli...the pix I posted is my unit before the recapping. Actually the ampli works fine, with a nice and warm sound. I was just curious how I could adjust the bias on this unit...from what you wrote, it looks like this ampli has fixed bias...am I right?

Well, the amps I've owned all had potmeters for setting the quiescent current. But the service manuals always give one particular value, so effectively it's fixed too.

Yes, I edited my post. I saw "issue 1" vertical far left in your schematic, but realized this issue is of the service manual itself, not the amp.
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Old 26th April 2012, 08:41 PM   #10
jitter is offline jitter  Netherlands
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Quote:
The question why cyrus deviate from the schematics is the key question...
Perhaps with the resitors in the schematic the right quiescent current couldn't be achieved with the used transistors. Variation between transistors is why there usually is a potmeter for the qc-setting. After replacing the output transistors with new ones, the qc usually needs to be corrected.

Quote:
what I could add is that my unit actually was assembled in Taiwan...so maybe they messed up the things...but why?
So the proud "Made in Scotland" sticker only applies to the toroid it's stuck on...
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