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Old 28th April 2012, 09:32 PM   #21
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by Ian Finch View Post
This is what sreten pointed out; R81,82 has been removed completely
and still the bias is too low, so replacing it as I first thought, like you,
won't work. You only have R83,84 to reduce or R85,86 to increase.
It's a voltage divider in a sense.
Hi,

I thought I'd pointed out also the two test points are a dreadful place
to attempt to measure the bias, I mentioned two other better options.

Shorts are never dead shorts due to finite track and soldering resistances.
The test points are useless in terms of measuring bias without the 130R.

rgds, sreten.
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Old 28th April 2012, 09:41 PM   #22
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by jitter View Post
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.
Hi,

Well IMO it clearly has, as it has in my Audiolab. Otherwise simple wire
links would be cheaper if hand assembled, and the board layout would
not need to stick to standard resistor lengths for the wire links.

rgds, sreten.
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Old 28th April 2012, 10:14 PM   #23
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Yes sreten, picked that point up in post#12. Without knowing what the OP read or understood of the procedure, it's difficult to know what he measures. However, it's not likely that with R81(130R) + R83(680R) in that arm of the divider that V(130R) was only 5-8mV in the version referred to by the manual, so I can't imagine those trimming posts were identified as test points.

I assumed he had found the output emitter resistors and they would surely have been specified in the manual. I also understood his comment in #1 to mean he was simply testing what the 0R links were. Perhaps not - we await his return.
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Old 29th April 2012, 07:20 AM   #24
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by Ian Finch View Post
The trim resistor R81,82 has been removed in manufacture so the service manual is simply wrong for this board. The bias sub-circuit increases current in the bias transistor to reduce bias voltage across bases Q27,29 and Q28,30.
But R81/82 have not been removed! R81/R82 are NOT the 220R resistors that we have seen sitting high up in between the pins in the very first picture (close up) because in the overview picture in pampalini's second post, you can see they're not there.

The way I see it is that the silkscreen R81/82 is closest to the 0R resistors, and that the pins themselves have no reference designator. They are parallel to R81/R82, though, that's where they're drawn in the schematics.
That makes the situation as follows:

R81/82 schematics: 130R - but here: 0R;
R83/84 schematics: 680R - but here 920R;
R85/R86 schematics: 3k6 - but here 2k6.

The fact that R81/R82 are 0R explains why parallelling them as no effect and why pampalini measured 0R between them, had they been 130R, the instructions in the service manual would have been useful. That's why I suggested replacing all resistors first. And yes, it's also why measuring there would be no good.
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File Type: gif cyrus1pwr_mod.GIF (23.0 KB, 66 views)

Last edited by jitter; 29th April 2012 at 07:45 AM.
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Old 29th April 2012, 07:25 AM   #25
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by sreten View Post
Hi,

Well IMO it clearly has, as it has in my Audiolab. Otherwise simple wire
links would be cheaper if hand assembled, and the board layout would
not need to stick to standard resistor lengths for the wire links.

rgds, sreten.
The fact that 0R resistors are in place of simple wires means very little. I work in industrial electronics manufacturing and we use these 0R resistors all the time. And no, we don't have a component stuffing machine for through hole devices.
Finding 0R resistors on a pcb might mean no more than that the client has specified them in their BOM rather than wire.

Last edited by jitter; 29th April 2012 at 07:49 AM.
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Old 29th April 2012, 07:52 AM   #26
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Just a quick reply....First of all thank everybody fo te help!
I did mesaured, as stated in the service manual, the bias across the big white .22 ohm resistor (part A).


The 220 ohm you see in the pix is my attempt, once again as stated in the service manual, to increase the bias. But, as the 81/82 resistor in my unit is 0, putting a 220 ohm resitstor in parallel, provide no effect to the bias current.
..so, once at home, I will measured the resistors, compare them with the list in the SM, and then think what to do....
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Old 29th April 2012, 08:01 AM   #27
jitter is offline jitter  Netherlands
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Beware that measuring in-circuit of resistance values probably leads to different readings than what is expected on the basis of the colour-coding on the resistors.
The values I stated are based on what I could see in the very first close up pic in this thread.

Last edited by jitter; 29th April 2012 at 08:04 AM.
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Old 29th April 2012, 12:27 PM   #28
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Sure, the actual values of R83,84 and R85,86 appear to be different from the schematic - I cannot be certain what they are but R81 is gone. It is now a link.

My reasoning regarding the manual refers to the original design which it appears to do also so the bias adjustment according to the manual and attempted by the OP didn't do anything. I think that's clear enough.
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Old 29th April 2012, 01:02 PM   #29
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Tiefbassuebertr. has just bumped up this old thread Cyrus One with a familiar pic.

To simplify the whole discussion and make it easier for Pampalini, assuming values are correct, just place a 10k resistor across R83 (briefly) and post result.
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Last edited by Ian Finch; 29th April 2012 at 01:30 PM.
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Old 29th April 2012, 09:01 PM   #30
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by Ian Finch View Post
Sure, the actual values of R83,84 and R85,86 appear to be different from the schematic - I cannot be certain what they are but R81 is gone. It is now a link.
OK, sorry, I misunderstood the meaning of "gone" here.
Found in another thread started by pampalini that C53-C56 are also not according to the schematics.

I can't say that I like this practice of not updating the service manual with every new issue. Or is it that "issue 1" in the sideline in the schematic posted by pampalini actually does refer to the Cyrus One issue 1 and that a service manual issue 7 also exists?

Last edited by jitter; 29th April 2012 at 09:10 PM.
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