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Old 21st December 2004, 07:55 PM   #1
karl.s. is offline karl.s.  United Kingdom
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Default Trying to fix my Cyrus II amp

Hi

I am trying to fix my Cyrus II amp (using Cyrus I schematic, which does seem to match for the bits I am interested in) I've done quite a bit of measurement with a 1KHz sine wave and a 'scope, but being mainly into microcontrollers started to get stuck (more than 3 transistors and feedback too!) Wondered if anyone had any ideas...attached is schematic I got from the web..

The problem is that the right channel has a much reduced power - it still drives the speaker, but at about 1/5th of the amplitude of the left channel.

I have done a preliminary check with a 1kHz sine wave input - and comparing the signals on left and right channels with an oscilloscope, I have the same input levels through the selector switch up to R49/R50. The volume control looks as if it controls both channels equally. The collectors of Q7 and Q8 show the same amplitude on each channel…but the base of Q40 has a much reduced amplitude compared with Q39… but analogue electronics is not my strong point...
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Old 21st December 2004, 09:24 PM   #2
anatech is offline anatech  Canada
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Hi Karl,
Replace C41 & C42. Those are the orange coloured capacitors under the ribbon cable.
-Chris
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Old 21st January 2005, 02:36 PM   #3
karl.s. is offline karl.s.  United Kingdom
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Hi Chris

Thanks for your suggestion.

It's taken a while to be able to fix it, as originally I looked at your suggestion to change C41 and C42 and couldn't figure out why you suggested those 2! I think you meant to say C43 and C44 (i.e. the 470u non polarised electrolytics) ?

Changed those and WOW! It blew me away! So much more power it's unbelievable! Checked the values - the right one was down to 6nF and the left down to about 200nF - no wonder it was bad! And I had put it down to being past 40!

Haven't yet got hold of 2 non polarised 470u caps, so improvised with 2 electrolytics in series (joined negative terminals).
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Old 22nd January 2005, 03:15 AM   #4
anatech is offline anatech  Canada
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Hi Karl,
My schematic indicates those part numbers, but you got the ones I intended. Those caps only have 100mV across them or so. Polarised may be okay and if they go, change them.
You circuit gain was close to unity before. Now it isn't.

-Chris
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Old 16th September 2005, 08:20 PM   #5
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Thanks anatech for your post. It was very helpful in fixing my Cyrus 2 as well!

After a lot of poking with a scope I decided that C43/44 was probably bust on one channel. Once I got it off the board I saw the case was cracked with electrolyte leaking out - so a more careful visual check would have been useful.

Those 440uF non-polar caps are strange things. I've never seen them anywhere else. You can still order them from Cyrus as spares BTW if anybody else cares.

The Cyrus 1 schematic here is also very useful. The power amp from the Cyrus 2 appears almost completely identical (though I am told the output transistors are different). One other point worth noting is that R65 is 750R not 1k as shown on the schematic. As this controls the -ve feedback it's rather important. I spent ages trying to work out why the gain on the working channel didn't match what it should be from the schematic!
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Old 16th September 2005, 08:55 PM   #6
anatech is offline anatech  Canada
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Hi alphabeta,
Glad that helped. I have also noticed that all the capacitors in the low voltage regulator section need to be changed. Pay attention to the heatsink to regulator contact. I have to retension the clamps and grease the heatsinks. That lowers the temperature on those poor reguators. They run all the time, even when you switch off.

-Chris
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Old 17th September 2005, 12:33 PM   #7
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Thanks. I did notice the regulators ran very hot and had tiny badly fitted heatsinks. As I only use it as a line-level amp though I didn't bother to fix them.

I was half thinking about taking all the MC/MM preamp out and fitting some kind of remote-control volume circuit in the space instead! Probably not worth the effort, but might be an interesting project if I get bored.
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Old 17th September 2005, 08:51 PM   #8
anatech is offline anatech  Canada
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Hi alphabeta,
In that case, consider pulling the LV rectifiers. Shuts that section down and reduces any commutation noise from the unused supply.
-Chris
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Old 18th September 2005, 11:55 AM   #9
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Chris

Thanks. I might well do that. Looks like I am going to have to do some more work anyway.

In its new role it's got too much gain. Everything useful is in the first two clicks of the volume. What's the best adjustment to change the range? I was thinking of changing R65 (the bottom of the -ve feedback divider) to something lower.
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Old 18th September 2005, 02:07 PM   #10
anatech is offline anatech  Canada
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Hi alphabeta,
Wow, you are sending it a hot signal, or you don't want very much output. Either way you don't want to change the feedback ratio. You would need to increase that resistor to reduce gain, increasing the feedback ratio.

Why not just attenuate the signal before the amp?

-Chris
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