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Old 22nd October 2009, 10:29 PM   #1
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Default Mission Cyrus III

A few years ago I blew one channel on my Cyrus III (SR 32151) and had it repaired by Mission. The problem now is that 1 channel of the amp doesn’t really work until you turn the volume up to about 11 o’clock when both channels snap back to near equal levels – using an attenuated audio lead to avoid the need for ear muffs .

I believe there may be a fault in the HPA7 volume control section and note it contains a ST 89340 chip. I’ve been unable to get any information on this chip from the ST website and wondered anyone might be able to suggest an option, given that I now live in New Zealand.
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Old 1st December 2009, 04:52 PM   #2
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In this case this thread could be of interest:
Cyrus III power amp schematic here, but schematics of the later models wanted
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Old 1st December 2009, 06:48 PM   #3
anatech is offline anatech  Canada
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Hi sonokeling,
Quote:
I believe there may be a fault in the HPA7 volume control section and note it contains a ST 89340 chip.
I doubt it.
ICs do not fail to be intermittent. What you have there sounds more like an intermittent connection.

So, where do you think you may find intermittent connections in there? The speaker output jacks are a natural, and it's easy to create a broken connection by having a speaker connector get banged sideways. Also, since it's been repaired before, try re-seating the ribbon cables between the input selector / volume PCB and main PCB. Be careful with these, you don't want to ruin the connection. It's also possible that you may have a cable or input jack problem. Further testing on your part can eliminate those problems.

Now for the really important part you should be prepared for. If you manage to remove the main PCB, you have created a problem (assuming the previous repair was done right). These use a special "plastic" transistor insulator and the heat sink is rough. You must do one of two things now. Either purchase the exact same insulators (they are not cheap) and completely clean the mounting area. If you are going to use any other type of transistor insulator, you must reduce the surface roughness of the mounting area to close to mirror smooth. This means starting with a course file and working down to a fine file to almost finish. This ensures a true surface and you must take your time. You will be tired after one side. Finally, a careful finish sand with 600 grit wet/dry paper using a true block. This is also how I actually do this, so I'm not recommending anything that I don't actually do myself. Once the mounting area is both really smooth and clean, you may mount the outputs as you normally would using thermal compound (not the stuff for computers!).

If you are not prepared to do this, do not open it up.

-Chris
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Old 6th December 2009, 06:28 AM   #4
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Thanks for taking the time to explain some more about these amps Chris,

I've had the thing apart before to replace all the diodes with Schottky's. This was before the problem I mention occurred, and I must have got lucky because it worked just fine.

The exact cause of the problem probably goes back to the damage I needed the thing servicing for, when the volume was accidentally turned up by the remote to almost full when it got stuck down a cushion and not realising this adjusted the volume down on the unit and caused some sort of power surge.

Having gone for ultimate fidelity I'd bypassed all the fuses and switches in the back and well, the rest is history.

Many thanks again for your help. As you suggest I will carefully check all the cables and connectors; otherwise, if it was possible to follow the audio tracks, I was thinking to swap out the remote volume for a standard volume control like Mission used in the Cyrus SL, or whatever it was called.
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Old 6th December 2009, 08:06 PM   #5
anatech is offline anatech  Canada
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Hi sonokeling,
Quote:
I've had the thing apart before to replace all the diodes with Schottky's.
Soft recovery types? If not, you were better off with the original parts. The other thing to watch for is the reverse voltage rating, Schottky diodes tend to be rated for lower voltages.

Understand that different components were created for specific jobs. This doesn't make them better, just suited to a specific purpose. The normal rectifiers are about the best for the job they perform. A sharp cutoff characteristic is going to generate more noise than the original parts.

Quote:
Having gone for ultimate fidelity I'd bypassed all the fuses and switches in the back and well, the rest is history.
I wouldn't call this "ultimate fidelity"!
In fact, this has possibly caused your insurance to become null and void. Your equipment no longer meets and safety codes and there is no performance gain to show for it. I'm very concerned of course, from a warranty person's viewpoint and also simply because these changes are not well thought out. Where ever you heard of this change, mentally mark them as a source of faulty information. I hate to say it, but people recommending this are completely irresponsible. They are the people I have been cleaning up after for years and years.

Please, for many reasons, return your amplifier to stock configuration immediately! I'm going to insist on this.

-Chris
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Old 6th December 2009, 11:12 PM   #6
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Hi Chris,

I can only agree that it was a lesson hard learned! The amplifier was returned to its near original configuration before being sent off to Mission and has remained that way since. I purchased the schottky diodes from Russ Andrews, rated for the positions they were fitted. However, I agree completely with your comments on diodes and only use well-rated bridge rectifiers in all my new builds. I guess I meant to put that ultimate fidelity comment in quotes.

I see the restoration of the Cyrus 3 as an interesting project, which has become a lot more likely following your guidance and the links to schematics posted by tiefbassuebertr.
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Old 19th December 2009, 08:17 PM   #7
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So, I carefully checked all ribbon cable connections and the obvious stuff like the speaker output jacks Chris mentions. Replugging the unit, there was no difference. From switch on the Cyrus III starts in mute mode, rotating the volume takes it out of mute with a small click through the speakers. On my unit this happens on the right channel immediately, but the left channel stays at a much lower volume until a specific point at 11 o'clock on the volume dial, when it too has a slight click and the volume balance returns. Obviously at this volume setting an unattentuated input would be extremely loud. I attentuate the signal through a pair of cermet pots in-line with the interconnect, which actually allows me to enjoy the amp at a fair level of fidelity - it pretty much matches the quality of my Adcom unit. I'm pretty certain the problem is in the pre-amp section, which is chip based. Does anyone have experience of a similar issue or any suggestions how I might diagnose this preamplifier issue. Thanks for all the help provided so far.
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Old 17th April 2010, 08:33 PM   #8
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Guess not! I think the word I was looking for was attenuate!
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Old 17th April 2010, 09:34 PM   #9
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Quote:
Originally Posted by sonokeling View Post
So, I carefully checked all ribbon cable connections and the obvious stuff like the speaker output jacks Chris mentions. Replugging the unit, there was no difference. From switch on the Cyrus III starts in mute mode, rotating the volume takes it out of mute with a small click through the speakers. On my unit this happens on the right channel immediately, but the left channel stays at a much lower volume until a specific point at 11 o'clock on the volume dial, when it too has a slight click and the volume balance returns. Obviously at this volume setting an unattentuated input would be extremely loud. I attentuate the signal through a pair of cermet pots in-line with the interconnect, which actually allows me to enjoy the amp at a fair level of fidelity - it pretty much matches the quality of my Adcom unit. I'm pretty certain the problem is in the pre-amp section, which is chip based. Does anyone have experience of a similar issue or any suggestions how I might diagnose this preamplifier issue. Thanks for all the help provided so far.
The whole voltage gain (if volume control stands at full power) must be arround 500
In general I hate cascading of a line amp with voltage gain factor arround 10-times and a power amp stage with voltage gain factor between 30 and 60 (for todays program sources with output voltages between 1Vss and 5Vss). Long before you will reach full power position, clipping point is present (and damaged tweeters in much cases).
Until this day I haven't understand this often to observe practice.

Therefore I go on and remove the active line amp in much cases and put the output wire from volume control direct to the input of power amp. In case of the Cyrus III you will not find a mechanical potentiometer (attenuator) like an Alps. Instead of this there is a electronic attenuator and a custom programable operating controller (MCU).
If you have a schematic of this part, you can try to remove the active line stage (an operational amp e. g.) and instead of this you must now introduce a bypass between the output of the attenuator and the power amp input. But please note, you will find mainly surface mounted devices and therefore you need a lot of experience, even with presently schematic.

Your mentioned solution with the pre-attenuation at the input is also helpful. However then you have still an (discrete or integrated) op-Amp with high gain factor (and additional THD source) in serial to the signal.

Last edited by tiefbassuebertr; 17th April 2010 at 09:39 PM.
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Old 18th April 2010, 07:12 AM   #10
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I like the solution you offer, which fits with my own minimalist approach. I was a bit shocked by all the smd on inspecting the inside of the Cyrus III, and have not yet been able to locate a schematic which would allow me to do exactly what you describe. There was a cheaper version of the Cyrus III which may have had a normal pot for the volume - Cyrus SL..? If I could get a look at the insides of one of them and this showed how to replace the flashy IC-pot for a Sfernice or some such, it might be all that's needed.
However, I get the feeling the best long term solution would be a return to base or the local cyrus agent, but thanks for your advice.
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