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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 25th Jan 2020, 1:45 pm   #21
ajgriff
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Default Re: Cyrus One

If David is right, and I'm sure he is, issues with the voltage ratings of the output transistors are most likely to arise if someone starts hot plugging the speakers. Cyrus made a point of warning against this in the owners manual.

Alan
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Old 25th Jan 2020, 2:18 pm   #22
Radio Wrangler
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Default Re: Cyrus One

Use ultrafast diodes made for SMPS rectifiers. They are quick to switch (1N400x aren't quick) and they usually have a controlled breakdown region designed to absorb a specified amount of energy.

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Old 25th Jan 2020, 7:04 pm   #23
ajgriff
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Default Re: Cyrus One

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Originally Posted by Radio Wrangler View Post
The amplifier circuit doesn't have the usual catching diodes from the output to the power rails that are usually fitted to limit transients should a speaker be disconnected while in operation.
Information about the Cyrus 1 is sketchy on the web so the following may not be entirely definitive. However, it would seem that the amplifier was subject to ongoing revision during the production run and later versions were fitted with 'catching' diodes (1N type). Despite this Mission/Cyrus continued to use the BUV28, probably because the company had a bin full of them marked up with its own PT7 code. It could be, therefore, that the voltage rating of replacement transistors is not as critical as might be imagined depending upon when the particular amplifier board was made. Not sure whether this will help Dave (OP) or not.

Alan
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Old 25th Jan 2020, 7:39 pm   #24
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Default Re: Cyrus One

Voltage rating is not really the issue as I have a box full of BUT11A (450V) and similar. The only doubt is that 5A is a bit marginal for 4 ohm speakers at full volume. If it is always used with 8 ohm speakers that is not an issue either.. I have the feeling that BUT11A's will do the job.

Back on the job on Monday.
Dave
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Old 3rd Feb 2020, 6:48 pm   #25
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Default Re: Cyrus One

Cyrus 1 has catching diodes on the outputs, clearly shown and described in the service manual (which is readily available on the web). Nothing 'sketchy' about it.

A couple of BUT11A's fitted along with one of the driver transistors and a new 0.22 ohm resistor. Now working fine again, did need to use some switch cleaner on the noisy mode selection switch but otherwise fine. All the caps also measured fine on my Bob Parker ESR meter.

Thank for the help.

Dave
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Old 3rd Feb 2020, 7:13 pm   #26
ajgriff
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Default Re: Cyrus One

The reason for using the word 'sketchy' was that the only manual I could find starts at Issue 6 and I couldn't track down any hard information about earlier versions some of which I believe may not have been fitted with catching diodes. Might be mistaken of course. Anyway it's good to know that the amplfier is working well again and that it has the necessary diodes.

Alan
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Old 4th Feb 2020, 7:46 am   #27
dsergeant
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Default Re: Cyrus One

Then look again. Can't remember where I got my manual from, easily found with Mr Google, and covers all versions with detailed discussion of the differences between versions and how to identify them. 3MB so too big to upload here.

Dave
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Old 4th Feb 2020, 8:20 am   #28
Ted Kendall
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Default Re: Cyrus One

I found it on elektrotanya, and that manual states that v6 was the first of three versions marketed.
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Old 4th Feb 2020, 8:40 am   #29
ajgriff
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Default Re: Cyrus One

My mistake then although I did find the manual on elektrotanya but clearly didn't study it closely enough. Apologies for the confusion caused.

Alan
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