Assistance with late issue Cyrus One. Repair output stage and eventual upgrade.

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Hi guys,
Long time lurker and -- I believe -- first time poster. I haven't contributed earlier and I'm embarrassed to ask for help in my first post. Well...

I bought new a Cyrus One in 1991. It was my main and later bedroom amp for years. Suddenly I began moving around Europe and most of my things went into storage. After many years, now I suddenly need that amp. Badly.

It had (when put in storage) been acting weird for a little while and I realized already 10 years ago that something was wrong with it but I had other things to worry about. I supposed it was the power supply (old capacitors and all). I tried it two or three years ago and it was (surprise) still acting a bit strange. The symptoms were that it worked fine for a while and then suddenly bad distorsion for a second and then silence.

It is now in parts in my workshop and I have checked (following the advice from the service manual) the rail voltages -- perfect. I lack all the beautiful instruments I dream about (at least after wine and cheese) and my electronics workshop consists of a small case with a few tit bits and a multimeter ...and a soldering iron. (I can show you some nice joinery planes though!)

Then I measured the output transistors and the fusible resistors. The resistors have survived but the transistors (Q41-44 + Q31-34) are **** (one seems to be fine). I measured them on the PCB and also desoldered.

I suspect that I have done the bad thing with the speaker wires at some point. There were no bangs and lightning though. I realized today that the fuses (FS1-2) were blown. Last time it misbehaved must have been the final straw.

I have a 26 year old background in electronic engineering, I can read a simple schematic, and I am not totally lost with solder. However, I lack experience and I wouldn't call myself a real Audio DIYer. Electronics dilettante is more appropriate. You know, one of the guys who reads a lot about what others have done... :)

Sorry about the rambling.


1. This is a popular amplifier, at least in the UK -- and Scandiwegia. I have the service manual and I have searched this forum. There is a lot of help on the web. But: Could you guys recommend me more things to check on the board. What usually breaks when one shorts the speaker wires? Apart from the fuses... sorry, transistors in the output stage (and sometimes even the actual fuses). I shall order parts tomorrow and I might as well order more.

2. When fixed I will (sensibly) upgrade -- change -- the electrolytes for fresh ones. Any tips? If none, I'll go with Nichicon golds.

3. If all goes well I'll put bigger caps in the PS (C65-66). Would it be sensible to also put bigger caps in the phono preamp? (C67-68) It doesn't "feel" right.

4. I will already change the burnt BUV28s for BUV48s. But, is it OK to substitute the MJE243/253s for BD237/238s? NOS MJE243 are pretty polly (+ shipping) and BD237 is 50 cents (+ tax). "I read somewhere" that it is fine. Any thoughts?

5. I listen almost exclusively to "vinyl" (hate the word). When searching I can't seem to find anyone who has populated the Cyrus-two-MC-pre part of the Cyrus One board and done a full conversion. As far as I can see it takes a few components and moving the connectors for the switch. Am I wrong? I have scored some LM394s so i might try it. I might as well do this rather than build another cheap mc-pre. (I can build a fancy one later!)

6. While I am at it I might as well change the volume pot. Is it worth shoehorning an ALPS RK271?

Sorry about the many words. I hope someone wants to read my questions and give me some advice.

If I go through with the larger upgrade I shall obviously post the results here. I have a new turntable cooking and that'll also go here.

/Chris, Stockholm
The output transistors usually go when speaker output is shorted.
They usually pop quicker than the fuses will.

Hopefully that is all that is wrong.
Sometimes driver transistors can be taken out with output transistors.
I would be checking all the transistors with a multi-meter.

When you do finally come to power it up again put a mains lamp in series with mains to limit current. If the bulb comes on and stays on there is still a problem. If it just flickers a little on power up then things are probably OK.
Joined 2010
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Don't substitute semis in an output triple such as in Cyrus amplifiers, if it is possible to avoid. "Fine" is a nice, breezy term that could mean anything from 'seems OK to me' or 'measures ok', doesn't get hot etc. to even the summing up of an engineer's analysis or an audiophile listening comparison. The source of the suggestion will matter and on forums where you might read such, there may not be a technical overview or membership who are informed enough to comment and correct.

Some salient differences between BD237/238 and MJE243/253:
Maximum current rating is half.
Ft(min.)is only 3 MHz compared to 40 MHz
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Joined 2010
Paid Member
There are several existing threads on recapping Mission Cyrus1 and 2, including this recent one.
No single thread will cover every issue correctly but you will get the idea that what is important is really simple and and not very expensive. Some folks become obsessed with replacement part brands and grades that have extremely low ESR though, even where it is unhelpful. A little education can be dangerous, it seems. :rolleyes:
Yes Ian,
I agree. Audiophilia quickly becomes esoterica. I try to go with "well made and tested". I don't have the resources, audioalter stress syndrome, or technical talent to go with excessive component rolling.

However, I'm still a romantic. I like the look of a few nice looking components, nice layout, and good looming!
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