Cyrus One
 

This first edition of the Cyrus One amp has one amp shorted. In fact just Q43 and the MJE243 driver Q33 in the upper half of the amp are shorted but best replace them all. R95 the 0.22R sense resistor is badly burned but measures correct. I have the service manual.

The original PT7/BUV28 transistors seem largely unavailable at sensible price. One thread here suggested a MJE3055T but that at 60V max Vce looks a bit marginal to run off the amps +/-30V rails. What is the recommended replacement nowadays?

Thanks
Dave

Re: Cyrus One
 

What about this?

https://www.ebay.co.uk/itm/BUV28-TRA...gAAOSwO7pc0X8Q

Seems pretty reasonable for a power transistor.

Alan

Re: Cyrus One
 

Hi Dave, I worked at Mission-Cyrus from the late 80's until 1999 in their service dept. I saw many Cyrus amp's returned for service that had been repaired with substitute output devices. Usually, they had failed again. After the correct types were fitted (BUV28 for Cyrus One), no more problems occured.
The two smoothing capacitors in the phono stage psu (just behind the volume control) were very prone to failure causing hum. By now, I would think many of the other electrolytics would also be due for replacement too.

Re: Cyrus One
 

Mind you, picture shows an ST device but text says "unbranded"......

BUV28 is a high voltage (400/200V) high speed switching device- an odd choice for audio unless it's class D.

Alltransistors lists possible subs, but they're probably all as obscure. Littlediode lists ST ones though.

Re: Cyrus One
 

An alternative in a larger package is https://www.onsemi.com/pub/Collateral/MJL3281A-D.PDF

A sustained beta high power audio device. £2.64 plus vat from Farnell.

Re: Cyrus One
 

I appreciate that the BUV28 switching transistor isn't the obvious choice for audio but it was the device used by Mission as confirmed by Andrew (PYE 405) who used to work at Mission. I'd stick with the specified device as he has suggested.

Alan

Re: Cyrus One
 

If you can buy one that is not a fake, that is. Obsolete device used in a classic piece of audio gear is open season.

I've been stung by that on three or four occasions, once with massive collateral damage, once when I put a micrometer on the package (too small, too light), then used a hammer to get the chip visible (it was 1/16th the area of a genuine device), and most recently just two weeks ago with jfets.

Heaven knows I ought to have learned by now. Just don't repeat my mistakes.

Re: Cyrus One
 

Fair comment of course but I've not yet found a sure fire way of avoiding fakes even when buying from an apparently reputable supplier.

Alan

Re: Cyrus One
 

Thanks for all the comments, including those from Pye 405 'from the horse's mouth'. I am sceptical about some of the alleged sources of BUV28's as it is now a discontinued product. However looking at why they used a switching device I think it comes down to what was available in high voltage NPN power transistors. A big market for these is in TV switch mode power supplies where 400V and higher transistors are widely and cheaply available. I have compared the spec of the BUV28 with the common BUT11A of which I have a pile. Very similar spec, 1000V even. Lower collector current rating of 5A against 10A for the BUV28 but otherwise identical or better. I am tempted to use them.

MJE3055T definitely fails the comparison, only a 60V device and if these are the substitutes fitted in the items Pye 405 mentions I am not surprised they failed.

Dave

Re: Cyrus One
 

Just wondering if a 5A collector rating is a tad risky. Craig's MJL3281A (15A) might be a safer bet if you don't fancy trying a BUV28 for the reasons stated. Trouble with amplifiers like this is that output transistor failure can take a number of other components to the grave at the same time. Let us know how you get on anyway.

Alan

Re: Cyrus One
 

From a 'reliable' source (ie tried and tested), it seems the BUV48 is a possible replacement, if you can get original old stock, manufactured at a 'similar' date to the ones you are replacing.

From what I can see on the available spec sheets, these (BUV28) are not tightly specified devices so their (important in this design) parameters are likely to vary rather a lot. A much newer manufactured device, even if its called BUV28, is just likely to cause problems and as any plucked out the ether transistor.

dc

Re: Cyrus One
 

Both the BUV48 and Craig's MJL3281A are in the larger TO264/TO3(P) style package so not easy to fit. I shall have to study the circuit to see if there are any important parameters. Of the various TO220 devices I have looked at it seems 10A collector current is unusual in this size package, 5-6A seems more the norm. At normal use I suspect 5A is adequate.

Dave

Re: Cyrus One
 

TIP41C is 6A/100V and are available from Farnell, but a bit risky into a 4 ohm load.

Re: Cyrus One
 

BD743C or BD911 have an IC of 15A and are TO220.

Re: Cyrus One
 

Are the voltage ratings of the junctions sufficient with these TO220 transistors? Certainly not comparable with the BUV28 although I still don't understand why Mission specified a device with such high ratings.

Alan

Re: Cyrus One
 

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.

Maybe not having transient protection is a MacGuffin for audiophiles? or a cultural/religious thing for the designer? or maybe they found ludicrously high voltage transistors to be cheaper than a normal transistor plus a diode?

A look at the circuit while asking the question 'Where could such voltages come from?' gets the answer 'Only from outside' and suggests that some more reasonable transistors, plus a couple of fast diodes would do the job better.

David

Re: Cyrus One
 

Thank you for that David. I'll have a closer look at the circuit diagram.

Alan

Re: Cyrus One
 

I fixed a pal's Cyrus amplifier which had blown transistors on one channel.
I used OnSemi D44H11 & D45H11.

Andy

Re: Cyrus One
 

Don't think the original Cyrus 1 used complementary pairs so Andy's Cyrus may have been a later model. Same issue with the voltage ratings too.

Alan

Re: Cyrus One
 

Sorry, my mistake..
I'd got some complementary pairs at the time I was doing the repair (it was over ten years ago..) but of course, only used the NPN ones in the Cyrus amp.
It's still working fine but is not used for listening at high volumes !

Andy

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

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.

David

Re: Cyrus One
 

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

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

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

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

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

Re: Cyrus One
 

I found it on elektrotanya, and that manual states that v6 was the first of three versions marketed.

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