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