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Old 20th August 2008, 07:01 PM   #1
tuck1s is offline tuck1s  United Kingdom
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Default Oh dear, I killed my Cyrus 2. Where to start faultfinding?

Looking for an intermittent phono socket connection on my Cyrus 2, I removed the PCB, cleaned it up, and put it back. I didn't tighten up the heatsink power transistor mounting "bars" as I thought to check for specific torque settings first.

Needless to say, I then did a very dumb thing - forgot, and put it back together. After around 30 minutes use, it stopped.

Checked the PSX, and both 4A quickblow fuses had gone. Replaced those, and they just went again. So something is now shorting out the power rails.

I am guessing the the power transistor pairs (PT77s) are probably what's gone. My questions are really around where I should begin. Should I

- Just replace the PT77s with equivalents (richie00boy recommends MFE340/MJE350 on another thread here)
or
- desolder the PT77s, power up the rest of the amp. Will it be stable (i.e. not for music purposes, but just to make some simple tests e.g. fuses no longer blow)?
or
- desolder the PT77s, test them on my multimeter Hfe tester, but not power up the amp until I have installed replacements?

All contrib's welcome, particularly from you guys who really know the insides of the Cyrus.

It's a rev07 by the way. Can post pics if it will help.

Steve
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Old 20th August 2008, 07:12 PM   #2
Did it Himself
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The MJE340/350 are for the drivers not the output devices.

I can say now that you will have blown the output devices by doing what you did.

You will need to replace the drivers as well as the output devices as usually the output transistors blowing will take them out or at least damage them.
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Old 20th August 2008, 09:01 PM   #3
tuck1s is offline tuck1s  United Kingdom
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Thanks for your reply, I will do as you suggest.

For the output devices I think you mentioned in the other post that the Cyrus recommended replacement is BUV48A, and an alternative is TIP35C.

Looking at the datasheets on the Farnell website:
BUV48A
http://uk.farnell.com/9936122/semico...tronics-tip35c

TIP35C
http://uk.farnell.com/9936122/semico...tronics-tip35c

BUV48A is rated for higher Vce0 (higher than needed), and DC current gain of around 25-40 (temperature related). As it's a switching part, it is fast (but specified in terms of switching time, not Ft).

TIP35C is a general purpose device. Vce0 looks to be adequate (100v) for the +/-40V rails but without too much to spare. Looks to have higher current rating, Ft=3MHz.

Not that much to choose on price, given the 20 minimum order value. The TIP35Cs are 1.21 each, BUV48A's are 2.63 each.

Apart from price, are there any sonic differences?

(Just going to look for the driver devices now).
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Old 20th August 2008, 09:01 PM   #4
tuck1s is offline tuck1s  United Kingdom
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Just been to look at my amp. Driver devices appear to be MJE243 / MJE253.

Comparing specs for the suggested replacement MJE340/MJE350, seems like the existing parts are slightly more robust in some respects - Ic max = 4A, rather than 500mA for the replacements.

Farnell seem to carry all of the above, so I wonder if it's best to go for MJE243/253 as per originals.

MJE243:
http://uk.farnell.com/jsp/search/bro...questid=132845

MJE253:
http://uk.farnell.com/jsp/search/bro...questid=132605

I've not heard of ON Semiconductor, are they OK?

BTW I am just using Farnell as a reference point here, because (a) they sell to individuals and (b) the parts seem to be precisely identified and described, with manufacturer's own data sheets. I'm sure there are other good suppliers as well.
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Old 20th August 2008, 09:24 PM   #5
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If you can get the right devices then go for those. I would go for the BUV parts as they are better.

ONsemi is Motorola's semiconductor brand since about the last 10 years, they are good.
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Old 20th August 2008, 09:56 PM   #6
Nordic is offline Nordic  South Africa
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Just before you go crazy and paranoid, things like this can be repaired systematicaly.... look up transistor diode test in google... once you understand it, remove and test transistors one by one, solder good ones back in board and mark them as you progress... blown ones need replaceing.
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Old 21st August 2008, 01:20 AM   #7
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That is a nice idea to try diode tests, but I've seen drivers pass this before only to fail down the line due to the stresses caused when the outputs blew. For the sake of pennies and while it's all apart already it seems a good idea to me to just replace anything suspect.
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Old 21st August 2008, 01:51 AM   #8
tuck1s is offline tuck1s  United Kingdom
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Sorry I gave the wrong link to the BUV48A, it should be

http://uk.farnell.com/jsp/search/bro...questid=133516
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Old 22nd August 2008, 12:04 AM   #9
tuck1s is offline tuck1s  United Kingdom
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Have ordered the parts.
Will it be necessary to check or trim the quiescent current in the output stage, after replacing the devices? I'm an analogue novice, but I guess that R81, 130R plays some trimming role.

Presumably also worth checking output for DC offset?

I found some tips by Destroyer_x in the forum, regarding his testing of his own designs - to temporarily replace PSU rail fuses with 10R power resistors using croc clips. Then flick the power on, make a quick check, then flick it off again - avoiding meltdown of parts in case of secondary faults etc. Is this a good idea?

Also purchased some 50W 8R2 resistors, which I can put on a spare heatsink. Once it's running OK with a clean signal coming out, I thought to try it into these, before connecting real speakers. Is that also a good idea?

Any other tips?
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Old 22nd August 2008, 08:14 AM   #10
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Both of those are good ideas that wouldn't hurt.

The bias will need adjusting most likely, can't remember the value offhand. Maybe about 75mA. Seems like you are on the right track.
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