The original PT7/PT77 parts are (supposed) to be switching transistors, as weird as it may be.
If it were class D they would be perfect but it's not.
The Hfe is too low to be of any use, as all power switching transistors are and I doubt they would fit 1000Volt transistors for a little amplifier like this.
Any of the many power transistors that use a TO3p body should do if the Hfe is over 50. 2SC5200 comes instantly to mind. Cheap and reliable.
Check the drivers.
I also had a look at these
http://cpc.farnell.com/on-semiconductor/mjl3281ag/transistor-npn-to-264/dp/SC06820
any idea where the driver transistors are on the board ?
I also had a look at these
http://cpc.farnell.com/on-semiconductor/mjl3281ag/transistor-npn-to-264/dp/SC06820
any idea where the driver transistors are on the board ?
so , although cyrus pt7 were switching transistors , i don't need to use switching transistors?
If you search the internet you will see the the PT7 is BUV28, a switching device, and the PT77 is the BUV48/BUV48A, also a switching device.
Yes the Hfe is low, but that would probably explain why the original Zetex drive transistors, which have pretty low ratings, are prone to failure.
I have Cyrus One's running better spec'd switching devices, with no issues. Driving 4 ohm loads such as B&W CM10's and Dynaudio Emit M20's; they sound incredible for such a "low power" amplifier. And nothing has gone bang.
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I have Cyrus One's running better spec'd switching devices, with no issues. Driving 4 ohm loads such as B&W CM10's and Dynaudio Emir M20's; they sound incredible for such a "low power" amplifier. And nothing has gone bang.
what "better spec'd switching device did you choose ?
As in the previous post, the STW13009 (TO-247) in one, and FJP13009 (TO-220) in an earlier model.
The drive transistors I used are the MJE243 and MJE253 as specified by Cyrus in the later versions. Fitting these in an earlier model designed for Zetex parts requires some creative lead bending!
The drive transistors I used are the MJE243 and MJE253 as specified by Cyrus in the later versions. Fitting these in an earlier model designed for Zetex parts requires some creative lead bending!
I have heard of the MJE3055T being used for replacement output transistors on the Cyrus. Cheap and readily available (CPC). BD139/BD140 for the drivers.
0.22 ohm/3watt resistors,
http://cpc.farnell.com/multicomp/mcknp03sj022ka19/resistor-wirewound-0r22-5-3ws/dp/RE07407
You need to check the value of all the resistors marked with an exclamation mark in the faulty channel. These are low value resistors and can be checked in circuit initially. If you don't see the reading expected then try reversing the meter leads. If it still looks wrong then lift one end and check. The shorted outputs/drivers may skew the result of some of these so remove the faulty transistors first.
The Cyrus appears to have non adjustable bias and this will be a possible issue using different non original semiconductors.
You may have to change the value of R63, a 680 ohm (hard to make the ref number out), perhaps even replacing it with a preset to give continuously variable adjustment.
Also you must use a bulb tester when working on this as it will save damage should anything go amiss.
0.22 ohm/3watt resistors,
http://cpc.farnell.com/multicomp/mcknp03sj022ka19/resistor-wirewound-0r22-5-3ws/dp/RE07407
You need to check the value of all the resistors marked with an exclamation mark in the faulty channel. These are low value resistors and can be checked in circuit initially. If you don't see the reading expected then try reversing the meter leads. If it still looks wrong then lift one end and check. The shorted outputs/drivers may skew the result of some of these so remove the faulty transistors first.
The Cyrus appears to have non adjustable bias and this will be a possible issue using different non original semiconductors.
You may have to change the value of R63, a 680 ohm (hard to make the ref number out), perhaps even replacing it with a preset to give continuously variable adjustment.
Also you must use a bulb tester when working on this as it will save damage should anything go amiss.
that is really very helpful thank you. I think some of the cyrus amps had adjustable bias because adjusting it is mentioned in the service manual i was sent last night. maybe some versions did and some didn't. this is an issue 07 as far as i can tell.
I'll make a bulb tester today before doing anything else. this is so if the bulb lights i can switch off pronto ?
I felt a bit daft asking you guys about this given you all know loads and I know very little but I have to say I've already learned a few new things and I'm bowled over by how helpful your replies have been. thank you. I appreciate it must be a bit irritating trying to explain stuff to me.
When I order the transistors do they need to be matched or is that only for another type of amplification circuit ?
I need to figure out where the driver transistors are from the circuit schematic. Thanks for the BD139/BD140 advice. is that either / or ?
If the bulb lights, it is telling you that significant current is continuing to be drawn by the transformer primary.
That can mean lots:
a.) That the output stage is turned on and all is working OK.
b.) That there is a fault in the secondary winding - not OK.
c.) That there is a fault in the primary winding - not OK.
In all three cases the power arriving at the primary is massively reduced and the risk of damage is very low.
If your bulb flashes briefly and then goes very dim or off, then the next step is to measure the PSU output voltages. If these are up where you expected and there are no signs of distress (heat or smoke), then it is safe to power ON directly.
If your bulb flashes brightly and dims (a clear glass envelope allows one to see the filament) but not off, then you should find that the PSU output voltage is significantly lower than expected. This may indicate a fault or case a. above
You have to determine which before proceeding.
If the bulb comes on bright, there will be a big drop in voltage across the bulb, leaving a small voltage across the primary. In this condition the power and voltage from the secondary can only be low. Measure the PSU output voltage. It may be zero Volts or a small voltage between 0.7Vdc and 3Vdc.
In this condition you need to take further measurements to find where the excess current is being drawn. Because the voltage is so low, you can leave the transformer powered ON while you investigate.
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Thanks. I fixed the cyrus 2 by checking the psu side of the preamp but that was much easier as the amp essentially worked on the line inputs but not on the phono side, it was a dodgy track on the phono circuit in the end , i changed the 18v regs and caps but may not have needed to. this I realise is far more complex. At the very least I think I'm going to learn a lot even if i fail ...
I checked the diodes on the output side and that seems ok. The fuse on the IEC plug input had blown ( a good thing probably? )
I was wondering if i could learn anything by taking the fuse out on the hot channel ? would the other channel still work as it was only one side that got hot ?
going to print out the service manual as it's awkward looking at it on my phone. hopefully i can figure out where the driver transistors are. One post here mentions checking all the resistors that have "an exclamation mark" but i didn't understand that bit, was there a diagram or photo i should have been seeing ?
A dual 0.22 ohm can be replaced by two singles. They are just wired in series with the junction being what would be the middle leg of the original dual. The CPC ones in the link should be fine.
There is no need for any matched transistors in this amp.
The bias could be a real issue and so I would suggest ordering a 1k and 2k preset to play about with.
The drivers are the pair of transistors directly feeding the outputs, the MJE243/253 pair. BD139/140 should be fine for these. Other commonly used parts for these would be MJE340 and MJE350.
There is no need for any matched transistors in this amp.
The bias could be a real issue and so I would suggest ordering a 1k and 2k preset to play about with.
The drivers are the pair of transistors directly feeding the outputs, the MJE243/253 pair. BD139/140 should be fine for these. Other commonly used parts for these would be MJE340 and MJE350.
ah i think i get it. so you need one of each. hopefully the component list and the schematic will guide me to those. thanks very much
When you begin work on this it is important to have things in a known state, in other words don't just power it up.
To begin with I would always recommend forcing a zero bias condition, and then only when the amp is working do we begin to think about setting and adjusting the bias.
Forcing zero bias is easy, we either short C51 out (the 22uF cap) or snip one of those two series resistors connected to Q30 ? (all the text is very blurred on the diagram).
To begin with I would always recommend forcing a zero bias condition, and then only when the amp is working do we begin to think about setting and adjusting the bias.
Forcing zero bias is easy, we either short C51 out (the 22uF cap) or snip one of those two series resistors connected to Q30 ? (all the text is very blurred on the diagram).