Cyrus 2 Resistors

[mannotanumber]

Martin

Your right - exactly the same situation - the more the merrier!!! I'll keep you all updated on my progress.

Dave

[Geoff]

Dave & Martin

It's rather late now (and too much alcohol :-) to give you a detailed reply but I will be happy to do so via individual emails, or via the forum, tomorrow (though I might need a reminder as my memory is not what it used to be - I blame the whisky for this!).

Geoff

[Geoff]

Originally posted by mannotanumber
Geoff

While your here I went to the farnel website to order the resistors 0.5W or 0.6W metal films for R84 but got stuck with the actual resistance rating that I need - I'm so sorry I cant work out how to determine this.

Thanks for you patience.

Dave

Dave

The resistance is given on the schematic I sent you. R84 is 120R (120 ohms). Unless you are ordering something else from Farnell it would probably be cheaper to get odd resistors like this from your local Maplin store.

Geoff

[mannotanumber]

Geoff

Thanks again, I was having a mad moment last night I've got the resistance now 120ohms and the 270ohms both 0.6w.

Try as I might I cant find Resistance value for R89-R96, Don't know if its my eyes ?

You have been a fantastic help mate, and I'm off to test the transistors now.

Kind Regards

Dave

[Geoff]

Martin

I can send you a copy of the earlier v6 schematic if you haven't already got one, just send me an email.

You ask "Is there a foolproof way to get it working?" and my reply has to be no, but with some care and a logical approach they is no reason why you cannot succeed.

Start at the output and work backwards, testing the transistors for a short between collector and emitter (the usual failure mode) and measuring the resistors. The resistors marked with a triangle on the v7 schematic are fusible types which are supposed to protect the associated and preceding semiconductors but, as with output transistors blowing before the rail fuses, it is more usual for the silicon to go first.

I have been lucky in that I have only lost the output transistors, the drivers have been intact and no resistors have blown. However your case may be different. If the drivers test OK then it should just be a case of replacing any failed output transistors. If the drivers have failed then you will need to check the pre-drivers (Q27/Q29 on the v7 schematic). Keep working backwards until you reach the point where nothing has failed to a short (transistors) or an open circuit (fusible resistors).

As for setting up (re-biasing) after components have been replaced, the earlier v6 has a preset for adjusting the output stage quiescent current but the v7 relies on fixed resistors with the facility to link one out to adjust the bias. When the repairs have been done and the amp re-assembled (don't forget to replace the isolation pad between the output transistors and the heatsink as this becomes less effective when reused), it is a case of checking the output stage quiescent current by measuring the voltage across one of the 0R22 resistors (or one pair of the paralleled 0R47 resistors in v6) to see that it is in the range of 50mA to 100mA.

I have not seen a recommended figure for this current published by Cyrus but my Cyrus 1s run at about 75mA. The method of adjusting resistor values on v7 to alter the quiescent current has been covered in another thread (I think the title is 'Cyrus 1 Project')

"Is it that easy or is it as hard as I have imagined?" It is as easy or as hard as you care to make it! Proceed logically in a step-by-step fashion and you should achieve a satisfactory outcome. Rush in and you may find your time and money has been wasted.

If you need further clarification on any aspect don't hesitate to ask, either here or by email.

[Geoff]

Originally posted by mannotanumber
Try as I might I cant find Resistance value for R89-R96, Don't know if its my eyes ?

Dave

Again, it's on the schematic but perhaps not a clearly as it should be. R87-R94 are labelled R47 which should be 0R47 or 0.47 ohms. R95/96 are shown as R22 which is 0R22 (0.22 ohms).

Geoff

[Geoff]

Originally posted by Geoff

Another candidate in a larger package (TO-3P or TO-247 depending on manufacturer) is the BUV48/48A which last time I checked was significantly cheaper than the BUV28.

I shouldn't have relied on memory! The BUV48/48A is only cheaper than the BUV28/28A at Nikko. I should add that the BUV48A has been supplied by Cyrus as a replacement for the PT77 used in the Cyrus 2 v7

[poiuytre111]

As a first post, may I offer the tip that has served me well in the past faulting solid-state amps. Generally these will run with around 20 to 30 volts rail positive and negative and will have fuses which will probably keep blowing until you fix the fault.

Ideally you have a current limited dual-stabilised power supply to use for testing purposes, great if you have one, no help if you don't.

My suggestion is that you pick up some old dual filament 45watt P45 headlight bulbs from a car breaker - shouldn't cost much and you may already have some. These will have two 12 volt filaments in each glass envelope. If you ignore the common connection and connect to the other two you have a light bulb that takes 24 volts and needs about 4 amps to light up. Put one (i.e. a separate bulb with two filaments in series) across where each of the low voltage fuses should go. Do not try this in place of a mains fuse - you have been warned.

These will not go bang if there's a fault, they just light up. If the current is <<4 amps they won't affect normal operation due to the temp coefficient thing viz the resistance is very low until they carry 3 or 4 amps. I sometimes found that you could change a Transistor and all seemed to go well until you put a load in place and then you draw excessive current so I would leave them in until you are 100% through with testing into a real load. The two filaments in series might also be used as a makeshift test-load at about 6.5 ohms, though clearly a lot less than this until current starts flowing.

One other tip - use any NPN power transistor to replace the dud and only when it all works worry about getting an exact replacement and finalising the bias settings etc. - you may take more than one attempt to get it right.

[Sonusthree]

Well I am both happy and slightly embarrassed! I opened the Issue 7 Cyrus and found no shorted transistors. I saw a few suspicious solder joints so reworked them. I also noticed that the ribbon cable wasn't looking very secure at the point where it joins it's push fit connector so I removed the connector and soldered directly to the PCB. It now works!!!

As a side issue I usually run an Issue 7 Cyrus 2 with PSX. These have been 'upgraded' and modified but the stock Cyrus 1 has the best 'speed' by a country mile. My usual Cyrus 2 rig only wins out in terms of lower frequency authority. I WILL find a way to Biamp with both Cyrii. I've heard many arguments about speed and PRAT but in this case I am referring to the clear superiority of the Cyrus 1 when, for example, following very fast and intricate piano. My C2/PSX sounds dull by comparison. I am, therefore, scared to mod the Cyrus one with more capacitance or a better pot. I guess I should start a whole new thread about this ....

Back on topic:
My plastic Cyrus one has a more serious problem. It keeps blowing fuses and two of the output transistors are missing entirely. (Yes, seriously) I think I'll stick to BUV28/48 if they are supplied by Cyrus but I cannot see a preset to adjust the bias but I haven't had a proper look yet. I'll have a good poke around later and post my findings.
Geoff: Please could you send me a v6 schematic to fivelitrebucket(at no flippin spam please)aol.com. I will also try to contact you by forum mail but I find it to be terribly unreliable.

Kind regards,
Martin.

[Geoff]

Originally posted by Sonusthree
As a side issue I usually run an Issue 7 Cyrus 2 with PSX. These have been 'upgraded' and modified but the stock Cyrus 1 has the best 'speed' by a country mile. My usual Cyrus 2 rig only wins out in terms of lower frequency authority. I WILL find a way to Biamp with both Cyrii. I've heard many arguments about speed and PRAT but in this case I am referring to the clear superiority of the Cyrus 1 when, for example, following very fast and intricate piano. My C2/PSX sounds dull by comparison. I am, therefore, scared to mod the Cyrus one with more capacitance or a better pot. I guess I should start a whole new thread about this ....

I have found exactly the same. The Cyrus 2+PSX only comes into its own when in a large room and/or feeding inefficient speakers. That is of course unless one is using the phono stage when the separate power supply for this can be a benefit.

Back on topic:
My plastic Cyrus one has a more serious problem. It keeps blowing fuses and two of the output transistors are missing entirely. (Yes, seriously) I think I'll stick to BUV28/48 if they are supplied by Cyrus but I cannot see a preset to adjust the bias but I haven't had a proper look yet. I'll have a good poke around later and post my findings.

The presets RV2/3 for setting the quiescent current are hidden under the ribbon cable.

Geoff: Please could you send me a v6 schematic to fivelitrebucket(at no flippin spam please)aol.com. I will also try to contact you by forum mail but I find it to be terribly unreliable.

Done.