Cyrus One

[hacknet]

hey guys!

i just got my hands on a spoilt cyrus one int amp. from what i've checked, the output transistors have given up. the originals are buv28, but i cant seem to get them from my local retailers. i have been considering getting the BD711 to take its place, with alittle more current and power dissipation abilities i think it would be better off.

i only face one problem, i did try swapping in a pair of 3055s and the played for about a minute before resigning to the grave. i suspect the HFE is too high thus drawing too much current. i was hoping to lower the Iq but there isnt a pot to do that.

i intend to replace the lower resistor in the vbe multiplier with a 1k pot to see how it goes.

is this sensible?

another thing, i was thinking of scrapping the original supply altogether for a psx-lookalike. from what i gather, all i need to do is remove the fuses that connect the original psu to the output devices and add another one. the question here is what voltage does the supply have to be? the markings on the PCB say +/-40v, but the buv28 would not be able to that kinda trashing especially with the increased current that can be drawn with the new supply.

there are holes in the pcb for a larger output device. can anyone help me out in locating a device that would be able to take that added power or would the bd711s do.

thanks a million!

[anatech]

Hi hacknet,
Don't use TIP3055's.

Your new transistors may have failed due to other faults in that channel. Check the drivers and bias transistor. Post your findings.

You always need to check your bias current and DC offset when powering up a repaired amplifier. A limited power start wouldn't hurt either. I am suggesting you use a variac or lightbuld in series with teh amp as you start it up.

Don't mess with other power supplies until you understand how these work. You can only cause reduced performance or smoke without a good understanding.

There are other things that need work before you are ready to add external power supplies. The regulated low voltage supplies for one.

-Chris

[AndrewT]

Hi,
I think the rail voltage on zero output will be a bit lower than +-40Vdc

I would aim for about +-35Vdc and with a stiffer supply it will hold closer to that when loaded.

Does the Cyrus One have 1pair or 2pair of output devices?

It's a pity an equivalent to the 15003/4 is not available in plastic package. It would cope with almost anything.
Can you find an equivalent say 150W to 200W, 15A to 20A, 100Vce to 140Vce, Ft 4MHz, hFE 40 @ 5A and 3A at 40Vce.

Fitting a temporary pot in place of the base emitter resistor will allow you to set a sensible Iq. Replace it with a permanent resistor once you have tried it.

I don't think hFE caused you second failure. Higher output device hFE draws less current from the driver which in turn suffers less stress. 3055 may have a lower hFE than buv. Either excess Iq or some other damaged component that caused output stage overheating.

[hacknet]

there was no dc offset on the amp when powered up, the bias transistor is fine, drivers have all been taken out, measured and put back, they are working fine too.


i cant seem to find what the original Iq was? has anyone help me on this, please?

[Geoff]

Originally posted by hacknet
i cant seem to find what the original Iq was? has anyone help me on this, please?

The measured Iq on one of my Cyrus 1s was ~75mA.

[anatech]

The Cyrus 1 runs about +/- 31 VDC for rails. If you have the older type (bias transistor mounted on heatsink) adjust for a voltage from 2~6 mV across R95/96. The newer type has the bias transistor mounted to an output transistor. Adjust the bias for 10~14 mV across R95/96. Let the amp idle for 1/2 hour to settle down to these figures. R95/96 are emitter resistors, 0R22, 1W.

The PSX will increase the supply voltages to +/- 40 VDC. Readjust the bias currents to compensate. If you really want to build a new supply, then try a regulated version like the PSX-R. You may want to reduce the voltages slightly as they will not droop under a heavy load. You don't want to overheat the little amp do you?

These are the factory figures and procedure.

-Chris

[AndrewT]

Hi Anatech,
the bias voltages; are they across each emitter resistor or across the pair?

[hacknet]

oh! thanks!

i had something like 30mv across those 0.22R resistors! far too much!

i was thinking of using the naim regulators from the mcbride website.

i just ordered BD711! lets see how it goes!

[anatech]

Hi Andrew,
Just one I think. I'll have a read. It may be Tuesday before I can respond though.

Hi hacknet,
Get it working properly first. Replace all outputs and drivers in that channel.

-Chris

[KBK]

Anniken:

Yah think the Big Guy's gonna stay dead?

There's two decent tranny's in that unit. And a few upside down heatsinks. Such a shame.