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!
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!
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
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
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.
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.
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
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
with 75ma running through the output pairs, you should drop 16mv across each emmiter resistor or 32mv across the 2.
let see what happens when the new devices go in. apparently there is going to be a delay in the delievery because of the july 4th holidays!
what wrong with useing a 3055 for the output pairs, i thought they are just a spec under what the buv28s are rated. i am waiting for the mje243/253 to come in, is there any harm in running them off bd139/140 temporarily?
let see what happens when the new devices go in. apparently there is going to be a delay in the delievery because of the july 4th holidays!
what wrong with useing a 3055 for the output pairs, i thought they are just a spec under what the buv28s are rated. i am waiting for the mje243/253 to come in, is there any harm in running them off bd139/140 temporarily?
sreten.
Hi sreten.
I do know the supply inside the "1" powers the low voltage regulator section with the PSX connected. I am pretty sure the entire amplifier section is powered from the PSX on the Cyrus1 and 2.
To connect the PSX, just pull the fuses on the board first. Do not fail to remove them!
-Chris
I'll check on that as well. The factory PSX delivers 40 VDC, the unit's own power supply is listed as 31 VDC. A quick look at a series 1 amp board shows the drivers share the same supply as the outputs.
I do know the supply inside the "1" powers the low voltage regulator section with the PSX connected. I am pretty sure the entire amplifier section is powered from the PSX on the Cyrus1 and 2.
To connect the PSX, just pull the fuses on the board first. Do not fail to remove them!
-Chris
Hi hacknet,
-Chris
Do you have a variac you can use? A variable bipolar tracking supply would work. If you can get it running at + / - 20 VDC at least, you should be fine at the 31 V normal supply. Double inspect your work before you apply power, remove the flux from where you solder so you can actually see if you have a short.
-Chris
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