Hi all,
Hoping I can get some help with this old Cyrus One. The left channel sounds weak and has mild distortion. I’ve been through with a scope with a 1khz sine wave and the Voltages and wave form look great and voltage at the power transistors are identical. I’ve checked and cleaned pots and even applied the signal directly to the power amp circuit, bypassing the controls. Also there was a bias issue on this channel, I’ve reduced a resistor in the bias circuit and got 8mv. Also recapped this circuit. Any ideas guys?
cheers
Hoping I can get some help with this old Cyrus One. The left channel sounds weak and has mild distortion. I’ve been through with a scope with a 1khz sine wave and the Voltages and wave form look great and voltage at the power transistors are identical. I’ve checked and cleaned pots and even applied the signal directly to the power amp circuit, bypassing the controls. Also there was a bias issue on this channel, I’ve reduced a resistor in the bias circuit and got 8mv. Also recapped this circuit. Any ideas guys?
cheers
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Have you checked the two diodes connected from the rails to the output?
How about the emitter resistors?
https://www.hifiengine.info/cyrus/mission_cyrus_one_sm.pdf
How about the emitter resistors?
https://www.hifiengine.info/cyrus/mission_cyrus_one_sm.pdf
Thanks Rayma, yes diodes ok and a 😡 to get at. Do you mean the huge emitter resistors at the end of the circuit?
Ok, then best to compare the good and bad channels and look for differences
in voltage or resistance. Do any of the outputs get unusually hot at idle?
in voltage or resistance. Do any of the outputs get unusually hot at idle?
I’m getting very similar readings from right v left. Nothing getting hot. Very baffling, I’ve been trying to bottom it out for 2 weeks. I’m gonna take a look at thd tomorrow
I would look at the waveform on the output at a watt or so, with load. Use a 1kHz triangle signal.
If you can easily hear it, you should be able to see it. I'd say either crossover distortion, or else
clipping on one peak.
If you can easily hear it, you should be able to see it. I'd say either crossover distortion, or else
clipping on one peak.
Hi again, I’ve tested as instructed and got totally different results under load. I only have the top of the waveform. I have to add, I’m only an amateur at this but really enjoy the challenge and learnt a lot on here on various models.
could this be a transistor?
could this be a transistor?
Could be almost anything, bad part, bad connection.
Are both power supply voltages on the bad board? Are the rail fuses good?
Are the emitter resistors ok?
Start over, using a load and the inputs shorted. Compare all DC voltages on the bad channel
with those in the good channel.
Are both power supply voltages on the bad board? Are the rail fuses good?
Are the emitter resistors ok?
Start over, using a load and the inputs shorted. Compare all DC voltages on the bad channel
with those in the good channel.
Yes, power supply and rails are good, fuses too and emitter resistors. Please can you explain how to short the inputs and also what % tolerance (DC( would be acceptable. Thanks for the schematic 👍
Cut an old pair of RCA cables a few inches from the ends and short together the inner wire to the shield.
Plug the shorted RCA plugs into each input, for zero noise pickup. Keep a resistor load on the speaker outputs.
The two DC supplies should be within a couple of volts of each other, opposite in sign, and may be a little higher than spec
due to the higher AC line voltages these days. Careful, high voltage is present. The other two regulated supplies
should be about equal and opposite, and will be lower in voltage than the unregulated supplies.
Measure the DC voltage across each emitter resistor. Usually most amps have 26mV or so there.
All readings should be about equal, within 10% or so.
If you know how to test the transistors in circuit (with the power off), do that also.
Plug the shorted RCA plugs into each input, for zero noise pickup. Keep a resistor load on the speaker outputs.
The two DC supplies should be within a couple of volts of each other, opposite in sign, and may be a little higher than spec
due to the higher AC line voltages these days. Careful, high voltage is present. The other two regulated supplies
should be about equal and opposite, and will be lower in voltage than the unregulated supplies.
Measure the DC voltage across each emitter resistor. Usually most amps have 26mV or so there.
All readings should be about equal, within 10% or so.
If you know how to test the transistors in circuit (with the power off), do that also.
That’s great, I understand and I’ll do more testing tomorrow. I’ve been going the transistors one by one today. I’ve had some success with these old Cyrus before but this one is on another level. While testing earlier I’ve started get a high current draw at higher volume, lighting up my bulb current limiter. I’ll definitely learn and improve by the time this one is sorted. I’ll keep you updated and thanks so much again
The light bulb tester is only for the initial turn-on, to limit the AC line current enough
to prevent a major meltdown. After it passes that first test, all further testing must
be done without the bulb tester in the circuit.
to prevent a major meltdown. After it passes that first test, all further testing must
be done without the bulb tester in the circuit.
Ok, I understand, is it normal for it to light up when under load then, is the extra load current going to the bulb then instead of the output transistors? I wonder if this is part of my problem
All that the series light bulb tester is intended for, is to light up and drop the AC line voltage,
if the DUT amplifier tries to draw much more AC line current than the bulb requires at the nominal AC line.
Then it will light up.
The tester bulb's wattage is chosen to be significantly larger than the normal power draw of the DUT.
Note that the AC current through the bulb, and through the amplifier, is the same since they are in series.
A normal amplifier cannot function properly if a light bulb is in series with the AC line. It is only for fault protection.
Again, never measure the amplifier with the bulb tester connected. It is solely for protection during the initial power up,
in case there is a DUT fault that would draw excessive AC line current, which would damage internal parts in the DUT.
For example, if the capacitor after the rectifier were shorted, turning the amplifier on could possibly damage the
rectifier and the power transformer, especially if the amplifier is not properly fused.
if the DUT amplifier tries to draw much more AC line current than the bulb requires at the nominal AC line.
Then it will light up.
The tester bulb's wattage is chosen to be significantly larger than the normal power draw of the DUT.
Note that the AC current through the bulb, and through the amplifier, is the same since they are in series.
A normal amplifier cannot function properly if a light bulb is in series with the AC line. It is only for fault protection.
Again, never measure the amplifier with the bulb tester connected. It is solely for protection during the initial power up,
in case there is a DUT fault that would draw excessive AC line current, which would damage internal parts in the DUT.
For example, if the capacitor after the rectifier were shorted, turning the amplifier on could possibly damage the
rectifier and the power transformer, especially if the amplifier is not properly fused.
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