Cyrus 2 amp distort when crank up volume

[cslaw]

Hi

At low volume, both channel sounds ok, when i crank up the volume above 2.5, the left channel distorts. ( sounds like sore throat vocal & farting bass )
Distortion disappear when volume is reduce.

I have changed the 2 brown cap 470uf/6V below the ribbon cable, and all the output resistors 4 X 0.47ohm, 1 X 0.22ohm / channel.

I have run out of ideas.

Please give me your advise in a simple way as i have limited knowledge in electronics.

Is the amplfier output cap faulty or any signal cap faulty,
or the worse case: transistors fault?

Regards,

CS Law

[cslaw]

Serial number 204073c.
Amplifier case plastic.

Some measurement i just taken, hope its helpful.
R96(R channel), R95(L channel), these are the 0.22ohm output resistors.
I measured the voltage across R96 is 1.1mv, and 1.8mv accross R95.

[godfrey]

Can't help with advice, but I found the service manual. Here's the pages relevant to your model.

[cslaw]

Thank you. I have the schematics.

As the R channel is ok, i adjusted the voltage across R95 (L channel) to be same as R96 (R channel). Both are 1.1mv. No improvement.

Update : Distortion from L channel is present all the time regardless of soft or loud volumes. At louder volume, distortion gets more apparent.

Please help.

Thank you.

[godfrey]

Some random thoughts...

Are you sure the problem is with the amp, not the speaker or speaker cable and connections? It's easy to check this by swapping the speakers from left to right.

If it is the amp, I assume the problem is with the amplifier circuit itself not the power supply, since it looks like a single power supply is shared by both channels. Is that correct?

If the input coupling capacitor (C33/C34) is electrolytic (which I doubt), it might have died of old age. Alternatively, the volume control potentiometer may be faulty. You could check by bypassing those and applying the input signal directly to where R45/46, C33/34, C35/36 etc are all connected together.

To do that, you'd need a source with it's own volume control, though. Maybe it would be easier to connect the 2 channels together at that point and see if you get the same sound from both speakers.

C41/42 in the feedback network probably is electrolytic and may well be the culprit, unless those are the ones you already replaced.

I'm assuming the problem with distortion started before you replaced various components, rather than as a result of that?

Did the problem with distortion start suddenly, or gradually get worse? If the problem is blown transistors, that would probably have happened while the amp was being played at very high volume, or if the output was accidentally short-circuited.

Edit: BTW, the bias seems low. According to the manual, there should be 8 to 20 mV across the 0.47 ohm resistors. I don't think that would cause bad distortion though, and only on one channel.

[cslaw]

Confirm its amplifier problem as i tried with 2 pairs of different speakers and source.
Believe its ciruit problem not power supply.

The distortion arise suddenly after playing louder.

I swap C41/42 with new cap 470uf/16v but no difference.

I check all the 8 output transistors with digital multi-meter seem to display similar readings between the 3 leads.

You could be right about blown transistor as it happen suddenly at loud volume.
But physically there is no sign of it, how can i further check.

[jaycee]

As you have a good channel, measure voltages at certain points in the good channel, and measure the same points in the bad channel. If you see significant difference that can point to a fault.

A good "starter for ten" would be the collector voltage at Q5/6, and across the resistors in the LTP - R51/52, R55/56, R53/55, R59/60.

[cslaw]

compared measurements on Q5/6: no difference, resistance across R51/52, R55/56, R53/54, R59/60 are similar. I will compare the rest of the transistors.

Whats the use of part X31, is it a diode? Its near the PSX connector and heatsink. there is small crack line joinning to the leads.
How can i replace it as its not listed in the part list.

[anatech]

Hi cslaw,
X31 ~ X34 are all jumpers. You should read a short if you measure continuity. They connect the headphone jack to the speaker outputs through a dropping resistor.

When trying to diagnose a fault like this, it's really helpful to have an oscilloscope so you can observe the waveform. It can give you great clues to where your problem might lay. For one, you'll know what the fault type is for starters. Your voltage readings and resistance measurements are only a shot in the dark. Since you don't have a DC offset, the voltages will most probably all be in the ballpark. In other words, right now you're only spinning your wheels without the use of a 'scope and an oscillator. AC hum might possibly work as a test signal. Also, if you plan n checking levels, you must run the volume control completely to the maximum position. Otherwise you may see differences that are due to the control itself and not the circuit.

-Chris

[cslaw]

Chris

I have to turn the volume to max when comparing values at all transistors.
What about for voltage across R95/96.