I've started working on a Luxman L504, the preamp first. Putting the amp on test first I've noticed that on one channel when using the tone controls the trace moves horizontally like it's going out of phase. Any thoughts?
The schematic is the same, just a few different values.
The schematic is the same, just a few different values.
It sounds like the DC level is shifting causing the trigger point of the CRO to shift. A leaky electrolytic capacitor in the tone circuit? Compare voltages at the ends of the control between good and bad channels.
Well, tone controls set off flat always shift phase. But if the 'scope is on auto-trigger the change of amplitude will also change the trigger point and, without an external reference, shift the sync.
Apparently the L-504 was introduced as the SQ-505, for which there is a service manual on hifiengine.
All the caps have been changed, 2.2 - 4.7uf coupling caps are now wima film and the larger new electros, verified by tracing the circuit that there in correctly, I'll measure the dc and get back.
Ok so strange behaviour, to me at least connect on oscilloscope channel to either preamp out and the trace doesn't move when adjusting the tone controls, connect 2 channels of the scope to L & R pre out and the right channel (always) waveform moves left to right? What could be causing this.
Its the same when I auto or manually adjust the scope.
Its the same when I auto or manually adjust the scope.
Yep reversed the probes used the other channels on the scope (4 channels) still does the same. When tone controls are defeated the traces don't move, just in case that's not obvious
Is it because the two channels of tone control are not perfectly matched? Tone controls introduce phase shift; filter components have minor variations in value, an so do multi-gang potentiometers. More information is needed to work out if this phase shift is within the bounds of 'normal'.
What is the test frequency? What is the maximum phase shift between channels in µS? What happens as the tone control reaches the end of travel at full boost or full cut?
The end stops are likely to be where the error between the two gangs of a potentiometer are minimal. If the phase at the end of travel doesn't line up, then differences in the values of the filter components in the tone circuitry are implicated. If at the end of travel the traces do realign, then differences in the tape of the potentiometer are implicated. If towards the end of travel the phase converges but doesn't quite line up then both the filters and the potentiometers are implicated.
What is the test frequency? What is the maximum phase shift between channels in µS? What happens as the tone control reaches the end of travel at full boost or full cut?
The end stops are likely to be where the error between the two gangs of a potentiometer are minimal. If the phase at the end of travel doesn't line up, then differences in the values of the filter components in the tone circuitry are implicated. If at the end of travel the traces do realign, then differences in the tape of the potentiometer are implicated. If towards the end of travel the phase converges but doesn't quite line up then both the filters and the potentiometers are implicated.
Ok scratch the last couple of messages, so to make things clearer,
Both probes attached, scope showing sine from both channels,
Switch tone controls into circuit and adjust. Trace on channel 2 on scope moves left to right, channel 1 remains unchanged
Swap probes over and the would be fault stays on channel 2
My scope has four channels and this behaviour is the same on any combination of channels
You need to trigger the scope from the input to the tone control so you can see the absolute phase shift on both channels. If you trigger from one of the outputs of the tone control (which is the default condition) then you are only seeing relative phase between the two channels. See my post above.
Ok, so stupid question and baby step time, in the attachment is the input to the tone control circled in red?
I will add that if I just use a probe connected to channel 1 and swap the probes between both channels it doesn't exhibit the described behaviour when tone controls are operated, why is that?
I will add that if I just use a probe connected to channel 1 and swap the probes between both channels it doesn't exhibit the described behaviour when tone controls are operated, why is that?
The point in time where the CRO starts to trace is triggered from one of the waveforms you are looking at, so the waveform that is triggering the trace will never seem to move. You need a third probe to use for the trigger that is not affected by the action of the tone control circuit, i.e. connected ahead of the tone circuit at P305 I think.
Thanks John,
So to confirm I'm connecting a probe at p305 on both channels, that's the input to the tone controls. Then connecting a third probe for the trigger say p302?
So to confirm I'm connecting a probe at p305 on both channels, that's the input to the tone controls. Then connecting a third probe for the trigger say p302?
Not quite. Put the amp in mono mode to ensure that the same signal going into each channel's tone control. Connect a probe to P305 to trigger the scope, You don't need to see this waveform, it is just to ensure that left and right waveforms are triggered before the phase shifts inherent in the tone control circuitry.
Connect two probes, one at the output of each channel's tone control P402. These are the signals you will look at on the scope to see the effect of the tone controls on phase.
Connect two probes, one at the output of each channel's tone control P402. These are the signals you will look at on the scope to see the effect of the tone controls on phase.