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A501 / Z501 Luxkit Amplifier schematic
A501 / Z501 Luxkit Amplifier schematic
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Old 12th March 2019, 08:39 AM   #21
Mooly is offline Mooly  United Kingdom
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A501 / Z501 Luxkit Amplifier schematic
You should be measuring the voltages between chassis and P1 and P2, not the voltage across or between P1 and P2.

Both should be zero.
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Old 12th March 2019, 08:51 AM   #22
SONDEKNZ is offline SONDEKNZ  New Zealand
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Default GREAT INFORMATION!

Thanks MOOLY!

A trap for young players there! So...

P1/Ground measures slightly negative voltage. Negligible amounts.
P2/Ground measures 6.7V.


So I guess we have now narrowed it down to the RIGHT Channel, according to the manual at least.

There's progress, right there!
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Old 12th March 2019, 09:01 AM   #23
SONDEKNZ is offline SONDEKNZ  New Zealand
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Default Could wildly inaccurate DC OFFSET settings be the culprit?

One thing that has always made me curious, is that this amp barely gets warm during either CLASS A or CLASS AB operation. This seems highly unusual given the way CLASS A works - and based upon the amount of highly efficient cooling engineering that this machine has been endowed with.

All begging the question:

As I have never previously attended to the DC OFFSET (nor the BIAS CURRENT), is it possible that these adjustments are so far of the mark that it is causing these DC Protection Circuit problems?

(Just a thought?)

Anyways, the LEFT CHANNEL DC OFFSET is now perfect at at solid .1DC. The RIGHT CHANNEL DC OFFSET is sitting at a solid - and undesirable - 6.7V.

Something in that right channel is very wrong...

Last edited by SONDEKNZ; 12th March 2019 at 09:19 AM.
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Old 12th March 2019, 09:31 AM   #24
pitbul is offline pitbul
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Chek what are the voltages on 4558 opam pins 1 & 7 on both channels. These IC's regulate DC offset voltage. As mentioned, measuring should be done regarding the gnd.
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Old 12th March 2019, 09:49 AM   #25
SONDEKNZ is offline SONDEKNZ  New Zealand
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Default OPAMPS 4558D Measured...

PITBUL

OPAMP 4558D (Assume it's the LEFT CHANNEL, from the balanced results) measured: -
LEG 1: .87V
LEG 7: .87V

OPAMP 4558D (Assume it's the RIGHT CHANNEL, from the unexpected results) measured: -
LEG 1: -13.17V
LEG 7: -7.54V

Seems like the latter OPAMP is not happy. Yet it shows no sign of wear, overheating or damage...???

Last edited by SONDEKNZ; 12th March 2019 at 09:57 AM.
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Old 12th March 2019, 10:01 AM   #26
Mooly is offline Mooly  United Kingdom
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A501 / Z501 Luxkit Amplifier schematic
Bias adjustment will have no effect on DC offset.

You would have to look at all the voltages on the opamp for it to make sense but I would doubt there is an opamp issue providing the supplies to it are correct (on pins 4 and 8).

Jump in at the deep end...

Switch the amp OFF and do a quick resistance check on the four 0.22 ohm resistors located on the emitter of the output transistors. Is any pair open circuit?

Also check the output transistors by measuring resistance from collector to emitter. Are any short circuit.
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Old 12th March 2019, 10:03 AM   #27
pitbul is offline pitbul
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If you are novice to electronics repair, I would suggest that you try to replace opamp on the right channel and after you do replacement adjust vr3 or vr4 (do not know which one coresponds to replaced opamp) to correct value on the output (0 V of the right channel).
If that doesn't help, you should check other components in the amplifier by measuring good amp channel with bad one. Maybe you should recap your amp, how old it is?
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Old 12th March 2019, 10:06 AM   #28
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@mooly,
I think that VR5 and VR6 are for bias adjustment, please check, VR3 and VR4 regulate DC offset through OPAMPs.
Power supply for both OPAMPs is the same.
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Old 12th March 2019, 10:18 AM   #29
Mooly is offline Mooly  United Kingdom
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A501 / Z501 Luxkit Amplifier schematic
I would say those presets are simply to trim the opamp bias currents. Normally a DC servo would use a FET opamp rather than a 4558 type.

The range of the preset will be minimal... look at the resistor values from the wiper of the pot. Its acting via a 10k across a 100 ohm.

Its really in place of an offset null adjustment for the opamp.
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Old 12th March 2019, 12:08 PM   #30
druidaudio is offline druidaudio  Wales
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A501 / Z501 Luxkit Amplifier schematic
Hi SONDEKNZ

Do you have the service manual? If not, you can download from the following link, once you have registered;

Avance Z501 - Manual - DC Stereo Power Amplifier - HiFi Engine

The manual explains how to set the dc offset (and bias), you adjust the trimmer pots on the driver board to adjust the dc offset. If you cannot adjust the faulty channel to around +/- 0.1V there, you have a problem.

You can measure from ground to the following points on the output board, which are the inputs from the driver board;

U1 & U2 should be around +1V
D1 and D2 should be around -1V

If you measure anything excessive here, it is likely that you have a problem on the driver board. Go around that and check dc measurements as they are indicated on the schematic.
This way, you can at least narrow down the fault area.
Good luck!
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