Another Marantz 18 issue.

[nickb]

So far all I have seen is the voltage. A flat line up to 5 volts then low. I will look some more over the weekend.

Tony

I just can't explain the hum but I realized I've been assuming it's loud. If not then I'm looking under the wrong rock and it may be just a side effect of the low gain during start up.

OK. So there is no instability. What we can do is get some control over this by putting a variable resistor say 100K across C21. As you lower it's value you can monitor the output voltage and set it for max. Then you can measure the DCV on Q1001 thru 1006 and see what;s going on.

[tonebells]

I just can't explain the hum but I realized I've been assuming it's loud. If not then I'm looking under the wrong rock and it may be just a side effect of the low gain during start up.

OK. So there is no instability. What we can do is get some control over this by putting a variable resistor say 100K across C21. As you lower it's value you can monitor the output voltage and set it for max. Then you can measure the DCV on Q1001 thru 1006 and see what;s going on.

The hum is like when you put your thumb on the input connector of a powered subwoofer. Not so loud that you think it is going to damage anything. I will go through it this weekend and try to get the peek voltage at turn on.

Tony

[tonebells]

driver.pdf

Here are some voltages.Let me know if something looks wrong.

Tony

[nickb]

[ATTACH]48515[/ATTACH]

Here are some voltages.Let me know if something looks wrong.

Tony

Since it's off at turn on and working OK when steady that doesn't tell us anything much I'm afraid. Please repeat but with it set up in the failing state using the pot as described in post #46. Do not connect a speaker.

One thing, the schematic shows that R1007 is 5.6 ohm 2 Watt. The measurements suggest that it's more like 5.6k which makes more sense. Could you please inspect it to check what the value actually is?

[tonebells]

Now I see what you are getting at. It may be a few days.

Tony

[tonebells]

One thing, the schematic shows that R1007 is 5.6 ohm 2 Watt. The measurements suggest that it's more like 5.6k which makes more sense. Could you please inspect it to check what the value actually is?

Good call 6.5K.

Tony

[tonebells]

With a trimmer across C21 and adjusted to keep the amp muted I get:
The high voltage at 10K and 10G stay high longer and fall more slowly with no click of the relay.
Voltage at speaker out goes to 6 volts then drops down and stays at +.5 vots.

	At turn on		When stable		Schematic     SM notes
									
1001	e	27		e	same		e	29	
pnp	b	33.7		b	same		b	28	
	c	0.3		c	0		c	1.1	
									
1002	e	36.4		e	same		e	40	
pnp	b	36		b	same		b	39	
	c	6		c	1.4		c	1.3	
									
1003	e	0.3		e	0		e	1.1	
pnp	b	7		b	0.38		b	0.45	
	c	-36.4		c	same		c	-40	
									
1004	e	-36.3		e	same		e	-38	-33.5
npn	b	-36		b	same		b	-40	-33
	c	35.5		c	same		c	4	
									
1005	e	0.3		e	0		e	1.1	
pnp	b	-0.5		b	0		b	0.55	
	c	-36.3		c	same		c	-37	
									
1006	e	-36		e	same		e	-38	
npn	b	-35.3		b	same		b	-37	
	c	6		c	0.5		c	-1.3

[nickb]

Sorry, that is still not what was asked for. You need to adjust the trimmer for maximum voltage at the output. It MUST be steady. No speaker. See post #46 "monitor the output voltage and set it for max".

[tonebells]

With the trimmer set at 4K ohm the voltage at C21 goes to the highest point of 34.35 volts the amp mutes and the voltage in the stable column would be the result.

Tony

[nickb]

No. The max voltage on the output i.e. R46, not C21. Take care not to set the trimmer to to low a value else you'll nuke the diode CR16. The object of the exercise is to get it into the failing state so we can see where the problem lies.

[tonebells]

Red at the junction of R40 and R46 and black to ground voltage goes to +.4 volts. The trimmer is at the same spot.

Tony

[nickb]

So, we not seeing the +6.5V reported in the original post. A maximum offset of +0.4V is not a big deal. I suspect we may be chasing a non real problem. The speaker protection is very insensitive as it requires a few volts for the relay to operate. A small 0.4V offset will never open the relay so letting the turn on transient be heard thru the speaker. Why you hear hum while that happens I really don't know, you would need to use your scope to trace that out.

You could try doubling the value of C21 to give more time for the power supply to stabilize before the mute is released.

[tonebells]

The +6.5 volts only shows up when first turned on and amp is in the muted state. I guess I will try swapping the drivers to see what happens.

Tony

[tonebells]

I cleaned up and replaced missing parts on a spare driver board and swapped it in with no change.

Tony

[nickb]

The +6.5 volts only shows up when first turned on and amp is in the muted state. I guess I will try swapping the drivers to see what happens.

Tony

The point of the trimmer is to reproduce turn on conditions yet when we did that we only saw 0.4V worst case. We should have been able to get +6.5V. So, there is another factor at play. I wonder if one or more of the power supplies to this channel is stable during the power up time. One way to test for this is to put a 2.2k resistor across C21, power up and then remove when you know the power supplies are stable and see if you still get the output voltage bounce. Or just observe what is happening on then at power up with your scope.