Another Marantz 18 issue.

[tonebells]

Hi all first post here. It looks like there is some pretty sharp minds here. I have been reading the other Marantz 18 thread and it looks like a few of you have a good knowledge of the amp section. Mine has a turn on hum in the right channel for about five seconds. DC on that speaker goes up to around 6.5 volts. Then the relay opens and closes DC goes near 0 and audio plays fine. Poking around the amp and driver I have found that Q2 C goes to ~+7 volts then to +1.5 volts, E goes to ~+6 volts then -1.5 volts and B goes to ~+5.5 volts then to -.6 volts. Before I bought the unit Q10 was replaced with SK3083 and Q14 was replaced with SK3027. The problem was still there with the outputs removed. Any ideas of what to look at would be a big help.

Thanks

Tony


amp page.pdf

[tonebells]

I put the drive and amp together on the same page in hopes of making it easier to read.

Tony

[g1]

Seeing as you said Q2 is acting funny, I assume you compared it to Q1 and Q1 does not do that?
Try this: measure the resistance of R27 and mark the setting.
Clean it, turn it back and forth and reset it to the resistance you measured originally. Setting should look close to where it was before.
See if the turn on hum is still there.

[Jazz P Bass]

It appears to me that you have a driver board problem.

Specifically around Q1006.

[tonebells]

Thanks for looking. Q1 powers up without the excessive voltage. R27 checks out fine and bias adjustment is spot on 80mv. The 18 does have a turn on delay. From the manual "After turn-on, the constant current supply for the pre-driver input stage provides a 4 to five second delay before the circuit is activated. This eliminates extraneous turn-on pulses and noise." The hum stops at the same time as audio starts to play in the left channel. Makes me think there could be something weird with the delay.

Tony

[tonebells]

It appears to me that you have a driver board problem.

Specifically around Q1006.

Replaced Q1006 need to let it set for fifteen minutes to see if the hum is gone.

[tonebells]

No Joy

Tony

[Jazz P Bass]

The question should be 'do the voltages at Q2 get better'.

That 7Vdc on Q2 collector is coming from somewhere.

[Jazz P Bass]

Oopsie.

"It appears to me that you have a driver board problem.
Specifically around Q1006. "


I mistyped Q1006: I meant to type: Q1002.

That is feeding Q2 base/ collector.

driveramp.pdf

[tonebells]

Oopsie.

"It appears to me that you have a driver board problem.
Specifically around Q1006. "


I mistyped Q1006: I meant to type: Q1002.

That is feeding Q2 base/ collector.

[ATTACH]47943[/ATTACH]

We can rule that one out. I was born in Pottsville.

Tony

[Jazz P Bass]

Speak up.

What is going on at Q2.
Do you still see that odd +7Vdc at startup?

[tonebells]

Speak up.

What is going on at Q2.
Do you still see that odd +7Vdc at startup?

Sorry for the lack of detail. I replaced Q1002 with no change. Yes +7 still there.

[Jazz P Bass]

And Q1002 also?

[tonebells]

And Q1002 also?

I have not. I need to find a good sub. Do you have a recommendation?

Tony

[Jazz P Bass]

Not too sure what the part number is.

But, it appears to be a small signal PNP, so anything would work at this point (as long as the pinout is correct and the voltage ratings are met.)