Another Marantz 18 issue.

[tonebells]

I will dig something out when time allows. Thanks for your time.

Tony

[tonebells]

I misspoke in post 12 I had change out Q1002 not Q2. Now I have Q2 swapped out with KSC1845 with no change.

Tony

[tonebells]

I read somewhere the Q5/Q6 where for over current and could be left out of circuit for testing. I removed them with no change in the turn on hum.

Tony

[Jazz P Bass]

I misspoke in post 12 I had change out Q1002 not Q2. Now I have Q2 swapped out with KSC1845 with no change.

Tony

It's really hard to tell from the smudged schematic but Q2 appears to be a PNP transistor.

KSC1845 is an NPN.

[nickb]

Try testing the power amp in isolation by disconnecting the input to the driver board at C1010 (pin 10E) or lift one end of C1010 whichever is easier. Then check to see if the hum and voltage bounce at the output is any different.

[tonebells]

It's really hard to tell from the smudged schematic but Q2 appears to be a PNP transistor.

KSC1845 is an NPN.

Q1/Q2/Q4/Q5 are all the same type of NPN.

[tonebells]

Try testing the power amp in isolation by disconnecting the input to the driver board at C1010 (pin 10E) or lift one end of C1010 whichever is easier. Then check to see if the hum and voltage bounce at the output is any different.

I will give that a try.

Tony

[tonebells]

Still the same with input to driver disconnected. Added a little better scan.

Tony

[nickb]

At least it narrows this down to the power amp. BTW the attachment isn't working. Disconnect the speaker for now.

Try replacing C1005 100uF. If you haven't got one to hand, just short it out for a quick test.

[tonebells]

Base of Q1003 also goes to ~+3 volts at turn on.

Tony

[tonebells]

At least it narrows this down to the power amp. BTW the attachment isn't working. Disconnect the speaker for now.

Try replacing C1005 100uF. If you haven't got one to hand, just short it out for a quick test.

C1005 and C1002 are both new.

Tony

[nickb]

Base of Q1003 also goes to ~+3 volts at turn on.

Tony

OK new caps..

It's all in a feedback loop so the voltages will swing everywhere and that makes it very hard to narrow down.

Try this, monitor the voltage at the output and try heating the each transistors one at a time for about 5 seconds using your soldering iron, or better if you have it, cool using freezer spray. Note which if any cause a big voltage swing. Failing that I'd use a scope to check for instability.

[tonebells]

Replaced Q1003 with no change. Tried cooling with no change. Will try heat.

Tony

[tonebells]

Heat showed no change. I looked at the left channel on turn on both + and -1.4 stay low until after the turn on delay. On the right channel after the unit has played and can be turned on with no hum the +1.4 volts comes on as soon as the power is switched on.

Tony

[nickb]

Heat showed no change. I looked at the left channel on turn on both + and -1.4 stay low until after the turn on delay. On the right channel after the unit has played and can be turned on with no hum the +1.4 volts comes on as soon as the power is switched on.

Tony

Sorry, I'm not really sure what you are saying.

+ and - 1.4V where? We need to know where you are probing. Guessing won't be helpful.
When you say "goes low" what do you mean? Do both go towards zero or negative?
"The turn on delay" what is that? As far as I know there is no such thing.
Are you saying that if you turn off and then immediately back on you don't have the problem?