|tonebells||3/29/2018 4:15 PM|
|Another Marantz 18 issue.|
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.
|tonebells||3/31/2018 4:57 PM|
|I put the drive and amp together on the same page in hopes of making it easier to read.|
|g1||3/31/2018 6:56 PM|
|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||3/31/2018 8:42 PM|
|It appears to me that you have a driver board problem.|
Specifically around Q1006.
|tonebells||3/31/2018 8:49 PM|
|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.|
|tonebells||3/31/2018 9:31 PM|
|tonebells||3/31/2018 9:54 PM|
|Jazz P Bass||3/31/2018 9:58 PM|
|The question should be 'do the voltages at Q2 get better'.|
That 7Vdc on Q2 collector is coming from somewhere.
|Jazz P Bass||3/31/2018 10:05 PM|
"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.
|tonebells||3/31/2018 10:49 PM|
|Jazz P Bass||3/31/2018 10:54 PM|
What is going on at Q2.
Do you still see that odd +7Vdc at startup?
|tonebells||4/1/2018 8:21 AM|
|Jazz P Bass||4/1/2018 11:37 AM|
|And Q1002 also?|
|tonebells||4/1/2018 3:22 PM|
|Jazz P Bass||4/1/2018 4:36 PM|
|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.)
|tonebells||4/1/2018 4:47 PM|
|I will dig something out when time allows. Thanks for your time.|
|tonebells||4/1/2018 8:34 PM|
|I misspoke in post 12 I had change out Q1002 not Q2. Now I have Q2 swapped out with KSC1845 with no change.|
|tonebells||4/7/2018 4:52 AM|
|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.|
|Jazz P Bass||4/7/2018 7:15 AM|
KSC1845 is an NPN.
|nickb||4/7/2018 7:15 AM|
|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||4/7/2018 6:37 PM|
|tonebells||4/7/2018 6:43 PM|
|tonebells||4/8/2018 10:21 AM|
|Still the same with input to driver disconnected. Added a little better scan.|
|nickb||4/8/2018 11:42 AM|
|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||4/8/2018 11:44 AM|
|Base of Q1003 also goes to ~+3 volts at turn on.|
|tonebells||4/8/2018 11:45 AM|
|nickb||4/8/2018 11:53 AM|
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||4/8/2018 12:14 PM|
|Replaced Q1003 with no change. Tried cooling with no change. Will try heat. |
|tonebells||4/8/2018 1:04 PM|
|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. |
|nickb||4/8/2018 1:23 PM|
+ 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?
|tonebells||4/8/2018 2:32 PM|
Yes after the unit plays for ~ five minutes you can turn off then back on with no hum. If you leave it off for ten minutes the hum comes back.
|nickb||4/8/2018 2:56 PM|
|Thanks for the clarification. |
The delay comes form the 350uF cap C21 and associated resistors. I'd missed that. The funny thing is that it feeds both channels so you'd expect them to behave the same. The idea is that initially C21 is discharged and so no current flows in the driver and so turns of the power amp. The time constant is not long enough when turned off to account for ten minutes so it has to be something after that. That sort of a time constant strongly suggests a thermal effect yet no transistor showing any sensitivity. Since you have freezer you can try it on other components especially the ceramic caps.
If that doesn't show anything I think I try defeating the delay by removing C21 and no speaker. You can then power up and down and compare the two channels to see what shows up.
|tonebells||4/8/2018 5:09 PM|
|The 28 volts at 10A and the 26 volts at 10B both turn on when power is applied. So I would guess there is something wrong in the delay. I will order some 350UF caps to replace C20/21/22 and see what happens. Thanks nickb.|
|nickb||4/8/2018 10:55 PM|
|tonebells||4/9/2018 7:56 PM|
|nickb||4/10/2018 1:13 AM|
|Are you saying that both channels now behave the exactly same way?|
|tonebells||4/10/2018 3:39 AM|
|tonebells||4/10/2018 6:45 PM|
|I may be wrong but here goes. The delay is caused by the base of Q1001 being more positive then the emitter so no 1 volt at collector. Looking around Q1001 to see what's up.|
|tonebells||4/10/2018 7:20 PM|
|Going to try replacing Q1001.|
|nickb||4/11/2018 1:28 AM|
I would not suspect Q1001 since it works OK when you don't have C21 fitted. I would consider possible that one of the differential pair transistors has gone low gain, i.e. QQ1006/Q1004 or Q1003/Q1005 or you that have instability. The heating/cooling was a rather inexact test for low gain transistors.
But stepping back a bit it's the hum that worries me. This suggests that far too much current flows through the output transistors during the bounce and it could be very close to destroying them. This is one of the reasons I said not to connect a speaker i.e. to avoid damaging that too. What I'd really like to know is what is happening to the current in the output transistors during the bounce. To do that you'd need to use a digital storage 'scope to measure the volts across R36. Or it could be a sign of instability.
I raised the possibility of instability before and that remains to be checked as far as I know. Again you need a scope to see that. There is a real likelihood that the presence of the speaker could make it switch from stable to unstable so making the whole thing rather tricky to work on.
|tonebells||4/11/2018 4:46 AM|
|I do have an analog scope and a PC based scope. Do you have an idea what good subs for Q1006/Q1004 ans Q1003/Q1005 would be? I would guess they need to be matched?|
|nickb||4/11/2018 7:32 AM|
|They might work. |
With C21 installed, use the analog scope to check for instability on power on. not having storage might make it hard to see. The PC scope might also work if the bandwidth is high enough 1MHz+ ought to do. The key thing is to set the trigger to normal and pick the level to grab what you want.
Use the PC scope in differential mode to measure the voltage drop across R36. Diff mode is where you use two probes, one at either end of R36 and both ground leads to OV. You use the math function on the PC scope to subtract the two. You'll be measuring a small voltage in the mV range.
|tonebells||4/12/2018 4:41 PM|
|Well the PC scope is junk. The channels are not even close to reading the same voltage. I hooked up my analog scope and R36 is as it should be (same as left). I looked at a few things on the output and it all looks OK. Is there some thing specific I should be looking for?|
|nickb||4/13/2018 1:09 AM|
|What we are looking for on the output is a burst of oscillation and note the max (+ or -) DC volts during power up.|
|tonebells||4/13/2018 4:57 AM|
|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.|
|nickb||4/13/2018 5:10 AM|
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||4/13/2018 8:41 PM|
|tonebells||4/16/2018 8:14 PM|
Here are some voltages.Let me know if something looks wrong.
|nickb||4/17/2018 1:30 AM|
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||4/17/2018 4:52 AM|
|Now I see what you are getting at. It may be a few days.|
|tonebells||4/17/2018 6:52 PM|
|tonebells||4/22/2018 12:15 PM|
|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||4/22/2018 12:33 PM|
|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||4/22/2018 2:42 PM|
|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.|
|nickb||4/22/2018 3:24 PM|
|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||4/22/2018 3:59 PM|
|Red at the junction of R40 and R46 and black to ground voltage goes to +.4 volts. The trimmer is at the same spot. |
|nickb||4/22/2018 5:00 PM|
|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||4/22/2018 5:51 PM|
|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.|
|tonebells||4/22/2018 7:46 PM|
|I cleaned up and replaced missing parts on a spare driver board and swapped it in with no change.|
|nickb||4/23/2018 12:34 PM|
|tonebells||4/23/2018 3:02 PM|
|nickb||4/23/2018 3:06 PM|
|tonebells||4/23/2018 3:20 PM|
|2.2K across C21 makes the voltage on the output take much longer to drop.|
|nickb||4/23/2018 3:29 PM|
|tonebells||4/23/2018 3:44 PM|
|The .5 volt on the output drops as soon as the amp unmutes with no bounce.|
|nickb||4/23/2018 3:49 PM|
|tonebells||4/23/2018 3:56 PM|
|One channel on the PC scope read voltage wrong by about 5 volts but we can give it a try.|
Well today it's working.
|tonebells||4/23/2018 4:06 PM|
|I have the PC scope setup.|
|tonebells||4/23/2018 6:11 PM|
|With the +/-36 volts unhooked from the driver board I still get the +5 volts and hum on the speaker output.|
|tonebells||4/23/2018 7:45 PM|
|I went with my gut and pulled Q9. It test low gain and high leakage. I will most likely not have it back together until tomorrow.|
|tonebells||4/24/2018 6:38 PM|
|I got it back together and tested for operation. Powered down for thirty minutes and powered up without DC on the outputs. Will try with speakers next.|
|tonebells||4/24/2018 7:25 PM|
|No hum and no dc on the outputs. Thanks for the help.|
|nickb||4/25/2018 1:28 AM|
|tonebells||4/25/2018 5:59 PM|