Marantz Nineteen 19 distortion repair

[ausman1000]

Hoping someone can help guide me in the right direction. My 19 has distortion in the right channel. I have localized it to the driver board. With a clean sine wave in this is what I get out (see pic). All incoming supply voltages to that board measure correctly per the schematic so I suspect a component failure. I have placed an order for Q1201, 02, 03 because some say these are prone to failure or become mismatched beta wise, however they do test good using the diode function on my meter.


For those of you that may be having intermittent tuner issues here is something I found that was the problem on my unit. There is an inductor/choke connected to a tag strip that was causing the problem. One end of the tiny wiring the makes up the coil was not connected properly to the lead of the device itself. My scope confirmed this and so a quick hit with the soldering iron and we are now back in business. I also hit the other end for good measure. See pic for culprit.

 

[dr*audio]

The distortion on your scope is classic crossover distortion. It means there is no bias current to the output transistors. Check and adjust it per the instructions in the service manual. If you can't adjust it, suspect the bias transistor or bias pot first.

 

[ausman1000]

The distortion on your scope is classic crossover distortion. It means there is no bias current to the output transistors. Check and adjust it per the instructions in the service manual. If you can't adjust it, suspect the bias transistor or bias pot first.

No I cannot adjust the bias regardless of position, it just stays at 2mv and should be 20mv. Other channel adjusts fine. Why would the bias to the output transistors create crossover distortion at the output of the driver board? Yes I have thoroughly deoxed the trimpot, and thank you very much for your input it is much appreciated.

 

[dr*audio]

No I cannot adjust the bias regardless of position, it just stays at 2mv and should be 20mv. Other channel adjusts fine. Why would the bias to the output transistors create crossover distortion at the output of the driver board? Yes I have thoroughly deoxed the trimpot, and thank you very much for your input it is much appreciated.

What you see at the output of the amp is reflected back to the driver board via the base emitter junctions of the output transistors. Check the bias transistor, the resistors in the bias circuit, the bias trimpot (check it with a meter, make sure the resistance changes when you adjust it) and if those are ok check the HFE of the driver transistors and the output transistors. I don't have the schematic in front of me but I'll download it and take a look at it. I expect to be getting a 19 soon.

 

[dr*audio]

More things to check, Q1208, Q1209, Measure the voltage on the collector of Q1201. It should be 12 - 18V. Bias transistor is Q1301. Check it with a meter on the diode range.

 

[ausman1000]

More things to check, Q1208, Q1209, Measure the voltage on the collector of Q1201. It should be 12 - 18V. Bias transistor is Q1301. Check it with a meter on the diode range.

Once again thank you for the info. Tomorrow morning I will study the schematics again and take your suggestions to see if I can zero in on the culprit. OK the reflection back to the driver board causing output distortion, I see that now since you explained it, excellent. I will keep you informed of my findings. I ordered 6x NTE234 to replace the first 3 transistors in both channels of the driver boards since at the moment there is a mix of old and newer populating those positions and I would rather get them all on the same page. Easy to replace thanks to the original push in sockets!

 

[ausman1000]

What you see at the output of the amp is reflected back to the driver board via the base emitter junctions of the output transistors. Check the bias transistor, the resistors in the bias circuit, the bias trimpot (check it with a meter, make sure the resistance changes when you adjust it) and if those are ok check the HFE of the driver transistors and the output transistors. I don't have the schematic in front of me but I'll download it and take a look at it. I expect to be getting a 19 soon.

Expect to be getting a 19 soon? Is that a definite maybe?!

 

[ausman1000]

What you see at the output of the amp is reflected back to the driver board via the base emitter junctions of the output transistors. Check the bias transistor, the resistors in the bias circuit, the bias trimpot (check it with a meter, make sure the resistance changes when you adjust it) and if those are ok check the HFE of the driver transistors and the output transistors. I don't have the schematic in front of me but I'll download it and take a look at it. I expect to be getting a 19 soon.

What kind would a good HFE typically be? 50, 100?

 

[ausman1000]

I'm leaning towards finding a bad bias transistor, as you quoted Q1301. My inputs on the collector and emitter of 1301 should be sitting at 1.15v per schematic but my values measure 500mv. Anyway I will await till morning to further investigate!

 

[ausman1000]

If trimpot R1304 is good then I should get a variable voltage on the base of Q1301 as I adjust?

 

[dr*audio]

If Q1301 is shorted you may not get a varying voltage. Test transistor and pot.

 

[ausman1000]

One thing I failed to mention earlier. I took recent possession of this unit and the distortion was not very bad. Yesterday while fixing the tuner problem I noticed the trimpot R1235 (constant current adjust) was way off its original paint mark so I thought that's odd and moved it back, that's when the distortion became a lot worse. Obviously someone had been in there before me and done their best to cover up the problem without repairing the real problem.

 

[dr*audio]

One thing I failed to mention earlier. I took recent possession of this unit and the distortion was not very bad. Yesterday while fixing the tuner problem I noticed the trimpot R1235 (constant current adjust) was way off its original paint mark so I thought that's odd and moved it back, that's when the distortion became a lot worse. Obviously someone had been in there before me and done their best to cover up the problem without repairing the real problem.

Or maybe you screwed it up? NEVER just arbitrarily adjust things without measuring! To do that adjustment properly you need a distortion analyzer. If you don't have one you are going to have to adjust it by voltage. As I said earlier, Measure the voltage on the collector of Q1201. It should be 12 - 18V. Adjust R1235 so you get 15V. That should put you in the ballpark and I bet you will have bias. Turn the bias pot all the way down before you make the adjustment, or you may cook the output transistors.

 

[ausman1000]

Or maybe you screwed it up? NEVER just arbitrarily adjust things without measuring! To do that adjustment properly you need a distortion analyzer. If you don't have one you are going to have to adjust it by voltage. As I said earlier, Measure the voltage on the collector of Q1201. It should be 12 - 18V. Adjust R1235 so you get 15V. That should put you in the ballpark and I bet you will have bias. Turn the bias pot all the way down before you make the adjustment, or you may cook the output transistors.

R1304 bias adjust works ie it changes resistance. Q1201 has 28.9v emitter, 28.3v base, 8v collector.

 

[dr*audio]

R1304 bias adjust works ie it changes resistance. Q1201 has 28.9v emitter, 28.3v base, 8v collector.

Does the collector voltage change when you adjust R1235?

 

[dr*audio]

Expect to be getting a 19 soon? Is that a definite maybe?!

I'm waiting to hear back from the seller.

 

[ausman1000]

Does the collector voltage change when you adjust R1235?

yes it does, the range is 4v-19v and on the good channel it is 5-28v (initial measurement 14.8v which is where I have returned it to)

 

[ausman1000]

This is strange. I turned the unit off for a while and then came back to measure the bad side collector range again and now it does go up to 28v (20 minutes earlier it would only get to 19v). I have set it at 15v and will see if it stays there after cycling power for a while. If it does I will attempt to do bias and offset. I flexed the board to see if perhaps there is an intermittent connection but voltage stays steady. Also the trimpot is now very close to its original white paint mark.

 

[ausman1000]

At the moment it is behaving itself. Q1201 collector is holding steady at 15v, dc offset adjusted to 0mv, bias adjusted to 20mv and strangely R1235 is back to the original factory paint mark (this is not where I found it prior to popping the lid). I am beginning to think that Q1201 maybe flaky and perhaps after exercising the voltage on the emitter it has come good for the time being. Oh and presently no distortion detected.

 

[ausman1000]

Or maybe you screwed it up? NEVER just arbitrarily adjust things without measuring! To do that adjustment properly you need a distortion analyzer. If you don't have one you are going to have to adjust it by voltage. As I said earlier, Measure the voltage on the collector of Q1201. It should be 12 - 18V. Adjust R1235 so you get 15V. That should put you in the ballpark and I bet you will have bias. Turn the bias pot all the way down before you make the adjustment, or you may cook the output transistors.

Just to make myself clear, the unit already had right channel distortion when I received it so no I did not screw it up. The only thing I discovered was that moving R1235 back towards its original paint mark made the distortion increase. Original voltage measured on collector of Q1201 was 8v prior to me ever touching R1235, clearly someone had turned this way off its original mark to "reduce" the distortion. In addition (prior to me ever touching R1235), I was able to get dc offset and bias within spec. After turning R1235 to bring the collector voltage within range the distortion got a lot worse. At that point I started this AK thread.