Idling current adjustment help - Marantz PM7200

[Homer4beer]

Hi guys,

I managed to fix this amp and am now trying to adjust the bias. I am measuring the voltage across the two legs as per the service manual.

I've attempted to adjust the bias now. For the bias for AB, set from cold, I can set both variable resistors to measure 0.4mV < 1 min.

For the class AB measurement I'm testing from cold across the two outer legs of 3335. I'm adjusting to get 0.3mV - 0.4mV at 1 minute. Then I turn off and wait for 5 mins (to make sure I'm starting from cold again) and then swap to 3336, repeat. Same process for 3337 and 3338.

I'm going off my multimeter for the readings.

When I leave the amp on for 30 mins I am only reading 10mV when the manual says it should be 18mV+-3mV. Odd.

Question....when making the Class A measurement do I turn the amp off, then back on again in class A then measure? Do I wait for 10 secs or so before turning back on? Longer? The service manual does not say to turn the amp off, just "Next, switch to class A operation." Now when you switch between class A and AB, going by the manual, you are supposed to turn the amp off first.

With the amp on for an hour if I turn it off, wait for 15 secs, turn it on and set the class A bias I can get it to get into the 65mV - 70mV range at 30 secs. It then climbs to 85 ish then drops to 80 - 82 after another 30 mins? The manual says it should go above 90 and then fall to stabilize at 90mV +- 5mV

What on is going on? Both the Class AB and class A voltages are not stabilizing to what they are supposed ot.

I can only assume I'm doing something wrong?

Thanks guys

Pics of what I'm connecting to and the manual.

 

[creeve]

Good morning,
Although I have never worked on this particular amp before I can give you some general guidance about some of the peculiarities when setting bias. First off when you are doing your class AB setting, you say you do one channel leave off for 5 min then do the other. You really should have two meters to do both channels at once. 5 min really won't bring you back to a stone cold state. If the SM says measure cold then that typically means off for an hour, you want to do both channels at the same starting point if that makes sense. Cheapest way to go here is get two cheap identical meters from harbor freight for doing bias and dc offset, this way both channels will be read by the same type of meter and you can work on both at once. My guess here is this initial setting will dictate how everything else settles in at the end. This may be why you aren't getting quite your 90mv on the last step. Also bias and dc offset do interact with each other to a small degree so I would also make those adjustments in the same order that the SM tells you to. For example on older Marantz amps the SM will tell you to set dc offset, bias, then readjust offset. I will use 4 meters for that LOL. Hope this helps somewhat.

 

[llwhtt]

The line voltage will make a difference as well as room temperature. I would monitor and adjust the AB idle current for ever how long it takes until it settles at 18mv. Then it says to immediately adjust A idle current, monitor and adjust it until it settles in at 90mv.

 

[Homer4beer]

So even though the service manual tells you to set from cold between 30s - 1min your saying let it reach operating temp and then readjust to achieve a stable 18mV?

Is that ok to do?

Thanks!

 

[llwhtt]

The time they mention would be at the specified line voltage and a particular room temperature which they didn't specify. They do specify the FINAL idle current and that's what you should shoot for, after all you will be listening to it for longer than that period of time won't you? I just finished a complete ground up rebuild of an SAE 2600 but I couldn't get the bias to adjust correctly because my shop was too warm, close to 90F. With the bias pots at min. it was right at the final spec. of .04% THD@20KHz. Once my shop was down to 70F it would align just fine and I was able to adjust the idle current and have some leeway on the pot.

 

[Homer4beer]

Thanks IIwhtt.

Let the process BEGIN!

Left Channel class AB

Right channel class AB

 

[Homer4beer]

Well guys this amp is done. I also modified the phone stage to make it sound better as per below.

My question is that I read that the marantz amp had too much capacitance installed on the phone stage to stop EMI

Page 9 C2553/4 should not be fitted and C2555/6 should be 150pF, not 680pF
That will give you ~200pF plus the cable capacitance

The class A bias adjustment did vary quite a bit as the heat cycled on and off. I set both channels as close together as I could and then waited 15 mins, checked again, realigned, checked again in 15 mins, adjusted again as necessary then left then for an hour. During the time they did vary above and below the 90mV I set but never more the -+5mV as per specification so I'm happy.

I also set the speaker DC offset voltage again and managed to get them pretty much the same at +1mV stable, minor fluctuations only.


Left class A adjustment

Right class A adjustment

Thanks again to everybody for all the AMAZING help I've received.
Cheers!

 

[creeve]

Nice work.

 

[Homer4beer]

Nice work.

Thanks Creeve...learning something new every day

 

[tonebells]