Mitsubishi DA-A7DC Bias and DC Questions

[gkwolfman]

This is revised to state.....That I am stupid........the snipping of zip ties was NOT NECESSARY....
If you have the right type of flat head? screw drivers, you can access your ADjustments from the bottom side of board.
Plus, you have better access to any measuring points...........maybe not stupid, but, careless for jumping in at it without knowing. Maybe the same thing?

I am going to attempt to adjust my DC and Bias.
My DC is 31.0 on one side, and about 11.0 on the other side.
Just got the amp opened to attempt the adjust and find the Bias points and
measurements.

Anyone have.... what the values are for the Bias and where to + and - for the checks.?

I should be able to figure out the DC, very carefully. That one is a "No Brainer", I would assume. I use the blue cap Variable Resistors (VR202=Right and VR102=left) for the DC off set. I hope that it was the right ones. It did adjust the DC values for both sides of the speakers.

I am puzzled on the Bias part. I always had just two VR's for bias, this looks like there are two on each side, VR101 VR103, and VR201 VR203.

Plus, I need to find out where to put meter ends + and - , to read the bias.
any help would be great.

Sorry about the photos, thought they would be larger, but , you can still click on them.

 

[TerryS]

I'm looking at the schematic for the A10DC, but I bet it is the same.....

VR102/VR202 set the DC Offset. VR103/VR203 also set the offset. Think coarse adjust/fine adjust. The manual says +/-5mV max.

VR104/VR204 set the bias current.
Measure the bias across R139/R239 or R140/R240. These are 0.33 Ohm 5W in the A10DC. Might be different values in the A7DC.
The test points are 10/15 and 40/45. If you chose to use the test points instead of the power resistors;
10/15 are left channel across BOTH 0.33 Ohm resistors, so you are measuring across 0.66 Ohms.
40/45 are right channel.

As far as the value, that is a bit confusing for me. The manual says set it to 50mV +/- 3 mV (again, for the A10DC). This means that it would be 76 mA (across the 0.66 Ohms). But the accompnying chart shows that it should be set to 50 mA. I suspect the label on the chart is wrong and that it should be set to 50mV = 76 mA. I think that's how I set mine. It is the same for both the A10DC and the A15DC, so I'm guessing it would be the same for the A7DC, but to be sure you should search out the right value.

Obviously, if you just measure across one of the power resistors, the voltage would be half (25 mV).

Be patient. Try to keep the bottom cover in place and let everything stabilze 20 minutes between SMALL adjustments.

Terry

 

[gkwolfman]

I'm looking at the schematic for the A10DC, but I bet it is the same.....

VR102/VR202 set the DC Offset. VR103/VR203 also set the offset. Think coarse adjust/fine adjust. The manual says +/-5mV max.

VR104/VR204 set the bias current.
Measure the bias across R139/R239 or R140/R240. These are 0.33 Ohm 5W in the A10DC. Might be different values in the A7DC.
The test points are 10/15 and 40/45. If you chose to use the test points instead of the power resistors;
10/15 are left channel across BOTH 0.33 Ohm resistors, so you are measuring across 0.66 Ohms.
40/45 are right channel.

As far as the value, that is a bit confusing for me. The manual says set it to 50mV +/- 3 mV (again, for the A10DC). This means that it would be 76 mA (across the 0.66 Ohms). But the accompnying chart shows that it should be set to 50 mA. I suspect the label on the chart is wrong and that it should be set to 50mV = 76 mA. I think that's how I set mine. It is the same for both the A10DC and the A15DC, so I'm guessing it would be the same for the A7DC, but to be sure you should search out the right value.

Obviously, if you just measure across one of the power resistors, the voltage would be half (25 mV).

Be patient. Try to keep the bottom cover in place and let everything stabilze 20 minutes between SMALL adjustments.

Terry

Greetings Terry, and thanks for your reply.
On the DA-A7DC, VR102 and VR202 are the blue cap Variable Resistors, and those are the two I used for my DC off set adjustments.

Well, they measured a value adjustment with the meter set on DCV at 200m. I was able to bring both sides down to (unstable) left=02.2/00.5......right=02.5/00.7

I may hold off adjusting further, until I know for sure that I am on the correct VR and find a DA-A7DC schematic. I did have a very good hunch, though, that the VR102 and VR202 was for the DC off set. I won't even mess with Bias, too much more critical than the DC, until I definitely know for sure.

I did search HiFiEngine, and they only had the DA-A10DC and it was in German. Thing is, my German went out long ago with my grandparents passing away and from school. But, because of the mono differences between the 7 and 10/15 models, I wonder if they would show the same procedures and measuring?

 

[TerryS]

I'm pretty sure that even on the A7DC (I have one of those also), the offset is set by VR102 and VR103 for the left and VR202 and VR203 for the right. There are two pots for each because one is coarse adjust and one is fine adjust. I don't remember which is which.
I also have the German version of the A10/A15DC manual. That's the only thing I could find. I know I did the adjustments on mine, but it was a while ago and I don't remember the details. I'll search my paperwork at home and see if I documented what I did by any chance.

Terry

 

[gkwolfman]

Greetings Terry

I combed the board for your reference with the Bias VR104/204, and I am not seeing it. This board is only showing the VR101 to 103 ....VR201 to 203

 

[TerryS]

I bet they are nearly in the same locations as the A10DC. I think the pots may have been mounted on the back side, with only a hole to adjust them visible.

Terry

 

[gkwolfman]

Terry

The position of your VRs are similar to the position of the VRs on the a7dc.
With the exception of, yours do not start with VR101 / VR201 respectively.

I will check the test points on mine, in your location , to see if they match some what close.

 

[TerryS]

Ah, that's right! The A7DC does not have the input level set adjustments that the bigger amps have. These are VR102/VR202 on the big amps. So all of the pot numbers may be one number lower on the A7DC.

Sorry it took me so long to catch on to what you were saying.

Terry

 

[gkwolfman]

Well, no apology necessary.
This is all too confusing to me.

I will have to say this for sure, that for being 30 years plus old , the DC off set on this model was not too bad off.....
Left was hitting around 30mv
Right was hitting around 13mv ...and I have seen worse.


I don't believe that the DC off set was ever done on this Amp, because you would definitely have to snip off the zip ties to free the wires to lower the board down to access the VR's and test points.:thmbsp:

The more I wait and search for the test points, the more it is killing me.
I may get impatience and wing it. But, usually test points are Numbered...like T101....as variable resistors are Numbered VR101. I just havent seen it yet.
Plus, I have to use a magnifying glass some of the times. My poor ole eyes.

Meanwhile, I have my Sansui AU 9500 running a Mitsubishi DP-103 through some ADS L-710s, listening to the four disc set of the Doobie Brothers. Thank god for having more than one system, while another is being repair/tuned up, which ever the case may be.

 

[TerryS]

I don't believe there are actual test points. Just the pads that wires are attached to.
Terry

 

[TerryS]

I dug through my paperwork and found this schematic of the DA-A7DC that I drew from tracing out the circuit many years ago. As you can see, VR101 and VR102 both set the offset. VR103 sets the bias. From my notes on the schematic, I set the bias to 15mV across one of the 0.47 Ohm, 5W power resistors (R131 or R132). This set the bias at 32 mA. I think that was just the value it was set to. I don't know if it is correct. I don't think I've ever seen a service manual for the DA-A7DC. All I've ever found was the German version of the A10/A15DC.

Terry

 

[gkwolfman]

thanks Terry, I have opened the .pdf and studying it now.
I am not too electrical savvy , sometimes, but, I try to make a go of it.
I have been always spoiled having a service manual and reference points with....VR101...VR201......T101.....T201 to adjust and measure. Never have tried other measuring points with the meter. Such is the case with my Sansui au 9500. Checking DC off set , for the most part, is easy for me. But the bias on the Sansui AU 9500 stumps me, like this Mits.
Gary

 

[MitsuMan]

I have the service manual for the A10/A15DC if you'd like me to post the bias and offset procedure. I don't have one for the A7DC but it may be helpful.

 

[gkwolfman]

thanks MitsuMan

hold on for now......trying to figure out Terrys' method right now.

I am measuring the mv across the resistors r131... r132 ...r231...r232
checking the values first to see how they look. before making any adjustments.

If it is alright, maybe I can PM you later?

Gary

 

[gkwolfman]

resistor measurements

Okay, I have the values for my resistors

Meter set on DCV at 200m

R131.......18.4 minus
R132 .......17.7 plus

R231.........17.2 minus
R232.........17.4 plus

if these are suppose to be...15mv.....are my readings bad?
For the first couple of hours, my amp is feeling cool.
When it reaches around three or so hours , it starts feeling warm.
Then , after at least 4 or 5 hours, a little bit more than warm, but, consistent, never approaching hot.

What do you think Terry.? Do you remember what your readings were before you did your adjustment? I have some amps that I can leave on for days and never get as warm as this A7DC does.

By the way....my DC off sets for both sides are banging around on the plus side for both around 1.0 mv.

 

[TerryS]

It sounds like you are already plenty close. The voltage across the resistors is caused by the bias current multiplied times the resistance. To figure the bias, take the voltage you measure and divide it by the resistance (0.47 Ohms). So 18 mV (0.018 V) divided by 0.47 Ohms = 38 mA (0.038A). This agrees very closely with the 32 mA I measured. It looks like all of your readings are essentially the same. A few mA either way is insignificant.
A bigger issue is the thermal stability. Like I said earlier, the right way is to have the unit buttoned up (board in place, bottom on, in normal orientation) and let it sit for 20 minutes or more before measuring the bias current. This is because the bias current causes heat in the heat sinks. The heat affects the bias circuit. The feedback loop is very long. Usually at least 20 minutes. So any change takes 20 minutes to evaluate. And it is important to have the cover on. I think I clipped wires on and fed them out through the cover to my meter.

1.0mV is excellent for the offset.

Terry

 

[TerryS]

I think warm is perfect. As long as it does not approach hot. If you can comfortably leave your hands on the heat sink for at least 10 seconds, it is good. Ideally, the idle heat would be about the same as when you are playing music like you normally do. This means the bias current is stable at the value that it will be when playing music.

Terry

 

[gkwolfman]

I think it is break time now

Thanks Terry.

I will compare notes and see what kind of angles and rigging I can come up with.

But, I think it is break time now.

I got some ribs calling my name....and my belly says ""get them"

Will continue later on

Gary

 

[BenWaB3]

I have the service manual for the A10/A15DC if you'd like me to post the bias and offset procedure. I don't have one for the A7DC but it may be helpful.

I'd appreciate it if you did that. I have the A15DC & have wondered about how to do it. Thanks.

Ben

 

[MitsuMan]

I'd appreciate it if you did that. I have the A15DC & have wondered about how to do it. Thanks.

Ben

Sorry for the delay, I've been away. I hope this helps.