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Mitsubishi DC-A10 Service manual confusion!

Kahoona

Kahoona

Super Member
Hi
I have recently gotten back a DA-C10 that helped blast me into orbit during my extended and misspent youth. I am wanting to set the Bias and DC offset and am very confused by the service manual. I have done this many times on other amps and will actually replace a couple of trimmers to make it easier. I am unable to make sense of the service manual and I know some of you have these machines ans have done this so I hope you can explain! If not perhaps someone might get a chuckle.

There are three trimpots per side 102,103 and 104, and a line level control mounted on the back of the cabinet.

Problem one:
Here are the instructions for setting the Bias



Easy enough remember that 103 sets the bias. Now we look at the DC offset instructions




Could the same pots really be used for both? I have to add that on the DC-A7 there are 2 pots each side to do large and small adjustment on the Offset. This diagram does not look a lot like the board but it is clearly labeled.

On top of this there is a pot # 104 that is not mentioned. I am guessing that in reality pot 104 does the bias and 102 and 103 do the offset but it is a guess.
Here is a diagram found on AK which does show the parts in the right places. I don't know where I found it. Sorry.



___________________________________________________________________________________
That is the end of confusion number one.
 
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so vr202 doesn't get a mention ..typo there me thinks . maybe idling side of things.look at schematic should clear things up .
extra pots maybe for meters ..
 
Now here is confusion 2. I separated them to make it less confusing.
There are a pair of full sized pots on the rear of the case. Some feel that they are VR101.
Here is a schematic from the service manual



Here is the parts list from the manual. They show only101,102 and 103.




These values don't correspond to the real ones on the board where 102 is 100kohms,103 is 680ohms and 104 is 2kOhms. This is just plain weird. I can replace them same as the originals so not so bad but am I missing something?
 
petehall347 said:
so vr202 doesn't get a mention ..typo there me thinks . maybe idling side of things.look at schematic should clear things up .
extra pots maybe for meters ..
Click to expand...
I wondered so I checked that. I posted the schematics while you were posting. Actually the meters come as a separate cabinet that could be bought and they might tie in there somewhere but I have no idea how they connected!
It is almost as if there were two DA-A10's and the manual never changed or half changed. I was careful to make sure I did not confuse it with the DA-A15
 
okay VR pots many are unreliable because of age even from 4 years ago. many can be cheap plastic. but many are good if vintage. but what else have you done? Have dbt tester built?

I would try to supply low power to the unit and see if 'cleans up on the rails etc. power supply is about 95% of common audio main circuits. all the relish is filtering and add on's. caps can reform. on and off.. restarting to idle and no speakers etc. you can add a little input audio or gain after about 5 hours.

no joy post back.
bink
 
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Binkman said:
okay VR pots many are unreliable because of age even from 4 years ago. many can be cheap plastic. but many are good if vintage. but what else have you done? Have dbt tester built?

I would try to supply low power to the unit and see if 'cleans up on the rails etc. power supply is about 95% of common audio main circuits. all the relish is filtering and add on's. caps can reform. on and off.. restarting to idle and no speakers etc. you can add a little input audio or gain after about 5 hours.

no joy post back.
bink
Click to expand...
Interesting. I always wondered if running at Idle could help. Since this was kept. This was stored in a friends basement do that might drive the demons out. I have have the DBT but I did take out one of the VR's to check it. I'll see if I can find it and warm it up! This one ran but I want to get the Bias and offset right.
 
petehall347 said:
cant see numbers on pics but bias pots in middle with box around . dc offset on left .
Click to expand...
Closer pic of board. Here is what I guessed but if Bias is in the box that would be different. I need to research. The odd thing is that VR103 is the first thing after the signal input so I would think it was an input level but there already is a pair of pots on the scan for the inputs.

 
your vr104 is vr103 .. and for bias .
vr102 dc balance / offset
vr101 input sensitivity .. bit like an attenuator .
 
The DA-A10 and DA-A10DC are different. The DC version does have coarse and fine offset adjustments. It also has true DC coupling stem to stern and is flat down to zero.

Couldn't find a C10, a C7 but no C10.

In general they do not put gain pots in power amps like that. If they have them they are on the front. Coarse and fine DC balance adjustments are much more common.
 
JURB said:
The DA-A10 and DA-A10DC are different. The DC version does have coarse and fine offset adjustments. It also has true DC coupling stem to stern and is flat down to zero.

Couldn't find a C10, a C7 but no C10.

In general they do not put gain pots in power amps like that. If they have them they are on the front. Coarse and fine DC balance adjustments are much more common.
Click to expand...
Thanks. I didn't know there was a C10 so I abbreviated the DA_A10DC which is what I have. That explains the difference. My Service Manual is for the A10 ans therefor the Parts list is too. So as Petekall says everything comes down 1 number and it is what it looked like by eyeball and experience rather than by the book. I am glad I ordered the same trim that was in there rather than the part list. I am glad that I have the real DA model. I had always heard it praised for the true dual mono nature when I originally bought it so many years ago.
Soon as the parts get here I will install the new pots which are the DC offset ones and do the setup as recommended by Binkman with the DBT and a good 8 hours to let everything settle down.
Sad to say I have pulled a muscle in my back and, like an idiot I kept right on hoisting these big amps up onto the bench ect so now I am hurting. It may take a little while. I'll report back though.
Thanks all.
 
Hello and Merry Christmas / Happy Holidays
I replaced the VR's for the bias in my A10DC using VR104 and set it. It is stable. I ran it for 8 hours on the DBT with a 75w bulb then tried the DC offset adjustments.
Here is what is happening. Part numbers are the ones on the board.
-VR103 has no effect.
-VR102 does have an effect and would adjust the DC Offset except that the speaker protection kicks in right around 6mv so that I cannot get to 0.
-The DC current at the speaker terminals will begin to rise and eventually pass 300 +mv.
-Both sides behave exactly the same.
-I found and repaired a crack in the board. It only broke one trace which I bridged with a wire and soldered. It ended at a hole so it won.t get longer. I bonded it from both sides and continuity tested the repaired trace. This changed nothing.

Question- Am I right to assume that the problem has to be in the power supply because it is the same on both sides?
Question- Since this is supposed to be a pair of independent Mono amps their power supply should me mostly independent so I should have a very small area to check. ?

Picture-
On the right are the trimmers identified by number. I think the older 102 and 103 are fine. I replaced 104 but it was probably fine too.
-C306 circled in blue and within the red area, has some blue goo on top. Does not look like a leak though what is it? Could it be the problem if it was a leak?
- The area circled in red has the only parts not duplicated by both sides. Would it be OK to look at that area? Part of it is the speaker protection and that is OK but I assume that part of it is involved in power supply also. Am I right to assume that I am looking for something that affects both sides and that the chance of an identical failure in both sides is unlikely? Something that fails to regulate voltage? A diode? Perhaps a resistor? None look blown.

I am doing my best to figure out as much as I can so that I can learn. I don't know 1/100th of what some of you do so I sure could use your input. My schematic is for the A10 not the A10DC so it won't help.

 
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I can't find any bad solder joints or traces. Should I simply start un-soldering a leg on everything and testing it? I won't know the specs on most of them but I can see if they are shorted or open. Perhaps someone could tell me if that would be the way to go?
Thanks
 
It is not uncommon for the DC balance control to have little or no effect in an amp with the bias set too low.
 
Just in case anyone needs them I have sent the Owners manual and service manual in English for the DA-A7,10,and 15DC's. I have the originals from Mitsubishi and I scanned them at 200dpi. It may take a little time for them to appear in the database. They are very zoomable. Thanks again.
 
Just signed up today! Hoping to get info on my DA-A10 DC. Looking for service manual or schematic. Thanks!
 
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