Mitsubishi DA-A10DC Need some help to bring it back to life.

[Kahoona]

Hi
This is an amp that I bought in the late 70's/ early 80's and used for 10 or 20 years. I recently got it back and want to make it work well again. It played when I got it back but the bias and offset were way out of whack. I had planned on doing a minor repair but it looks like at lease a recap is a better idea. I have the solder skills and equipment but don't know what makes things work exactly. I can repopulate a board but troubleshooting can be hard. I am learning though. Before I begin swapping parts en mass I would like to find out where the actual problem lies. The good part is that the Bias is set-able and solid on both sides. I replaced the trims for the bias before I set it. The problem is that the offset cannot be set on the right side. The trims do not seem to have any effect. The left can be set but is unstable. I am guessing there is a problem n the right side.

I have a German Service Manual which I can not understand much of. The schematic is good though. What it does not have is the voltages for the test spots on the board. Does anyone have these? Here is what I get

Left-
#8- Orange wire +59v
#6- Orange +5.8v
#11- Gray 4v
#13- Blue -58v
#15-Brn +4.7v
#10- Grn +4.7v

Right-
#38- Orange -58v
#36- Yellow +.7v
#41- Gray **
#43- Blue -58v
#45- Brn **
#40 Grn **
** indicates a variable reading of less than a volt that I am assuming to be 0.
I am guessing that I have a problem with 45 and 40 and I can start checking things near them on the schematic.

Here is the schematic



And a shot of the board which should be vert large but I am not sure how big it will come through.



The board has been cleaned up a bit since it was taken. The funny spot where the board is notched is a repaired crack. It only passed through one trace so I bridged it with wire, soldered it to the trace. The crack ends at a hole so won't get longer. I repaired the physical crack with super glue then with Bondic. I love Bondic!!

I just uncovered a schematic with voltages on it. It is for the DA-A10 which is a similar amp but it may help.



I will get to checking voltages and seeing what I can deduce. Please let me know if you have any suggestions.

Stay tuned and I will post plenty of pics.
Thanks

 

[Kahoona]

I took a look at the transformer voltages that I was able to find on the schematic and I found that nothing matches those on my machine. Since the both mismatch the same and I am able to set the bias I am guessing that it is not a blown transformer nut I will check some other points and perhaps un-solder the wires and retest. Does anyone know anything about this?

Ordered all of the Caps from Mouser. Whoa! Those big boys aren't cheap. My German language Service Manual has no Capacitors in the parts except for the big filter caps. Very strange. Stiii it is easy enough to just read the values off of them.

 

[elnaldo]

Hello. I can't find the pins on your first post in the schematics you posted, also, the 1st schematic and the one you wrote the voltages are not the same, speacially the transformer, one has a different secondary, even if the concept is the same: you have a 2nd secondary (3.5V AC in one schematic), (the one at the bottom of the main secondary), with a small voltage referred to the "-V" rail (if you follow the schematic, you'll see the smaller secondary "ground" is connected to the "-" of th emain filter capacitor on the "-V" rail).

Let's choose a schematic, just one, and work from there. Also, if you find the PCB pins in the schematic, please mark them in some way, to see what you are measuring.

 

[Kahoona]

Hello. I can't find the pins on your first post in the schematics you posted, also, the 1st schematic and the one you wrote the voltages are not the same, speacially the transformer, one has a different secondary, even if the concept is the same: you have a 2nd secondary (3.5V AC in one schematic), (the one at the bottom of the main secondary), with a small voltage referred to the "-V" rail (if you follow the schematic, you'll see the smaller secondary "ground" is connected to the "-" of th emain filter capacitor on the "-V" rail).

Let's choose a schematic, just one, and work from there. Also, if you find the PCB pins in the schematic, please mark them in some way, to see what you are measuring.

-The one with the red voltages if from a forum so it may even be from a DA-10 (not DC).I did not write the voltages on it. Don't trust them. I assumed that they were original and would be good to check against. The first one is from the shop manual I have, the German one.
-The Transformer diagram is from the German Manual and may be the correct one. I have those voltages and will add them to the diagram as quickly as I can. You have explained why my measurements are different from the second schematic. They are balanced and I suspect that the transformers are fine. It did play when I got it but did not sound right in the right chan.
-I would like to be able to post clearer pics but the new format makes it hard. I am working with huge 14m pics and big BMP screenshots that I can magnify forever but I can't seem to get anything that can be blown up past either AK or Photobucket. I'll keep trying though.

-I have to go out now but when I get back I will try to put what voltages I have onto the schematic and the Diagram. They have some pins numbered but none of the ones I need. There are voltages on there too but no notice of which pin. Would it be easier to read if I added it in quarters for better Resolution?

-I have ordered the replacement caps just because I want it to sound better but I more suspect the problem is elsewhere. I'd love it if it was something like the DA-A7DC but no such luck so far.
Thanks

 

[elnaldo]

OK, so let's forget about the pict with red voltages on them. They are wrong, perhaps somebody tracing a problem, showing his measurements.

Let's start from scratch with the German schematic. 1st, DC voltages at the power supply output (filter capacitors). If all the Power Supply rails are OK, let's move forward, but we need to be sure the power is OK.

We don't need 14MP picts to have a good reading on a screen. If you can manage to downsample to something like 2000 pix wide, it will be enought. We can always have a close up of some part, from your 14MP original.

To upload a pict here, I use Dropbox, and post a link to the picture using the "Image" icon on the tools bar. It will link to the original pict, regardless of it's size. I think you can do the same, a link to a pict uploaded to any web server.

 

[dr*audio]

Try reading your voltage offset at L101 and L201. If the speaker relay contacts are flakey you will get bad readings at the speaker jacks.

 

[Kahoona]

. 1st, DC voltages at the power supply output (filter capacitors).

OK. From the very terminals of the big caps
Right chan
Black dot 42.2V
Red dot 42.7
Left Chan
Blk 42.8
Red 42.7
Looks good to me so far!

We don't need 14MP picts to have a good reading on a screen. If you can manage to downsample to something like 2000 pix wide, it will be enought. We can always have a close up of some part, from your 14MP original.

2000p and image it will be. I was using 1400.

While reading values on caps I found c305 to be 109uf instead of 220 and c 306 off by a similar amt. I was able to replace c306 temporarily but not 305 yet. For what it's worth. Wife wants to eat dinner now.

 

[elnaldo]

What about the other secondary?

Also, I always follow Dr Audio's advice, so I'd check the offset again as suggested

 

[Kahoona]

Hi
Here is a 2000p image of the board in case it helps.



Should be back shortly after checking that offset and measuring the other secondary.

 

[Kahoona]

Try reading your voltage offset at L101 and L201. If the speaker relay contacts are flakey you will get bad readings at the speaker jacks.

Hi
Thanks for helping. I would love it if it was something simple. I just tested it with the same results but I don't know if I did it right. Did you mean checking voltage between the points I circled in blue and ground? I was not sure if one side or the other was correct.
What are those copper coils for anyway?

 

[Kahoona]

I have some sort of short going on I believe. The dim bulb will brighten, The right chan relay also clicks. It happens when I move the board to the closed position and it happens when the board is in the "out" position and I shift it slightly. Working with the board may have worsened a fault. I'll track it down and get back.

 

[Kahoona]

A small purple wire led to a thermal sensor connected to the Right heat sink. It's solder joint failed. After re-soldering it I was able to set both offsets to a solid 0mv without a problem.
I did it using the L101 and L201 because the right relay is now out. Strangely when I measured it between the + speaker terminal and the ground it measured 0mv also.

I may need a relay. The other side was replaced already. I'll research how to test them.

 

[elnaldo]

When a relay receives voltage on the coil, it engages. So you should check if the voltage is reaching the coil. You can compare both relays.

A protection circuit will block the voltage from reaching the relay, so if the relay is not powered, perhaps the amp is protecting itself..

The relay circuit is at bottom right of the schematic.

EDITED, I see Relay A manages Speaker set A, and Relay B manages Speaker set B, not CHANNELS L and R.

Channels L and R are connected at the same relay.

 

[dr*audio]

Hi
Thanks for helping. I would love it if it was something simple. I just tested it with the same results but I don't know if I did it right. Did you mean checking voltage between the points I circled in blue and ground? I was not sure if one side or the other was correct.
What are those copper coils for anyway?

Either side of the coils are ok to measure. The coils are part of a Zoebel network. They help to isolate the amplifier from the effects of capacitance of a long length of speaker wire. In an amplifier that is not 100% stable, a capacitor across the output can cause the amp to oscillate. If the oscillation is strong enough, it can blow the output transistors.

 

[Kahoona]

It keeps getting more interesting by the minute.
elnaldo

When I heard this I took a fresh look at them. I know that it is best to never assume and in this case it was true. The front set of contact plates on these are a rest position or possibly a ground. What looks like closed contacts are actually open. They move to the rear and contact a second pair of contacts when they are open and working. Exactly the opposite of what I expected. So when I thought I had a Chanel out I actually had both A channels working. That bad solder joint was causing them to kick in and out together. Not they work correctly. Switching from A to B causes the other one to come on. Tomorrow I will test with speakers just to confirm. The Caps are on the way including the Big Boys. I got 2 Nicholson for each side and they are narrow and tall so I can fit them side by side. They have lugs. I may have to make a spacer to keep them steady. Is it safe to assume that the diodes and semiconductors are OK if it runs well?
Dr Audio
That is very interesting about the Zobel network. Now I know! I also know not to trust the relays.

Do either of you have a suggestion for what else I should test? I know to check the heat paste on all of the semiconductors and check all solder joints. Do these output transistors hold up well?

 

[dr*audio]

I think you've done all that needs to be done.

 

[Kahoona]

This morning before dawn I awoke to the noise of someone on my front porch followed by the sound of a Grumman sliding door closing. I looked out of my window to see the back end of a retreating USPS vehicle headed down my driveway. Just then the motion detector lights came on (hint: don't buy Zenith motion detector lights) and it refused to go off again so I got up. It was a nice heavy Mouser package! Mouser boxes are like Christmas to me. Always some adventure in there. So I have all of the electrolytics. They will probably be a simple thing to swap out. Thanks Doctor for letting me know that the rest is probably fine.



For those of you contemplating a recap power amps are often the simplest because they don't have a ton of tiny parts. In this case since the sides are discrete I am going to do them one at a time so I have a reference if I get confused. It happens. I have to clean up the bench and put away the tools. Mouser is great but the package still gets inventoried in case I screwed up. Once everything is clean (as it ever gets here) and ready to go it will be time to remove the old caps, solder and clean as much as possible. I will also test some of the parts that come off out of curiosity. The 2 caps I have tested so far have read only half what they ort so I wonder. I also hope to keep the old filter caps intact if I can. I can't test them so I will get a friend to do it. I am guessing they are good and could be used bu a fellow AKer or sold "at that nasty place." They may be unobtainium!

 

[Kahoona]

Please Elnaldo or Dr, audio could you take a look and see than my reasoning is OK for polarity. Edited to prevent damage due to my ignorance.






Here are the new






The wiring will be with crimped eyelets into the caps and then probably soldered, shrinkwraped to the old once they are in. The minus poles will be connector and common. Here you can see why I was not able to mount the new ones flush. The mounting bracket has too small a hole.





Here is the schematic with the most recent voltages written in at the numbered test points that apply.



If you are doing this job yourself don't plug anything in yet. I want to confirm that this is right and then I will test the voltages on the Dim Bulb before anything happens. Hope to be back tomorrow. The world gets in the way sometimes. You might also want to wait until I get my actual (promised) shop manual in English! The friend who was storing these had it. I'll upload it to AK because I know these are insanely hard to find.

 

[elnaldo]

Stop, you are connecting "-" to "-" on your new caps, but the old Nichicon clearly says "-" / "earth" / "+".

Look at the schematic... C 11 is marked, both "+" pointing UP, that means the positive of the bottom one is connected to the negative of the one on top. The same at C51.

Also, I wouldn't "drop" them into the hot glue. If you want, cut a small piece of the needed height of the hot-glue bar, glue it at the bottom of the old case, then, glue on it the new caps. They won't "escape" from the case. Just add some drops of glue. I prefer hold them with plastic cable ties, and just a bit of glue (if needed) on one side, or a piece of eva rubber holding them. Think they will be in a hot evironment, hot glue could melt a bit. Caps need air, on top, and all around. They shouldn't be "compressed", just free all around.

Another idea, connect a solid wire from one - to one +, a bridge, and tie there all the ground wires.

 

[dr*audio]

Elnaldo has it right. The caps need to be connected - + to -+ with the blue wire to the - and orange to +, and center to ground.