Build Thread:MS76

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karloff70

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So I have arrived at the blend pot. The pot has the three normal pins, which I have wired to the 3 pin Molex that says blendpot. Assuming that isn't wrong already, what do I do with the two contacts named A and C on the switch and how do I get the relay involved? I see the hole marked 'blend switch return' on the board, but have no idea what to do now?
 

rtron

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May 16, 2014
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So I have arrived at the blend pot. The pot has the three normal pins, which I have wired to the 3 pin Molex that says blendpot. Assuming that isn't wrong already, what do I do with the two contacts named A and C on the switch and how do I get the relay involved? I see the hole marked 'blend switch return' on the board, but have no idea what to do now?
Wire a 12V wire (from the 12V header on the main pcb) to the blend pot/switch contact (doesn't matter which one). Then run a wire from the other contact on to the blend switch return of the the main pcb.

Also, for the blend pot connections, I did board pin 1 (red wire) and went to pot pin 3, board pin 2 (brown wire) to pot pin 2, and board pin 3 (black wire) to pot pin 1 and it worked out correctly. Note that I define my pot pins like this: https://sound-au.com/pots-f5.jpg
 

karloff70

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Wire a 12V wire (from the 12V header on the main pcb) to the blend pot/switch contact (doesn't matter which one). Then run a wire from the other contact on to the blend switch return of the the main pcb.

Also, for the blend pot connections, I did board pin 1 (red wire) and went to pot pin 3, board pin 2 (brown wire) to pot pin 2, and board pin 3 (black wire) to pot pin 1 and it worked out correctly. Note that I define my pot pins like this: https://sound-au.com/pots-f5.jpg
Amazing, thank you! Looks like I need to swap 1 and 3, but now I know what to do with the 12V. Thank you very much!
 

JMan

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Hey All,

I ordered one of Gustav's last spare boards -- now just to wait out the long shipping times between Germany and the US... 🙃

I took a look at the schematic, which I think is the only way for me to put together a BOM. (I guess since this was previously offered as a full kit, there is no published BOM?). I see that the M/S encoding and decoding sections use THAT 1240 chips. I was just looking things up on Mouser to kill time, and I see that there's a 0dB version and a -6dB version of this IC. Is one or the other preferred? EDIT: Never mind, I see that the -6dB one has a slightly different part number that I didn't catch in the sea of results that Mouser gave me.

More questions to come, I'm sure, whenever the package arrives and I have a chance to start working on it.
 
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karloff70

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Ok, so I have it powered up. It passes signal, the VU's work. But it does not compress. At all. And when you put the meter in GR the needle goes to the bottom and stays there as opposed to zero. Everything else seems to be working.

If you smell a specific diagnosis of what fudgery I have committed based on that description I would very much appreciate learning about it. :)
 
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karloff70

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Looks like I am on my own and with the 'schematics' provided my tech can't help either. Great. Any chance you could help me finish this, Gustav?
 

rtron

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Ok, so I have it powered up. It passes signal, the VU's work. But it does not compress. At all. And when you put the meter in GR the needle goes to the bottom and stays there as opposed to zero. Everything else seems to be working.

If you smell a specific diagnosis of what fudgery I have committed based on that description I would very much appreciate learning about it. :)
Did you follow the calibration procedure? My meters did weird things too, before I calibrated it...
 

karloff70

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I didn't but the tech did, so technically it is calibrated. But mostly it doesn't compress at all regardless what the meters are doing.
 

Gustav

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Looks like I am on my own and with the 'schematics' provided my tech can't help either. Great. Any chance you could help me finish this, Gustav?
HI Karloff

There is a full schematic in the folder on the site - its separate for the main board/aux boards, and you can "overlay" the aux boards on the main board to see connections and continuity in signal flow.

If your tech cant read them, I would recommend trying to find someone else to try to help you.

Unfortunately, its impossible to solve your problem based on the symptoms you are describing. They are simply too general. From what you are saying, the GR is triggered hard, regardless of setting, by indication of the meter, so we can suspect you have something pushing it. It could be a short to a supply, so you have DC flowing in, but it could also be a number of other things.

For trouble shooting, start by verifying all voltages. The main supply voltage values are printed on the board - and in addition to that, you can check the voltages around the transistors, which are printed on the board (we dont expect them to be spot-on, but in the general area of what is printed).

Thats always the first step for trouble shooting, so do that, get back, and we can help you look further into the problems you are experiencing with the build! :)

Gustav
 

Gustav

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I didn't but the tech did, so technically it is calibrated. But mostly it doesn't compress at all regardless what the meters are doing.
After replying to your earlier post, I saw this.

I dont know if this means, the GR meter is no longer pegged, and your are seeing compression action on it!? I sort of assume it does.

If so, please note, the metering here is done by shadowing what goes on in the signal path. The transisor in your signal path may still be bad/blown, even though the meter is responding. I would start crudely by replacing the 2N5457 in the signal path. (its less likely, if you are experiencing this in both channels, of course).

There are test points scattered throughout the signal path.

1. pure input after the 1206
2. after the input gain
3. after the output gain.

(double check these on the schem, Im going by memory, but it will be obvious when working with the unit).

Check if those are working/if you are seeing any compression.

Another reason why you may not be seeing compression could be, that the blend circuit is not set up properly - double check that as well.

Hope that helps!

Gustav
 

karloff70

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Hi Gustav,

Well, the tech is Mick Hinton at DAV/ex Decca, so I don't think I 'need a new tech'. He seemingly can't work with those schematics though, unfortunately.

The meter is not pegged, just slowly falls to the bottom when switching to GR after working as expected on in or out. There is no compression happening.

I will try your suggestions best I can as soon as I can, as about to start moving house. Maybe this thing can finally come alive in the next address.
 

Gustav

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Hi Gustav,

Well, the tech is Mick Hinton at DAV/ex Decca, so I don't think I 'need a new tech'. He seemingly can't work with those schematics though, unfortunately.

I understand its frustrating, that your unit is not working, and you need help.

I would suggest you find a tech who can help. Alternatively, I would be happy to lend Mick a hand.

He may have just missed the tag on the end of the input section/start of the amp, end of the amp, start of the output section.

All sections are named, and drawn in sequence, and as mentioned, you can overlay the aux.

I will try your suggestions best I can as soon as I can

Sounds good. I hope you get it working! :)

Gustav
 

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