Build Thread:MS76

[James Meder]

Hi all,
A week ago I had some time to visit my friend James (above) who was having issues with his build similar to others here. The problem turned out to be oscillation on the -12V line which was confirmed with a scope.

Hi all, I'm back yet again. @mrbutterskin helped me get 99% of the way there with this mod on the power supply but I'm still not 100%. I'm able to calibrate everything correctly and it's compressing nicely. The weird thing I'm experiencing now is that the output gain becomes noisy when turned up past 3oclock (like between -20 and -25db on my +4 converters)
@mrbutterskin recapped all the 0.1uF to 10uF on the power supply when I visited hime last... just hoping to mitigate any irregularities there. When probing the power supply, I see the smallest amount of noise but he's thinking it's not enough to contribute to my issue (or at least not obviously). He's helping me remotely for now so it's a bit tough to troubleshoot.

You can see the noise on the video. The probe is attached to pin 2 on the output xlr. This happens exactly the same on both channels.

I will note that I can achieve unity in and out with my calibration so this issue is just to get the unit 100% and hopefully I can rack it up permanently. I can use it as is but it would be nice to be free of issues like this.

Has anyone else experienced this or something similar? Thanks!

 

[c1184701]

Hi James, I had oscillation on my unit when the output pots were near to full. The mod suggested by mrbutterskin cured this for me, a 100pF cap across pins 2 and 3 of T5.

I would have expected that swapping the 7912 cap to 10uF would fix all stability issues, and therefore eliminate the need for the 100pF. I left it on mine anyway.

 

[James Meder]

Hi James, I had oscillation on my unit when the output pots were near to full. The mod suggested by mrbutterskin cured this for me, a 100pF cap across pins 2 and 3 of T5.

I would have expected that swapping the 7912 cap to 10uF would fix all stability issues, and therefore eliminate the need for the 100pF. I left it on mine anyway.

Thanks so much for chiming in! I have yet to try the 100pF cap on T5 so I plan to do this soon and report back. Will need to order one.

 

[aquarian_tapes]

I have been working on building an MS76 for a friend. He bought it from someone who had held onto it for some years but never gotten into building it.

It has been slow going for me. This is the most complicated kit I have put together so far and I have been encountering some missing parts (some might been lost, hard to say). For the most part sourcing these parts hasn't been a problem. However, I am looking for advice on two different THAT chips that seem to be missing. There are two THAT 1206 (input) and four THAT 1240 (M/S encode/decode) that I need to source.

The only THAT 1206 I can find is through Newark. Can anyone speak to suitable alternatives?

As for the 1240s, I can get them through Mouser which I would prefer to buy all the parts through if possible. But I am not sure whether they should be the -6dB, -3dB, or 0dB versions. Does anyone know? I have a similar question about the 1206; the one I can get through Newark is -6dB.

Sorry if this is an annoying, novice question, and any advice is greatly appreciated. This build has been stalled out for too long now!

 

[TwentyTrees]

The rev 2 BOM I have specifies 1200 and 1240 for the THAT chips, and has a note against the 1200: "(board marking for 1206 is wrong)".

Shouldn't matter which retailer you use, as long as they're credible - THAT Corp is the sole manufacturer so it shouldn't matter where you get them from, apart from the price.

Good luck with the build! It's a good bit of kit once you get it working.

 

[aquarian_tapes]

The rev 2 BOM I have specifies 1200 and 1240 for the THAT chips, and has a note against the 1200: "(board marking for 1206 is wrong)".

Shouldn't matter which retailer you use, as long as they're credible - THAT Corp is the sole manufacturer so it shouldn't matter where you get them from, apart from the price.

Good luck with the build! It's a good bit of kit once you get it working.

Thanks for the reply. I just ordered 2x 1200. Unfortunately 1240s are on backorder but I got that order in the pipes. I ended up going for the unity gain 1240s; I'm assuming this is correct (would love for anyone else to weigh in if they know).

 

[TwentyTrees]

THAT 1240 is unity gain. The other variants have their own designations: 1243 for +/-3dB; 1246 for +/-6dB. The BOM calls for 1240, so you've ordered the right parts.

https://www.thatcorp.com/datashts/THAT_1240-Series_Datasheet.pdf

 

[zappo1234]

THAT 1240 is unity gain. The other variants have their own designations: 1243 for +/-3dB; 1246 for +/-6dB. The BOM calls for 1240, so you've ordered the right parts.

https://www.thatcorp.com/datashts/THAT_1240-Series_Datasheet.pdf

Hello. I cant find THAT 1240, You know any equivalent? Thank you.

 

[gyraf]

perhaps INA134 and the SSM2141? (better check yourself for details)

 

[TwentyTrees]

Was just about to post something about other balanced line receivers out there, but Jakob beat me to it! Basically, what he said. You'd want to check datasheets for pin compatibility, power rating, impedances, gain and such.

https://thatcorp.com/balanced-line-receiver-ics/

 

[christophemo]

Well done, Frederick!

If you dont mind, can you tell me how you are measuring that mono is not completely down center in MS, have you traced it back, and how much is it?

In theory, anything that is M content will per definition be center, anything that is not will per definition be S, even if they are not in complete balance as far as what you are putting into the unit.

I usually have to have a unit on the bench to double check, but I dont see how mid shift could happen anywhere but after the decoding, and I dont see how it could be fixed by swapping the IC outputs to transformers.

Hope to understand this, and see if we can get you 100% there!

Gustav

Hi Gustav,
I have the same problem with MS mode, stereo image drift on the left.
As you asked Frederik to send an email to solve that issue, i'd like to know what's going on.
Also, i bought the kit at the same period (mid 2016) Is it possible that a batch of IC could be guilty? I measured +15,22V and -15,08V at the PSU, is that could be the pblm?
Cheers

 

[Gustav]

Hi Gustav,
I have the same problem with MS mode, stereo image drift on the left.
As you asked Frederik to send an email to solve that issue, i'd like to know what's going on.
Also, i bought the kit at the same period (mid 2016) Is it possible that a batch of IC could be guilty? I measured +15,22V and -15,08V at the PSU, is that could be the pblm?
Cheers

'
"Stereo image drift" can be within spec, so please quantify or elaborate.

for example +15.22 and -15.08 is perfectly within range of what we expect on the +/-15V supply lines, and I cant know if what you are experiencing on the drift is more or less.

2016 is so many years ago - I wouldnt be able to remember an error in a kit, or something specific to that time. Which ICs are you putting in for the encoder and decoder part of the unit?

Gustav

 

[hiegdk]

Hi all, a while back I bought a set of MS76 boards from Gustav when he was winding down PCBGrinder. Since then I've started putting together a BoM for my own use since I couldn't find one. It's a work in progress and I may update it as I complete my build but I thought it might be useful for others.

MS76 BoM:

 

[c1184701]

Hi all, a while back I bought a set of MS76 boards from Gustav when he was winding down PCBGrinder. Since then I've started putting together a BoM for my own use since I couldn't find one. It's a work in progress and I may update it as I complete my build but I thought it might be useful for others.

MS76 BoM:

If you go for the 7912 for -12V, make sure to increase C18 as suggested in this post:

Hi all,
A week ago I had some time to visit my friend James (above) who was having issues with his build similar to others here. The problem turned out to be oscillation on the -12V line which was confirmed with a scope. The datasheet of these regulators does state that a minimum output capacitance is needed for stability and this design only has 0.1uF on each line. Clearly the other voltages are fine but for whatever reason this one has instability without the extra capacitance. The fix is to switch out the 0.1uf cap on the 7912 output with a 10uF aluminum or tantalum electrolytic. Remember since we are dealing with a negative voltage rail here, the positive side of the capacitor should go to ground, and the negative to the output of the 7912 regulator.

I hope this helps anyone else with this specific issue.


View attachment 96908

 

[timpanic]

Hey folks. I have a problem that the release pot changes the GR. When I turn it to slower the GR kicks in.
Unit can be calibrated on fast release and works so far.
Like here:
https://groupdiy.com/threads/1176-build-release-knob-affecting-gain-reduction-without-signal.78828/
It's a common error but people have all kinds of solutions or they just never answered what was the reason for this error. It´s obvious DC Leakage. Often it seems to be the fdh333 or the tantals or the leaky 220n caps at the input stage but I changed these.. I even took two fdh333 out of a working mono g1176 but they show the same problem on this build here and the fdh300 I used are working in the mono G1176 so the diodes are okay.
I liftet c27 and c5 but it is still the same problem that by turning the release slower the gain reduction needle drops.

 

[Gustav]

If you go for the 7912 for -12V, make sure to increase C18 as suggested in this post:

This! - wether you use the 7912 or 7910.

When I did this, I was a bit under the influence of the idea of using poly caps whenever possible, and had it verified working.

But yes, it is beneficial to up that cap to 10uF, using an electrolytic, which means you will avoid the possible ripple on the regulator output.

In essense, it wont always be a problem if you dont, but it will never be a problem if you do.

Gustav

 

[Gustav]

Hey folks. I have a problem that the release pot changes the GR. When I turn it to slower the GR kicks in.
Unit can be calibrated on fast release and works so far.
Like here:
https://groupdiy.com/threads/1176-build-release-knob-affecting-gain-reduction-without-signal.78828/
It's a common error but people have all kinds of solutions or they just never answered what was the reason for this error. It´s obvious DC Leakage. Often it seems to be the fdh333 or the tantals or the leaky 220n caps at the input stage but I changed these.. I even took two fdh333 out of a working mono g1176 but they show the same problem on this build here and the fdh300 I used are working in the mono G1176 so the diodes are okay.
I liftet c27 and c5 but it is still the same problem that by turning the release slower the gain reduction needle drops.

Ive only seen it that bad once. I think I swapped the pots or reversed the wiring for the att/rel or something.

Ages ago, not sure, but since someone had luck rebuilding it in the thread you linked to, it seems like some easily-missed error that was avoided on the rebuild.

Gustav

 

[timpanic]

Ive only seen it that bad once. I think I swapped the pots or reversed the wiring for the att/rel or something.

Ages ago, not sure, but since someone had luck rebuilding it in the thread you linked to, it seems like some easily-missed error that was avoided on the rebuild.

Gustav

Hi Gustav. There are several links to that same prob caused by many thing creating these leakage errors. I only grabbed this one for demonstrating, where the 220n cap was causing the issue. The guy who rebuilds appears in the G1176 help thread.
Thing is, att/rel are working correct here so far. Direction (fast/slow) and funktion are doing right with the audio ( even tried to swap the 22k to a 47k, which I find better). But I will check the pot wiring again. Also Calibrating works and voltages at the testpoints are all on point, no hum, only this. Maybe it´s something with the charge of the 2Ns Fets I have here... Headscratch.

Update: checked the pots. Wiring and orientation are correct.

 

[c1184701]

Hi Gustav. There are several links to that same prob caused by many thing creating this leakage errors. I only grabbed this one for demonstrating, where the 220n cap was causing the issue. The guy who rebuilds appears in the G1176 help thread.
Thing is, att/rel are working correct here so far. Direction (fast/slow) and funktion are doing right with the audio ( even tried to swap the 22k to a 47k, which I find better). But I will check the pot wiring again. Also Calibrating works and voltages at the testpoints are all on point, no hum, only this. Maybe it´s something with charge of the 2Ns Fets I have here... Headscratch.

Have you increased C18 to 10uF as mentioned in the above posts? My unit had several issues, one of which was release pot affecting GR on meter. Granted your situation seems different, with your compressor calibrating/working correctly otherwise. But If I were you I would swap that cap before changing out anything else.

 

[timpanic]

Have you increased C18 to 10uF as mentioned in the above posts? My unit had several issues, one of which was release pot affecting GR on meter. Granted your situation seems different, with your compressor calibrating/working correctly otherwise. But If I were you I would swap that cap before changing out anything else.

Thank you for your help. Yes I even changed this cap even though I used a 7910 here. Didn´t change it sadly. Maybe I should put in a 7912 and then it works little joke...I also tested it with my bench PSU with only the 30v and -10v connected and it is even doing this release-gr.drop thing this way. I ordered now some BF245 to see if they act the same.
So I changed the diodes, tantals, checked the pots, psu, lifted the input and all while the voltages at testpoints are on point. Last thing are the FETs themself then I will through it out of the window .

What I found out is that when I switch off the compressor the output level decreases with turning the release to slow BUT when I pull out the 2N5457 before the TL071 it´s not, so the DC leak should be at the Meter Circuit, right? So maybe the 2N5457 produces the leakage back to the release knob? I wait for some new ones.

Update: I ordered some new 2n5457 and some bf245A. Still the same Problem. The VU Driver Circuit still leaks into the circuit. When I pull out the transistor before the tl072 the GR is NOT effected because the ouput does not drop when turning the release, but when I put it back in the prob starts again. Really really annoying. So maybe ALL transistor I have a leaky but I give up the project now.