Studer 928 Story...

I think I have likely found the cause of the intermittent R channel on stereo input module #28...I pulled it out of the frame with the intention of looking for an obvious bad solder joint. Then I noticed this is one of the two modules the previous owner recapped. Knowing that the two-sided glass fiber PCBs can require a little more care and patience when replacing components, the new caps became a prime suspect. I did some crude analysis of how the board might flex when plugged in and while applying pressure next to the stereo width control, which led me to an area of the board with several new caps. Out came the flashlight and jeweler’s loupe, which revealed this:

27F9BED1-0500-431F-9411-9A97F4C46A6D.jpeg

I may have time tomorrow night to resolder that and look over the rest of the board. It feels like an accomplishment, even if unconfirmed and small.
 
Stonkingly clever find Beats! Do I see some browning of the PCB and track damage south of those resistors? The mixer also seems to use the "four band" colour code? I am way to old to get my head around those! Out comes the trusty Fluke 83.

Dave.
 
Stonkingly clever find Beats! Do I see some browning of the PCB and track damage south of those resistors? The mixer also seems to use the "four band" colour code? I am way to old to get my head around those! Out comes the trusty Fluke 83.

Dave.

No, no discoloration whatsoever I’ve seen anywhere on anything I’ve looked at so far. I think what you’re seeing might just be shadow and an area of trace.

You know I didn’t really look at the color bands on the resistors but I see 5 bands on most. I usually use the online calculator at Digikey for calculating values, though I do try to figure it out myself first and then verify with a calculator.

Fluke 83? I have an 85 myself. It’s been handy and reliable.
 
Out came the flashlight and jeweler’s loupe, which revealed this:

View attachment 104223

:D

That's exactly the same as my Otari MX5050 BIII issues...intermittent issues on the left channel when I first got the machine several years back...and more recently, on the right channel. In both cases it was a cap with one poorly soldered connection. Took awhile to find them with the magnifying eyeglasses I have) but once found...easy fix. :)
 
"Fluke 83? I have an 85 myself. It’s been handy and reliable. " Yes, had it a long time and it is one of the few digital meters I have found to have a flat response over the audio band (-3dB at about 25kHz iirc). Most start to droop as low as 2 even 1 kHz, even the "true rms" meters which in fact are only "true" at power frequencies.

So, note to all you potential electronic dabblers..Do not trust a DMM at audio frequencies, almost any DAW can generate sines so check the calibration.

Dave.
 
Module #28 is fixed. It didn’t end up being the solder joint I pictured earlier...there ended up being dozens like that one, so I just went over all of them on both sides of both boards. It was a little tight in some spots with the iron, but it came out alright, and it works! :D
 
Hi, I just created an account to reply to this thread...

I bought a 928 (for € 3k) that was used in a studio at RTL Netherlands 3 weeks ago, fully untested, as there were no power cables :facepalm:
I built 2 and then saw that everything was working (as far as i could test it).
C3AEF819-4976-4735-98B9-03FD5FF6EC74.jpg

I started recapping the whole desk with enhanced specs (at least one step up in voltage rating) with Elna Simic 2 and Panasonic FC series. The second stereo channel I recaped I additionally paralleled every electrolytic cap with a 0.1 uF smd foil cap (Panasonic ECHU(X)).
34E88F0C-E021-4905-ACC1-90C274377B00.jpgA3B3C6D2-B326-4B62-AC1C-C48925564C75.jpg
I have to say, the differenced between original and recapped are enormous. The "vintage" sound is gone. Crisp and clear aftwerwards. Quick test: inserting the eq in neutral settings does not change the sound anymore. With old caps the effect was like hanging an additional towel over the sound.
The channel with the additional foil caps is even better, sounds not so harsh, wider imaging, more punchy bass etc. (I do have to change it in all channels (laughs insane)

You definitely want to recap the desk, especially the blue Phillips brand 100uFs are crap. I measured some of them, around 60-80 uF and 5-10(!) ohms esr! And these are used as decoupling caps between the seperate stages all over the channel. So its reasonable that if an audio signal travels through 10 to 20 of this sound "vintage":laughings:

The mic trim potis are the only ones in the entire desk, that are not capsuled. I have to clean them, hard to find a replacement (47k lin, 3mm axis).

Love to see your project moving on
Michael
 
[MENTION=201181]michael1991[/MENTION]

Hi, I just created an account to reply to this thread...

This is great! Glad you did...glad to be able to correspond with another 928 owner! :D Seems hard to find other Studer console owners, and far less the 928...not as popular as the more vintage Studer consoles.

I bought a 928 (for € 3k) that was used in a studio at RTL Netherlands 3 weeks ago, fully untested, as there were no power cables :facepalm:

Oh I saw that one for sale...it looked a bit had...but it’s okay?

And it didn’t come with a manual, correct?

No mono group modules but 30 :eek: of the mono input modules, and 10 of the stereo input modules too, yes?

What meter modules are loaded?

And it looks like yours wasn’t outfitted with the integrated patchbay?

I built 2 and then saw that everything was working (as far as i could test it).

Does yours utilize three power supplies?

I started recapping the whole desk with enhanced specs (at least one step up in voltage rating) with Elna Simic 2 and Panasonic FC series....

I’m curious why you increased the voltage rating. From what I see the voltage rating of all the caps is very appropriate. I was schooled that going too high above what is called for by the power rail is not necessarily beneficial and potentially detrimental. A good safety margin is wise, but that’s what I was seeing as far as what Studer specified. What are you seeing?

The second stereo channel I recaped I additionally paralleled every electrolytic cap with a 0.1 uF smd foil cap (Panasonic ECHU(X)).

Every coupling cap, or literally every electrolytic cap?

I have to say, the differenced between original and recapped are enormous. The "vintage" sound is gone. Crisp and clear aftwerwards. Quick test: inserting the eq in neutral settings does not change the sound anymore. With old caps the effect was like hanging an additional towel over the sound.
The channel with the additional foil caps is even better, sounds not so harsh, wider imaging, more punchy bass etc. (I do have to change it in all channels (laughs insane)

You definitely want to recap the desk, especially the blue Phillips brand 100uFs are crap. I measured some of them, around 60-80 uF and 5-10(!) ohms esr! And these are used as decoupling caps between the seperate stages all over the channel. So its reasonable that if an audio signal travels through 10 to 20 of this sound "vintage":laughings:

I’ll just say you and I are having two totally different experiences with our 928s...mine sounds wide open...I definitely wouldn’t describe it as “vintage” at all. It’s pretty neutral but pretty much everything sounds better through it so far, and it doesn’t get in the way of doing what I want with the sound like for instance with the EQ...imaging is fantastic...I’ve got more clear bandwidth than I know what to do with...low end is clean and clear with plenty of punch if I want or need it...I just switch in the HPF and boost the low shelving filter at around 50Hz...and the high end is just...sweet. And I can hear no difference at all pulling the EQ in and out with the controls zeroed out. No “towel over the sound” anywhere on this thing. And I’d always understood the Philips caps to be good quality. Studer really didn’t skimp on the components anywhere is far as I can see. I mean, the 928 *was* their more budget, if you can call it that, large format mixing desk, but the price point seems like it was dictated more by the feature set (like compared to the 980 for instance)...more scaled down...as opposed to skimping on the quality of the components...or even the values. Like the coupling caps are 100uF parts which is not what you typically see when a company is trying to cut costs...I don’t have the schematics in front of me at the moment so I don’t have the calculated LF knee point on a 100uF coupling cap in the signal path on the 928, but I’m pretty sure its sufficiently good...like probably more than it needs to be but not so much there’s to much compromise with the HF performance. I usually see 47uF or 22uF for coupling caps...sometimes *gulp* 10uF, which is usually too small but purposely chosen because the circuit doesn’t have the headroom to really handle the subsonic content. Here I think we’ve got plenty of headroom throughout the signal path, AND coupling caps that stay out of the in terms of LF bandwidth. Regardless, mine sounds great. I mean, my console sold for $89,000 in 2000. That about $132,000 in 2019 dollars. That’s not a budget console! I don’t think they’d put garbage caps in there...their mantra was “Studer ‘good sound’”. I’m not doubting your caps were shot, and that is cool (and how exciting it must be for you) to swap those caps and have a totally different animal on your hands...a relief in a way I bet...but I bet your situation is something of an exception. What year is your 928?

One of my stereo input modules was recapped last year by the previous owner with Nichicon KA 105C caps...I can’t hear any difference between it and a stock module.

I don’t plan on doing any recapping for some time even though the console came with all the caps to do the whole console...the aforementioned Nichicon 105C parts..KA series are “audio-grade” for what that’s worth. Your post does make me curious...might pull some sample stock caps and measure to be sure.


I wonder...was yours stationed in a harsh environment?

The mic trim potis are the only ones in the entire desk, that are not capsuled. I have to clean them, hard to find a replacement (47k lin, 3mm axis).

Love to see your project moving on
Michael

Right back atcha!

Let me know how servicing of the mic trim pots goes...that’s really the only issue with mine is those are a bit scratchy.
 
[MENTION=201181]michael1991[/MENTION]



This is great! Glad you did...glad to be able to correspond with another 928 owner! :D Seems hard to find other Studer console owners, and far less the 928...not as popular as the more vintage Studer consoles.



Oh I saw that one for sale...it looked a bit had...but it’s okay?

It looks kind of used, but this is all visual, just some missing colour.


And it didn’t come with a manual, correct?
No manual, but as you know, you can find it on the internet.

No mono group modules but 30 :eek: of the mono input modules, and 10 of the stereo input modules too, yes?
No groups, but perhaps I will try to find some, or try to build some in the future :D:D
and yes, a insane number of mic inputs


What meter modules are loaded?
Sum A, B, Monitor Plasma with gain reduction and Aux led meters, and correlator
IMG_3179.JPG

And it looks like yours wasn’t outfitted with the integrated patchbay?
No internal patchbay, but i think from what i read it would be limited in use for me...


Does yours utilize three power supplies?
there are two of them. but they generate a high amount of line noises, so i will replace them at some point.
IMG_3167.JPG

I’m curious why you increased the voltage rating. From what I see the voltage rating of all the caps is very appropriate. I was schooled that going too high above what is called for by the power rail is not necessarily beneficial and potentially detrimental. A good safety margin is wise, but that’s what I was seeing as far as what Studer specified. What are you seeing?
A colleague here at university (member of the digital signal processing group, i work at the chair of communications engineering btw) who had done some testing recommended this. Higher voltages reduce distortions. so i didn't compared them a b with same and higher specs, just chose one step higher...


Every coupling cap, or literally every electrolytic cap?
I think it would be sufficient to just search for the coupling caps, i don't know if or how much of a different this makes, but i bypassed/will bypass every electrolytic cap. Yes, thats another 3500 caps :eek:


I’ll just say you and I are having two totally different experiences with our 928s...mine sounds wide open...I definitely wouldn’t describe it as “vintage” at all. It’s pretty neutral but pretty much everything sounds better through it so far, and it doesn’t get in the way of doing what I want with the sound like for instance with the EQ...imaging is fantastic...I’ve got more clear bandwidth than I know what to do with...low end is clean and clear with plenty of punch if I want or need it...I just switch in the HPF and boost the low shelving filter at around 50Hz...and the high end is just...sweet. And I can hear no difference at all pulling the EQ in and out with the controls zeroed out. No “towel over the sound” anywhere on this thing. And I’d always understood the Philips caps to be good quality. Studer really didn’t skimp on the components anywhere is far as I can see. I mean, the 928 *was* their more budget, if you can call it that, large format mixing desk, but the price point seems like it was dictated more by the feature set (like compared to the 980 for instance)...more scaled down...as opposed to skimping on the quality of the components...or even the values. Like the coupling caps are 100uF parts which is not what you typically see when a company is trying to cut costs...I don’t have the schematics in front of me at the moment so I don’t have the calculated LF knee point on a 100uF coupling cap in the signal path on the 928, but I’m pretty sure its sufficiently good...like probably more than it needs to be but not so much there’s to much compromise with the HF performance. I usually see 47uF or 22uF for coupling caps...sometimes *gulp* 10uF, which is usually too small but purposely chosen because the circuit doesn’t have the headroom to really handle the subsonic content. Here I think we’ve got plenty of headroom throughout the signal path, AND coupling caps that stay out of the in terms of LF bandwidth. Regardless, mine sounds great. I mean, my console sold for $89,000 in 2000. That about $132,000 in 2019 dollars. That’s not a budget console! I don’t think they’d put garbage caps in there...their mantra was “Studer ‘good sound’”. I’m not doubting your caps were shot, and that is cool (and how exciting it must be for you) to swap those caps and have a totally different animal on your hands...a relief in a way I bet...but I bet your situation is something of an exception. What year is your 928?

One of my stereo input modules was recapped last year by the previous owner with Nichicon KA 105C caps...I can’t hear any difference between it and a stock module.

I don’t plan on doing any recapping for some time even though the console came with all the caps to do the whole console...the aforementioned Nichicon 105C parts..KA series are “audio-grade” for what that’s worth. Your post does make me curious...might pull some sample stock caps and measure to be sure.
The same colleague was at an intership at Studer during studies in 2002, one of the enineers at studer told them the Phillips electrolytics weren't the best caps, and they replaced them somewhat around 2000. I don't know the exact sell date, and i did not remove every module so far, but the ones i saw seem to be produced 1997 for the stereo and 2002 for the mono inputs, so no more light blue phillips any more on the latter. (ALL inputs are transformer coupled btw)

Another anecdote to the philips caps. We have a Rhode&Schwarz UPV audio analyzer where the power supply was faulty some time. that mentioned colleague looked at it, pointed at the phillips electrolytics saxing "they are crap", and in the end it was one of them blown up....

At some point i will capture some measurements as well, since we have the equipment:cool:



I wonder...was yours stationed in a harsh environment?
To be honest i do not know much about this desk. I found inventory stickers of RTL (european broadcaster) on top of every module. The dealer did not know anything as well and bought the desk from an other person, so i assume i am the fourth owner but the second that does not only storages it.


Right back atcha!

Let me know how servicing of the mic trim pots goes...that’s really the only issue with mine is those are a bit scratchy.
I start with the stereo channels (try some other opamps as well (yes i am just a little bit crazy :guitar:), then the master section, and then the mono channels, so it takes some time, but if I reach that point, I will share my experiences...

Hope I didn't messed up with the text editor and it is readable...
 
Most hints and suggestions for the upgrades i get from a freind that works at a small company who design and manufacture some of products for RME as the flagship M-32/M-32 pro. He builds highest end HiFi equipment as well. Together with an well experienced engineer at this company they not only recommended the bypassing but replacing the TL072 with an OP275 and the MC33068 with an LM4562 ICs. I will try that on the next stereo channel i recap, so i can report afterwards...
 
[MENTION=201181]michael1991[/MENTION]

It looks kind of used, but this is all visual, just some missing colour.

Oh okay...good deal. ;)

No manual, but as you know, you can find it on the internet.

Right, but if you've read this thread you also know the manual you can find online is the base manual and does not include a lot of documentation. I've been able to find some of the missing sections, but others I will never find because each manual is specific to each individual 928 console. Since each console is custom ordered, so is the manual customized. The part that I'm really wishing I had is the technical documents to the power supply. You and I both have the early version of the power supply, and the available manual only includes documents related to the later version power supply.

No groups, but perhaps I will try to find some, or try to build some in the future and yes, a insane number of mic inputs

Keep in mind if you find some, the motherboard and backplane will need to be significantly reconfigured to allow the group channel to work. Each module will only function in the frame slot designed for it. You can't even put the mono mic input modules into a stereo input module.

Sum A, B, Monitor Plasma with gain reduction and Aux led meters, and correlator

Your SUM A, SUM B and MONITOR meter modules are a little different than mine...yours are pretty cool because they can be configured between two different balistics via jumper settings and you have LED indicators to show which configuration is active. I *think* mine can be configured, but mine have no indicators to which configuration is active.

No internal patchbay, but i think from what i read it would be limited in use for me...

Gotcha. Yeah its pretty much the access for the input module and master section insert points, and then there are 12 tie line access points. How do you access your insert points?

there are two of them. but they generate a high amount of line noises, so i will replace them at some point.

There's only two? Oh wow. Huh. Are the fans noisy? I bought some new quieter fans for mine that also move more air. Hey, if you ever replace your keep me in mind...I might be interested in one or both of yours.

A colleague here at university (member of the digital signal processing group, i work at the chair of communications engineering btw) who had done some testing recommended this. Higher voltages reduce distortions. so i didn't compared them a b with same and higher specs, just chose one step higher...

Actually on second look I see what you are talking about...they're all 16V parts. The power rails are 15V, and actually are stepped down slightly on each module for reliability. So there's still a margin there, but 25V parts is not a bad idea.

[/QUOTE]I think it would be sufficient to just search for the coupling caps, i don't know if or how much of a different this makes, but i bypassed/will bypass every electrolytic cap. Yes, thats another 3500 caps[/QUOTE]

Yes it *would* be sufficient to just do the coupling caps, or at least that's where you would get the most benefit. So many of those caps are related to the logic and are not in the audio path...adding a small value parallel cap to those is of no sonic benefit.

Another anecdote to the philips caps. We have a Rhode&Schwarz UPV audio analyzer where the power supply was faulty some time. that mentioned colleague looked at it, pointed at the phillips electrolytics saxing "they are crap", and in the end it was one of them blown up....

Well capacitors are ultimately as good as the circuit...if the circuit or the physical assembly is poorly designed and the caps are either electronically or environmentally stressed, they will be short-lived. I trust there's value to what your colleagues are telling you. At the very least it makes me curious to test a sample of mine, but its difficult to fully accept Studer would but inferior components in critical circuitry...these are the same caps used in all the 900 series consoles it looks like...900, 928, 963, 980, 990...

I start with the stereo channels (try some other opamps as well (yes i am just a little bit crazy ), then the master section, and then the mono channels, so it takes some time, but if I reach that point, I will share my experiences...

That would really be great. Thanks! :D

Most hints and suggestions for the upgrades i get from a freind that works at a small company who design and manufacture some of products for RME as the flagship M-32/M-32 pro. He builds highest end HiFi equipment as well. Together with an well experienced engineer at this company they not only recommended the bypassing but replacing the TL072 with an OP275 and the MC33068 with an LM4562 ICs. I will try that on the next stereo channel i recap, so i can report afterwards...

That would be interesting. I'm really happy with how mine sounds. But maybe someday it would be interesting to socket some of the key stages and try different parts. I need to see what Studer did about power rail bypassing to prevent oscillation with faster chips. It may need to be added.
 
[MENTION=201181]michael1991[/MENTION]

Right, but if you've read this thread you also know the manual you can find online is the base manual and does not include a lot of documentation. I've been able to find some of the missing sections, but others I will never find because each manual is specific to each individual 928 console. Since each console is custom ordered, so is the manual customized. The part that I'm really wishing I had is the technical documents to the power supply. You and I both have the early version of the power supply, and the available manual only includes documents related to the later version power supply.[\QUOTE]

Ok, but there is not much that i can not find in the standard documentation. Since the powersupply has pinout printed on it, it was not that big deal, especially after i found out that the connector is this hirose jr type.


Keep in mind if you find some, the motherboard and backplane will need to be significantly reconfigured to allow the group channel to work. Each module will only function in the frame slot designed for it. You can't even put the mono mic input modules into a stereo input module.[\QUOTE]
Another idea was to change stereo channels to be fed from the busses. Since the rails are inside the connectors as the busses are used as sends, this could be perhabs be a possibility. Not sure about that right now:D






[QOUTE]Gotcha. Yeah its pretty much the access for the input module and master section insert points, and then there are 12 tie line access points. How do you access your insert points?[\QUOTE]
Mine is equipped with insert points on DB25 connectors on all channels. The ones that you miss on the back.



There's only two? Oh wow. Huh. Are the fans noisy? I bought some new quieter fans for mine that also move more air. Hey, if you ever replace your keep me in mind...I might be interested in one or both of yours.[\QUOTE]
Only two powersupplies, only two Siemens connectors on the back, desk splitted in two power regions. One of them drives 5, the other 6 amps at the 15 V rails. Noisey fans, i will replace them, and swap the caps. Think they will reject more power line noise afterwards. If I find a good replacement for the supplies, perhaps you would be interested in an insane designed power supply, instead of a standard one :p




Well capacitors are ultimately as good as the circuit...if the circuit or the physical assembly is poorly designed and the caps are either electronically or environmentally stressed, they will be short-lived. I trust there's value to what your colleagues are telling you. At the very least it makes me curious to test a sample of mine, but its difficult to fully accept Studer would but inferior components in critical circuitry...these are the same caps used in all the 900 series consoles it looks like...900, 928, 963, 980, 990... [\QUOTE]
My guess would be, since they replaced (at least the 100uF Philips caps at some point, they were not putting bad components at will inside their products, but did not (i assume nobody did) know that they are not the best ones. I don't know when this series was released, but in the beginning nobody can know in what way the caps age. I think the other caps in my desk were a bit off specs, but not as far as the 100uFs...


That would be interesting. I'm really happy with how mine sounds. But maybe someday it would be interesting to socket some of the key stages and try different parts. I need to see what Studer did about power rail bypassing to prevent oscillation with faster chips. It may need to be added.
Thats the things I need to figure out, curious to see how things figure out:)

---------- Update ----------

And still have to figure out how this editor works:laughings::laughings::laughings:
 
[MENTION=201181]michael1991[/MENTION]

...there is not much that i can not find in the standard documentation. Since the powersupply has pinout printed on it, it was not that big deal, especially after i found out that the connector is this hirose jr type.

Right, but I'm looking for the schematic for the power supply itself, not just the pinout...all the cable and connector details are in the base manual, but nothing deeper than that for the early version power supply.


Another idea was to change stereo channels to be fed from the busses. Since the rails are inside the connectors as the busses are used as sends, this could be perhabs be a possibility. Not sure about that right now:D

I actually believe that is possible. The motherboard connector pinouts are out included on the schematics for each module. It would mean cutting a bunch of traces and putting jumpers in their place to source the inputs to the group busses, but I think that might be feasible.

Mine is equipped with insert points on DB25 connectors on all channels. The ones that you miss on the back.

Ah okay. Interesting. Those were used for multiple different things then. Those are the kinds of things that would be incluced in the original manual for each specific console...all the specific wiring for the frame as configured.

Only two powersupplies, only two Siemens connectors on the back, desk splitted in two power regions. One of them drives 5, the other 6 amps at the 15 V rails. Noisey fans, i will replace them, and swap the caps. Think they will reject more power line noise afterwards. If I find a good replacement for the supplies, perhaps you would be interested in an insane designed power supply, instead of a standard one :p

There are always two Siemens connectors on the backplane for each set of three frame bays. So, like mine has three frame bays, but the three power supplies power them through only two of the Seimens connectors on the backplane. Look at the pinouts for the Seimens connectors in the Power supply section of the manual.

Hey can you let me know when your 928 was manufactured? The date should be on a label on the inside of one of the legs toward the back.

ALSO...

Do your output transformers like on your master A and B modules get warm when the console is operating? You'll have to power it down and pull the module to check, but I'm curious.
 
I can't find a production date on the sticker, only the sn (1021), but the production dates on the modukes are dated 1997 and 2002.

i will chack the temperature after the desk running some time...
 
I can't find a production date on the sticker, only the sn (1021), but the production dates on the modukes are dated 1997 and 2002.

i will chack the temperature after the desk running some time...

Thanks for checking the temp of the transformers. That’s helpful info.

Yours is s/n 1021?? Mine is 1014! :D

The production date on mine is August 30, 2000.
 
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