Studer 962 "option 1" Tape Return Path add

[corgan4321]

Hello!

I'm working on a Studer 962 that sounds very nice. I was combing through the schematics and discovered the 961/962 channels strips have a vestigial unbalanced tape return input from the console's big brother the 963. It appears that by adding just a few components, the "off" position selects a tape input path. This "modification" is labeled as "Option 1" on the schematics so presumably you could order strips from the factory this way. I'd like to explore adding this function to at least 8 of my channel strips for my 8 track r2r returns. I've highlighted the areas in question on the schematic below. My questions are, how matched should JFETS be in this situation? I have a Peak Atlas Pro that can read Vgs and Idss easily but only at 5V. To what degree should the JFETs be matched in this circuit? For this type of circuit, is it important for the FETs so be tested in a similar circuit with similar sized resistors/voltages??
Also, it looks like to me that D18 on the bottom right of the page, and everything past Q13 is for signaling and to light a "Mixdown" LED that exists on the 963 when you have this tape return path selected. Is this a correct assessment?

The most difficult part will be to find a place for the input to go physically. There are two banana jacks that go unused (broadcast signaling jacks), so perhaps they could be removed and replaced with some more Bantam jacks.

 

[zamproject]

Hello

Those FETs are used as audio switches, I don't think you need to care about any matching or whatever.
Use J111/112 as used all around for this purpose by Studer.
The PCB is ready, you just have to feed the few components, input is done at pad/TP 1 and 20 (everything is just above P5 connector)

I just have a quick look so maybe double check the logic around, but to me:
-Q13 R194 and D18 seem to come from a mix-down signaling bus to force/flip all channels at once to mix-down status with a single button. Which can be useful to add depending of you work flow.
-R56 D5 and DL1 seem just for visual signalling, so not a conditional request for audio switching to work.
Frontpanel probably differ with this led option factory fitted, with one more hole drilled...

Regarding connectors, as a start you can try to use two signaling banana (if you don't use mic/line signaling) to keep mechanical untouched. I do this on a 369 (for direct out) and it work fine.

Cheers
Zam

 

[corgan4321]

Zam, thanks very much for your insights!
Okay, I’ll pop the parts in next week and see what I get.

That’s interesting regarding the Q13 and makes a ton of sense. It also explains why I’ve seen the “Mixdown Defeat” button on some 963 strips. That button must disconnect the logic from Q13.
I will look into it to see if that logic is present in the 962 schematics, maybe if I see where that pin 13 goes from D18 it will be clear.

That’s what I was thinking for the moment, just disconnect the existing banana plugs and reconnect pads 20 & 1. I haven’t looked closely, are these like standard banana plugs for a speaker?

Thanks again!

 

[zamproject]

Okay, I’ll pop the parts in next week and see what I get.

Check bom in the service manual, all values/ref are listed, including optional component

are these like standard banana plugs for a speaker?

If it's like the previous series x89 x69 yes it's standard 4mm banana.

13 look like free pad, probably wire connected around the non used DIN connector except in the 963 verssion (extra bus/signaling)

 

[zamproject]

Ok... I can help myself, I have to look

You better catch the 963 service manual for details.
https://www.reeltoreel.nl/studer/Public/Products/Mixing_Analog/963/Manuals/963_Op_Serv.pdf
Page 68 you'll see that tape input have symmetrical input/buffer (5532) before point 1 at the complementary 963 PCB...
Also you'll see that pin 13 go to pin 20 at this extra PCB, with the mixdown defeat (to extract a channel from global mixdown flip bus)

 

[corgan4321]

Ah hah! Thank you for sharing this.
It appears the global Mixdown control signal that would flip all channels is normally routed through P7 connector on the 963 extension PCB which of course I do not have on my 962 modules.
Perhaps I will get lucky and there will be an unused pin on the modules that runs to all channels from the Control Room module… I could repurpose one of the unused monitor switches there…

I don’t see any issue with foregoing the 5532 symmetrical input do you? I could always add a JLM Tardigrade in the future if I needed them to be balanced, but for now, my Otari MX5050 is already unbalanced I/O so no big deal. I suppose the biggest risk would be if DC ended up on that input somehow I’d be passing that through to the rest of the channel. Would it be worth considering a coupling cap of some kind?

 

[zamproject]

Don't know for coupling cap, maybe you should measure DC when the tape playback line out is hooked, to see if you need one, as the polarity.
About sym input, you probably can do without, but maybe adjust R14/R15 value ? someone else will have better answer to this regarding load etc ...

Thinking out loud... If you need a buffer and don't use the direct out OR symmetrical I/O insert, you can configure the desk with the asym insert option, and probably repurpose the direct out buffer as line/tape in buffer, as this section have jumper/wire at input and output it should be possible without many component change.

 

[radeng]

This is really interesting, I too would like to route my 8T output to dedicated 3rd inputs so to not tie up the line inputs. The global flip is a cool feature too. If the OP gets this mod working I'd like to implement it on my 961.

Some years ago, I came up with an upgrade to the red mute LED. By swapping it for a tri-color, it still works as originally designed-turning red when the mute is on. When the channel is active, it now shows signal presence by turning green @-20db, then yellow at 0db (adjustable). I haven't got around to making any PCB's past the prototype. My idea was that it would be helpful in live situations when things are pumping and you have a bunch of faders open, and all of a sudden you lose a channel, you could see right away which one is not showing signal. Based on someone tripping over a cable and breaking an XLR at a snake extension location at a live gig I was recording. I used the PFL to figure it out but it would have been nice to see instantly what channel had dropped. Hence my channel signal indicator mod. If there was some interest, I would have some more boards made.

 

[corgan4321]

This is really interesting, I too would like to route my 8T output to dedicated 3rd inputs so to not tie up the line inputs. The global flip is a cool feature too. If the OP gets this mod working I'd like to implement it on my 961.

Some years ago, I came up with an upgrade to the red mute LED. By swapping it for a tri-color, it still works as originally designed-turning red when the mute is on. When the channel is active, it now shows signal presence by turning green @-20db, then yellow at 0db (adjustable). I haven't got around to making any PCB's past the prototype. My idea was that it would be helpful in live situations when things are pumping and you have a bunch of faders open, and all of a sudden you lose a channel, you could see right away which one is not showing signal. Based on someone tripping over a cable and breaking an XLR at a snake extension location at a live gig I was recording. I used the PFL to figure it out but it would have been nice to see instantly what channel had dropped. Hence my channel signal indicator mod. If there was some interest, I would have some more boards made.

Hi radeng, that’s a very interesting idea regarding the tricolor LED. Did you end up implementing in on your channel strips? I’d be curious to know how you achieved it and would definitely be interested in trying it out on my strips if you’re willing to share/develop a PCB for it!
I’ve got all the parts to populate a strip to test out the tape input option, but work has been so busy I haven’t gotten around to it! I’m hoping to get to it before the end of the month and will update here as soon as I do.

 

[radeng]

I only did the one channel, which works fine. Like you, work has been busy so I have not pursued it. Also, my little 961 has been supplanted by an ex-public radio On-Air 2000M2, so the 961 is only used for remotes now. They have the same number of Mic/Line modules (8) but the 961 only has 2 stereo line inputs vs. the On-Air's 10.

 

[corgan4321]

I finally got around to populating one of my 962 channel strips with the missing components for the tape input option. I disconnected the banana plugs from their original points and used them as the inputs for the tape input. It works great while playing audio into those points from my iPhone. I can just select the "OFF" position and hear the audio from my iPhone.
The funkiness began when I plugged into the output of my computer. I did the same test from the headphone out of my studio compute. I get good signal when switching the selector switch to "off" (as expected), however I also get signal when selecting the "generator" input. I suspect this is cross talk because the negative input of the tape input mod is shared with the generator input - does this make any sense? I've been looking at the 963 schematics (attached) and it's clear the add on module has an opamp buffer going into the tape input (simple 5532 circuit). I suppose the tape input would work as is if whatever you're plugging into it is totally isolated from ground...?

Am I on the right train of thought here? Also, I haven't figured out how to force the input to switch as in "mix down mode"

 

[corgan4321]

Another data point:

Connecting ONLY the positive end of a signal to the positive end of the tape input (pad 1) gives good signal and no crosstalk onto the generator input so clearly audio ground is being completed elsewhere. If I connect signal ground to the tape input (pad 20) i get more signal (hotter) but then get the crosstalk on the generator input.

 

[13engrsapper]

Hello

Those FETs are used as audio switches, I don't think you need to care about any matching or whatever.
Use J111/112 as used all around for this purpose by Studer.
The PCB is ready, you just have to feed the few components, input is done at pad/TP 1 and 20 (everything is just above P5 connector)

I just have a quick look so maybe double check the logic around, but to me:
-Q13 R194 and D18 seem to come from a mix-down signaling bus to force/flip all channels at once to mix-down status with a single button. Which can be useful to add depending of you work flow.
-R56 D5 and DL1 seem just for visual signalling, so not a conditional request for audio switching to work.
Frontpanel probably differ with this led option factory fitted, with one more hole drilled...

Regarding connectors, as a start you can try to use two signaling banana (if you don't use mic/line signaling) to keep mechanical untouched. I do this on a 369 (for direct out) and it work fine.

Cheers
Zam

Zam is correct - that whole generation Studer console was loaded with Jfet switches.