Studer 961 micpre channel strips in a rack

[mikedoubleu]

Hi there,

Wondering if anyone could lend me some brains for a quick problem.
Been trying to get a pair of channel strips from a studer 961 to work in a rack I made.  So far, I've managed to activate the mute light but not much more...

The psu I'm trying to use is a also Studer. It's a 1.915.100  which gives me + and - 15vdc, 48vdc and 30vdc unregulated,  which seems like it ought to do the job and more if necessary .  I used an LM337 to get the 30v down to  -6v for the logic supply and my  multimeter is telling me that's what I'm getting,  but perhaps it's not clean enough? Or is the regulator circuit reacting differently under load?  I'm guessing there's enough current for only two strips, half an amp for logic....
Maybe (...likely) I am overlooking something in that department?

Anyway, it seems to me that I might have a problem in the logic domain because signal doesn't seem to be passing the FET switching gain selector section (R44 and downstream).  I haven't done a whole lot of probing beyond signal tracing yet, but I feel this part of the circuit is truly not in my league. I don't quite understand the role of the logic chips.

Perhaps it's all just a grounding problem?  I've got OV Audio, OV Logic, OV reference, 0V External and "chassis" to think about on the strip side.  I connected the OV A to the common 0V for the + and - 15V on the psu, OV logic to the other side of the regulated -6V, OV reference to 0V External and "chassis" on the strip,  as well as the rack unit chassis.

Schematics for strip:
1 http://s7.postimg.org/5tqmprl23/961_mic_pre_1_Page_1.jpg
2 http://s4.postimg.org/705izr6ql/961_mic_pre_2_Page_1.jpg
3 http://s27.postimg.org/zai6lv8gz/961_mic_pre_3_Page_1.jpg

PSU
http://s28.postimg.org/t2rtb3ta5/psu_Page_1.jpg

Any help would be great. I'm learning lots, but I certainly feel like I could benefit from a push in the right direction at the moment.

 

[PRR]

Welcome to the forum.

Your mic_pre_1 image is too fuzzy to read comfortably, and too complex to read uncomfortably.

 

[mikedoubleu]

Heh, thanks.

Erm, maybe this is better?

and, more of the switching section I'm confused about:

 

[bxt403]

I've racked a stereo 961/962 channel strip (with the third/extra bus connector) and am only getting the MUTE switch LED to respond (when the button's pushed in).

I've wired the XLR inputs directly to the PCB (as there aren't any panel mount connectors; i.e. they're connected to the extra bus connector).  The strip was advertised as being a 963 but the 961/962 manual is the one that has been consistent (with the excepting of a few minor details like the lack of on-board connectors and a the blank bus connector on the schematics being populated on the circuit board).

I've daisy-chained all the Ground pins to the power supply ground- with the exception of the chasis P5-pin 1: Chasis Ground (which is wired to the chasis).  The grounding scheme seems to be consistent with a wiring table posted in a different 900 racking post (that was addressing hum issues).

I'm taking the output from the AFL1 & AFL2 pads on the PCB (and removed all the wires from  a third BUS connector that seem to be connected to the pads what would otherwise connect to the XLR and 1/4" connectors on the PCB). 

I've fed audio in but am not getting anything out.  Furthermore: I don't seem to get the OVERLOAD LED to respond to any inputted signal that's being fed in (albeit it did light up when a DC voltage was fed). 

I'm feeding power through a Five Fish 2448 power supply that's connecting to a JLM TRAX (where the -15V from the Five Fish supply is supplying the voltage for ~ -6V from the JLM).

I've checked all the N5532's outputs and they all seem to be a few 0.5 to 125 mV at various location s (i.e. they don't seem to be blown).

I'm wondering if the problem was sorted through or not, as I'm trying to get audio in/out and am not concerned about any noise or hum issues.

The LEDS lighting imply that the TTL chips are working but I'm perplexed to why/where the weak link is.

The fader's and mute switch are seem to work (i.e. there's no noise when the former is off and the latter is enabled) so am wondering what I might be overlooking.

 

[bxt403]

Looking at the schematic: the version I have actually requires a jumper to bypass the (otherwise open) inserts.  That is:  jumping across C127-128/C227-228 to R153-154/R253-254 respectively.

Also I noticed that the Pan/Balance pot has a pull-out switch on it.

I'm getting faint sound out of the Left channel (i.e. the right channel isn't producing sound).  The level isn't getting loud enough to get the Overload LED to react but it looks like it might be an input issue (as the fader & EQ are all working- albeit faintly).

I'm still getting sound so at least I know the power supply's wired correctly (albeit I'm unsure of the grounding).

 

[abbey road d enfer]

I used an LM337 to get the 30v down to  -6v for the logic supply and my  multimeter is telling me that's what I'm getting,  but perhaps it's not clean enough? Or is the regulator circuit reacting differently under load?  I'm guessing there's enough current for only two strips, half an amp for logic.... 

From what can be seen it looks like the -6 rail is not necessary for proper audio operation; anyway, if you checked the voltage to be present and about correct, that should not be an issue. 

Anyway, it seems to me that I might have a problem in the logic domain because signal doesn't seem to be passing the FET switching gain selector section (R44 and downstream).   

This is quite normal since the opamp IC8a operates as a virtual earth amp, so no signal there, but there should be signal at its output (pin 1); the only reason there would be no signal there would be the mute FET Q12. The voltage on its gate should be negative, but I don't recommend you measure it directly because the voltmeter will interfer. Check voltage at point N (junction of R50 & D40); should be close to 0v (slightly negative but less than 0.5V. 

  I haven't done a whole lot of probing beyond signal tracing yet, but I feel this part of the circuit is truly not in my league. I don't quite understand the role of the logic chips. 

They "read" the rotary switch S3 and provide the relevant signals to the switching FET's. 

Perhaps it's all just a grounding problem?  I've got OV Audio, OV Logic, OV reference, 0V External and "chassis" to think about on the strip side.  I connected the OV A to the common 0V for the + and - 15V on the psu, OV logic to the other side of the regulated -6V, OV reference to 0V External and "chassis" on the strip,  as well as the rack unit chassis. 

It seems correct to me.

 

[bxt403]

I figured out the traces at the component end need to be broken (w.r.t.the insert C27-R153 (etc.) on the stereo module schematic- given the traces go to unpopulated pads where the connectors terminate the signal). I'm getting both channels (L&R) producing output using a stereo signal in (using my iPhone and custom cable). 

I'm also using a pair of Neutrik NTL1's for the output and had to ground pin 1 of the transformer to the primary's -ve pin (given Studer tends to use them for line input and they seem to ground the the secondary -ve pin to chasis/transformer pin 1).

The only issue that's left is getting the signal to increase so that the Overload LED will respond: I also noticed that I need to connect a 1500 pF capacitor (C156/256) between the L & R input XLR-pin 1 and chassis Ground. 

I managed to get the Overload LED to respond (i.e. the input level to increase) when I shorted XLR pin 3 to the input transformer 's casing (transformer pin 1) but am cautious not to solder or short anything (given I'm going first install the 1500 pF capacitors, as they're missing). 

I should note that I'm using an adapted mini-plug to stereo XLR connector (that has a resistor connected between pins 3 and 2) from my iPhone (and haven't had any issues using the aforementioned as a test setup with my Studer 069).

In the event the capacitor doesn't fix the level issue (and knowing shorting pins 3 to 1 work for both the Left and Right inputs)- I'm wondering where in the grounding scheme the issue might be (given there was the resolved  issue involving the output transformers being grounded to the module).

 

[bxt403]

I forgot that the custom cable has a 150 or 220 Ohm resistance; that is:  once I plugged my Studer 069 (with the iPhone into the mic inputs) into the channel strip, everything's working the way it should.

The strip with the EQ disabled is transparent but the EQ seems clinical.  I'm still impressed I could hear the sublteties but it's definitely comparable to SSL's Duende Channel Strip: it's just as transparent (when it's bypassed or in a neutral setting). 

I'm looking forward to racking up the 980 module I have.

 

[mikedoubleu]

Anyway, it seems to me that I might have a problem in the logic domain because signal doesn't seem to be passing the FET switching gain selector section (R44 and downstream).   

This is quite normal since the opamp IC8a operates as a virtual earth amp, so no signal there, but there should be signal at its output (pin 1); the only reason there would be no signal there would be the mute FET Q12. The voltage on its gate should be negative, but I don't recommend you measure it directly because the voltmeter will interfer. Check voltage at point N (junction of R50 & D40); should be close to 0v (slightly negative but less than 0.5V. 

Thanks for the response!  I hadn't noticed this thread came back to life there for a moment.  I've definitely been avoiding thinking about these strips,  but maybe now is the time to give them another try...

Since my last post there I've tied the grounds together as bxt403 did and had some better luck. I'm now getting signal all the way to Q21 right before the fader, but two problems remain.  Firstly the mute seems to be stuck on, the led is lit whether or not the switch is in or out and no signal gets by the fader.  Secondly, I only get signal to that point if the input selector is in the second of four positions OR if it is in the "tape" or "gen" positions.  Nothing at all if it is set to either line or the other mic spots...  In the positions where signal does pass, I get roughly -250mV at the point "N" you mentioned, and when it is set to any of the positions which don't allow signal I get roughly -300mV.  Not sure what this means.

Input trimmer and eq seem to be functioning fine.

I also just noticed that there's a jumper on the bottom of the pcb between pins 14 and 8 on IC5.  Seems like it could be a factory bodge, but i'm not sure I understand it...

Thanks again, any help is appreciated.

 

[abbey road d enfer]

  In the positions where signal does pass, I get roughly -250mV at the point "N" you mentioned, and when it is set to any of the positions which don't allow signal I get roughly -300mV.  Not sure what this means.

Now what is the voltage at the collector of Q34?

 

[mikedoubleu]

  In the positions where signal does pass, I get roughly -250mV at the point "N" you mentioned, and when it is set to any of the positions which don't allow signal I get roughly -300mV.  Not sure what this means.

Now what is the voltage at the collector of Q34?

On all the positions passing signal I'm getting -13.15v, and on all of the non working ones it is -12.77v.

..I'm not sure I understand the function of Q12. It is some sort of mute when it's turned on? 

Thanks!

 

[abbey road d enfer]

..I'm not sure I understand the function of Q12. It is some sort of mute when it's turned on? 

Yes, it's a mute.

On all the positions passing signal I'm getting -13.15v, and on all of the non working ones it is -12.77v.

In both cases, it's negative enough to make it open, so the cause for absence of signal is elsewhere.