TAC Scorpion Overhaul Odyssey

[Potato Cakes]

The traces are not masked, so you can solder directly to them. For the record, the flux on the solder joints was there when I got here. I double checked to make sure the traces were indeed broken and the ring and sleeve on a TRS connector inserted in the jack were shorted and not to the tip.

 

[Potato Cakes]

To make this operation easier, I took two 1x4 pieces of lumber and clamped them together with the opposite edge of the rear console panel I was working on in between them so it would stand on end. I did have to remove the JP280 card that has the monitors and slate connections, but there is plenty of cable to allow the wires to remain attached and be moved out of the way.

I also removed the resistors for the main monitor jacks and am going to impedance balance those, but it seems this will require a 22R resistor to match R15/16 on the S4001 schematic.

I moved ahead prepped the main XLR to be used with an ACA-Bo. Going to order that this week. Parts for recapping are showing up tomorrow and then the real party starts.


Thanks!

Paul

 

[Eliani]

Hi Paul,

I can't believe I never picked up on your overhaul thread, show how ocupied I've been lately...
I'm happy it's all working out for you, and like always if you've got questions or doubts installing the Eli-Audio mods, or the ACA-2-BO mod... I've been there done that, no need to re-invent the wheel right...

And thanks for the support... 
Yves

 

[Potato Cakes]

Hi Paul,

I can't believe I never picked up on your overhaul thread, show how ocupied I've been lately...
I'm happy it's all working out for you, and like always if you've got questions or doubts installing the Eli-Audio mods, or the ACA-2-BO mod... I've been there done that, no need to re-invent the wheel right...

And thanks for the support... 
Yves

Hi Yves!

Installing your boards was pretty simple, so I feel that when I power these up I shouldn't have any trouble with at least that section. I would be curious if there were any resistors you had to change on the ACA-BO to accommodate the buss resistors values on this console.

Thanks!

Paul

 

[Potato Cakes]

I started recapping the master channel today, and when I was almost done installing the 10uf/40V when I happen to notice that they were bipolar caps. I had been going off the leg lengths for polarity, but on one I had to trim it down to fit and that's when I discovered I might have an issue as there was no stripe for the negative leg. There were black rectangles with the letters NP in them that almost looked like the indicative black stripe, but the lettering was a clear giveaway of what I had. These came up when I did a search for 10uF caps and I didn't see anything in the data sheet or product description about being non polar, but when I went back and did a search for the product number (Mouser# 140-RN100K1HBK0811P) it shows Bi-Polar in the filter section but still nothing attached to the product.

So the question is can I still use these? I feel the answer is no, but I'm not totally sure. I also have a bunch of 33uF caps that are the correct size and voltage so I'm wondering if they would work. I'm hoping they would be fine as I have the ACA-Bo parts ordered and my plan was to power through getting this thing back together by the weekend.

I also decided to impedance balance the direct outs just in case I do need to use them when all the new patch bay wiring is completed.

Thanks!

Paul

 

[jensenmann]

You can use them except in the PFL section. There you´ll find a 10uF cap parallel to a 5,6V Zener diode. In this spot the supply voltage for the HEF4053 multiplexers is created. You need a polar cap here. But if you have 33uF then that can be used here.
I mentioned before that it´d be VERY beneficial to increase all coupling caps. Our studiomonitors nowadays go an octave lower than back in the days when the Scorpion was new. If you double all couplingcaps then you will have a cleaner lowend, less distortion and phaseshift as well as an extended lowend. There´s no penalty if you increase the coupling caps even more (except at the mic input). Particularly the summing amps want to see a lot larger caps than those 100uF, e.g. 1000uF or as large as mechanically possible.

 

[Potato Cakes]

Well, that is good news on the 33uF, but not so good news on the 100uF's as I have already installed all of those. I had missed the part of one of your post about increasing the value of the coupling caps. I've got to order some more parts anyways so I suppose I'll see about changing those out as well. I'm a bit bummed out because I was hoping to wrap this up this weekend, but I want to do this right, so this will have to wait another week I suppose. I'll start researching to see how much capacitor I can fit in there. I do appreciate the continued input from everyone on this project.

Thanks!

Paul

 

[jensenmann]

Just reuse the 100uF in the spots of the 10uF coupling caps.

 

[Potato Cakes]

Just reuse the 100uF in the spots of the 10uF coupling caps.

The 100uF I have are rated for 35V and the 10uF in the schematics are 40V. I know that the PSU is only doing +/-17V, but will having a lower voltage rating than what was originally in the design be an issue?

Thanks!

Paul

 

[jensenmann]

No, it´s fine, no problem to be expected.

 

[Potato Cakes]

Got the back panel back on so now that's not taking up space in my working area anymore. I went ahead and impedance balanced the direct outs because there isn't a reason not to if you have the back panel off as they are also easily accessible and don't require removing the cards to perform this operation. Simply remove R2 and 3 from the card then put a 47R in at the R2 position and that's it. Be advised that these boards are cheaply made and the solder pads and traces like to come off the PCB.

I have to order another round of caps per the information provided by jennsenmann. I've started working on placment for the ACA-Bo and calculating changes that need to be made to work with the 10k buss resistors. I briefly toyed with the idea to change the buss resistors in the console for the L/R sends to 47k to match the ACA-Bo to possibly get better cross-talk or something like that, but my desire to be finished quickly put that to rest. I do wonder now that 30 years have passed since this console was made if "better" buss resistor values have been implemented in newer designs.

I'm going to be wrapped up in work this week but hopefully the following I will wrap this guy up and start testing. There really isn't that much more do, at least compared to other tasks/timelines within which I've had to operate.

Thanks!

Paul

 

[Potato Cakes]

I now have all of the parts in to finish everything. I got back super late on Sunday from doing some shows so today I've been taking it pretty easy. I did build a small power supply for another project and the ACA-Bo that is replacing the master summing buss. I did some test fitting with it built up and you can see from the photo there is plenty of clearance under the buss channels towards the meter bridge. The PCB card for the master channel is a bit longer so I am making sure it is slid over enough to not be directly under it. I'm going to test the circuit and make sure it is working as expected with the changes to make the top of the master fader unity gain and then I'll install it. Then the fun begins with finishing the recapping and adding bypass caps. I'm off all this week so the plan is to have this done and do some testing. I still have to procure another patch bay and make all of the XLR to DB25 cables, but that won't be too much trouble after I know that this guy is working.

Thanks!

Paul

 

[Potato Cakes]

Got the ACA-Bo on the bench and made adjustments to make sure it works before I commit to installing it. It's being set up to do unity at the top of the fader throw, which I found that requires a 2k74 resistor. To keep the corner frequency the same as per the schematic, this requires a 480pF capacitor, which I would have to install a 470pF parallel with a 10pF. With just the 470pF it raises the corner frequency to a little bit over 123k, which might be fine, but I don't know high one can go with this value before a DOA isn't happy anymore. I'm going to mount it tonight and pull some more caps from the channels and tomorrow do a marathon of recapping. That is pretty much all that is keeping me from wrapping this guy up at this point.

Thanks!

Paul

 

[Potato Cakes]

I finished installing the ACA-Bo and Master Channel tonight. This proved to be a the more tedious part of this project for some reason or at least it felt that way. It should be all down hill from here.

I did the electrolytic in addition to the ceramic bypass caps per jensenmann for the master channel. I replaced the 10uF caps with 100uF and the 100uF caps for 330uF as that is as big a value that was small enough to fit in there.

For the channels, I used 1000uF for the all of the caps except for the 2.2uF and C27, as there isn't enough space at it's position for something like that, so I used a 330uF. I'm going to power through as much as I can with bypass caps this evening so I can start doing some testing tomorrow. I'll have one channel with ceramic and electrolytic bypass caps to test against one without. Pending any major failures, I hope to be done on Sunday minus cables.

Because I can't count, I found I don't have enough 1000uF caps to do all of the buss channels, but I do have enough to do 2 per. It was mentioned above that the summing amps like to see bigger coupling caps. Since there are 4 coupling caps for U4 on the buss channels as it is a dual op amp, would there be any benefit to just doing it's outputs or inputs. I have plenty of 330uF caps to finish this project. If there is a benefit to strategically using 1000uF over 330uF on the buss channels I would like to do so, but if not I won't worry about it.

Thanks!

Paul

 

[jensenmann]

330uF is already a lot more than originally, so don´t worry.

Regading bypass caps I hope I have mentioned before that ceramics are the choice for supply rails only. They don´t sound good if they are in the signal path. Film caps are better suited here.

 

[Potato Cakes]

Correct. I am only bypassing using the ceramic caps for bypassing the VDC rails on the chips. For the audio path only electrolytics are used for coupling and film for EQ. At least that is what I can see.

Thanks!

Paul

 

[Potato Cakes]

As promised, a pictorial guide for replacing the Omeg shafts. I found these on eBay listed as "Potentiometer shaft to repair amek tac recording console pots." They're a little shorter than the ones in this desk but the knobs attach to them just fine and you'll never know the difference.

First, you have to get the front metal cover off. Take a small flat head screw and gently pry the four tabs holding the cover in place.

 

[Potato Cakes]

Once, the cover is removed, carefully pull out the broken shaft. It may give you a little resistance coming out, but there is nothing else holding it in so all you have to do is pull it. When removed you'll see this.

 

[Potato Cakes]

The replacement shaft has a slot in the back that mates to the index inside the potentiometer. Before you insert the replacement shaft turn one of the adjacent pots all the way CCW or CW (it doesn't matter) to use a comparison of which way the flat side of the side should be facing. Insert the replacement shaft and turn it the same way the adjacent pot and verify that the new shaft is oriented the same way as the rest of the them. If you don't do this you can potentially insert it in backwards making the indicator on the knob to be also backwards, and then you get to start this process over.

 

[Potato Cakes]

Put the metal cover back on and gently press the tabs back in place. You'll see there are notches on the corners of the plate where the tabs are supposed to be pressed on to lock the cover in place.

Too easy...

Thanks!

Paul