Recapping Questions - Teac model 5 mixer

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ChunkyBustout
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Recapping Questions - Teac model 5 mixer

Post by ChunkyBustout » Thu Mar 24, 2011 2:56 am

Hello all,

I am in the process of creating a shopping list of parts and wanted to know if there a standard as far as recapping different sections of a 1975 mixing board. I'm doing the Monitor/Master module first since it's needed more there. Then it's on to the power supply then input channels and last, the sub module. I've heard on a mixer's power supply, you can replace the caps with the largest capacity that will fit. For the signal path, I don't think this is as simple. Do I use the exact capacity I'm replacing or ...? I don't want to have to return inside the mixer to do more recapping because I didn't take care of it the first time.

There are electrolytic, ceramic, mylar and polystyrene caps on the Monitor/Master module - any suggestions on exchanging one type for another to maximize improvements to the circuit?

All suggestions, ideas and info regarding this would be very appreciated.

I would especially like to hear from former and current users of this mixer!

CB

aophoto
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Post by aophoto » Sat Mar 26, 2011 9:08 am

Chunky,

I have one of the Teac 5 boards, and the extender section as well. I recently had the 5 serviced by a local repair shop (I'm not much of a tech, myself). I will try to dig out the invoice and see if I can help with some values.

I can tell you that the biggest sonic improvement, for me, was getting the power section sorted out. I had them install some other things as well that improved the headroom a bit.

Shoot me a message if you get the routing figured out on this thing....I use the 5 frequently, and I still get in fights with the signal path! Maybe I'm just too dumb for this board...

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Post by ChunkyBustout » Sat Mar 26, 2011 11:33 am

Hi aophoto. I'd appreciate looking at what a repair shop would use for recapping that board.

The routing hasn't been an issue for me. However, I can't speak confidently about this until I can test it using the studio feed outputs. Other than that, everything works great. The service manual is a good resource if you are completely in the dark about the routing.

You mention having your board done by a local repair shop. Other than recapping, what were the other mods done on it?

CB

kslight
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Post by kslight » Sat Mar 26, 2011 1:47 pm

Unless you have specific requirements for modifications (different frequency response, etc) I'd honestly go with stock values which still ought to be a significant improvement over what's in there...

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Post by The Scum » Sat Mar 26, 2011 6:20 pm

My recapping guidelines:

Electrolytics are the worst culprit - they're full of chemicals that degrade over time & exposure to heat. Eventually, the value drops, and the parasitic resistance goes way up.

Other types of cap are more stable over time. The doesn't mean that a new one won't be better than a 30 year old one. Particularly where cheap ceramic caps are concerned.

And since electrolytics are the biggest culprit, start with them. And the biggest ones are in the power supply, so start there, if you're confident (if you need to polish your skills, do some smaller ones, and work your way up). Be very careful not to get them in backwards, because they'll do some damage if they explode.

(Side note - that assumes the supply is a reasonable design to start with. 1975 is the very dawn of modern, low noise, solid state supply design. Lots of threads here about supply swaps and upgrades.)

As far as lytic values, sticking in the biggest that will fit is probably asking for trouble - it sounds like the inverse of the rule I use: if a given value isn't available, use the next larger value as a sub - for instance, some makers make 47 uF caps...others make a 50 uF. Swapping the 50 for the 47 is a fair trade...as might a 100 for 68, etc...putting in a 1000 where a 10 was is more risky - don't let size guide things, because modern caps are often smaller than their vintage counterparts.

You need to understand how the caps are being used. For lytics, there are 2 common applications: supply decoupling, and AC coupling of the audio signals (soaking up any offset that might creep in).

If you bump up the supply decoupling caps, they present a bigger load when the board is powered up, called "inrush current." Most supplies are designed with a particular inrush load in mind - if you exceed that inrush, you'll probably blow fuses as it powers up.

Bumping up the AC coupling caps is usually OK, you're shifting the cutoff of a highpass filter lower. Some consoles of that vintage are known for undersized coupling caps, which leads to weak bass.

Alternatively, if you can eliminate DC offset (with modern precision opamps), you don't need the AC coupling at all.

ChunkyBustout
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Post by ChunkyBustout » Sun Apr 03, 2011 2:02 pm

Thanks guys! I really appreciate the info and it helps get my head around it.

Please don't stop - feel free to post relevant links to more info, articles and vendors (other than Digikey, Mouser and Newark) of quality parts with minimum purchases!

Thanks so much!

(and I have been searching for a while but I know there are those with lesser known resources that are worth their weight in gold!)

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Post by Jim Williams » Mon Apr 04, 2011 7:00 am

Remove and replace those 4558 opamps, nothing worse except a 1458.
Install machine sockets. Add a small 10~22 pf cap across any feedback resistor of 3 k ohms or more. Add a pair of .1 uf mono ceramic caps from pin's 4 and 8 to ground. Now you can use some of those good opamps.

The LME49720NA is good and it's a DIP package. Then check DC offsets from the opamp's output pins 1 and 7. If DC is below a few millivolts, pull out that electrolytic coupling cap.
Jim Williams
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ChunkyBustout
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Post by ChunkyBustout » Tue Apr 05, 2011 12:25 am

Thank you very much, Jim.

I've been sitting on some sockets I ordered when I got the mixer ;) I'm not familiar with "feedback resistor" (I assume that has to do with where it is in the circuit?). Also, I assume "mono" means monolithic?

I've been considering the LME49720NA for the Master module and your suggestion is appreciated.

CB

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Info about the 5A vs 5B

Post by Monarke » Sun Mar 11, 2012 8:44 pm

I found this information about the differences between the 5A and 5B while looking through stereomanuals website to find a 5EX svc manual:

We do not have Model 5B material, but have read that the difference betwee 5B and 5A is mostly the color of the knobs and that the performance and features were essentially the same. And this from Tascam... The Model 5B mixer incorporated a new Integrated Circuit (IC) chip which was four times faster than the previous Model 5 and offered a cleaner sound. It appears that you should be able to use our Model 5 Mini User #2 for some operations type info and our Service Manual #2 as an appropriate, if not perfect substitute. We do not guarantee this information, so make up your own mind.
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Post by Jim Williams » Wed Mar 21, 2012 9:51 am

If you replace coupling audio path caps, use good stuff like Panasonice FR series. It's rather safe to hang a large value on the output of an opamp. 220uf/25v is usually good. That assures any low impedance load it hits won't roll-off the low end.

Input cap values are dependent on the input impedance of the circuit. If 10k ohms or so, anything at 47 uf or so is ok. If it's the wiper return of a 10k ohm fader hitting a 100 k input impedance, 10 uf is plenty.

The general idea is to set the low end roll-off at 2 hz or below, that removes any phase shift that might accumulate. If you use the standard 2xpi x rc/1 formula you can determine the -3 db roll-off points accurately.

If you use opamps like National's LME series or BB's OPA161x series you can usually just remove them.

In power supplies, I go big, really big. I see no problems with 20 or 30,000 uf's on console mains filter caps. If the in-rush current is high, use higher current rectifiers. I also use very large caps on the outputs of the regulators, typically 1000 uf. I also mount 1000 uf caps on every module after the fuse resistors. Then you get big low end.
Jim Williams
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Monarke
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Post by Monarke » Wed Mar 21, 2012 7:04 pm

Jim Williams wrote:
In power supplies, I go big, really big. I see no problems with 20 or 30,000 uf's on console mains filter caps. If the in-rush current is high, use higher current rectifiers. I also use very large caps on the outputs of the regulators, typically 1000 uf. I also mount 1000 uf caps on every module after the fuse resistors. Then you get big low end.
This would be fine only, if you increase the size of the filter capacitors and also the rectifier bridge, the inrush current may be substantial to be more than the fuse(s) are rated for and they will pop upon turn on.
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Post by Jim Williams » Thu Mar 22, 2012 8:31 am

In that case you use the next higher fuse current rating or switch to slow-blow.
Jim Williams
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Post by The Scum » Thu Mar 22, 2012 1:08 pm

Or limit the inrush with some 100 Ohm power resistors, and use a time delay relay to short them out once the inrush is over.
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