Studer B67 recap: Part II, other studer work, some guitar work

I edited this post on September 22nd, 2015. If you’ve referenced it before that day, you might want to re-read it.

The next step after replacing the caps on the motor control boards was to check the voltages in the power supply. There are test points on the stabilizer’s PCB, so that’s easy enough to do. I did that and the -12V rail was reading 0. So I decided to take out the power supply and recap it. Removing it is a bit of a pain. There are two screws on the top, one horizontal screw that you need a 6″ T-Handle wrench to undo, a bunch of connectors that need to be removed, and the 50/60hz switch on the power supply.

The PCB is different than what the manual shows: The -12V regulator is a Motorola MC7912CK while the manual shows National LM320K-12. The manual shows some SAL caps next to the regulators while my PCB has all tantalums there. Turns out, my PCB is the same as what they used on the Mk I of the B67 (figured that one out after downloading the Mk I manual). The Motorola regulator is a variant of the LM320K-12, so no problems there, and the tantalums can be replaced with high grade electrolytics (which I’m going to do). They layout of my PCB seems to otherwise be identical to the Mk II manual, and the Mk I manual also shows a very similar layout. The only differences I could see are that D4 and D5 are interchanged (shouldn’t matter since they’re oriented the same way), and that the Mk I uses a 4.7mF cap for C3 where Mk II uses 10mF.

While waiting for high grade electrolytics to use instead of the tantalums, I replaced the axial lytics and a couple tantalum caps. I ordered a 1000uF filtering cap instead of 10000uF, so I’m waiting on the big caps until it’s all here.

IMG_20150919_202850

See those blue tantalums? Those are the ones that should have been SALs.

I also recapped the little monitor board under the front panel. The one that drives the built in speaker. No pictures of that because it was late and I forgot.

I also cleaned the front panel of the machine real nice. Someone said that toothpaste is good for that, and holy cow – it is! It’s much better at removing years old grime than isopropyl alcohol. I also poked around the card cage and noticed that the stabilizer card has a blown capacitor. I’ve never seen anything like it! The is torched and got black sooth all around it. According to my french friend who’s a Studer expert, this is the result of it being really close to the heat sink. I’m going to replace those tantalums with electrolytics also.

As far as guitar work, I think I mentioned making cauls to fix my F30. Trying to shape a single piece of wood to have 12″ radius did not work. I did something similar when I built my neck rocker, and it came out somewhat ok, but required a lot of rasping, chiseling, and filing. I thought I’d have better luck here, but no dice. It was too hard to maintain the radius along that block of wood.

I was at the hacker space trying to figure out how to do this, and a guy suggested cutting the radius on a bunch of small pieces of wood and gluing them together. Then another dude suggested doing that with the laser printer/cutter. I complied, and the thing does cut a very accurate radius, but it can only cut slightly thicker than 1/8″ pieces of wood, and since I need a 5″ long caul, that meant having to cut (and glue) lots of pieces. I tried gluing all the pieces at once and it was a disaster. They kept slipping, then the glue was drying real fast on me (what the hell, it’s Titebond Extend which is supposed to have longer open time).

So I googled and found this page. I followed his advice and cut 7 pieces from pine on the bandsaw. Here’s what one piece looks like:

IMG_20150922_190212

I’m gluing them up in pairs (to avoid what happened last time), and it’s working out well. Only one piece left to glue.

I hope this guitar will be fixed by the end of this week. I still get filled with sadness every time I think about it.

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