Not a lot of updates. I’ve been rearranging my workspace/studio and there’s nothing too exciting about that. However, I did clean the faceplate of the B67 as well as the knobs with hot water and dishwashing soap. It made a pretty big difference:

Under the knobs there was this grease that I couldn’t identify. It wasn’t grime from years of neglect, but rather grease that was put in place either at the factory or at some point when someone serviced this machine. I cleaned it off for now and I’m going to figure out if it actually needs to be replaced.

Other than that, I noticed that the tape path is a bit misaligned and the tape doesn’t pass right over the heads the way it should. Will deal with that when I’m done reorganizing. Then I hope to order capacitors for the audio electronics if I have time before the week ends.

Studer B67 Mk II fixed.

This time for real. The SN75462 replacements got here yesterday and I popped on in IC3’s socket and the machine works. The sensor lock in STOP and the motors are both responsive. Here, see for yourself:

There are a few things I need to figure out for next time I look under the hood:

Does J3/4-6 provide the +24V to the solenoid?

Does K-EDIT deliver exactly the same voltage as what comes from the +24V rail, or is it something like ~25V (the Zener diode “trick” so that it looks like the output of IC3 is ~25V instead of 5V)?

Does the voltage at IC 4 pin 1 equal to the voltage on pin 7? That is, does it work like some kind of a comparator? See previous post and notes I left in the manual.


Studer B67 Mk II problem fixed*!


I was in the middle of writing a progress post and then I went to probe around IC3 in the transport and realized that IC3, was put in the other way around. That is, pin 1 is where pin 8 should be, etc. I flipped it around and it is now fixed! Here’s a more detailed explanation of what I’ve done in the past few weeks:

First a recap of the problem: Upon turning the machine ON, only one motor will be responsive to the transport controls. For instance, pressing PLAY would make the take up motor spin but not the supply. Then if I spun the roller on the right clockwise, the take up motor would turn off and the supply would turn on and start spinning. Spin the roller counterclockwise and now the take up would run and the supply motor would shut down. Even weirder was that disconnecting the J5 connector from the pre-divider board caused the machine to operate correctly.

So when I picked up troubleshooting the machine again I decided to go over the counter’s schematic to try understand better what’s happening with QP-DIR1 and QP-DIR2. To be honest, I’ve done that before, but this time I also graphed the waveforms along the way from where the signals come into the board, and their way to becoming Y2-FORW and Y2-REVS. I also keep these drawings with the manual for future reference. That didn’t tell me much other than that when the roller on the right is spinning clockwise, Y2-FORW goes HIGH and when it’s spinning counterclockwise it’s going LOW and Y2-REVS goes HIGH. I also realized (might have before as well) that the pre-divider carries these signals FROM the counter TO the transport.

Advice on the reel-to-reel forum directed me to the tension sensors, so I started learning the circuit comprised of ICs 4, 1, 6, and 2. That made sense to me because that circuit is responsible for generating the pulsating signal that controls the motors. My understanding of how IC4 is used is that it is some kind of a comparator. So depending on YAN-TT1 and YAN-TT2 it’ll go positive or negative. That’s how the lower half of IC4 (YAN-TT2) was acting and that made sense. However, the voltages from YAN-TT1 weren’t enough to cause the comparator to work the right way. Instead it went from +12V to something like -2V. (By the way, that’s still how it is!) I thought that since this voltage is dependent on the displacing of the tension arm, that maybe its mechanical settings are out of wack.

So I took out the left sensor and adjusted it per the manual. I put it back in the machine and now both left and right sensors were locking in place. This one made me scratch my head, but it led me to start figuring out what’s controlling the solenoids of the two sensors. This stuff isn’t mentioned in the manual, so it took a bunch of probing and continuity tests to realize that the solenoids are fed the unregulated +24V and the respective outputs of IC3 (SN75462). I mentioned that to a friend and he said that IC3 goes bad often, so I put in an order for replacements. Meanwhile I went to check the voltages at the inputs and outputs of IC3 to see if it’s working right and that’s when I realized that it was put on backwards. I flipped it and now not only are the sensors not locked, but BOTH motors are responsive to the transport controls!

However, the sensors should lock when the machine is stopped. A quick check of the voltages in and out of IC3 showed that it’s not functioning right – it’s a NAND gate but when both inputs voltages are high (IC7 pin 7 is HIGH meaning the machine is stopped, and then a signal derived from pin 7 also HIGH) I get +24V at its output, but it should be 0V. It’s a good thing I ordered some SN75462 so I’ll drop a replacement in and see what’s up.

FMR RNC Repair

I bought a broken RNC online for $40 shipped. The seller literally tried every possible wall-wart with the unit except for what’s written on the back of it: 9VAC @ 500mA.

He mentioned smoke coming out at some point, so the first thing I did when I got it was to open it and see what burned, but nothing looked burnt. So I ran it on a 9VAC wall-wart I have, but before I plugged it into my interface, I decided to look at the outputs to see if spitting out a high voltage. Both were putting out 4VDC. I took another glance at the unit and noticed two bulging 470uF capacitors.

I tried to trace the power supply circuit, but it didn’t look like the usual rectifier and regulator circuit. I called Mark at FMR Audio and he explained that they use a charge pump to generate +/-15VDC. Now things were making more sense, especially when the two bulging capacitors are the rectifying caps. I replaced them with other 470uF 25V capacitors and no more DC at the outputs. The compressor seems to work fine, but the capacitors get kind of warm and the regulators get even warmer to the point where I can’t leave my finger on them for too long. I also noticed that the DC voltage on the two caps I replaced is a bit over 25V. Weird since the factory caps were also rated at 25V. Another weird thing I noticed is that my wall-wart puts out 10-point-something VAC even when it’s loaded.

I called Mark at FMR again and he explained that since my wall-wart is rated at 1.5A, it’s probably too stiff a voltage source for the RNC. We then calculated that the extra voltage is what’s causing the higher DC voltage I was seeing on the capacitors.  He also suggested swapping the 25V capacitors for ones rated at 35V so that they’ll last longer. He did say that the regulators getting warm is normal and expected considering the voltage drop and current through them. He also tipped me to the wall-warts they sell with their units (Jameco part no. 100061). Lastly, he said that the two distortion trimpots should be left alone. They dial in the distortion with an Audio Precision to .005%. He then mentioned that the units keep their calibration for the longest time.

As you can tell, Mark was extremely helpful. He is happy to answer question and help probe a broken unit, and never even said that I should mail it to them to repair it. If you’re looking for a new compressor, consider the RNC because it’s awesome and cheap, and their support is excellent.

December – March updates

Nothing fancy. Worked this week on an Aria Precise bass (a P-Bass knockoff). It’s a pretty neat bass, but needed a new volume pot and I felt like the neck was weird – I ended up maxing out the truss rod for acceptable relief.

I also had to replace the pickguard on that bass (owner wanted a different flavor, is all), and that required a lot of filing, scraping, and sanding to get it to fit right by the neck and over the pickups. A razor blade with a burr works really well for scraping here. Files are not great. The best tool for getting the pickup openings right is to take one pickup cover, put sticky sandpaper around it and use it to shape the openings. This maintained the right radii of the corners.

I also set up a friend’s Guild M-120. All Mahogany, made in China, and it sounded and played great. Fretwork was pretty much excellent, and the neck is straight with a very responsive truss rod. For that price, you really can’t go wrong with those guitars.

Also re-wired an EV RE 664 with an XLR connector and at the same time eliminating the Hi-Z output. I’ll make a post about that later.

I also finished (in December) my second rack. It turned out pretty well, about as good as the first one did. I had a few mishaps, the biggest of them was that even though after I dry-practiced for more than 5 times, when I came to glue it together, things weren’t square so I had to take it apart while the glue was starting to set. It was incredibly stressful and I had to clean the glue out of the joints with a lot of water, which warped the the wood a bit, but like I said, it turned out fine. I’ll probably write a whole post about the new rack, but for now here are some pictures:


Also worked on a few other guitars in the last few months, as well as my own Stratocaster. I “inlayed” a piece of mahogany in the neck pocket:

Also leveled its frets. Also it probably needs a new nut.

I’m also building some drawers for my works area, but more on that when they’re done next week hopefully. Maybe I’ll write a whole post about it.

That’s pretty much it other than little jobs.

Re-foaming Beyer M380

I’m lucky enough to own two Beyer M380, and since they’re both probably over 20 years old, their foam needed to be replaced. In one microphone (gold M380) the foam was almost completely gone, but for the other it was more of a precautionary measure – the foam had holes in it, but wasn’t exposing the whole capsule. Anyway, these guys can go bad if even a small hair gets in the capsule, so it’s a good idea to replace the foam as soon as it starts showing signs of breakdown.

Greg from Electrical Audio tipped me off to McMaster Carr and said to search their website for “the thinnest Reusable Polyurethane Foam Air Filter”. It’s this one, and in case the link goes dead in the future, the catalog number is 9803K301. The thinnest one is 1/8″ thick, and that’s the same thickness of the foam that’s already in the microphones.

The next question is the porosity, 30 or 60 pores per inch? I got both and compared them to the existing foam in the microphone. The answer is 60 PPI. See for yourself!

The difference between 30 and 60 pores per inch is very obvious:

One sheet was enough to re-foam two microphones with some extra material left over.

There’s really not a lot to it once the microphone is open. One side the body comes off completely and the other is holding the capsule in place with a couple of screws.

I started on the gold microphone because its foam was in worse shape. However, it was deteriorating so badly that I couldn’t use the old foam as a template. So in retrospect, I should have started with the black M380 whose foam was in a better shape. Anyway, it took a bit of experimenting to figure out how to cut the foam. Like I said, it’s better to use the old foam as a template, but if it’s in rough shape, start with a square that is bigger than the grill. Fit the square of foam in the “tub”, and cut the four lines for the corners. Now the foam is overlapping at the corners and need to be trimmed. This is kind of tricky and here’s what I learned: the body of the mic has these vertical slats, right? They go all the way around to the rim of the tub, so if you cut to much foam along the length of the tub, you might leave holes in the foam that overlap with a slat. That’s bad. So the trick is to cut the excess at the top and bottom of the tub. Also, make sure to first cut the lines for the posts where the screws go in. Anyway, that’s basically the only tip I have for how to actually cut the foam. Here are a couple of shots of how my foam pieces turned out.

And the two M380s fully re-foamed.


Old foam (from the black M380):


Important: Don’t try to glue the foam to the grill, that’s not how it’s supposed to be and the glue will probably eat the foam. When I got my black M380 I was dumb and decided to glue it to the grill, and when I re-foamed it I had to clean out a bunch of old gunk. It sucked.

(This was done in September, but I only now remembered to post about it)