Studer B67 fix explained

I explained two posts ago that the problem with this tape machine was IC 3, an SN75462 that was put on backwards and therefore fried. That IC was responsible for interpreting logic from the transport and operating the tape sensors’ locking solenoids. Here’s the data sheet for the SN75462, and I can’t say I understand it all, but the gist of it is that it has two NAND (or AND) gates followed by an NPN transistor whose collector is taken to a pin on the actual chip, so it’s not necessarily connected to Vcc powering the chip. This is called an open collector, and here’s a very good explanation of what it is, and what are some applications. Here’s a little drawing of what this looks like in this particular case (can also be seen in the data sheet):

open collector


Here’s the portion of the schematic that shows IC3 in the circuit, the anode of the zener (D59 in this case, but all of them do) goes to ground.

IC 3 circuit

Now, when I was measuring the voltage at pins 3 or 5 of IC3 (after it’s been replaced with a functional one), what I saw was that it was “delivering” +24V when it was supposed to deliver +5V (Vcc), and that was very confusing because how could it put out such a high voltage when Vcc is 5V? Not only that, the +24V was identical to what I was seeing on the other side of the relay (that should have tipped me off..)

The answer to this is the open collector but also a KVL of the circuit. I was thrown off before because someone mentioned off-hand that the reverse diode pulls up the voltage, and that sent me on the wrong path for a while. I finally understood what’s going on when I drew the NAND gate + the transistor + diode + relay together:

open collector + relay

Here’s how I understand it. When the transistor is ON, the collector is pulled to ground through the transistor, so current flows through the relay. When the transistor is OFF, no current flows through it so the collector is free to being pulled up. What does it mean, though? KVL shows it. There’s +24V on one end of the relay, but no current flows through it (the transistor is OFF and the zener is reverse biased), so there’s +24V on its other end. That other end is the collector, and that’s why I was seeing the exact same voltage on both ends of the relay.

Now, one might ask why use the zener at all? Pull it out of the circuit and you get the same behavior. However, when you open the switch connected to a relay, the relay shoots out a spike of high voltage. Without another path to ground, that voltage will fall on the transistor and burn it. Instead, the zener starts conducting as soon as the voltage across it is higher than 30V. A regular diode reverse biased won’t work because it won’t recover from being pushed to its breakdown region. And of course, a forward biased diode will always conduct so current will always flow through the relay.

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)

EV RE20/PL20 Repair, Part II

I took the top grill off and of course there was disintegrated foam everywhere. I cleaned the top grill and fixed the actual grill a little (it was pushed in).

The capsule has this little black plastic basket that protects it. It comes off by undoing the two little screws on the perimeter of the capsule. Be careful to not drop the screw driver into the capsule, though. I didn’t take a picture with the basket on, and I also started cleaning the capsule before taking pictures, but here’s the capsule with quite a bit of dirt still, and the basket before cleaning it.

A guy I talked to through the TapeOp messageboard said that he didn’t have to clean the capsule too hard to get the bass reponse back. I tested it in the condition shown above, but I still got no bass. So I kept cleaning.

At first I was cleaning with a q-tip dipped in 99% isopropyl alcohol. It worked, but I was told that the q-tip is a little too rough for the delicate membrane. So I switched to a fine artist’s brush dipped in 99% IPA. That worked well and it was better for when cleaning around the tiny coil wires or getting into the crevices. Also, the IPA is only necessary for getting the gunk loose. It can be brush off with a dry brush, and remember to brush towards the outside, away from the center.


Clean membrane, but notice the off-center pole piece.

There are still some specks of dirt under the membrane, but now the bass response was back. Check it out. I don’t have a very boomy voice, but for comparison, you can check my friend trying out the microphone on his voice prior to handing it to me.

At this point I was ready to move ahead and order new foam, clean the whole thing and work on the wiring (low cut switch is not working). However, if you’ve spent any time reading about these microphones and the ways in which they become dysfunctional, you probably saw mentions of “baby rattle”. There are examples of it online, but basically the rattle is there when the microphone is used on low end sources with fast transients. So I decide to test it on bass, and sure enough, with some low notes that were loud enough, there was this rattle that almost sounded like static. Here’s what it sounded like.

Honestly, I don’t know if that qualifies as baby rattle, because baby rattle is the result of the pole piece being loose, and this one is not loose. I also noticed that the buzzing is still there, although to a lesser extent, when the microphone is facing away from the speaker cabinet.

I asked a friend who worked on a couple of RE20s about it, and he said that he had one with a similar problem and he believed that it was dirt dirt in the motor and that some notes would make it resonate against the coil. Whether this is the problem, or it is a variant of the baby rattle or another pole piece related problem, this is the end of the road for me. I do not know how to remove the membrane, I do not have the tools to do it, and I don’t think the owner of the microphone will want to pay to learn how to do all this on his microphone. So I contacted Ben at He’s universally recommended for fixing EV microphones and his rates are supposed to be lower than EV’s. I still haven’t heard from him, so I don’t know what are his rates yet. If he’s too expensive, the owner of the microphone said that he’ll have me replace the foam, and then the microphone at least will be usable on voice.

EV RE20/PL20 Repair, Part I

A friend of a friend brought to me a very beat up PL20 to repair. At the moment it has no bass response and the capsule moves around. The foam had obviously deteriorated and needs to be replaced. I should say that I never worked on an RE20 or a PL20 before.

The first issue was trying to open the thing. YouTube has videos and people show how to do it, but basically there’s a small hex screw at the bottom of the top grill.


Location of the screw

It supposed to use uses a 0.035″ key (confirmed via the Ampex Mailing List). The screw was stripped and neither my 0.035″ or 0.9mm keys would grab onto it. I thought that maybe the screw was seized, so sprayed some WD-40 on it. When you do that, cover the grill with a paper towel or a rag so no WD-40 gets inside. That didn’t help and the key kept slipping.

Removing a stripped set screw that is sunken in its socket is tough. Some people say to use a very small flat head screw driver. I tried with one but I didn’t have a good grip on it. I went to the makers space I’m a member of and found a Pittsburgh Precision tool that had a 0.9mm bit (which I was sure was square, but looking online it says it’s hex). I put the bit into the socket and tapped it lightly so it grabbed into the screw. Then I used a handle to turn it slowly while applying some pressure. The screw came out.

I should really get that Pittsburgh tool thing, it’s cheap and it came in handy. It seems sturdy, too.

Harmony Bobkat gets a Tune-O-Matic bridge, clean up, and a setup

My friend’s guitar that I worked on over two months ago. I just didn’t have the time to write about it in detail until now.

It seems like it sat in someone’s closet for a while after someone made an interesting modification to it – the neck was glued to the neck pocket. Also there was glue all over the body of the guitar under the pickguard. I forgot to take “before” pictures, but basically, the neck had one very small point of contact with the neck pocket, and the screws were still used to hold it in place. If the screws weren’t used, the neck would have snapped out under string tension. No doubt about it.

I saw that the point of contact was tiny, so I decided to just dislodge the neck out. That’s dumb. Don’t do it. Use steam or heat or how water. This is what it looked like when I did that:


Not horrible, but it could have been cleaner maybe. Also, this is a testament to the bad gluing job.

I pulled out that big splinter and glued it back to the neck. It was a weird fit. Actually taller than the neck.


I shaved it down with a chisel so that it’ll be flush with the neck.

Next I cleaned up the neck pocket and body of the guitar from all the glue. I wish I took pictures after it was done because as you can see from the first picture in this post, there was a lot of glue.


The next step was installing a Tune-O-Matic. That was a bit of a big deal for me because I’ve never done it before and it’s also kind of a big modification to the guitar, but it turned out well and made the guitar more playable.

My friend wanted a Kalamazoo style TOM installed. These are the ones with the “skinny” posts and require less precision to install. However, I saw that StewMac makes a fancy and expensive tool for installing these, so I thought I’ll make one myself. It took some drilling tapping, and playing around with it, but the result turned out ok (at first).


My tool in action.

I set to get the posts 3/4 of the way in, and the tool worked well at first, but as the posts were getting deeper, the tool ended up turning in place instead of pushing the post further into the guitar. At some point I mangled one post and realize that I what I’m doing isn’t working. I googled around a bit and found a really simple and cheap solution. I can’t find the links, but here’s the gist of it:

The posts have 6-32 thread count/size. Get a a 6-32 nut and the appropriate nut driver. Now take the thumbwheel and thread it on the post. Now take the nut and thread it against the thumbwheel (coming from the “top” side of the post). Tighten the two against each other real hard. Like, really hard. Now you use the nut driver to drive the post into the guitar. Super easy and simple, and would have saved me a lot of time and trouble. Seriously, I don’t know why anyone would use the StewMac tool, which admittedly is nice but very expensive.


All good!

Next was grounding the guitar. The original bridge was made of wood, so no one thought to ground the guitar. What it meant was that the strings and the hardware were at one potential that was different from ground (0V). Things weren’t going to be any different with the TOM installed because it too has no path to ground. One solution is to drill through the body cavity into where the posts go. That was going to take a lot of precision and a special drill bit. Instead I opted for something much simpler. I used copper tape to make a path between the treble side TOM post and one pickguard screw hole.


Then I wired a little ring to the electrical ground.


The following electrical path is formed: bridge -> copper tape -> pickguard screw -> electrical ground (via the ring and wire). Now the guitar is grounded! Here’s what it looks like with the pickguard on. The only thing to keep in mind is to not over-tighten the one screws that holds the ring in place or the pickguard will break.


You can barely tell it’s there!

Lastly, what was left was to set it up. My friend uses 52 strings and tunes down to C, and course, I compensated for that + intonation before drilling the holes for the TOM posts. Anyway, the neck had a bit of a backbow because the wasn’t a lot of string tension on it. It’s also possible that not being glued in changed something, but I doubt it. So I was set to adjust the truss rod, but couldn’t get nut driver in there. The nut looked like it’s 1/4″ but it won’t turn. I tried 5/16″ driver following the suggestion of a friend, but that didn’t work either. After further inspection, I realized that I can’t really fit the driver in the little “bed” under the nut. So I pulled out my tiny chisels and excavated a little bit of wood from that area so I can fit the tool in there.


Very small amount of material removed.

Now that I was able to fit the tool in, the nut wouldn’t budge. I know better than to force it, so I talked to a friend who’s the most knowledgeable person I know when it comes to guitars. He said that there’s a good chance the rod is broken, and if not – it could break easily. He admitted to breaking one before and said that 50% of Bobkats he saw had a broken truss rod. He didn’t have any tips for undoing the nut, but I appreciated him telling me that these truss rods are fragile.

I squirted a tiny bit of WD-40 on the nut and rod and let it sit for maybe 10 minutes. Then when I came to adjust the nut, it turned without having to apply force to it. Great! So yeah, if you have a truss rod nut that’s seized, give it a squirt of WD-40.

I loosened the rod and the guitar played really well.

These guitar are made horribly, but the pickups sound absolutely fantastic. Really made me want to get a couple of goldfoils.

Here are some pictures of the guitar. Other than the ridiculous gluing mod, it was in a decent condition.