The BTD-12S is a superb tonearm - period. That said, it is important to keep in mind it's a product of it's time (designed in the 50's). By that time most low effective mass tonearms and high compliance moving magnet cartridges that we know today were still, if at all, on the drawing boards. High / medium mass tonearms were the sign of times and the BTD-12S is no exception.
What makes the BTD-12S a remarkable precision instrument is the fine tolerance machining and the highest level of craftsmanship of the entire apparatus. The vertical pin type bearing is an early version of the same bearings used with the TP-16 tonearm (TD-125 / TD-160 variants). With the BTD-12S however, the bearing pins are adjustable on the lateral axis, they are detachable and probably replaceable. The pins rest in two high precision ball bearings (same as the TP-16) that allow for more lateral leverage and fine tuning of the bearing tolerance. It is remarkably crafted.
The VTF fine tuning is adjustable via a spring loaded lever mechanism that applies downward force on a brass/copper leaf plate that's attached to the tonearm bearing. The copper leaf is of a 'floating' design and the degree of compliance is also adjustable by tuning the gap between it and the set screw it's attached to. This can synchronize between the actual force applied and the markings on the tonearm assembly cover. It can be set pretty accurate.
The counter weight slides across an extension at the back of the tonearm and is locked by a set screw. This is not the turn-to-slide type of counter wight, but rather an earlier and more rugged type of counter weight. It is also relatively heavy (more than the 100g my Ortofon DS1 scale can handle) and was obviously designed for the heavier BTD-12S MK-I black (SPU style) headshell. I guess that by the norms of the times it was probably expected to accommodate a heavier MC phono cartridge, something in the order of the 8.5g - 10g. The extension on which the counter weight slides is attached to the tonearm by a long screw from the back, which is connected to a dampened aluminum tube at the back of the tonearm. This tube piece is mounted on rubber isolation that tends to rot and fall apart through the decades. It is a well known short coming of the BTD-12S design, but it is fixable and can be replaced with appropriate diameter rubber tubing.
The lateral motion of the BTD-12S relies on a ball bearing design which varies by type. The standalone BTD-12S design is well documented in the respective manual (available here on vinyl engine) but the TD-135 variant is a little obscured. The TD-135 comes with a fixed BTD-12S tonearm that's directly mounted in the turntable's deck. This mounting includes two sets of ball bearings, each composed of a cup, balls, brass/copper washer, shield washer and tightening spring.
The upper cup, at the base of the tonearm, is a mirror image of the lower cup (located under cue lift mechanism), such that the entire pillar is suspended vertically between the two springs that compress the contents of the bearing cups. Each cup contains 9 small 2.8mm steel balls. The TD-135 variant of the BTD-12S lateral bearings are not user friendly and if one is not careful, it is very easy to lose part of the 18 balls when the tonearm is freed at either end.
Decades of use (perhaps even abuse in some cases) may cause scratches and etched bearing cups. I guess the balls can engrave the bottom of the upper cup when too much force is applied to the tonearm or, God forbid, something drops on it from above. The good news are that it can be easily made smooth again with 2000 grit sanding paper and a dremel tool. The alloy is soft enough. Once the cups are scratch free, and the 9 balls are inside and retained by the brass/copper washer - it's as good as new. The springs are important too - do not forget them, one on each end.
It is well known that the BTD-12S does not, by design, have provision for dampening it's decent. However, a simple mod can change all that. On the bottom of the cueing pin theres a little spring held in place by a washer and a 'C' clip, that will pull the tonearm down as soon as the cueing lever is turned. In fact, by design, the user is expected to control the decent speed using that lever. Turn it slowly, and the tonearm will drop slow. On the other hand, if you just flip that switch - the tonearm will drop like a rock.
To dampen the tonearm decent you could remove the spring from the bottom of the cueing pin, and coat the entire pin length with thick, high viscosity silicone oil. You will need at least 500K cSt oil and you can go with 700K and up. If you have 300,000cSt silicone oil for SME of TP-16, it will not do! This tonearm is heavier and you will need high viscosity oil here. You may want to get as much of the substance inside the pillar too. You can jiggle the pin inside the pillar a few times to release air pockets and then to coat the pin with the stuff. Repeat the process a few times until an even, air free layer of thick silicone oil surrounds the cueing pin inside the pillar. You can keep the washer and 'C' clip at the bottom of the springless cueing pin after this mod, but it's not necessary.
Another issue with the BTD-12S is the lack of bias adjustment (aka anti-skate). That's not entirely true. I doubt the designers of the tonearm intended for this to happen, but the original wirings can deliver enough twist torque to counter skating forces. I have discovered this by mistake, and in fact, due to a too curvy angle of the wiring at the base of my BTD-12S, I had encountered enough torque to make the tonearm skip. I had to adjust the angle at which the wires exit the tonearm and make sure there's enough slack of it before it disappears down the small hole to the other side of the turntable, as to not affect the lateral motion of the tonearm. I then managed to adjust that angle and to make the wires apply enough anti-skate force as per my test record.
So, to summarize, once the BTD-12S is maintained properly and adjusted with accordance to the cartridge it accommodates in it's headshell, it is of superb performance. It delivers an accurate and authoritative output, and can also easily handle warped old vinyl. It's few inherent cons such as the lack of anti-skate or rate of decent dampening, can be mod'ed and remedied. Even it's rotting rubber counter weight extension mounts can be replaced. These are not beginner's mods and they do require a fair amount of dismounting and disassembly of lots of small delicate and springy parts, but for the seasoned DIY'er or professional technician they should present no challenge.
On a personal note, the BTD-12S seems to like the Denon DL-103 a lot, rather more than the Shure M75 that's so commonly associated with it. Give it a shot. If it's properly set you won't be disappointed. I love mine!
we use it with an TD 135 with a Shure M 75 E type2 sounds really great. It was also used on TD 124.
Perfect arm. Easy to set up. Sounds great with any vintage old cartridge! Even with a DENON 103!