The mystery LED is an infrared LED. It works the servo that moves the arm. It shines through slots in a shutter, and then onto two photodiodes. They are on the verttical PCB in front of the cartridge. If the photodiodes get equal light, the tracking motor doesn't move. If one photodiode gets more light, the motor pulls the arm in one direction; or vice versa.. The cartridge can pivot a tiny bit in the arm. The shutter moves with the cartridge, as it pivots to follow the groove. A very tiny bit of non-zero tracking angle unbalances the light, activating the motor, moving the arm to null the error.
The arm-lift bellows will leak eventually - I replaced mine with a solenoid off a Revox B77 tape deck which is a common fix. Power supply caps will need to be replaced before they blow. It's a unique machine when all its issues are taken care of.
Same as mine, got the device from my brother who had it together with a B&O4002 stored on a high shelf in his kitchen. He bought them in 1981 and stored them since the mid 80s. The B&O had a major problem,the arm wouldn't go down after the decades long standstill. Cause was the dried up grease or oil in the armmechanism, cleaned and lubricated it and the machine worked. I tried the Revox but a capacitor went up in smoke. After removing it and the blown fuse it worked fine for some time but at a certain moment the arm started returning after a couple minutes. Opened the device and saw a damaged part, there are 3 PTC's and one was broken,think 50% of the PTC was broken off,the other 2 looked ok but might be faulty too. It might have caused the returning problem i think. It's a project i have to finish, the 3 PTC's must be replaced, some capacitors on the same board and other boards too. Think the many ic's in the sockets are ok while the machine worked fine for a while.
Good job keeping the repair simple. Most prefer manual s-shape or ULM. Q-lock is good. Way better than belts. Linear trackers are hard to sell. Most people want the classic look and the linear trackers are hard 80's styling...aka Fugly. Had a Technics SL that took over a year to sell.
Fugly or not, I have two of these turntables and nothing comes close to it. Ease of use, total lack of rumble. Like virtually nothing, even with a scope connected to it. It uses a very light cardridge and adjusting is a bit difficult, but once setup its a friend for life. And its build like a tank. And on top of that, all the schematics are easy to find and components are mostly off the shelf mouser electronics. Oh, FIRST THING TO IS REPLACE THE RIFA CAP!!! Otherwise its kaboom and a lot of smoke.
Hello, I have a Revox turntable. The stylus goes side to side and up and down but when I drop the stylus on the record it does not track. It stays in the same place. Can you help? Thanks.
Replacing that LED would have broken the arm angle sensor. It likely has a PWM drive so the average voltage is low. Being infra-red you would not see it. It may show up with some phone cameras. The end of record is sensed by the speed of motion of the tone arm at the end. The end stop is only used if you are commanding it with the buttons. That motor has a gear box at the top end so one drop of oil isn't going to do much. Fortunately the entire service manual is freely available on the web and includes the full schematics. It uses classic TTL logic chips. These are mechanically and electrically very simple machines. Much simpler than a typical mechanical nightmare automatic pivot arm mechanism. Much simpler than any CD player.
Sure it does. The optical part has nothing to do with any record, it just detects the position of the cartridge and regulates the tracking arm. There is no interaction with the record at all. The cartridge is mounted so it could tilt a tiny bit. If it starts to tilt to the inner or outer side of the record, the servo mechanism will get activated until tilt angle is exactly zero again (less than about 0.3mm or about 1/1000th of an inch will activate it!). This is why you do almost get no tracking angle misalignment of the needle at any position all over the record. You do get varying mis-angled positioning of the stylus of some usual tone arm when it swings over the record on it's circular track.
Hideous turntable, design wise. I could never understand how groove distortion would not creep in after years of use. I wouldn't want to play my LPs on it no matter how good the quality (of which Revox are renoun). Also, how does one select tracks, or you just don't?
Why should groove distortion creep in? Your regular swing-in tone arm may get stuck from oil of the bearing gumming up. The track mechanism will overcome any force your needle could otherwise have to pull your tone arm. The tracking is working with no switches but just by the photo-electric guard, so no force is needed to pull the carrier (or if you want to call it tone arm) to the exact position. And, BTW - this turntable is out of about 1984. What does that say to you, if it is the first service for it (or let's call it oil-change) after nearly 40 years? And, being Revox user on my own, I bet it is! ;-)
@@Slartibartfas042 its from 1977 until 1983. They made some small adjustments over the whole lifespan. Got mine in a not working state, but it was a blast working on it. The schematics and all the small changes they made over the years are well documented. And the speed is just dead on. At one point I had two working examples. Played pink floyd dsotm on both at the same time from beginning to end. Both were dead on the same speed. Like perfect. Used the same pressing of the album on both. Ugly or not (I think its beautifull because of its out of the box approach during the 70's) its one of the best tables ever made. And they are still fairly cheap if you compare the second hand prices to the performance it brings. SL-1200's are getting ridiculous, not to mention simple duals or thorens players. Long story short: the b790 is a best kept secret that people are affraid of to work on and lets keep it that way 🤣
Due to the tracking geometry a typical pivot tone arm causes a "skate" side force on the stylus. The anti-skate applied to the arm keeps the arm in place, but the skate force is always pushing the stylus against one side of the record groove. A linear tracking arm tracks perpendicular to a radius so there is nearly zero skate force. So linear tracking tables don't have this "groove distortion" that is a fundamental flaw of other turntable designs.
