(After posting this article to some Facebook RTR pages, Tim Millenaar from the Netherlands offered some useful feedback that I will transpose almost verbatim in the appropriate sections. Thank you Tim!)
Overall, the Revox reel to reel decks are solid machines once they are fully serviced, however the brakes in these units have given me nothing but grief until I took a number of hours, and invested them into thoroughly analyzing them, and how the braking system works. Interestingly, I posted a question about how to service them on a couple of Facebook pages, and received many differing and conflicting answers, so I decided to compile my own work in this article and throw it on the internet, to be ridiculed and critiqued. This method has worked for me throughout numerous decks now, and it’s saved me countless times over the ways that I thought were correct.
Depending on the Revox model number and revision, the service manuals give little information on how to adjust the brakes, however none of the engineers that wrote these manuals were expecting their decks to still be in service 40-60 years after they were made. Below are three pages of scans, taken from the Revox A77 and B77 service manuals. Fortunately, the information can be interchanged between these manuals, as the brakes are the same between the A77, B77 and PR99 models, and similar braking systems are used in other Revox/Studer models as well, with some revisions throughout.
Description
The braking system in the Revox decks is pretty simplistic, however without a full understanding how they work, they can be a nightmare to adjust if you have tape spillage when going from a fast wind mode to stop, or to play.
The key thing to keep in mind (which is what fooled me for many hours) is that the braking system is balanced, meaning that the supply and takeup reel tables need the same braking pressure applied to the reel tables at the same time, otherwise there will be problems. You can literally think of folding the braking system in half between the supply and takeup reels, and the action of the brakes, the brake bands and the solenoid that drives both brakes needs to be identical.
The braking system consists of:
- supply and takeup reel drum, that is covered with a fabric type of material that is somewhat coarse on its surface.
- supply and takeup reel brake bands, that apply pressure to the reel drums when the brakes are engaged
- a spring between the top and bottom of the brake bands, that applies pressure to the brake bands when they engage
- a single solenoid, that releases both brakes when power is applied to the solenoid. (located under the chassis)
- a brake assembly sub-chassis, that the brake bands and springs are mounted to.
Three pictures below identify the various brake parts, this deck being a Revox A77 MK III. (reel tables already removed)
To service and to inspect the brakes, the front cover of the deck needs to come off. Depending on the model, there are between 2-5 screws holding the transport cover in place. Remove, and put aside.
Assuming your reel tables are still in place (they can stay on for now), power the deck up, and without a tape threaded, press the rewind and fast forward buttons, to see how the braking system works. Regardless of the fast wind direction, the braking solenoid engages, pulling the centerpoint of the braking release mechanism up, which then loosens the brakes on the supply and takeup reels. Without a tape threaded, the transport functions will only work for as long as you are pressing the button, but this allows you to see how the braking system functions. It’s a pretty simple mechanical device.
Assuming you have poor braking, or you have tape spilling when you go from a fast wind mode to stop (especially as you get to the end of the tape), the brakes need servicing. Tape spillage is more likely to happen with 7” reels, so I always use a junk 7” reel of tape in case the tape gets damaged as the deck is being serviced.
Generally, tape spillage occurs in rewind mode when the supply reel is almost full, or in fast forward mode, when the takeup reel is almost full. The imbalance in weight between the supply and takeup reels, along with the difference in brakes between the two reel tables is what causes this annoying problem.
Also note that the tape is likely to spill off the reel and wrap itself around the brake drum under the reel table if you allow the tape to run out in fast wind mode, with the transport cover removed.
Comments from Tim:
Before we start, it’s important to note that tape spillage when braking can occur due to a dirty tape path, from SSS, or older shedding tape. Before assuming that the brakes are defective, make sure the entire tape path is clean, and is free of oxide and other nastiness that comes off old tape, and is deposited on the tape path. Don’t just clean the tape heads, also clean the roller bearing, the stationary disc to the right of the playback head that the tape passes over, and all of the other places that the tape touches.
Checking the Brakes
I’ve found that there are any number of causes for tape spillage, and more than one has nothing to do with the braking system itself, specifically the reel motor bearings. Often, the reel motor bearings will be worn, causing extra friction within one reel motor, and while it usually doesn’t affect the fast wind operation itself, the extra friction will throw off that balanced braking system, causing tape spillage.
There’s a couple of ways to check the bearings of the reel motors. The first method is to simply listen to the reel motors as they are in fast wind mode. A low rumble, or grumbling sound coming from the reel motors is an indication of worn motor bearings, and these should be changed before working on the brakes themselves (not covered in this article).
The second method is to see how freely the reel motors spin when the brakes are released by hand, and then spinning the reel table manually with your free hand. Remove the counter belt from the counter, so it doesn’t put any extra load on the takeup reel motor.
With power to the deck being turned off, you can manually push the braking mechanism to the right by hand, as if the solenoid was engaged. This releases the brakes. With your other hand, give each reel table a good spin. Each reel table should spin smoothly, and for about the same amount of time as the other one. If one reel table spins more freely than the other one, then the bearings of that reel motor are bad, and should be changed. There should be no grinding or rumbling sounds that are audible while the reel tables are spinning, either by hand, or when the deck is powered up, and the reel motors are spinning at a higher speed.
Note that early Revox A77 decks have open bearings, loaded with grease. Over time, especially if the deck hasn’t been run for decades, this grease hardens, and the amount of friction of the motor bearings increases exponentially. Change the bearings!
Once the motor bearings have been cleared of any problems, the next step is to remove both brake bands that are around the brake drums. The E clips (also known as C clips) undo with a small screwdriver. Be sure you hold your fingers close to the clips, as they tend to spring far away, never to be seen again, if you’re not careful.
You can undo the spring with a small pair of pliers from one end of the brake band. Leave the spring attached to the one end of the brake band, and remove the brake band. Do this on both reel tables.
Once removed, you’ll be able to see the entire brake drum, and you’ll be able to inspect the brake bands.
It is VERY easy to put a kink in the brake bands! Be very careful when removing the brake bands so that they remain straight. One good twist, and you’ll have a kink in the band.
Below are pictures of the two brake bands as removed from this Revox A77 deck. The bottom one shows some normal wear, the top one shows a kink in the brake band, which should be replaced.
Closeup of the kink in the brake band. It’s also showing some normal wear.
Cleaning the brake drums and the bands
Revox says to use isopropyl alcohol to remove built up grime and dirt from both the brake bands and the brake drums. I didn’t realize how important this is until I fully cured a problematic deck simply by cleaning everything thoroughly. If the surrounding area of the transport is also dirty, use a toothbrush or a paintbrush to remove any excess dirt and crud from the transport.
The brake drum. They can also be reddish in color, depending on the production year. This one is very clean compared to most.
If a brake band is bent, or very worn, it will need to be replaced. The B77 brake bands are slightly different in length than the B77 ones, and they do not interchange. I order new ones from Nagravox in Australia, however there may be other places online to order them from. The same goes for the brake drum linings. Nagravox is a good supplier of these. The old ones generally peel off, then the outside of the drum needs to be cleaned of the old adhesive residue, and the new brake linings will stick right on.
The brake drums/reel table base will come off the deck by unscrewing the single nut (or slot screw bolt in some decks) in the middle of the drum. Don’t lose any washers, and put the drum back exactly as it came off.
The Secret of the Brakes, not covered in the Service Manual
After fighting with brakes for many hours, I finally realized that there’s a crucial point that is mentioned in the service manuals, but is really the key to getting the brakes to work properly.
Both brakes must release and engage at exactly the same time!
Since the brakes are identical and balanced, any shift in balance will result in poor braking and tape spillage. While the service manuals recommend loosening the sub-chassis that all of the brake hardware is mounted to, I’d recommend NOT doing this.
Instead, I’d recommend adjusting the tabs of the brake bands that engage and disengage by the round pins on the brake solenoid levers.
The brakes can drift due to the following reasons:
- slight stretching of the brake bands
- weakening of the brake band springs
- brake band wear
- brake drum material wear
By observing the brake solenoid, you’ll see that at rest, the brakes are engaged fully. When the solenoid is engaged (pulled in), the brakes are disengaged. The crucial point is between the two solenoid extremes, where you need to verify that both brakes are engaging and disengaging at the same time. Here’s how to check that. The travel of the solenoid is about 1.5cm, or ½” or so. With the power off, move the solenoid lever with one hand a bit, while turning each brake drum by hand. Keep moving the solenoid lever ever so slightly until one reel table spins freely, meaning the brakes are disengaged. Verify that the other reel table spins freely at exactly the same time. If not (and they are usually very slightly different), bend the brake band tabs with needle-nose pliers ever so slightly so that both brakes disengage at exactly the same time.
The engagement point between the brake engagement pins and the brake band tabs determines when the brakes will be released. As the brakes age and wear, this point changes minutely, causing the annoying tape spillage. I went as far as changing both the brake bands and brake drum linings, as well as the springs (all purchased from Nagravox) on one machine, only to find out that tape spillage still occurred. The problem always comes back to the point where both brakes engage or disengage.
Since the travel of the solenoid is small, there’s a very narrow window between the brakes engaging and disengaging, depending on the solenoid location. As you manually hold the solenoid lever in place, make sure it doesn’t move as you check the brake engagement on each brake drum, otherwise you’ll need to repeat the procedure.
The brake band tabs are very malleable, and will take a ton of bending back and forth without breaking.
Solenoid Adjustment
The service manual says that the solenoid can be adjusted by loosening the two screws below, and move the solenoid back and forth a bit. I’ve found that this isn’t necessary, and while I’ve played with the adjustment screws, I’ve found that braking problems were always due to something else. As with the sub-chassis adjustment screws, I’d leave them alone.
Brake Springs
Before coming up with the above procedure, I used to change the brake band springs, and on occasion I would add a spring on one side or the other to improve braking. This does help on occasion, but despite ordering over $100 worth of small springs from McMaster-Carr, the only springs I found that had the same pull as the originals were from Nagravox. Over the years, I’ve kept similar springs from other RTR decks, however I’d suggest that if you need to add springs on one side or the other, the brake engagement point between the two reel tables is incorrect.
Again (hammering this point home!), the brakes are a balanced system. The springs need to be equal in strength in order to put the same pressure on the brake bands, ensuring equal braking. I’ve found that the actual strength of the springs is less critical, and as a test, I put on a pair of matched weaker springs than those of the original Revox ones. While it took much longer for the reel of tape to stop from fast wind as a result of these weaker springs, there was no tape spillage, confirming that as long as the spring pressure on each side is equal, there’s a range of spring pressures that will work fine.
Note however, that if the spring pressure is too strong, the solenoid lacks the ‘oompf’ to pull down on the brake levers, and the brakes will never disengage. Been there, done that!
Comments from Tim:
Adjustments of the springs
The springs can be adjusted on the top and bottom side of the brake bands. When at rest, the two metal bars at the end of the brake bands sit slightly wider than the two posts that are next to them, and by turning the brake rotor, the two bars should move side to side, changing which post touches one bar.
If this doesn’t happen, you don’t have enough grip between the brake drum and the brake band. This needs to be fixed FIRST, before messing with the springs.
IF this is working, the top adjustment is for the ‘spooling off’ direction (tape coming off the reel) and the bottom adjustment is for the ‘spooling up’ direction (tape being added to the reel).
The ‘spooling up’ braking must always be LESS than the ‘spooling off’ braking on the other side of the machine. This generally means that the top adjustment of each spring is almost all the way out, while the bottom adjustment is closer to the pivot point.
The bending of the metal bars at the end of the brake bands (if needed) should be done AFTER the point where they touch the pins. Tim does not recommend bending the metal bars (somewhat counter to my own recommendations above), but if they do need to be bent, do it at the end of the bar, after the metal pin that they hit.
Reassembly and Final Check
Once you are satisfied that the brakes are adjusted and working properly, reassemble the deck, and test the deck with 7” reels, running the deck in FF mode, and going from FF to stop mode several times, especially towards the end of the tape. Then do the same tests in rewind mode, stopping the tape several times along several points. Once complete, put the deck together, and make a mix tape!
More interesting links to check out below…
https://www.revox-online-shop.com/ersatzteile/?p=1
