Introduction

I wanted a Studer A721 tape deck but I ended up acquiring a Revox B215. Not that much of a difference but still a domestic machine with studio-class pedigree. While this is one of the (if not the) most robust cassette decks out there, it still requires basic service. Not at the level I plan to do but at least electrolytic capacitors should be replaced. I will be a little bit overreaching in my approach as I want to have the best reliability in time.

It might not be the best sounding deck of them all but I like it. In my opinion an absolute best sounding deck would be a SONY in range TC-K990ES, TC-K555ESA, or TC-K555ESJ. At least these models have the sound signature I like. Ups did I just wrote signature? I feel like I am stepping on moving sands here. So I will stop using epithets and making comparisons and let the engineering side to take over.

Technical Data

This cassette tape machine has the following technical characteristics.

Parts List

I have assembled a parts list for this tape machine. My restoration targets mainly the electrolytic and tantalum capacitors but also some other components. Because I am aiming for maximum reliability and low maintenance, some of the electrolytic capacitors were replaced with film counterparts. Below you will find lists consisting of the various parts in this unit.

The schematic value corresponds to what normally can be found in the electrical schematics but the actual parts could vary. Especially electrolytic capacitor voltages -- derating (!?). The recommended value is what I replaced the former part with. The TC inscription signifies a tantalum capacitor.

The Capstan Motors Control printed circuit board is coded 1.721.260.00 and is located in the left side of the machine mounted perpendicular on the System Control board.

Note that the red lines below don't need to be ordered. I did so because I had these parts available in my parts bin as leftovers from other projects.

The System Control printed circuit board is coded 1.721.220.00 and is located in the leftmost side of the machine, mounted vertically.

Note that the red lines below don't need to be ordered. I did so because I had these parts available in my parts bin as leftovers from other projects. The blue lines below should be ordered only if the existing ICs are bad.

The Capstan Motor Driver printed circuit board is coded 1.021.516.00. There are two independent PCBs affiliated to each capstan motor. No capacitors here but there are a few transistors which could benefit from replacing for future proofing. If you decide to swap these transistors then make sure you double order the parts below.

I decided not to replace these transistors but for the sake of completeness I'll be listing them in the table below. I'll be keeping the red color to signal that these are not mandatory for replacement.

The Input/Output printed circuit board is coded 1.721.270.00 and is located in the rightmost part of the machine. It is mounted horizontally and there are two other big vertical PCBs inserted into it.

Note that the red lines below don't need to be ordered. I did so because I had these parts available in my parts bin as leftovers from other projects. The blue lines below should be ordered only if the existing ICs are bad.

The Record Control printed circuit board is coded 1.721.300.00 and is located in the left part of the rightmost side of the unit, socketed in the Input/Output board.

Note that the red lines below don't need to be ordered. I did so because I had these parts available in my parts bin as leftovers from other projects.

The Noise Reduction System printed circuit board is coded 1.721.290.00 and is located in the right part of the rightmost side of the unit, socketed in the Input/Output board.

Note that the red lines below don't need to be ordered. I did so because I had these parts available in my parts bin as leftovers from other projects.

This is a complex cassette deck with a very service-friendly architecture. But like the majority of top of the line units, the restoration process takes a long time and involves good tools and even better skills.

Restoration

The modular construction of Studer and Revox gear is a joy for any service operation attempt. Whatever you need to fix implies minimal disassembly work. In contrast to Japanese gear that is filled with wires going in all directions, the Studer design philosophy is unique and highly avantgardist for the time. You have the impression that you're working on vintage computing hardware instead. Every logical module has its own PCB. Furthermore the PCB is either socketed in a mainboard or connected with minimal wiring. It took me less than half an hour to completely disassemble every PCB in the B215.

Servicing the PCBs however implies high soldering and desoldering skills and very good tools. Word is that the Studer PCBs are very heat sensitive and also have very thin traces. While this might be (and actually is) true to some extent, I've seen worse. Throughout the entire restoration I have yet to damage a single trace.

The Revox mechanical block is an engineering marvel with the four direct drive motors. The capstan motors are each PLL controlled while the reel motors are microcomputer controlled. But there is a catch. If you don't use these machines for a long time, oil tends to leak from the sintered bronze bearings. Then wow & flutter will increase drastically. And most of these decks suffer from this. The required oil is called Isoflex PDP 65 produced by a company called Klüber and it is still available. Also, the pinch rollers play a very important role in minimizing wow & flutter in this type of mechanical block. So make sure you replace them. Originally Revox used sintered bronze bearings for each of the two pinch rollers. Then they updated to nylon. Any roller you find will be good as long as it is brand new.

The restoration of a Studer or a Revox machine implies almost always electrolytic capacitors replacement and possibly integrated circuits. It is not a rule to replace semiconductors though. Studer used reputable brands for all of the ICs and transistors. But somehow restoration enthusiasts tend to overreact with parts replacements.

I have to say that I haven't tackled the trimmer resistors. As much as I wanted to replace them, I haven't studied what each of them does and if I have the possibility to adjust them to factory settings. Maybe one day I will also do this.

General Considerations

Working on this unit exposes you to electrical hazards. There are lethal voltages inside.
Severe accidents and possibly death by electrocution might occur. I am qualified and skilled at electronics and I have been doing audio gear repairs for over 20 years. If you lack experience, please take these articles as just a knowledge base. Do not attempt to repair something that you cannot handle as there is a high chance of doing further damage while also possibly suffering accidents.

Good tools are a must for a quality restoration. I use eutectic soldering alloy and a temperature-controlled soldering station equipped with various tip shapes. I have both standard and precision desoldering pumps and desoldering wicks of various widths. To clean the flux, I use isopropyl alcohol and high-purity acetone.

Every replacement part is brand new, from a reputable manufacturer, ordered from the U.S.A., Japan, or Germany. In addition, I only use parts that are suitable for specific circuit sections, after inspecting and comprehending the original schematic diagrams. Last but not least, I have years of experience backing up my choices and actions.

Sometimes, I like to take unnecessary actions, such as replacing perfectly working components for futureproofing, or completely stripping down PCBs of parts for a better cleaning. While tedious, I have fun while doing so, and I also learn a thing or two in the process.

Capstan Motors Control Restoration

This is a small PCB that implements the two phase locked loops (PLL) that control the two individual direct drive capstan motors. It contains a handful of ICs and a few capacitors. I decided to replace some electrolytic capacitors with WIMA film counterparts. The old tantalum capacitors were replaced with new Kemet parts while the remaining electrolytic capacitors were replaced with Nichicon PWM parts.

While totally unneeded, I decided to replace all of the ICs with new Texas Instruments branded ones. I also added sockets to each of the integrated circuits.

This is the board prior to servicing.

Everything was replaced and overhauled. Fortunately there is enough space on the PCB to replace the small value electrolytic capacitors with WIMA film parts.

Tilted-angle view.

The solder side looks good. Almost industrial-grade.

System Control Restoration

The System Control printed circuit board contains the different power supplies, the microcomputers that govern the functions of the deck, and various additional circuits such as the IR receiver and the serial link interface.

This board contains some unobtainable ICs and all of them are ESD-sensitive. Be very careful when working on this PCB.

This is the board prior to servicing.

The three microcomputers are removed from the PCB. Their sockets as well. Due to scarcity of these ICs, this operation is not for the faint of heart types out there. Keep in mind that these are CMOS devices, very sensitive to electrostatic discharge (ESD).

All pads have been throughly cleaned. The PCB is ready to receive new precision sockets.

All parts have been replaced.

The three Philips microcomputers are installed back in their new sockets.

Detail on the connector for the flexible printed circuit ribbon that connects to the IR detectors for the reels.

I have used high quality, high temperature axial capacitors for power supply filtering purposes.

The tracks side.

Detailed view on the tracks side. There is one resistor factory-placed directly on the tracks side.

Input/Output Restoration

The Input/Output board acts like a mainboard for the NR-system and the Record Control boards. It is relatively easy to service and doesn't pose any impediments.

This is the board prior to servicing.

And after.

Detailed view. Pretty high parts density in this section of the PCB. I wanted to preventively replace that relay but I had a hard time to source a similar part.

Detailed view of the input/output section.

Unfortunately I forgot to take a picture of the solder side. But all is good there.

Record Control Restoration

There are only a handful of capacitors to change on this board. However I replaced the ICs as well.

This is the board prior to servicing.

And this is the board after the servicing operation. All new film and electrolytic capacitors, integrated circuit sockets and brand new Texas Instruments integrated circuits.

Detailed view. This unit looks very factory-original with the new WIMA capacitors. Why? Because originally they used WIMA as well.

The upper right part of the PCB contains a lot of empty holes for parts with unknown purpose. But other than that I am happy with how I serviced the PCB.

And the solder side. As always, all good. No lifted pad, no uncleaned flux, no brittle solder joints. Everything is ready for many years of continuous playback and record service from now on.

Noise Reduction System Restoration

This PCB is full of capacitors of all types. I decided to replace all tantalum capacitors with WIMA film parts. Fortunately as with all these old STUDER layout designs, the raster for the small value capacitors is 5 mm which means they provided plenty of space to easily accommodate the modern film parts.

This is the board prior to servicing.

The board after the restoration. Or should I better say ... overhaul. Capacitors have been changed and all ICs received precision sockets.

Tilted-angle view.

Detailed view.

On the solder side everything looks great.

Detailed view on the manually soldered pins of the Dolby ICs.

I found this board kind of boring to service. Normally I don't get bored that easily but this one was pretty much linear and repetitive. But I like the end results.

Aftermath

Old parts that have been removed from this unit.

Electrolytic capacitors -- there is a strong electrolyte smell emanating from the pile.

Tantalum capacitors.

Integrated circuits and sockets.

Mechanical Block Restoration

The full direct drive mechanical block of Studer A721 and Revox B215 requires only minimal maintenance. However pinch rollers are very important and even the slightest imperfection causes audible distortions. I am concentrating on the following operations:

• oiling the capstan axles bearings;
• replacing pinch rollers;
• replacing defective IR-barrier sensor;
• cleaning the IR emitter and receiver for each reel;
• cleaning all the dust.

So let's proceed to oil the capstan motors bronze sintered bearings then. For this I used the Isoflex PDP 65 oil which I applied carefully with a small syringe. But first the capstans need to go out. Extra care should be taken to mark the position of the sprint that holds each capstan flywheel in respect to the factory mounted position. This ensures the minimum possible wow & flutter. Also, special attention needs to be paid to removing the axles themselves since these are high precision fine tolerance machined parts. Force them and you might break these tolerances. The result? Increased wow & flutter. Probably irremediable.

The capstan shafts are mounted back in.

The metal springy clips were mounted exactly in the same position they were originally factory set.

The IR barrier for tape detection was not working in my unit. I have one of the first series deck that does not use red light but IR light for the detector. So there is no way to tell by naked eye if it's working or not. I used the well known trick of observing the IR emitter through a digital camera. And there was nothing. Thus, I decided to switch the IR emitter to a deep red LED emitter around 660 nm.

This is the original IR barrier. The LED is easy to remove if you have a lot of patience and quick steady hands. The flexible printed circuit traces don't stand too much heat before they burn out.

The fragile flexible printed circuit traces.

The clear tinted deep red LED emitter is inserted in its place. The beauty plate is mounted as well. Besides the old pinch rollers, the mechanical block is ready to go.

The original pinch rollers in my unit were with sintered bearing hubs. The rubber was very dry and cracked. Wow & flutter is totally unbearable. So time to change them. I have sourced a pair of new sintered bronze bearing ones which I mounted carefully. A tiny drop of PDP 65 oil inside the bronze bearing does miracles in terms of audible distortions. In order to remove the small steel clips you need a set of pliers that can interface the small clips. If you don't have such a tool, I can't imagine a way to remove the clips without damaging them.

The old pinch rollers versus the new ones.

The old ones are sticky because somebody overreacted with the oil. But the shininess says that they have seen a fairly high tape mileage.

The new pinch rollers are mounted in their place.

More detailed view on the tape path.

This is all the work I did so far.

Impressions

I have used this deck for about two months, time during which I made myself a clearer idea on what the B215 can do and how it can sound. Overall I am pretty much happy with it. But let me go on with a deeper analysis.

In terms of tape transport, this is the best deck that I ever had. Period. The smoothness and precision of the mechanism is simply amazing. This machine operates the tape so gently that it is almost unbelievable. I have other decks with quality tape transports but this one is over them all. However I have noticed that I have to clean the pinch rollers as soon as they get dirty. Once they accumulate tape dust, the tape tends to slightly skew off the head, affecting high frequencies output. The audible effect is so small that most people wouldn't even notice this. But a trained ear can easily spot this caveat. Luckily I have only two tapes that leave dust. And both are vintage Maxell UR series with Ferric tape formulation. I don't use them that much. However with the BASF Chrome Extra II, Chrome Super II, and Chrome Maxima II, this deck operates wonderfully. Both in terms of tape handling and sound recording or reproduction.

Speaking about the sound, I find that my B215 machine records very well. There is a slight imbalance in the left and right channels when monitoring the source input. I have noticed that if I lower the input master volume to less than -14 dB then both channels are balanced. Both in input and record monitoring modes. I strongly suspect the digital to analog converters (DAC). I will replace these at a later point in time. Anyway I really like the way this machine records. The recorded program is not as bright as AKAI decks recordings, and not as smooth and laid-back as a TOTL SONY decks recording. But it sounds precise and and I cannot spot any difference between the source and the recorded audio program.

In terms of playback though, the machine sounds authoritative. Like an old Sansui amplifier. It doesn't track azimuth errors as good as an AKAI GX-F71 or a SONY TC-K555ESA but it sounds clean. Even though on badly recorded cassettes it sounds a bit muffled. I have heavily used it for tape playback in the last couple of weeks and I enjoyed every single second of operation. Also, wow & flutter distortions are very low. I can't spot any audible differences between the B215 and the TC-K555ESA tape transports.

What I don't like at all is the fact that I cannot rewind completely to the beginning of tape. I mean, sure, I totally agree that the light barrier stops the tape when it reaches the transparent leader tape. But a function to press again on rewind and slowly, applying low torque, rewind to the beginning of tape would've been great in my opinion.

As a final statement I'd say that this deck is a good addition to my audio system.

Replacing the DAC Integrated Circuits

I decided to change all Analog Devices branded ICs in this deck in order to correct the source monitoring and recording channel imbalance. This operation is most probably overkill and I should've replaced only the faulty IC. But since one IC was (partially) failing, I thought the others might follow sooner or later. Thus, I sourced these new ICs.

Which I used to replace the existing ones on the Record Control PCB.

And the Input/Output PCB.

There is one smaller DAC integrated circuit on the System Control PCB. I replaced this one as well.

While testing the machine I observed that monitoring a source program and even recording it is now flawless. Both channels sound equally good without signal deterioration.

Now I can set the source input signal attenuation to whichever value I want from the the entire available range without any loss on either left or right channel.

I (re)placed another brick in the wall. I have also updated the parts list in Article #3.

Please note that all the work presented on this site is non-commercial. This is my hobby, and I do this in my spare time. Since I freely share my knowledge, if you like my work, please consider helping me buy a transistor or a capacitor for my projects.

Thank you!