Revox B215: 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 impressions 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 (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 with 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 a standard and a precision desoldering pumps and desoldering wick in various widths. To clean the flux, I use isopropyl alcohol and high purity acetone.
Empirically, I found that working with a temperature of exactly 300 °C is safe for these vintage printed circuit boards. I have never lifted any pads and I never wait more than a couple of seconds with the hot tip on any pad. While working on the chassis, I use between 360 and 440 °C. Flux fumes are extremely toxic and should be avoided at all costs.
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 in 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.
If you need to work on the flex boards (e.g. on tape transport and on end-of-tape light sensor) then it is advisable to use the lowest temperature able to melt the solder. I went with 270 &C and I managed to keep the flex board intact; no damage whatsoever.
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.
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