Honest Repairs, Maintenance & Renovation #1: R&S BN1521/2
INTRODUCTION
The radio measuring instruments developed during the 1950s and 1960s represent the golden age of analogue electronic engineering. Designed long before digital technology became the standard, these instruments combined precision mechanics, vacuum-tube electronics, and outstanding manufacturing quality to achieve an exceptional level of performance.
Built for laboratories, research institutes, telecommunications, and military applications, they were expected to operate reliably for decades. Their robust construction, carefully designed mechanisms, and service-friendly layout explain why many of them remain fully functional even today.
Restoring such an instrument is about much more than cleaning and repairing old equipment. It is a way of preserving a remarkable piece of engineering history and paying tribute to the people whose knowledge and craftsmanship made these instruments possible.
Today’s story is about the restoration of a Rohde & Schwarz BN1251/2 Selective Microvoltmeter, a project I recently completed.
This impressive 27 kg precision instrument was carefully opened and completely disassembled. Years of accumulated dust and dirt were removed, the drum scale assembly was gently vacuumed, and every accessible area was meticulously cleaned using cotton swabs where necessary.
In the following photos and accompanying text, I will show the restoration process step by step and explain the work that was carried out.
Rohde & Schwarz USVH BN1521/2 Selective Microvoltmeter (10 kHz – 30 MHz)
This instrument had spent nearly 30 years stored on the floor of a dry basement. Although the storage conditions were relatively good, it had become heavily contaminated with dust and dirt over the years. Before any electrical testing could begin, a thorough cleaning of both the exterior and interior was essential.
STARTING THE CLEANING PROCESS
1. Preparation
Wise people—my parents among them—used to say that good preparation is 90% of the job. I have found that to be absolutely true. Preparation includes gathering the necessary tools, attachments, accessories, and cleaning materials that make the work easier and more effective.
For external cleaning, I use a standard vacuum cleaner fitted with a soft brush attachment. After finishing the job, I always clean and wash the brush with water before storing it.
To reach narrow gaps and hard-to-access areas, I made a simple mini dust-suction nozzle. It is attached to the vacuum cleaner using a nozzle adapter from an air pump for inflatable beach toys. I secure it with ordinary adhesive tape. If the diameters happen to match, the nozzle can simply be pushed onto the vacuum hose without any modification.
I also keep a good supply of non-aggressive wet wipes and various technical cleaning swabs for detailed work.
2. Initial External Vacuum Cleaning
Using the soft brush attachment, I carefully vacuum the entire exterior of the instrument to remove loose dust and dirt.
Work gently and avoid applying pressure. Pay particular attention to the areas around the knobs, push-buttons, ventilation slots, and other openings where dust tends to accumulate.
Image 1a. Measuring System – Basic View
Image 1b. Trimer Adjusting Stick / Use Only for Professional Calibrating Process.
3. Disassembly and Initial Cleaning
I remove all covers from the chassis before beginning the detailed cleaning process.
For general cleaning, I prepare a cup of warm soapy water. This solution is intended only for external use and only in areas where there is no risk of damaging electronic components.
For stubborn dirt, I sometimes use a mixture consisting of:
- warm soapy water,
- 6–7% denatured alcohol,
- 2–3% cleaning vinegar.
Take great care to prevent any liquid from reaching the printed circuit boards or electronic components.
I carefully clean every accessible square centimetre of both the inside and outside of the chassis.
Image 2. The image above shows the PCB and Components only vacuum-cleaned.
Image 3. My homemade mini dust-suction nozzle.
4. Cleaning and Lubricating Mechanical Components
The instrument contains numerous shafts, bearings, gears, and other moving mechanical assemblies.
Lubricate only those parts that originally require lubrication.
Important: Do not lubricate the steel drive wire or any surface that comes into contact with it. The entire drum-scale drive mechanism is designed to operate dry.
Image 4. Cleaning and Lubricating Mechanical Components
5. Cleaning Vacuum Tubes and Passive Components
Cleaning the vacuum tubes and passive components is one of the most time-consuming parts of the restoration.
Carefully remove the shield from each vacuum tube. Using technical cleaning swabs that are only slightly dampened with clean water, gently remove the accumulated dust from the glass envelope.
Avoid excessive moisture and unnecessary handling of the tubes.
6. Cleaning the Printed Circuit Boards (PCBs)
Cleaning the PCBs consists of two separate stages.
I.) Dust Removal
Take the chassis outdoors or to a well-ventilated area and use compressed air with a suitable nozzle to blow away all loose dust.
A soft toothbrush or similar brush can be used together with the compressed air to remove stubborn deposits.
II.) Detailed PCB Cleaning
The second stage is careful cleaning between the components and in all accessible open areas of the PCB.
For this work, I use numerous technical cleaning swabs moistened with isopropyl alcohol, taking great care not to damage delicate components.
Image 5. Before detailed cleaning of Coaxial cables, tubes & shields.
Image 6a. After detailed cleaning of Coaxial cables, vacuum tubes, and their shields.
Image 6b. After detailed cleaning of housing, vacuum tubes, and their shields.
7. Cleaning the Front Panel
The scale assembly is protected by a glass window, while the front panel contains delicate printed markings.
To avoid damaging the lettering, I use only warm soapy water with a small amount (2–3%) of denatured alcohol.
Every knob, push-button, pointer, and scale marking deserves careful attention during cleaning.
One of my favourite moments is comparing the very first photograph of the instrument, covered in dust after decades of storage, with the final photograph after the restoration has been completed.
The entire cleaning process is deeply satisfying. Watching a precision instrument regain its original appearance is rewarding in itself. Yet the most memorable moment comes after the restoration is complete.
As evening falls and the instrument is powered on for the first time, the warm amber glow of the vacuum tubes shines through the freshly cleaned glass. It is a quiet reminder of a remarkable era, when precision laboratory instruments were designed and built to serve for generations. That gentle light creates a unique atmosphere of warmth, nostalgia, and quiet admiration for the engineers who made such craftsmanship possible.
In the following section, I will share photographs taken during the various stages of the cleaning process.
Image 7. Almost Done
Image 8. Finished the process of cleaning
The Device is Available in Bulgaria, EU.