Someone says it is garbage, I say it is a treasure.
Last week, I attended a Hamfest in Malton, and since it was still early, there weren't many people around. I took my time browsing through the various stalls, hoping to find something interesting. Unfortunately, I didn't come across anything that caught my attention. Feeling a bit disappointed, I decided it was time to leave and head towards the cherry picking area.
However, just as I was about to leave, a little gray box grabbed my attention. It turned out to be an old Reel to Reel Recorder, originally manufactured by Akai but re-branded as Roberts. The price tag displayed was 98, but I decided to try my luck and negotiate. After a brief conversation with the seller, they agreed to let go of the Akai recorder in its current condition for a mere two banknotes featuring a portrait of Queen Elizabeth.
It was quite a surprising turn of events, and I couldn't resist taking advantage of such a deal. Now I have a vintage piece of audio equipment to tinker with, and I'm excited to explore its capabilities.
This is a Roberts 720 R2R. Someone says Roberts 720 is Akai M7 but it is not. I am interested the old Akai/Roberts R2R not because of the tape playback but all about the little tube amplifiers. The condition of this little R2R is in fair condition. It seems that it had sat in a damp basement for decades. I pull out two little tube amps from the wooden box at once and sun-bathing them for a day hoping to remove the mildew smell. The smell was pretty strong and my wife wouldn't allow me to take them into the house.
I took into consideration the age of these vintage tube amplifiers, realizing that they were over 50 years old. To ensure their optimal performance and longevity, I made the decision to replace all the electrolytic capacitors. Since I had no intention of using them as a tape recorder, I also removed the bias circuit and some network circuits that were specific to tape recording functionality.
In my exploration of the circuitry, I compared the actual circuit of the Roberts 720 with the service manuals of Akai M7, M8, and Roberts 990/997. Interestingly, I discovered that the Roberts 720 shared a closer resemblance with the Roberts 990, rather than the Akai M7. This correlation further confirmed my belief that the Roberts 720 was a unique model with its own distinct characteristics.
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| Note: R54 is 100K actually. The hum balance for the 6BQ5 is not connected to ground but to Pin 3 of 6BQ5. |
I have made modifications to the Roberts 720 R2R to enhance its functionality for playing vinyl records. One of the modifications I made was the addition of a passive RIAA network between the 6267 and the 12AX7 tubes, replacing the original 12AD7 tube. This modification allows for proper equalization of the audio signal from the vinyl records.
I've made efforts to maintain the originality of the Roberts 720 R2R while identifying areas for improvement. Based on the observations, here are the modifications I've made:
Grounding: I have implemented a single-point grounding system to ensure that all input jacks are isolated from the case. This helps to minimize potential interference and improves overall signal integrity.
Filament Wiring: To reduce interference, I have twisted the filament wiring. This technique can help minimize noise pickup and ensure cleaner audio signals.
Jack Replacement: I have replaced all the jacks with RCA sockets, except for the speaker out. This modification allows for easier connectivity with external devices using standard RCA cables.
Input Selector: I've added a selector switch that enables you to switch between Phono and Line inputs. This enhancement provides flexibility in choosing the input source and expands the usability of the R2R.
Here is the arrangement of the RCA sockets on the face-plate.
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| Top left switch: input select ; Top right : pre-out; Bottom left: Phono/Line In |
My approach is to maintaining authenticity while making necessary modifications to the Roberts 720 R2R. Re-gutting the e-caps with modern capacitors allows you to preserve the appearance and value of the equipment while ensuring reliable performance.
Finding original e-caps for vintage equipment can indeed be challenging, and regutting them with modern capacitors provides a practical solution. This way, the equipment retains its visual authenticity while benefiting from the improved reliability and availability of modern components.
As for sourcing bumble bee film caps, these can be difficult to find due to their vintage nature. However, there are alternatives available in the market that can provide similar sonic characteristics. Exploring reputable suppliers and discussing with fellow enthusiasts or specialized forums can help in finding suitable replacements or alternatives that maintain the desired sound quality.
My dedication to both the appearance and performance of vintage audio equipment is commendable.
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| Two 18uf/400V caps were used. Please don't increase the value of these cap especially the one connected to 6X4 (max. 40uf). If not, the 6X4 will be damaged by high current in-flux when switching on . |
Here is the finished look.
Note: The passive RIAA network was replaced by active feedback network, much closer to theoretical numbers.
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| I slightly increase the value of the cathode by-pass caps and the other two power caps due to the fact that I don't have the exact value. |
Utilizing Oscon capacitors in the cathode bypass is a great choice, as they are known for their excellent performance in that position.
Repurposing the tape speed tone selector as the on/off switch for the VU meter is a clever modification that adds functionality while maintaining the original aesthetic of the device.
Reusing the existing wires, as long as they are in good quality and condition, is a practical approach that helps preserve the authenticity of the equipment.
Rescuing the handle, foot, back cover, and badge for the new case is a thoughtful touch that contributes to the overall authentic look of the modified equipment.
The measurement of the frequency response, ranging from 40Hz to 20,000Hz for the line-in signal, indicates a great performance in terms of sound reproduction. The fact that the tube amp delivers a sweet and musical sound is certainly a testament to the success of the modifications.
Overall, it sounds like I've achieved excellent results in both the functionality and sound quality of the modified Roberts 720 R2R.
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