Sansui SC-1110: Restoration
Cassette decks from the 1970s are pretty much built for life. They don't have the specifications of a 1980s or 1990s machine, but they sound interesting. The cinematic transmission is mostly based on belts, idler tires, and felt clutches. If well maintained, they can go on forever. The DC motors are simple and still available in new condition. The electronics is generally speaking typical for that era, all transistor-based. Sometimes there are also ICs present, especially for Dolby processing sections.
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.
G-1218 Power Supply Circuit Board Restoration
This board is located in the upper-left part of the steel chassis. It's fairly easy to remove as there are only a few wires attached. There is one connector that interfaces to the motor control circuit board. It would've been nice to have connectors for all wires. The PCB is fixed with three screws. First, I removed the motor control board and next I desoldered all wires. The board was then free to be taken to the workbench.
The electrical schematic diagram shows a simple implementation of a classic series regulator circuit based on a Zener diode and a power transistor. There is an additional row of rectifier diodes and a ripple filtering capacitor that provides clean DC power for the motor control board.
Initial condition of this board. Somebody was here before; questionable quality Chinese capacitors signify at least a careless technician.
And what do you know, four lifted pads in a row.
First, everything had to go off the circuit board. I cleaned the PCB thoroughly. I also got rid of the corrosive glue.
On both sides. Unfortunately there is another lifted pad next to one part of a fuse holder. And an ugly longitudinal scratch. Nothing that I cannot repair, though.
Let's take a closer look at the damage.
I'm wondering why somebody would want to remove this part. In addition, why make that ugly scratch?
Next, I cleaned all mechanical parts, installed new Kapton tape under the connector, replaced all rectifier and Zener diodes, and added new Nichicon capacitors. For better thermal dissipation, I decided to reposition the series regulator transistor somewhere on the steel chassis.
The new Zener diode received glass standoffs.
While there was nothing electrically wrong with the old fuses, I decided to replace them as their terminals were very corroded.
Finally, this is the solder side, with all the repairs carried.
Fixed all issues in this area.
As well as this area.
The power supply is now restored. Let's advance.
G-1230 Lead-in and Shut-off Circuit Board Restoration
This is an interesting implementation of an electro-mechanical auto-stop feature, mainly based on a rotating magnetic switch. This circuit also implements the tape lead-in functionality by means of a simple timing constant obtained from charging of a capacitor through a resistor. While the lead-in function is quasi-questionable, the auto-stop functionality implementation is elegant in its simplicity.
This circuit board interfaces the power supply circuit board and the main circuit board through connectors; the same type used in the Sansui AU-20000. In addition, there are a few connection wires directly soldered on pads, located on the PCB outer edges.
This is the initial situation. Thankfully, this circuit board escaped butchering as its position is a bit awkward for non-skilled technicians.
A view from another angle.
I removed all components and cleaned the PCB.
The solder side is in a pretty good condition. No lifted pads and no scratches.
Making progress. I installed new Kapton tape under the connectors and new bridges. Then, I cleaned the original resistors and soldered them in their respective positions.
A view from a different angle better shows the Kapton tape. Very good for preventing the solder to climb on the connector terminals while soldering.
All components are soldered. I added a 470 uF / 16 V capacitor in the previously vacant C606 location. Examining the printed circuit board reveals that C606 is connected between terminals 5 (switched power supply rail) and 6 (ground). These are responsible for providing DC power to the motor. Since terminal 5 connects to terminal 7, a side effect is that once the PS701 relay coil is energized through TR606, upon auto-stop mechanism sequence, the small charge stored in C606 will keep the relay coil energized for a few milliseconds more. This effect is negligible compared to the additional ripple filtering benefit added to the DC motor.
C602 is not critical at all. It is part of the timing constant for the tape lead-in circuit. I specified a Nichicon PW series capacitor in the parts list but I ended up using a KZ series capacitor instead. It's an overkill 10 uF / 100 V rated part. But I just had it in my spare parts box without any other immediate use. I decided to use it in this project.
A detailed view shows that the reverse-biased suppressor diode for the relay coil received glass standoffs. Even though this diode does not get warm at all, I kept a consistent look throughout the various PCBs.
Resistor R614 is used for the tape counter illumination. That small bulb requires a voltage that is not readily present in this unit. Such an implementation is highly inefficient in terms of heat dissipation. I have plans to replace the incandescent bulbs with LEDs (yes, that's a first on Vintage Audio), but for now, I will keep the original 100 Ω resistor and tape counter bulb.
Here is a picture of the solder side after my restoration.
That's it for this PCB.
G-1217 Main Circuit Board Restoration
A classic '70-'80 Japanese cassette tape deck implementation: all circuitry on the same PCB, electrically symmetric but the layout is not. There is a forest of wires connected to this circuit board. Some wires have connectors while others are soldered directly. In addition, another wire loom connects various circuit sections locally in a complicated network. Thankfully, the PCB is clearly marked and each wire is color coded. It took me a while to free this circuit board.
The electrical schematic diagram shows the various construction blocks in a clear fashion. The first stage represents a very simple magnetic head preamplifier that doubles as a microphone preamplifier. It is followed by a single transistor preamplifier stage that couples directly to a textbook (exactly the schematic from the datasheet) implementation of a Dolby B processor based on the NE545B integrated circuit. The remaining blocks represent the record, VU meters, and headphone amplifiers. The bias oscillator, built around a specific component (XO601) and record indicator are depicted in the lower section of the schematic diagram.
This is the whole PCB. Although not visible in the picture, it is dusty. The former technician replaced a couple of transistors with new components. Otherwise, the board is in perfect condition. It is a bit warped but I've seen a lot of warped Sansui boards.
Let's start with the right channel magnetic head preamplifier section. I removed all parts and cleaned the PCB.
I installed new Kapton tape under the tape head connector.
First, I soldered all of the resistors, followed by the various film capacitors. Second, I installed the new electrolytic capacitors. Finally, I added the small-signal transistors, all hand-picked and with the same amplification factor. As always, I used glass standoffs where appropriate. As per my parts list, I used bipolar coupling electrolytic capacitors in the signal path.
Now, my attention shifts to the right channel preamplifier and Dolby B processor.
I removed all components and then I cleaned the circuit board.
On both sides.
Next, I soldered all components back along with the new capacitors.
Following up next is the last section of the right channel signal processing circuitry. This part of the PCB implements the record amplifier, the VU meter driver, and the headphone amplifier.
Again, stripping down all the parts first. The same mixture of isopropyl alcohol and acetone makes miracles on the dust caught within the old flux.
Tracks side looks great. Again, we can see Sansui engineers knew how to design PCBs, almost 50 years ago. Ground tracks from local ripple filtering capacitors emerge radially. In some places they even used guard tracks. Although the effect of each of these engineering bits and pieces is questionable or even measurable, together they contribute to the quality of the audio reproduction.
Both old resistors, coils, and film capacitors and the all new hand-picked hFE matched transistors and capacitors are in position.
Here is a picture with the PCB, halfway rebuilt.
And a view of the restored circuit board section, from another angle.
Let's move to the left channel circuitry. First, the magnetic head preamplifier.