The Bell Sound reel to reels have a modular amplifier that can be electrically disconnected from the rest of the system, leaving a functional tape transport mechanism. Since I need to repair the bent sheet metal on the amplifier anyway, I removed it completely. This has the added benefit of making the tape transport much easier to investigate.
I started with testing out the transport mechanism. It is clear that it needs new felt pressure pads all around and a new counter belt. After oiling up all the motor bearings, everthing else seems to work properly with me physically holding down the auto-stop switch. Even the counter came alive after I lubricated up the little bearings and moved everything back and forth a bit. It still could use a fresh belt, but it works!
Checking voltages on the transport power supply:
| Status | Expected | Measured |
|---|---|---|
| Trans. Input | 117VAC | 120VAC |
| Trans. Output | 35VAC | 30VAC |
| Standby | 25V | 23V |
| Run | 30V | 26V |
| Fast | 30V | 28V |
Note that the output from the transformer is less than exepcted, and consequenly all of the DC values are less than expected.
Although everything on the transport appears to work, a full bridge silicon diode rectifier will be a nice replacement for the aging (but neat looking) selenium rectifier. The newer silicon rectifiers are small enough that I have room to keep the selenium rectifier bolted in place, but electrically disconnected.
 |
| Note the black silicon rectifier hidden behind the large blue selenium rectifier, attached with a slightly longer machine screw. |
Investigating the electrics, there are two large CDE "Tiny Chief" film capacitors. In my limited experience, these plastic molded caps are prone to failure. This really isn't a high risk component like a coupling capacitor, but I am going to replace it anyway to avoid noise from the motors and problems with the dynamic breaking system. I was able to find some TRW branded film capacitors on ebay of the correct value which will make suitable replacements. Those are expensive, and a bit on the old side for NOS, but it seems too appropriate to pass up the opportunity to put TRW capacitors in a Bell TRW audio component.
 |
| Silicon rectifier is hidden while TRW film capacitors look right at home. |
After the changes, I measured:
| Status | Expected | Repaired |
|---|---|---|
| Trans. Output | 35VAC | 30VAC |
| Standby | 25V | 25.3V |
| Run | 30V | 27V |
| Fast | 30V | 60.7V(30VAC) |
The silicon rectifier brought the voltages up about 2 volts, which is a nice improvement. I am a little confused at the high speed (Fast) voltages though. I don't see how the voltage would have doubled to 60+volts, but I am pretty sure that is because there is no load on the rectifier since the dynamic braking is disabled at high speed. I will do some more testing to see if I need a resistor in series after the new rectifier.
After a new counter drive belt and new felt tape pressure pads, the tape transport is ready to test out!
