Tuesday, April 6, 2021

T-337 Tape Transport

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 ExpectedMeasured
Trans. Input117VAC 120VAC
Trans. Output35VAC 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 ExpectedRepaired
Trans. Output35VAC 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!

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T-337 Tape Transport

The Bell Sound reel to reels have a modular amplifier that can be electrically disconnected from the rest of the system, leaving a functiona...