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	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!

Bell T-337 Reel to Reel Restoration

After a long search, I recently was able to find a Bell TRW reel to reel tape deck that matched my Bell TRW 2420 amplifier and 2421 receiver. This model appears to be a T-337.

Unfortunately, the device was in bad shape and arrived in even worse condition after being dropped on my driveway by the delivery man. I have some serious concerns with the unit, both cosmetic and electrical:

• missing head cover
• missing control cover plate
• missing reel retainers
• missing all tubes
• original capacitors

Not to mention, the dirty, hardened rubber everywhere, the strong smell of what I would presume is a blown selenium rectifier, and bent chassis from the driveway drop.

This is going a long rehabilitation and I will go step by step in a series of blog posts. Typically, I use these posts to track my progress, retain reference photos, and document my understanding of a circuit for my own later reference and maintenance. But, I hope that someone else may find my work useful to restore their own reel to reel player. If you do, please leave a comment. I would love to see your project!

Bell TRW 2420 Amplifier Parts Cross Reference

Here is a list of modern parts replacements I have used on the Bell TRW 2420:

ITEM No.	Bell Part No.	Description	Modern Replacement	Notes
C1	ASTRON 23B87	40/40/20 @ 450V Electrolytic Capacitor	CE Manuf. C-EC4020X2-525	Higher voltage rating, extra 20μf capacitor. Same pin order.
C43	RMC .02 Z5U	.02μf Ceramic Capacitor	RIFA PME 265 MB	Y2 Rated Safety Capacitor ensures fail open so that chassis cannot be hot.

Bell TRW Audio Blog Debut

This blog is dedicated to setup, repair, and maintenance of vintage audio equipment, with a special focus on Bell Sound/Bell TRW component systems. I will be using it to record my projects, experiments, and discoveries. Hopefully, I can help some other people along the way.