How to Repair the Ten Tec Century 21 – A Great Classic Transceiver

Buying a “New” Radio

I look at Ebay from time-to-time, and lately I’ve been having a little too much fun. A Ten Tec Century 21 CW Transceiver (model 570) attracted my attention recently. Released in 1975, the “570” was a solid-state radio intended for Novice hams and covered five bands: 80, 40, 20 (and with accessory crystals) 15 and 10 meters.  Its “Double Direct Conversion” receiver could detect SSB and CW signals, but had only about 30 watts output. It was being sold as a “part only” device with at least two problems. The main tuning dial was frozen and it didn’t seem to have any output on transmit. Although the shipping was a little expensive, the auction price was modest price given the radio’s cult status with CW aficionados. I had always had a soft spot for Ten Tec products.

My History with Ten Tec

Back in the 80’s, I lusted after the Triton IV digital. It was expensive even though I had a good job engineering at a local TV station. Hoping for a deal, I went to the local dealer and mentioned my interest. He wondered if I could help him repair radios. He was both an amateur and CB radio dealer, and had a backlog of service tickets. Before long the deal was struck: if I would work on Saturdays repairing radios, the Triton IV was mine. I don’t remember the time frame, but I probably worked at least several months. I later traded that radio for an ICOM 720A, which I still have, but I always regretted selling the Triton IV. I worked a lot of DX on that rig.

Solving the frozen dial problem

Fast forward 40 years, and the Century 21 arrived from the Ebay seller. He had packed it very carefully, and the radio was in good cosmetic shape given its age and Ten Tec’s use of contact paper for exterior finishes. I opened the top cover and could see the reason for the frozen dial immediately: the plastic spur gear that connects the Permeability Tuned Oscillator (PTO) to the frequency dial had split in two. I took the front panel off  and I had the part out in no time. There were two clean breaks and both halves were complete. I could repair it.

A word about the plastic used for these gears. In the past, I’ve tried repairing them with super glue and epoxy with mixed results. Neither adhere well to the plastic. I recently discovered a product that works though: Loctite Epoxy Plastic Bonder. It adds a solvent that attacks the plastic, allowing the epoxy to bond. The result is strong, machinable and permanent. About 30 minutes later, the gear was as good as new. There was a some squeeze out in the center hub and around the gear teeth, but a drill bit and an x-acto knife cleaned that up.

Documentation for the Ten Tec Century 21

Manuals for these radios are not hard to find, with several helpful hams posting PDFs from their personal copies. I was able to find a copy at rigreference.com. The manuals  are very helpful with more than just installation and operation. They discuss the radio’s theory of operation and how each section works in detail. Voltage readings and schematics are also included. I was grateful for this help because I knew that the “no output” problem was a little more challenging.

Diagnosing the symptoms

I learned years ago that no matter what the problem, it’s a good idea to check power supply voltages first. The Century 21 is powered by an internal, transformer-isolated  power supply that supplies a regulated 12.6 volts. It’s got a beefy transformer and the supplied voltage was correct. Although there was audible white noise when it was powered, suggesting that the audio stages were working, connecting an antenna didn’t change the white noise much — even when changing bands or the PTO setting. Pressing the “set drive” button did mute the radio, and advancing the drive control did increase the reading on the “input power meter”, but not very much. Checking the output with an oscilloscope didn’t show much output, and it wasn’t anywhere near the expected frequency.

We have a diagnosis (With a little help from my friends)

There are several good resources online and I found specific help on the QRZ.com Century 21 Restoration Forum. A couple of things stood out: these radios often had bad final output transistors, and also often needed to have the PTO rebuilt. Thinking that the finals might be bad, I started working backwards from the the final amp. Checking the output of the “Low Level Driver” board, I was surprised to see very little output and at the wrong frequency. Going back to the “Mixer” board, it’s output also had an unrelated frequency at the output. This is a simple radio (something I love about Ten Tec’s products of the era) and there were only two other possibilities. Either the PTO was off frequency, or the Local Oscillator (also on the “Mixer” board) wasn’t right. It turned out to be the PTO. It was running, but at about 2.87 MHz, and it didn’t change when the tuning dial was moved.

The PTO is the only module in the radio where voltage readings are not  provided. The QRZ forum and a couple of YouTube videos encouraged me to dive in though, and  I decided to remove the PTO from the radio. It would be easier to work on, and should I need to rebuild it, I would have to remove it anyway. Supplied with power from a 9-volt battery, I looked at the output on a scope. Nice sine waves at about 2.87 MHz. No change when I turned the tuning knob, even though I could see the ferrite slug moving in and out of a coil. The frequency should have been changing between about 5 and 5.5 MHz. Was something wrong with that coil? I couldn’t see any damage, and the wire leads seemed firmly attached to both the coil and the circuit board.

Looking at the schematic for the PTO, I could see a continuity check of the  tuning coil  (“L3″ on the schematic) wouldn’t tell me much since it was connected in parallel with”L2”.  “L1” and “L2,” are connected in series and used to set the overall frequency and band spread of the PTO.  I unsoldered one lead of “L3” and tested it with an ohmmeter. No connection. That explained why there was no frequency change when the ferrite slug moved and why the frequency was so low. Without “L3” in parallel with the other two coils, the effective inductance would be higher, resulting in a lower oscillator frequency. Getting the coil out of the tiny box involved removing the tuning mechanism. I was going to rebuild the PTO after all.

Repair and Rebuild

I’ll spare the tedious details of repairing the coil except to say that the coil dope was difficult to remove. I ended up using a small file and an x-acto knife to abrade and chip enough off to reconnect the broken lead. Also, I had to repair both leads. Both were well connected to the coil dope yet had failed underneath. I haven’t seen that before.

There are two good videos from K5CYF showing how to rebuild the PTO and an excellent Web page from N5ESE (captured in 2005 on archive.org) with more detailed instructions. Both mention the use of a Ten Tec “PTO Rebuild Kit” which may no longer be available. With their help, I was able to reassemble my PTO using the existing parts. The good news? The PTO now worked, and was producing RF in the expected range from 5 to 5.5 MHz.  The vernier also seemed to work reasonably well. BTW, this PTO design was used in several products of the era, so knowing how to service them is useful for the Ten Tec collector.

Success! (or Back to the Future)

After reinstalling the PTO, the radio had output. 80 meters was a little low at 12.5 watts out, but the other bands came close to or met original specs of 25-30 watts out. The PTO was surprisingly close to frequency after having major surgery, requiring only a slight touch-up. The receiver was originally rated at 1 uV or less for 10 dB S+N/N, and I could indeed hear signals from a test generator set to 1 uV.

How is it? Like a trip down memory lane — and it has a few quirks. The direct conversion receiver hears signals on both sides of zero beat meaning that a CW signal will be heard in two places on the dial. This takes a little getting used to for us modern Superhet owners, but works fine in practice. The three section audio filters (2.5, 1 and 0.5 KHz) work well for voice and CW and I remember their audio quality fondly from the Triton IV. Here’s a sample receiving W1AW. First with the 2.5 KHz filter, then switching to 1 KHz and finally .5 KHz.

Best of all, and perhaps why they’ve become a favorite of CW ops, is the smooth, full break-in CW. This was novel in the late 70’s and sold quite a few radios. And as we’ve seen, it’s nice to work on a radio that it’s possible to understand. If you like CW, and have a chance to get one of these gems, don’t pass it up.  I think I’m going to have some fun with this radio.