Ten-tec Century 21 restoration

Discussion in '"Boat Anchor" & Classic Equipment' started by HAMHOCK75, May 9, 2016.

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  1. HAMHOCK75

    HAMHOCK75 QRZ Member

    I recently wrote on another thread that I was interested in restoring a Ten-tec Century 21 but found it difficult to find one for a reasonable price. A QRZ.com member, I will call him Matt", made me an offer I could not refuse. Matt requested I post a thread so he could follow the progress as this was his first rig.

    A brief history. This C21 has been stored for 20 years or more at Matt's mom's house. It was thought to have a bad output amplifier. It arrived last Thursday. The following was found on initial inspection.
    1. The PTO ( permeability tuned oscillator ) was frozen
    2. The power supply was dead
    3. The power amplifier was also dead.
    The power supply was up and running after the 2N3055 transistor on the rear panel was replaced ( $0.95 ). Turning the C21 off causes the +12.8 VDC output of the 2N3055 to drop immediately but the filter capacitor ( C1 ) drops from +23 VDC to about +13 VDC, staying there for a long time. There is no resistor to bleed the charge so it was done manually.

    The power amplifier is a module which is easy to remove. There are four RF transistors, the input being a pair of Motorola MRF472's and the outputs are a pair of Motorola MRF475's. The 472's were good but both 475's were dead.

    Bought a pair of MRF475's off eBay ( $20 for the pair including shipping ) but they will not be here until later this week.


    The PTO was the most difficult to tackle. This is a pretty good description of the PTO issues by N5ESE,


    Here are the Ten-tec instruction,


    The bad news is that Ten-tec appears to be in the process of being acquired.


    Doesn't appear to be any way to order parts at present.

    The PTO rebuild kit was not needed in this case. None of the parts that went bad or are wearing out are part of the kit.

    In the picture from N5ESE below,


    the Core Drive Rod ( K ) and the Pistol ( L ) had been glued together by the old grease. After removing the old grease, the PTO turned fine. The most worn part was the Wedge ( O ) which is not part of the kit. Spring ( P ) presses pretty hard against that little Wedge which is plastic.

    PTO alignment went smoothly. There are two coils, L1 and L2 on the same coil form accessed through one hole in the top of the PTO assembly. L1 adjusts frequency, L2 ( the core at the top ) adjusts span. It was pretty easy to get within 1 KHz at 5 MHz and 1 KHz at 5.5 MHz with a counter. Specification is within 5 KHz at both ends of the PTO range.

    A part that broke is shown in the N5ESE photo below,


    The upper gear seen in the photo was cracked pretty severely. It would have been better to replace it but for now, the part has been plastic welded together with strands of stainless steel wire embedded in the plastic.

    The receiver portion is now working fine. The PTO has been aligned. The PTO drift after an hour warm up was measured. It started at 5.100069 MHz, drifted up to 5.100101 MHz ( about 32 Hz ) after 10 minutes, stayed there at 20 minutes, dropped down to 5.100095 ( 26 Hz ) at 30 minutes, and 5.100084 ( 15 Hz ) at 45 minutes. It would move down in frequency if either RF/AF volume was increased by 5-8 Hz.

    More later.
    Last edited: May 9, 2016
    KL7KN, NL7W, KE4OH and 3 others like this.
  2. HAMHOCK75

    HAMHOCK75 QRZ Member

    C21 Service Manual


    276 Crystal Calibrator

    Matt also sent the Ten-Tec 276 crystal calibrator with the C21. It's a bit unusual in that it is pulsed on/off. You can hear it working on this video.

    While waiting for the output transistors to arrive, the circuit of the 276 was examined. What is is not described in the video is that the calibrator plugs into a 3 socket connector on the back of the C21 which supplies +12 VDC, ground, and a return wire for the calibration signal which is introduced after the solid state PIN antenna switch and before the receiver input mixer via a 20 pF capacitor. The 276 consists of a single transistor oscillator, a 7400 quad, NAND gate, and a 7476 dual, J/K flip/flop. Two of the NAND gates are wired as inverters to create an RC oscillator which goes to the input of a third NAND gate. The other input to that NAND gate is the 100 KHz from the crystal oscillator. The RC oscillator has an adjustment potentiometer so the duty cycle can be adjusted to 50%. The frequency of the RC oscillator is about 6 Hz. The pulsed 100 KHz output from the third NAND goes to the clock input of one of the J/K flip flops in the 7476. The 100 KHz NAND gate output also goes to the the calibrator output via a voltage divider composed of two 2.2 K resistors so the output to the C21 at 100 Khz is ½ the voltage at the NAND gate output. When the 7476 is enabled by the selector switch 25 KHz/100 KHz, the divide by four output of the two 7476 flip/flops is coupled via a capacitor directly to the 276 output so the 25 Khz output is stronger than the 100 Khz output.

    Sidetone Output

    The thread below had mention about the sidetone.


    You can hear the sidetone sound here.

    The C21 did not have a speaker when it arrived. The manual says a 4-16 ohm speaker should work. A four ohm was available so it was used to check the sidetone output. The output is about 350 Hz but very harmonic rich. It looks more like a series of alternating 200 microsecond spikes than a sine wave. Ten-Tec has no specifications for the sidetone output. There is an adjustment for volume but not frequency. This might bear some looking into if the sidetone is working properly.
    Last edited: May 12, 2016
    N4FZ, KN4LGK and NL7W like this.
  3. HAMHOCK75

    HAMHOCK75 QRZ Member

    Sidetone Output Analysis

    Analyzed the LM3900 current input op amp as an astable oscillator. It produces a slightly asymmetric square wave output at 350 Hz which is pretty rich in harmonics. 350 Hz is correct based on the values used. It gets worse. The .01 ufd coupling capacitor ( C2 ) on the audio amplifier board and the 1K ( R4) + 6.8K ( R5 ) = 7.8 kohm resistance it works against ( in transmit mode ) acts as a 2 KHz high pass filter which filters out the 350 Hz fundamental creating the spike like output seen on an oscilloscope at the speaker output.

    I tried shunting one of the frequency setting resistors ( a 22Kohm resistor at pin 10 of the LM3900 ) with a 18Kohm resistor that raised the oscillation frequency to 760 Hz. The output at the speaker was much less spike like and sounded smoother. I also momentarily tried holding a 0.1 ufd ceramic capacitor in parallel with the .01 mfd ( C2 ). That sounded better too since it lowered the high pass frequency to 200 Hz allowing more fundamental at 350 Hz though. Again the speaker output looked less spike like.

    The output transistors arrived today. I should be able to get to repairing the final soon.
    W3KW likes this.
  4. HAMHOCK75

    HAMHOCK75 QRZ Member

    The output transistors were replaced today. Some initial testing on 20M showed no problem with output power but the front panel meter indicating input power was pretty far off. When the Yaesu external power meter read 30 watts out, the internal meter was reading 38 watts in.

    I tried to set the SCR trip point at 6 A. per the manual but it would not trip at any setting of the internal potentiometer. The SCR was removed and tested bad. This probably explains why the finals failed since the protection circuit itself had failed.

    When working properly, the trip point can be set down to around 4A but it probably varies from unit to unit because the sense resistor for the current is not a separate high tolerance part. The resistor is just a wide pcb trace in a long serpentine pattern that takes up most of the power supply board.

    The factory 2N5060 SCR had been replaced before with a SY3950 equivalent. A local electronics store had an NTE5400 equivalent for $1.19. It worked fine. The SCR trip point at 6A was set with about 12.8 VDC at the power amplifier so the trip point for input power was 76.8 watts. The front panel meter was then adjusted to correctly read the trip point power. Measured output power was as follows with the input power set for 70 watts,

    Band Output Power ( watts )
    1. 80M 40
    2. 40M 40
    3. 20M 43
    4. 15M 34
    5. 10M 30 ( 28.0-28.5 MHz )
    6. 10M 30 ( 28.5-29.0 MHz )
    The above were measured with a Dentron dummy load and Yaesu YS200 swr/power meter.

    This seems to be much better than the specification of 25-30 watt, typical from the manual. It compares favorably with the Heathkit DX35 which runs about the same input power. The only difference was on 15M where the DX35 had 40 watts output and on 10M where it had 33 watts output.

    I am getting in a Ten-Tec 670 keyer that goes with the C21. More about that when it arrives.

    Last edited: May 15, 2016
    N4FZ, W3KW and KA0HCP like this.
  5. KP4SX

    KP4SX Premium Subscriber QRZ Page

    Thanks for the tour around this rig. I've always been interested in acquiring one and your posts may prove to be helpful.
  6. KA2TVX

    KA2TVX Ham Member QRZ Page

    Looking good - you are making a ton of progress! Not sure if I knew that keyer was available. Will look good with the rig.
  7. HAMHOCK75

    HAMHOCK75 QRZ Member

    I tried the C21 with my Nye Viking MB-I-02 antenna tuner and a 20M dipole today. I was able to achieve a perfect match with no reflected power on every band but 15M. The clicked stopped inductor resulted in the antenna capacitor all the way out on position 20 or all the way in on position 19.5. Still the reflected power was only 0.75 watt. Strange thing is on position 20 of the inductor, I could get up to 55 watts input power. Above that I could see on my oscilloscope that the final broke into oscillation. On position 19.5 input power went smoothly up to 70 watts input.
  8. HAMHOCK75

    HAMHOCK75 QRZ Member

    Last week the side tone volume control failed ( R6 = 1K open circuit ). Fortunately a local surplus store had an exact replacement for $0.25. The audio pre-amp board had to be removed to replace R6. I decided since the side tone oscillator is on the same board, to photograph and identify the side tone parts because the part values are not those in the schematic but they appear to be the original factory parts.


    This is a photo of the side tone output at pin 10 of the LM3900 and the voltage across C1. The side tone oscillator is an astable design which uses the LM3900 as a comparator to alternately charge and discharge C1.

    The frequency of the side tone is 344 Hz measured with a frequency counter.

    Not sure if Ten-Tec will be with us much longer. There is a discussion going on at eham.net that sounds not good.
    Last edited: May 31, 2016
  9. K4RT

    K4RT Ham Member QRZ Page

    I have a Ten Tec Century 21 and the frequency or pitch of the sidetone cannot be adjusted, only the volume. Good luck.
  10. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Ahem. Ignore the naysayers. There is a certain group that has made it their project to run down Ten Tec for the last year, in the most vicious way. Many of the same people can be found similarly denigrating everything that Flex Radio has ever done. It's pure blood sport.

    Actually, emails and posts from the new owner and one of his consultants indicate things are going well, that maximum effort is presently going into fulfilling lucrative DOD contracts; some repair work is being performed, and that at least three radios are being revised for production.

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