Ten-tec Century 21 restoration

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

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  1. WU7H

    WU7H Ham Member QRZ Page

    Huge thanks to Ed, W3NR, for mailing this to me! It is a hand written letter to 'Tom' (not sure who this is) from Ten-Tec dated Feb 13, 1978.

    From Ed:

    "I also have a hand written how-to on adding a pre-amp to the RX. I did
    this mod also and it made a remarkable improvement to the C21's
    sensitivity on all bands, most noticabbly on 10-15-20. This how-to is
    dated 2-13-78 and was written by Dick Frey W1FCC/K4XU then the Chief
    Engineer at Tentec."

    You will find the 3 hand written pages here:


    You will also find two interesting articles that Ed also sent me. Both are from 73 Magazine:

    *Little Extras for the C21 (73 Magazine, 08 August 1979)
    *AGC, PDQ (73 Magazine, 05 May 1986)



    -Josh WA7FPV
  2. HAMHOCK75

    HAMHOCK75 QRZ Member

    I read briefly what Dick said. It is all accurate. I used to design spectrum analyzers at HP. SA front ends are remarkably similar to the C21 front end. They also have a noise figure of about 20 dB. I have also used the 2N5109 and what he says is correct.

    Interesting what he says about the "birdies".

    His numbers on the last page look good. I used to do those in my head. Thermal noise in a 1 Hz bandwidth is -174 dBm. In a 1 KHz bandwidth that he mentions on the first page it becomes -144 dBm/1 KHz, a 5 dB noise figure degrades the noise floor to -139 dBm/ 1 KHz. Sensitivity is when the signal is about 10 above the -139 dBm noise floor. He uses .1 uV which is -127 dBm which is 12 dB above the noise floor. Pretty good analysis in my opinion.
  3. HAMHOCK75

    HAMHOCK75 QRZ Member

    Can the Century 21 run off an external +12.8 VDC power supply?

    The answer is not easy because some manuals like the one uploaded by WA7FPV say it can but the current manual on the Ten-Tec web site says nothing at all about use of an external power supply. The following are tests to determine if it is safe to operate a Century 21 from an external power supply.

    First from the schematics it was found that an unregulated +24 VDC is used to reverse bias the PIN diode during transmit. The PIN diode then blocks the transmitter RF from entering the receiver. The problem is that there is no unregulated +24 VDC if an external power source is used. The first experiment is to find what bias voltage is available to reverse bias the PIN diode when an external power source is used.

    An HP6205C power supply was connected to a rear panel +12 VDC aux output RCA connector with the Century 21 unplugged from the wall outlet. The unregulated +24 VDC line was measured with a surprising +11.7 VDC found. As time went by that voltage could be seen drifting down eventually settling at just above +3 VDC. This happened because C1 ( 13,000 mfd ) has no discharge resistor. The initial +11.7 VDC happened to be the voltage left on C1 from when the Century 21 was last powered on from the AC line. The picture below shows how the +3 VDC gets to the unregulated +24 VDC line from the external power source.


    The path through the MC1723 appears to be as shown below,


    The next question is if the only available bias for the PIN diode is +3 to +4 VDC, will this be adequate to protect the receiver? This paper might provide the answer "Establishing the Minimum Reverse Bias for a PIN Diode in a High-Power Switch".


    To answer that question requires knowing the width of the intrinsic layer for the PIN diode ( Micro-Semi UM9301 ). The answer tomorrow.
  4. HAMHOCK75

    HAMHOCK75 QRZ Member

    According to Micro-Semi, the thickness of the intrinsic layer is 12 mils. If we solve the equation,

    we get 713. Calculating the Vp for 40 watts is about 63 volts. Using these numbers and figure 3 from the MACOM article we see that it requires more than +24 VDC to keep the PIN out of conduction.


    But what does conduction look like for a PIN diode? The following are measurements at the anode and cathode of the UM9301 PIN in circuit.

    This first photo is of the voltages present during transmit with no drive. It shows the +24 VDC on the cathode ( yellow trace ) and about + 24 mV at the anode ( blue trace ) created by the "on" state of the driver.


    The photo below shows the behavior under low drive conditions. There is a leakage of the input at the cathode to the anode of about -52 dB = 20 long ( .112/43.2 ). The output signal shows distortion because this is before the output filters.

    The 24 mV offset of the leakage signal is also visible as an upward shift in the leakage sine wave.


    Under higher drive, rectification is taking place as can be seen in the photo below. Notice that even though the 24 mV shift of the sine wave up due to the PIN drive source is still visible, the time that the leakage signal goes negative relative to the 24 mV positive offset is now longer than the time it goes positive. Since the net DC value is equal to the sum of the areas under the curve, with there being more area under the negative curve a negative bias has developed from rectification although it is not dramatic as in a diode designed to rectify. It appears that the calculation of the MACOM paper is essentially correct.

    With rectification comes a decrease in the isolation of the diode. Notice below that we now have isolation of -45 dB = 20 log ( 0.34/63.2 ) which is a 7 dB degradation from low drive situation.


    In the photo above, Vmax(1) is the +24 VDC reverse bias voltage plus the positive peak of the RF input adding up to 63.2 VDC. A 40 watt output would be +24 VDC plus 63 Vp or about 87 Vmax(1). The specification for the UM9301 is 75 Vr. This is lower than Vb at which the diode goes into breakdown.

    In the photo below, we raise the power so Vmax(1) is about 86 V. Notice the wiggles circled in red below. The PIN is starting to break down under high drive. It does not damage the diode because of resistance in the circuit like resistor R3 ( 1K ) limiting the current that can flow during break down.


    This is not the first time I have seen a device operating at break down. One of the crystal oscillators in my Yaesu FT101EE finds its base-emitter junction operating at a current limited break down when the oscillator is turned off.

    So far, we find that operating the PIN with some conduction below what MACOM calls the "conditionally safe operating" region still works since the PIN diode slowly transitions into rectification. There is no half wave rectified RF suddenly at the receiver input with 63 volt peaks which would be very bad. The next post will be what happens to the PIN diode as the bias voltage is reduced from +24 VDC.
  5. HAMHOCK75

    HAMHOCK75 QRZ Member

    In these photos, the internal +24 VDC unregulated supply was replaced with an HP6205C variable lab power supply to set the reverse bias voltage at the cathode of the UM9301 PIN diode. The voltages on both cathode and anode are shown in the photos below with the yellow trace the cathode voltage. The blue anode voltage has about a +0.24 mV offset due to the saturation voltage of the driver.

    A 44 Vpp RF signal is applied to the PIN diode anode from the output amplifier. The amplifier distortion prior to the output filters is readily observable.

    First, V = +12 VDC. Isolation is -50.2 dB.


    V = +6 VDC. Isolation is -44.2 dB.


    V = +5 VDC. Isolation is -43.4 dB.


    V = +4 VDC. Isolation is -42.6 dB.


    V = +3 VDC. Isolation is -41.6 dB.


    There is some conduction resulting in a shift down of the DC component of the leakage signal as well as a degradation in the isolation of that signal but no rectification like one might expect from a normal rectifier. In the case of a rectifier like a Schottky diode, the diode would half wave rectifiy the negative 22 Vp excursions under low bias and apply that 22 Vp negative signal to the receiver input with likely damage to the receiver but nothing like that is happening here.
  6. KN4UD

    KN4UD Ham Member QRZ Page

    I have been reading all this great information on the C21 and remembered using one many years ago at a friends house. All this information sparked an interest for another project so I found one on line, not working, no electric cord and loose wires inside, kind of a pig in a poke. It will be here tomorrow and I will find out more. As far as the pto goes TenTec no longer has any rebuild kits and no plans by the new owner to source them. No big deal if I need any parts I can make them. I did take a gamble and ordered a new 14000 cap for the power supply which I am sure it will need.
  7. WU7H

    WU7H Ham Member QRZ Page

    Awesome, let us know how it goes. My C21 is still working fine - I used it for the Novice Rig Roundup this year and made some contacts. You should definitely be able to fix it - nothing particularly hard to find in there. FYI, if you suspect the power supply... you CAN run it on external 12V, just note that the drive meter won't work and there won't be any protection for the finals. Dummy load or well matched antenna, and don't turn the drive up much.

    The PTO rarely requires parts - mostly just cleaning / degreasing, and then fresh grease. There is all kinds of debate on what grease to use. Some people even skip the disassembly and just drip denatured alcohol followed by Tri-Flow in there. There is a thread around here somewhere on that method.

    Someone needs to start selling PTO rebuild kits since Ten-Tec isn't going to. Could be a nice little side business - there are tons of Ten-Tec rigs with PTOs out there!
  8. KN4UD

    KN4UD Ham Member QRZ Page

    As for power supply my only concern is the transformer. This may possibly be something I can get from TenTec, I remember going their years ago and going on tour through the plant and was told the build their transformers as well as for other companies. The PS board would be easy to build if needed. Yes in most cases the pto can be cleaned up and will work just fine, I did this to a C22 for a friend and it worked just fine with no parts required. I have seen a lot of discussion on the grease and from past experience back in the 60 ' s when most radios had mechanical tuning a little petroleum jelly was hard to beat . Lithium grease is very good but some types are not a good choice , some use an oil base that will evaporate in time and become very hard. Any light thin grease should work fine.
  9. KN4UD

    KN4UD Ham Member QRZ Page

    Well, got the C21 today and I sure hope they did not manage to connect up 120 ac up to it. Look at the picture they have it wired to where it is going into the PA. I need a gain control with switch on it, no luck yet. The PTO knob turns but the dial don't move I kind of figured this. Looks like I have my work cut out for me, LOL..

    Attached Files:

  10. W0GSQ

    W0GSQ Ham Member QRZ Page

    Ten Tec used to sell a Airpax magnetic circuit breaker so that the C-21 could be run off a external DC source, I don't remember the breaker capacity, perhaps someone else will chime in.

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