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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

    This photo shows the location of R1 on the audio preamp board ( 80356 ) which has been replaced with a 20K variable potentiometer. A good time to modify the side-tone is when servicing the PTO because the audio preamp board will need to lifted up as shown below to get to one of the screws holding the PTO to the main chassis. The proper value of the R1 replacement will depend on the actual values used in the side-tone oscillator as described earlier in the side-tone oscillator analysis.


    I found an error in the analysis. Where it says "The time that the LM3900 output is low is determined by how long it takes for C1’s voltage to drop so that v/R3 < 12 microamperes." should be "The time that the LM3900 output is low is determined by how long it takes for C1’s voltage to drop so that v/R2 < 12 microamperes.".
    Last edited: May 19, 2017
    W0GSQ likes this.
  2. WA7FPV

    WA7FPV Ham Member QRZ Page

    Some more info for this EXCELLENT C21 thread.

    Update on my C21 Digital's hum problem. I think the big power supply filter capacitor is bad. I measured 300mV P-P of ripple (measured at the big cap with an O-scope). Manual gives no spec, but it seems like a lot to me.

    To further test my theory, I powered my C21 with external 12V DC via one of the rear AUX ports. No hum on the audio!

    In the C21 Digital manual it says you can power the rig from external 12VDC for portable operations, but you really SHOULD NOT do this. If you look at page 3-25 of the Analog manual (page 3-31 for Digital manual), you will see why.

    The output PIN switch operates off the unregulated 24 VDC from the large capacitor. This higher voltage is needed to prevent high RF voltage from turning on the PIN diode during transmit. When running on external 12 VDC via aux port, the PIN diode will have no bias due to no unregulated 24 VDC. This is ok for a receive test, but I wouldn't try to transmit!

    Also, the power switch is not in the circuit when you power the radio via external power - the radio will turn on as soon as power is applied.

    Really weird that Ten-Tec included this as a feature in the Digital manual when clearly the radio will not work correctly on external 12VDC.

    I found a suitable replacement capacitor that is still available and will bolt right in with no modifications. The original capacitor was a GE 86F140M, 14,000UF, 25 VDC, 85C. 1.75" diameter, 5.9" length with 3/4" screw terminal spacing. The replacement that I found is: Mallory CGS143U025U3C

    Same specs as original part, except the length is 3.125". Should be a perfect replacement requiring no modifications. I will confirm once I've received and installed the part.
  3. WA7FPV

    WA7FPV Ham Member QRZ Page

    Filter Capacitor follow up. I received an email from an electronics supplier listing another in-stock capacitor that would also be a direct replacement:

    3186BD143U025APA2 BC COMP-PHILIPS CAPACITOR 14,000UF 25V ALUMINUM ELECTROLYTIC LARGE CAN COMPUTER GRADE screw terminals capacitor. High Post; Case Size: 1 3/4 X 3 5/8

    At the time of this post, the part is available for $16.

    I also found a listing for the same Mallory capacitor listed with a slightly older part number:


    This is identical to the part in my previous post, they just include the last 3 digits of the part number which seem to be omitted in the newer version.
  4. HAMHOCK75

    HAMHOCK75 QRZ Member

    I recently had to rebuild the PTO a second time but not because of the grease problem. Tuning in SSB stations started to become difficult. It felt like there was backlash in the tuning. Here is what the problem actually was.

    Ten-Tec's PTO rebuilding instructions, step 9, says " Using long nose pliers, remove bearing race (R) and retainer (Q). These parts are sometimes glued in and all the old glue must be removed before re-assembly PTO." Also, step 14 says "Apply a small quantity of super glue on each side of the retainer." Part Q and R support the front of the tuning shaft (K). Below is a photo of the retainer (Q) and bearing race (R).


    If the retainer is not glued to the PTO case, it can move. It's movement subtracts from the movement of the main tuning shaft (K). Once R and Q were glued, the PTO tuning was a pleasure to use again showing virtually no backlash.

    The rebuild kit also comes with four plastic washers (C) according to the rebuild instructions. It does not explain the purpose of these washers (C). They are used to adjust the tension of a spring that presses the tuning shaft (K) into the bearing race and retainer shown above. More washers decreases the pressure making the tuning knob easier to turn but at the same time, it makes it possible to push directly in on the tuning shaft which will change the PTO frequency without rotating the tuning knob. So the correct number of washers is dependent upon the age of the spring and the desired ease of rotating the tuning knob. These washers are shown below,


    The Century 21 dial drive mechanism does have backlash unfortunately. It consists of three gears with backlash of about 2 KHz. These gears are shown below,


    If not set up properly, the main dial on occasion may appear to not move with the tuning knob while rocking the tuning back and forth. The dial is held in place with a push nut. To attach the push nut, a socket was placed over the nut and it was tapped into place so it holds the the dial to the tuning shaft (K) securely. This causes the dial to always move with the tuning knob but you might notice that the movement is fast initially then slower while rocking the tuning knob back and forth. This happens because the tuning shaft (K) moves at a faster rate than the gears that drive the dial. The faster rate is imparted to the dial by tuning shaft K when the gear teeth have not yet fully engaged.


    There are also four plastic washers that go on the front of the PTO housing. These space the PTO back from the front panel so the two mounting holes for the PTO in the chassis line up with the right angle mounting tabs on the bottom of the PTO. It's also a good idea to attach the first dial drive gear to the tuning shaft (K) before installing the PTO otherwise the set screw for that gear lies directly under the front panel sheet metal making access difficult.


    One last note, the Selectivity switch used by Ten-Tec has a lot of positions perhaps 12 with one fixed detente and one adjustable detente. Two install the adjustable detente correctly, rotate the Selectivity control clockwise until it meets the fixed stop ( 500 Hz position ) then installed the adjustable stop as shown below,

  5. WA7FPV

    WA7FPV Ham Member QRZ Page

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