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
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  2. WU7H

    WU7H 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. WU7H

    WU7H 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,

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  5. WU7H

    WU7H Ham Member QRZ Page

  6. HAMHOCK75

    HAMHOCK75 QRZ Member

    Friday, May 21, 2017, I sent a message to Ten-Tec about parts for the Century 21. They responded yesterday with this message,

  7. WU7H

    WU7H Ham Member QRZ Page

    It appears I was wrong about running the C21 on external 12V power. Several people told me they ran C21's on 12V battery power many times, back in the day. I also ran across this:


    Current protection for mobile operation of Century 21 Models 570 or 574. 5 A operating current and 6.75 A trip current. Mount in a 1/2" diameter hole. Manufactured by Airpax Electronics.

    This part is still available: T11-1-5.00A-01-10A-V

    OnlineComponents.com has them for $13, but requires a $50 minimum order. Any other C21 owners out there who want to go in on a group buy? Would be around $15 per unit after shipping.

    The places mentioned in the Ten-Tec wiki no longer carry the part. OnlineComponents.com was the only source I could find.
  8. HAMHOCK75

    HAMHOCK75 QRZ Member

    Earlier the final transistors ( MRF475's ) were replaced in this Century 21 but recently, the output power dropped on 15 and 10M from about 30 watts to about 20 watts. The remaining bands stayed at 40 watts but on 20M the drive control had to turned up to near the max. I had similar symptoms with a Kenwood TS440SAT in which the problem was a failed intermediate stage transistor in the power amplifier assembly.

    When the final transistors were replaced, the power amplifier was removed from the Century 21. This time an attempt to work on the power amplifier without un-soldering it worked as shown below. After removing the four screws holding the power amplifier to the back, detaching the drive and output coaxial cables with phono jacks and moving the amplifier slightly left to allow the right phono connector to clear the rear panel, the power amplifier will drop out enough to allow removing the four screws holding the cover. A piece of egg carton is used to prevent the +12.8 VDC and other connections from touching the case.


    The failure this time appeared to be the lower MRF472. The class AB amplifier consisting of a pair of MRF472's should draw about 3 mA each in the emitter resistors which are 1 ohm. In this case the upper transistor drew emitter current but the lower device had almost none even though they share a common bias supply adjusted by the blue potentiometer in the lower left. The failure was not the transistor however. Instead a hairline crack had occurred in the collector trace of the pcb. When I went to remove the device by un-soldering and lifting the emitter lead, the collector lead complete with the pcb trace under it came right up. Some left over adhesive backed copper tape was used to do the repair as shown below,



    In the process of doing this, it was found that NTE makes replacements for both the MRF472 and the final MRF475's. The parts are NTE295 and NTE236, respectively. The NTE295 is stocked by Fry's Electronics for $2.49. NTE236's are reasonably priced. http://www.ebay.com/itm/NTE236-Fina...407640?hash=item1a2b3a8ed8:g:PJ4AAOSwzJ5XZs2D

    Output power is now back to 40 watts on 80/40/20M and 30 watts on 15 and 10M. On 20 M the drive control only needs to be about 1/2 way up for 40 watts out.
  9. HAMHOCK75

    HAMHOCK75 QRZ Member

    Worked on the backlash in the display dial ( not the PTO ). This backlash occurs because of play between gear 3 on the back side of the dial and the smaller of the double spur gears of gear 2 that engages it. There is little backlash between gears 1 and 2 due to the spring arm pulling the teeth together. The problem is easily demonstrated by placing a finger on the edge of the dial while it is in place and trying to move it. There is about 2 KHz of play. The inconsistent behavior of gear 3 while tuning happens because it fits over the vernier shaft which turns a faster rate. If gear 3 moves because of friction with the vernier shaft before it moves because of engagement with gear 2, the dial moves faster than normal. If there is little friction with the vernier shaft, then gear 3 will not move at all until its teeth engage gear 2. This is not so bad on CW but when rocking the tuning back and forth to tune a SSB signal it can be disconcerting if relying on the visual feedback from the movement of the dial. Several solutions were tried including felt washers and O rings but this is what worked best for me. Gear 1 has a setscrew to adjust its position. Gear 1 was adjusted so its face is a bit back of the face of gear 2. This allows the back face of gear 3 when it is placed over the vernier shaft to only touch the front face of gear 2 and not gear 1. Some automotive grease was placed on the front face of gear 2 and the rear face of gear 3. This grease is heavier than that used to rebuild the PTO. If gear 2 turns and the teeth between gear 2 and 3 are not fully engaged, the grease drags gear 3 at the proper rate. The grease has to heavy enough to overcome the friction of gear 3 with the vernier shaft and the cap nut that holds the dial in place. The cap nut should only hold the dial in place but not force the gear 3 against gear 1. The solution is not perfect but is a significant improvement.

  10. WU7H

    WU7H Ham Member QRZ Page

    Nice post. Two things:

    1. Probably best to use some kind of plastic safe grease so your gears don't get damaged over time.

    2. I don't have this problem with my C21 Digital.

    Sorry, couldn't resist =)

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