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Author Topic: Century 21 PTO-Quick and Easy Tuning Restoration  (Read 5499 times)
K4EJQ
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Posts: 146




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« on: May 13, 2012, 07:24:47 AM »

Hello All; The Ten-Tec Century 21  that has been unused for a while is known to have a problem with the grease that was used in manufacturing hardening with age. This causes the PTO tuning to become difficult if not impossible due to slippage of the drive due to the drag caused by the hardening of the grease. Several ideas for resolving the problem appear on various sites on the internet. I was about to implement one of those ideas when I ran into a problem removing the dial. I did not want to force anything to the "breaking point" so I came up with the following procedure.
 
With the power disconnected to the unit, remove the top cover of the transceiver.. Attach a 200 ohm-10 watt resistor to the top cover of the PTO using just thermal grease ( heat sink compound). Do NOT cover the PTO calibrate access hole. A flat sided resistor is preferred to optimize heat transfer. No Extra Holes-Please!! Using stiff solid conductor wire ( No. 12 or No.14 is fine) connect one terminal of this resistor to the ground lug on the mainframe chassis next to the right-rear of the PTO enclosure. Connect the other end to the positive terminal of C1 the large 13000 ufd filter capacitor which is attached to the PC board directly behind the PTO. Position wires to provide maximum contact of the resistors' body with the top of the PTO enclosure keeping the terminals of the resistor from shorting to the PTO enclosure ( ground).

Reinstall the top cover of the transceiver. Reconnect power to the unit. Turn on the unit and let it warm up for several minutes. After a period of a few minutes you should now be able to tune the PTO again. I have used various values of resistance. 200 Ohms should provide adequate heating to soften the grease. If not try a 100 ohm-10 watt resistor. Do NOT use excessive heat-less than 75 ohms-15 watt for the resistor value should be avoided.
As would be expected, the  additional heat in the area of the PTO will cause a slight shift in frequency of the transceiver-the less heat the better. For my use I did not find it necessary to recalibrate the PTO.

Your mileage may vary. This is not a Ten-Tec approved modification. Use at your own risk. Good Luck and keep the Century 21s talking.

Bunky, K4EJQ




 
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NA7U
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Posts: 72


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« Reply #1 on: May 14, 2012, 03:41:28 PM »

I like it!  Cheesy  Sort of like those dipstick heaters they use for cars that live in sub-zero winter climates! HI.
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N4OI
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Posts: 273




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« Reply #2 on: May 16, 2012, 05:34:37 AM »

Just curious if you have tried installing the TenTec PTO rebuild kit?  I bought a C21 with an unmovable PTO, rebuilt it using the kit, and now it tunes as smooth as butter...   

73 ES GOD BLESS U ES URS DE KEN N4OI   Grin
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W3TTT
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Posts: 66




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« Reply #3 on: September 11, 2015, 02:05:35 PM »

Warning: this topic has not been posted in for at least 60 days.
Unless you're sure you want to reply, please consider starting a new topic.

Well, despite the warning, I would like to add my two cents.
I just bought a Century 21, untested.  Got it home, and surprise the tuning was stuck.
In fact, someone had tried to force it, and the gears were off track.

I didn't first check the internet to see what everyone else had tried first...but here is what worked for me.

I dripped a few drops of "Liquid Wrench" from a screwdriver on the joints, and I opened up the PTO and dripped a few drops there too.  A little push with the screwdriver on the gears got them turning again.  I pushed the one gear on top of the other where it should be, and wouldn't ya know it, it started working and turning again. 

If you push on the little spring loaded lever located by the gears, it disengages the dial, and you can thus reset the dial.

Now I have a different problem.  Like I said, I bought the rig untested.  So anyway, as soon as I got the dial turning, I turned on the set, with an antenna.  Hiss but no stations. I thought that I had broke the PTO.  So I turned it off.  When I turned it back on, I heard the wonderful donald duck of a ssb conversation.  However, it faded out in about one second.  Off.  On.  Sound for a second.  Off. On.  sound for a second.  It fades just the opposite of a cap that is charging.  Anyone have an idea?
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W1VT
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Posts: 1837




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« Reply #4 on: September 14, 2015, 11:27:33 AM »

Do you hear hiss when the stations fade out?

You may be seeing something call squegging--which means self quenching.

https://en.wikipedia.org/wiki/Squegging

If you still hear hiss, the PTO may be doing the squegging, or some other RF oscillator.  If not, it could be in the audio amplifier.

Zack W1VT
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