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Author Topic: Ten Tec Century 21 CW rig low output power  (Read 3858 times)

4X1KS

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Ten Tec Century 21 CW rig low output power
« on: May 15, 2016, 07:18:50 AM »

I've got a Ten Tec C21 that used to put out 30 - 35w..input power is about 70 watts.

It hasn't been used in a while.  When I fired it up it was only putting out about 18 watts regardless of which band it is set on.  Using a low SWR Yagi.  Still the Bird wattmeter says 19 - 20 Watts.

Any ideas on what's going on with this radio?  Nice rig, but would be happier if I could get 35 watts out like the old days.

73,
Mark ku4m
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AC5UP

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RE: Ten Tec Century 21 CW rig low output power
« Reply #1 on: May 15, 2016, 09:07:12 AM »

If you're using the internal AC power supply my first guess would be the filter electrolytic(s) have lost enough capacity that the power supply is no longer holding regulation under peak load.

Compare the VCC voltage in TX against RX vs the factory schematic / service notes.  Considering the 70's vintage of the radio there's a fair chance one or more condensers are past their prime.

4X1KS

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RE: Ten Tec Century 21 CW rig low output power
« Reply #2 on: May 15, 2016, 10:01:47 AM »

The power supply was on my mind, but on the input power meter the meter is stable and I only have to crank it up not even half way to get to the limit on the input meter.  That may not mean much.  I was thinking finals, but this rig is from 1970??  I'm not sure I could even get the parts.
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AC5UP

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RE: Ten Tec Century 21 CW rig low output power
« Reply #3 on: May 15, 2016, 10:34:40 AM »

Late 70's...  Here's the January 1978 revision of the owner's manual:   http://www.k3jls.net/Analog_Century_21.pdf

Notice how the schematic drawings are dated January and February of 1977.  Yours could be late production closer to 1980, but even so, the radio is pushing 40 years old.  The chances of needing some technical TLC are greater than zero with the PTO in particular a common rebuild candidate.

HAMHOCK75

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RE: Ten Tec Century 21 CW rig low output power
« Reply #4 on: May 15, 2016, 01:05:10 PM »

I have been writing about restoring the C21 on this thread.

https://forums.qrz.com/index.php?threads/ten-tec-century-21-restoration.522748/

Got two final transistors off eBay for $20 shipped. Ten-Tec seems to be in the process of being acquired so their web site does not indicate any parts can be ordered from them at present.

I would test into a dummy load first just to eliminate the antenna as a potential problem. The power amplifier has a phono jack output to the low pass filter assembly. I would take that output to the dummy load next to eliminate the filter module.

I am doubtful the output transistors are bad. Both the MRF475's finals were bad in mine and they shorted the power supply.

With the new MRF475's, output power is 40 watts on 80, 40M, 43 watts on 20M, 34 watts on 15M, dropping to 30 watts from 28.0 to 29.0 MHz with front panel meter set for 70 watts input.

P.S. Verify the output power meter is reading correctly too. I had a case where it was the meter that was wrong. It needed a good cleaning.
« Last Edit: May 15, 2016, 01:14:31 PM by HAMHOCK75 »
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