Ten-Tec T-KIT 1260 6 Meter FM Transceiver final dying

In the picture above, ignore the Kenwood TM-261A 2 meter radio on top of the Ten-Tec. I got T-Kit as a Christmas gift in 1998 just after getting my license in October of the same year. I had a ton of fun with it the following summer on sporadic E band openings and I have QSL cards from up in the north eastern and eastern parts of the US to prove it. At some point though it stopped transmitting. Last I checked I could hear it fine on my Dad's handheld Icom IC-T81, but only really, really close by, like within half a block. Hooking it up to a MFJ-864 SWR Wattmeter shows *no* movement in needles when transmitting. I went through the manual and tested all the voltages at the test points and they were mostly within the expected ranges. So I called Gary at Ten Tec T-kit tech support and he said he was 99% sure it was the final and suggested replacing it. Unfortunately the original 2SC1971 is no longer available. The replacement for that is the NTE 342:

I got this and replaced it and tried it out and what I observed was that the needle on the MFJ-865 SWR Wattmeter moved once and then went down immediately. The T-Kit went back to operating the same way it had before I replaced the final.

Originally this kit came with a Supplement and a Tech Bulletin to the Supplment. In the Tech Bulletin it provided an extra capacitor and a resistor to create feedback "to reduce gain at lower frequencies, therby stabilizing the amplifier." I had alway suspected something might be wrong with this circuit that could cause the final to short. First, here is the tech bulletin:

Tech-Bulletin-1260-498-1 (click the arrow at the top for full size).

Now looking closer at the resistor in this circuit today I noticed it looks brown in the middle, like it is shorted out! Here, take a look:

feedback circuit 1
feedback circuit 2

Am I on to something? Before I replaced the 2SC1971 with the NTE342 I wanted to remove that circuit for fear it was at fault, but I didn't.

In addition to that you may have noticed in the tech bulletin from above it added a DC blocking circuit on the SO-239. I thought maybe something could have been wrong there, but I have no "smoking" gun like the browned resistor from above, but here are some pics of that:

DC Block 1
DC Block 2
DC Block 3

Do you think I should be worried about that?

Especially after finding that browned resistor I'm tempted to remove it completely or replace it and buy another NTE342 and try again! Any advice appreciated.

For reference here are some of the relevant documents from the instruction manual:

(as in the above links there is an arrow above and in the middle of the reduced size image that shows the full size scan)
Schematic 1
Schematic 2
Schematic 3
RF Section schematic and drawing

Circuit Board X-Ray
Mechanical Assembly

Top of circuit board after NTE342
Close up of RF section showing NTE342

Bottom of board after original construction
Top of board after original construction
Back of radio after original construction

I got some good advice from two different people. I have yet to try it out.

Larry Young from RARS

John: Some advice, and a few questions. My first rule when attempting a repair is to try and determine the exact failure and then its cause before attempting a repair, lest the same failure occur after the repair. Did you do any measurements with an oscilloscope to determine if you had drive to the solid state pa, and does it have a protection circuit, as that could be defective. Did you ask TenTec the cost of repair by them ?

No disparagement on you, but if you are not that experienced in solid state PA troubleshooting, the learning curve could cost more than it should. This kind of retrofit as well as repair could be very frustrating. Larry K4LXV

    • After looking at the schematic:
  • John: That Transceiver pa is not complicated as pa's go. Please check the following:

    1. D-19, 1N1418 diode. This establishes the the final pa bias. make sure it is not open or shorted.
    2. R-91 100 Ohm resistor. check its value to make sure it is not open or has changed value significantly.
    3. D-22, MI402. This is is for solid state RF TR switching.

    4. D-23, MI308 Receiver RF protection diode. is also a part of the solid state RF TR switch
    5. Q-16, 2SC1971 PA transistor or equivalent.

    I hope this helps.
    Larry K4LXV

    Stuart Rohre from tentec@contesting.com


    John, First of all something besides the final was bad which caused the new final to fail. The resistor is a good suspect. Do you have a scope?

    That would be a help in finding out if you need the "fix" for excessive low frequency gain, ie any off 6m signals.

    What was the purspose of the suggested series capacitor on the antenna connector, (I am thinking you said series). AC (and RF) coupling by the cap should not hurt. It does put a reactance in series with the antenna, but maybe they were worried about the final collector shorting to an external ground if an antenna cable failed?

    I would talk to Ten Tec again and determine the "why" of each mod. Obviously, some of these rigs worked without them. That need for a mod can come about because the gain of transistors may vary from unit to unit due to production variations. They may not find that out in prototype testing, but only after customer experiences are totaled.

    If you wanted a "beefier" transistor than the NTE replacement, you need to find the same polarity of transistor, NPN for example, equal or greater collector power rating, about the same Beta and F tau, to insure duplicating the operation parameters of the original. The voltage ratings need to be about the same, although higher Vce voltage rating can be useful if it does not increase the capacitance greatly. Not being able to look at a schematic right now, these are general comments aimed at any replacement of an out of production transistor. You could also check with RF Parts Co. who handle some power transistors that are less common now.

    One of the issues with power transistor troubleshooting, is that you must have sufficient accuracy in measurements to recognize an abnormally low resistance. A good visual inspection of all components for signs of heating should be the first thing to do; with high intensity light and a magnifier, upon starting troubleshooting before even powering on a circuit. After replacing components, do resistance checks to see it you have reasonable values, just to catch the rare solder bridge, or bad joint that can happen in replacement soldering.

    Good Luck to you; there should be many hours more fun with the 1260, once you get the final sorted out.

    Stuart Rohre K5KVH


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