When you buy older radio equipment, it is a good idea to check inside before getting on the air with it as this article details about my Ten Tec 277 Antenna Tuner. Sometimes things are lurking inside that neither you nor the person you got it from know about.
I have had this tuner for over a year and have done park activations with it. The 277 also sees extensive shack lately with radios like the sBitx and Penntek TR-35 and such. I had noticed the other day that the sBitx was exhibiting some odd behavior on transmit and when connected directly to a dummy load, it acted normal. So I pulled the cover off of the tuner to have a quick look inside and this is what I found.
These two photos show both problems I found inside when the cover was removed. The first thing I saw was this burnt resistor, which goes to the SWR meter circuit and the wires were broken on the balanced line transformer.
Fortunately for me, neither of these actually impeded performance in how I used the tuner except maybe from arcing on that transformer where the leads were broken if it was still close enough to enable an arc... Probably not though so I am not sure there. The resistor is the 68 ohm resistor on the print that I found online. It looks like it is a simple load resistor to allow the diodes to sample from and was probably overheated by pushing a little too much RF through the tuner at some point in the past. With 86mA of current flow through this resistor, you are at the power limit of 1/2 watt. Doing some simple ohms law, that turns into 5.848 volts across the resistor. I should count the turns on the transformer to see what the primary voltage would have to be to make that happen… lol. That is a whole different rabbit hole though so let’s continue.
When I found these things I decided to go to the internet and see if I could find a schematic of this tuner. Well, the internet did not disappoint. Below is the schematic for this exact tuner.
The bottom half of the page is the schematic for my tuner as it has the SWR meter circuit in it. I printed out the schematic so I could have it on the bench with me during the repair process.
Something I have noted about Ten Tec prints, they lack critical information about things like the transformer ratio or the turns count on the tapped inductor on the matching network or even the turns count on the balanced line transformer, or even the rating of the little meter movement, none of these are present on the print.
To replace the resistor properly involved having to remove the circuit board from the tuner. This involved having to remove all the knobs as well as desoldering several wires to all the board to lift out of the chassis. I will take photos of the areas where I remove wires like this so it is easy to see how to reconnect them later. This is a bonus of the iPhone for me, it allows visual records to be kept in real time of things like this.
Once dismantled, I set out to replacing the resistor first. Sometimes the power rating of the component is just as big of a factor as the value, and I didn’t have a 68 ohm 1/2 watt resistor in my parts bin. What is a guy to do? Well, it turns out that if you put two 330 ohm resistors and two 220 ohm resistors all in parallel that you get 66 ohms mathematically. Mine measured out to about 65.7 ohms or something like that and so I added a 2.2 ohm resistor in series with the other four and BAM! 67.86 ohms! .002% accuracy is good enough if I had to guess, and I upped the wattage rating to 1 watt as well since the four higher value resistors are all 1/4 watt each and they will bear most of the burden of the power dissipation work anyway. (Probably a little less actually since more current will obviously flow through the 220 ohm resistors than the 330s but it will still be way over 1/2 watt total capacity now…) I put it together like you see so it would more easily fit the hole spacing on the circuit board as well as stand up the four parallel resistors to allow better heat dissipation.
Once I had the new resistor installed, I remounted the circuit boar5d and proceeded to reconnect every thing that I disconnected. This is also when I decided to rewire the transformer to the balanced like connections on the backplane as well. With all that done, I reinstalled all the knobs and was ready to test it out.
One last thing I did was tuck the print inside the chassis so that next time who ever is in here will have the information I didn’t and will be able to solve their problems that much easier. Kind of a gift to the next generation as this thing is really robust to be honest about it.
All buttoned up and ready to go back into service in the shack. Thanks for following along on this little adventure.
73
WK4DS