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Old 28th Apr 2018, 6:54 pm   #1
Simon_V
Tetrode
 
Join Date: Feb 2014
Location: Pembrokeshire, UK.
Posts: 52
Default Trio TS711 low or no RF power output

I am NOT going to call this definitive as I may want to add to it but it is a good starting point.

These radios are starting to show their age (aren't we all) but that said they are superb radios and more than capable of holding their own against some more recent radios. Audio quality on both RX and TX is excellent.

As long as you are sure you have not been transmitting into a bad SWR and that you have hopefully not destroyed the PA block because they are rarer than dentures for chickens then this is for you.

The first step is NOT to dive in there with a trimming tool, no amount of twiddling will correct the problem.

I purchased my TS711 knowing that it had an RF power problem. The seller was super honest with me and said he had changed the PA block which had made no difference. He also reported that the fan did not run.

The fan was the easy bit, it was hardly surprising that it did not run as the PA was outputting less than 1 watt so it never got hot enough to trigger it on.

Remove the bottom cover and then remove the RF unit (6 screws) 3 coax connectors plugged into this board, 2 Coax fly lead connectors that are plugged into the IF unit and a 5 pin multipole connector that supplies power and various control signals.

The coax plugs and sockets will probably benefit from a good clean, a cotton bud and a bit of contact cleaner should be good enough.

I have attached a couple of pictures that you can use as a guide.

Replace the following ceramic capacitors

C30 - 7 pF
C34 - 3 pF
C36 - 1 pF
C40 - 0.5 pF
C37 - 2 pF
C41 - 2 pF
C33 - 8 pF
C38 - 8 pF
C42 - 8 pF
C44 - 22 pF
C46 - 470 pF
C47 - 100 pF
C45 - 0.001 uF

At this stage on mine from cold I have a really nice 25 watts but this falls away as the radio warms up or I transmit and falls to around 10 watts. I have some more capacitors on order and I have a feeling I am going to end up changing quite a few 0.001 uf caps.

Something else I have noticed on mine is that the RF unit does some odd things if I cool the output transistor Q6 (2SC2538). I do have a new one of these and next time I have the cover off I think I shall change this component regardless as it is not exactly thermally stable which often happens with transistors.

Before I add any more to this a word of WARNING.

The PA output device Q1 (M57727) is capable of around 37watts according to the manufacturers specification sheet. Experience from other radio amateurs suggests that this is not a good idea as it shortens the life of the device. As I said in the beginning, as these devices are getting rare it's not worth over driving them so be careful not to push the PA beyond the rated 25 watts.
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