October 2013
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Nautel V-10 repair

Not exactly sure how it happened, but one of our Nautel transmitters malfunctioned!  It is a pretty rare event, so I thought the exclamation point was needed.  One of the PA pallets went bad and the transmitter lost 1/2 a PA module.  Since the TPO for this particular station is 7 KW, they remained on the air at full power.  In the interest of staying on top of things, we fixed it anyway.

Diagnostics were simple:

  • Fault lights on front of transmitter observed
  • Press status button to find out faults, which were Module D failed
  • To to module sub-menu, find Module D and discover Q1 disabled, Q3 shutdown.
  • Problem is with Q3, order new pallet from manufacture

Upon removing module, I did not see the damage at first:

Nautel V series FM transmitter PA module

Nautel V series FM transmitter PA module

It is board A3, which for this particular flavor transmitter is a Nautel Part number NAPA16-B. Once I replaced the defective module with a new one, I discovered what looks like a symptom of the greater problem:

Nautel NAPA16-B defective board

Nautel NAPA16-B defective board

Over to the in the left-middle-lower section of the board, R10 and R8 are burned open. These are surface mount 2 watt, 20 ohm resistors.  A glance at the schematic shows that these are part of the bias supply.  A quick set of measurements with a DVM shows that Q1 seems to be intact and not shorted.  Interesting…

The question is: Is it worth trying to fix this board or should I just trash it an buy a new spare?

Update: Schematic diagram as requested:

Nautel NAPA16-A schematic

Nautel NAPA16-A schematic

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15 comments to Nautel V-10 repair

  • It would be easier to make the scrap vs. repair decision if the schematic were shown…

  • Eric Schecter

    @Paul-good example of a topic worth sharing! If the board isn’t too damaged, I’d replace those resistors just to have an on-shelf spare in case it happens again. OTOH, because its failure didn’t cause you lost air time or reduced power, it comes down to a cost vs. time decision. I saw a BE amp module with the guts of an electrolytic cap exiting the top of the component. Easy enough fix to tack a new one on in place of the bad, though they do use a special RTV silicon to assist with heat transfer. Enough babbling..

  • You could simply replace the burned out resistors and reinstall the module to see if it works. If it does, fine – problem solved. I’m guessing it will fail again. The resistors burned out for a reason. Knowing exactly where they are in the circuit might provide insight into the problem source. That’s why I mentioned getting a look at the schematic.

  • Eric-KC7ES

    Good point Rod. Yes, determining the failure mode is important.

  • Paul Thurst

    Rod, you are correct, resistors burn out for a reason, which is why I though Q1 might be bad. It is a pretty simple circuit but I didn’t too much time to troubleshoot beyond just basic DVM checks. I’ll put the schematic up, unless somebody from Nautel has an issue with that.

  • Is C12 shorted? (R5 and R9 should also show signs of excessive heating if that’s the problem.)

    Depending on parts cost and how nasty it is to change the transistor array (lost time), I’d just pull it and the bad resistors and check everything with eyeball and ohmmeter — a problem with the input transformers could unbalance drive levels, for instance. If it all checks out, replace transistor and bad parts and plug it back in!

    Does Nautel offer any kind of test fixture? Harris did for their solid-state VHF TV rigs. Their modules are kind of fiddly to change transistors in, so most users don’t bother, but it helps to have the thing out where you can (safely) see it and measure operating parameters.

  • (That was the Platinum series that was fiddly to fix at the component level. Pretty early solid-state TV PAs, so I’ll not ding Harris too badly over it.)

  • If C12 is shorted then the current into R8 and R10 from the bias supply would be limited to just a few ma since R1 is 15K. This is not enough to cause them to burn up. However, if that half of Q1 is shorted drain-to-gate AND C12 is shorted, then R8 and R10 would burn up.

  • Tom Osenkowsky

    I bought one of the early V5d transmitters. Just after the warranty expired I had a PA module fail. I noticed heated resistors on every PA module. The customer service tech at Nautel replaced all the modules at no charge. I recall telling him he gave the term “Customer Service” a new meaning. He said the resistors should not have overheated and they wanted to find the cause.
    I do know that the module currents are different when seeing loads other than 50+j0. I never heard back the results of their findings. I suggest contacting Nautel on this matter. They may have an answer.
    I’m not with that station so I can’t refer back to my notes.

  • Aw, rats, that is a K and not an omega. And looks like supply rail and bias voltages are nowhere near high enough to make it interesting. My blushes.

  • Paul Thurst

    Final test results: C-12 is good, Q1B is shorted drain to source.

    @Roberta, I may make a test jig for this module, we have many of these transmitters in the field, it would be nice to repair the pallets and stock them as spares.

    @Tom, I agree, Nautel customer support is excellent and I have nothing but good things to say about their products and the level of support I have experienced over the years.

  • I had 2 V-10 transmitters on-air from 2009 but not yet had symptom like that.
    Anyway, I feel not happy with PAs from V-10 series; the PAs from Z16 (Harris) working better, long life !

  • So, how bad is it to change that dual power FET? 🙂 (I remember the big fun in changing output transistors in a Mosely STL transmitter at a lighting-prone location. Eventually the mfr decided they weren’t field-replaceable and would only sell us new PA modules, but by then, we’d put in decent transient suppression in the coax.)

    FWIW, the other interesting Harris trick was the Platinum module test fixture could connect the module output to a dummy load instead of the combiner, so you could measure Po into a known-good load. That might be a handy thing to have if/when you build yours.

  • Kent Teffeteller

    Paul, why not see if you can repair it so you can have a spare? Never know when you might need it.

  • I think I must have “The Knack” – when I saw you mention the burnt resistors were in the bias supply I immediately thought “FET gate insulator punctured by static; FET shorted.”

    Is that… a good thing or a bad thing that I diagnosed it in three seconds flat? I’m doomed to be an engineer forever, aren’t I?

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