Gates 5 failure

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ChuckG
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Gates 5 failure

Post by ChuckG » Wed Mar 09, 2016 7:12 pm

Final voltage pinned full-scale, no final current, PDM and RF drive readings low. No power output. Plenty of smoke.

Kicked the standby on, opened the rear door and heard a loud CRACK when the shorting bars hit. Found a bad bleeder resistor.
Then found what's in the pic below: one of the mica caps failed.
This is in a group of three parallel caps mounted in the rear right corner (from the front).
Also found about half the fuses on the PA boards blown. I'm assuming the FET pairs behind the fuses are bad.
Fuses elsewhere, including the PDM amps are all OK.

Q's:
Would you replace all three caps, even though just one has visible damage, and
Is it a fair assumption the blown cap damaged the FET's, or should I be prepared to find other hidden issues once I power it back up? Anything in particular I should check before putting power back to it?

Fist time I've had to fool with the tuning net in a Gates series. Thanks for any advice.

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Chuck Gennaro
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Shane
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Re: Gates 5 failure

Post by Shane » Wed Mar 09, 2016 8:45 pm

If you replace all of them, you'll have two spares for next time! I know with some owners that's the only way you can get spares. :lol: I'm sure you already knew this but that's about all the help I can be on this one.
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Re: Gates 5 failure

Post by Deep Thought » Wed Mar 09, 2016 10:59 pm

I can't tell where it is in the circuit but those don't generally "wear out". Usually in an RF circuit Thor has had something to do with a failure, but it is easy to check the other two for correct value and ~0 ohm ESR by using a bridge. With all those fuses and FETs out I would also suspect something up the power line especially considering the blown bleeder resistor.

Then again, those transmitters are notorious for blowing fuses for no particular reason.
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ChuckG
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Re: Gates 5 failure

Post by ChuckG » Wed Mar 09, 2016 11:51 pm

Shane wrote:If you replace all of them, you'll have two spares for next time! I know with some owners that's the only way you can get spares. :lol: I'm sure you already knew this but that's about all the help I can be on this one.
Unfortunately they are each a different value, which foils that plan. However...having spares for two of the three means I'll know which one has failed next time. :lol:
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Chuck Gennaro
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ChuckG
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Re: Gates 5 failure

Post by ChuckG » Wed Mar 09, 2016 11:58 pm

Deep Thought wrote:I can't tell where it is in the circuit but those don't generally "wear out". Usually in an RF circuit Thor has had something to do with a failure,
I had a minor shift in the volts/amps ratio in that transmitter last summer and never could find the cause. All those caps up top were cool to the touch and measured correctly with a cap meter. I probably just found the culprit.
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Re: Gates 5 failure

Post by BroadcastDoc » Thu Mar 10, 2016 6:43 am

The Gates series were known to blow fuses by the simple act of walking into the transmitter building. :lol:
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Re: Gates 5 failure

Post by KPJL FM » Thu Mar 10, 2016 8:10 am

BroadcastDoc wrote:The Gates series were known to blow fuses by the simple act of walking into the transmitter building. :lol:
Or walking out...
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Re: Gates 5 failure

Post by PA_TUNE » Thu Mar 10, 2016 10:57 am

Deep Thought wrote:
Then again, those transmitters are notorious for blowing fuses for no particular reason.

That's for sure!

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Re: Gates 5 failure

Post by Kelly » Thu Mar 10, 2016 3:20 pm

ChuckG wrote: Q's:
Would you replace all three caps, even though just one has visible damage,
I'd say if you can get your hands on one of those capacitance meters, just check to see if the value changed, out of the circuit of course. If the value comes up +-10%, they are probably okay.
ChuckG wrote: Is it a fair assumption the blown cap damaged the FET's, or should I be prepared to find other hidden issues once I power it back up? Anything in particular I should check before putting power back to it?
If the fuses aren't blown on the PA FET's, they're probably okay. You can confirm with an Ohm meter, as generally they short and cascade.
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Re: Gates 5 failure

Post by ChuckG » Thu Mar 10, 2016 11:51 pm

C2 is three mica caps in parallel. The failed cap is part of that assembly.
Also found the copper strap connecting L1 to C1 burned in half. (pic below)
C1 is an interesting monstrosity, it's 15-ish doorknob caps sandwiched between two copper plates. Hopefully it survived.
11 blown PA fuses, 22 shorted FET's.

All of this precedes the directional coupler, which explains why I saw a minor shift in E/I before the cap. failed completely.. but no change in VSWR. Also explains how all this damage could happen without the rig shutting down.

Just FYI: Harris wants $1,300 for the failed mica cap, $750 and $800 for the other two. Commercial Radio in Vermont has them all for $495/ea. They should arrive on my doorstep Friday.
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Chuck Gennaro
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Re: Gates 5 failure

Post by KPJL FM » Fri Mar 11, 2016 8:24 am

Me thinks that schematic should show a spark gap just before the antenna....
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Re: Gates 5 failure

Post by PA_TUNE » Fri Mar 11, 2016 9:36 am

ChuckG wrote:
C1 is an interesting monstrosity, it's 15-ish doorknob caps sandwiched between two copper plates. Hopefully it survived.
.
Interesting thing about C1. Not sure if you have a daytimer or run 5kW 24/7. Stations that tend to power up and down have had problem with the "capacitor sandwich" assembly. After enough heating and cooling cycles, the copper plates start to pull the electrodes out of the doorknob caps, leading to failure.

Harris has designed modified plates for this assembly that you may want to install. The new design copper plates have reliefs in the copper around the mounting holes that allow them to flex, thereby relieving the stress on the doorknob caps during heating and cooling cycles. Give GatesAir a call about these.... I believe they aren't expensive.

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Re: Gates 5 failure

Post by kcbooboo » Fri Mar 11, 2016 2:12 pm

It looks like a plate with flexible tabs is already present in the photo above. Plenty of segments and screws that could be connecting those caps where the strap blew apart.

Bob M.

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Re: Gates 5 failure

Post by PA_TUNE » Fri Mar 11, 2016 2:40 pm

kcbooboo wrote:It looks like a plate with flexible tabs is already present in the photo above. Plenty of segments and screws that could be connecting those caps where the strap blew apart.

Bob M.
Well lookie there.... he DOES have the new style plates! So much for my attention to the photographs. :)

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Re: Gates 5 failure

Post by Deep Thought » Fri Mar 11, 2016 5:22 pm

"Also found the copper strap connecting L1 to C1 burned in half."

That's interesting, considering that it connects the PA to the rest of the world. Haven't seen something like that happen due to lightning that did not also take out C4 since it is the first connection to ground off the output connection, so you must have had something start an arc from the strap to something else that the transmitter (back panel with the black spots?) was happy to sustain until the strap burned through. This would also explain the dead FETs.

C2 is part of a tank circuit along with L2 tuned to the station frequency. When C2 abruptly changed value you may have had an inadvertent resonance which kicked the voltage at that strap up a few orders of magnitude... :shock:
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