Gates 5 PA Low Amps

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Gates 5 PA Low Amps

Post by radiovigilante »

I have a Gates 5 I have been asked to look at that all of a sudden PA Amps went down as did power..Usual amps is around 54 this dropped to 34..All other readings look good..No blown MOSFETS in the PA section..ideas where to look next? This one has us stumped and we are probably overlooking something..
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Re: Gates 5 PA Low Amps


Forgot almost everything I knew about the rig....and only ever worked on one.

Be sure to check all fuses in PA/PDM sections. Something is nagging me about a possible PDM module failure. Sorry I haven't got anything more specific. Stay tuned....there are plenty of experts here. Someone will pop in soon with better ideas.

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Deep Thought
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Re: Gates 5 PA Low Amps

Post by Deep Thought »

Can you peak the PA current by changing the tuning adjustment? There are parts of the output network which are "before" the VSWR detector and can shift the tuning point if they fail. L1/C1, L2/C2.

From the manual:

4.9. Causes for a PA Volts/PA Amps Ratio Change

This discussion covers causes which might not be apparent based on front panel indications and readings.
4.9.1. No +20 Volts to PDM Amplifier Loss of the +20 volt supply to a PDM Amplifier should cause one or more PDM Fault LED’s to illuminate. To be certain this is not the problem, measure the voltage on the fuses on each PDM Amplifier to make sure +20Vdc is present. If not, trace +20Vdc back to its origin via the Interface board.

4.9.2. Impedance Change

The Output network can cause a change in the PA Volts to AMPS ratio. The PA volts to PA AMPS ratio is affected by the impedance seen by the PA module. If the impedance seen by the PA module changes, whether from an antenna impedance shift or from adjusting the front panel TUNING and LOADING controls, the PA VOLTS/PA AMPS ratio will be affected. Small changes will only cause a ratio change. Large
changes will cause VSWR overloads.

Check the Detector Null reading on the multimeter. It should be zero

If the Detector Null reading is zero, but you suspect an Output Network failure, it will be in the L1/C1 and L2/C2 sections. This circuitry is ahead of the VSWR sensing. A failure of C1 will shift the PA Tuning. Rotate the PA Tuning control to peak the PA current. The PA Amps should peak within three small divisions on the PA Amps meter. If the PA Tuning has to be changed by a considerable amount to peak the PA current, it is indicated that there is a problem with L1/C1. Inspect the C1 capacitors for any physical signs of stress.

A failure of C2 will shift the PA load resistance. You can measure the impedance looking into the Output Network to determine if there is a problem with L2/C2. This is done by connecting an RF bridge in place of the A1 output connection. An RF resistance significantly different than the test data value for “Combiner Load Impedance” would be an indication of an L2/C2 problem if the impedance at the back end of L2 is verified to be 50 ohms j0. The parallel resonant frequency of L2/C2 may also be checked per the procedure in Section III (Maintenance).

4.9.3. PA Failure

Although a rare condition, an open PA transistor could cause a ratio change without blowing the associated fuse. One way to isolate this problem would be to disconnect PA transistor pairs by removing fuses to see if one particular pair is not drawing any


Pulling one fuse at a time to observe its affect on the transmitter operation is one way to isolate a non-working amplifier. If no change is observed after removing a particular fuse, then it is apparent that you have isolated a bad amplifier. Another approach is to remove transistors to test them out of circuit as described in the section on the PA modules. However, do not operate the transmitter with any transistors removed.
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Re: Gates 5 PA Low Amps

Post by TPT »

I have one of these things I have to take care of. Would be prudent to pull the PA boards to check the little three pin plastic terminals that hold the MOSFETS. You are likely to find some of them have burned. At the same time you can check for bad transistors.
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Re: Gates 5 PA Low Amps

Post by RodeoJack »

Deep has covered most of this.

A couple of other possibilities?

Most of those 5s are 3 phase. Is your "Supply volts" up around 260? If not, I'd check incoming power (don't know if it will light up with a phase down, but maybe...). If not, then a PDM amp or two could do it (should have alarm lights on the front panel though). There could be a problem in the power supply, though that's the one area I've never had a failure in.

Check the aluminum strip below the PA boards. If the screws work themselves comes loose, the area around them will burn (melt) and you'll eventually lose power to half the deck. Last time I needed one of these, Gatesair couldn't find it and declared the whole transmitter obsolete. I made one with some copper strap.

I have several of these transmitters. When they're happy, they run pretty well. If not, they can test your personal boundaries.

If / when you get this worked out, you'd be well advised to follow the service bulletin regarding periodic change of power supply capacitors. This doesn't sound like your problem, but those caps wear out over time, and when they do, the transmitter's operation becomes... "unpredictable".
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