Gates MW-5 Problem - FOUND IT

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CRippel
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Gates MW-5 Problem - FOUND IT

Post by CRippel » Thu Aug 10, 2017 7:45 am

Found the problem on Saturday. We could get the transmitter up to 2,200 watts out then it would shut down due to a DC overload. Turned out all the lights put one person behind the transmitter with the doors open and the aluminum cover off the PA section and repeated the test. Sure enough, at 2200W out, there was a flash which reflected on the overhead above the modulator compartment. Stood on a ladder, lights off and looked down into the modulator compartment and at 2200 watts, there was a fat arc from the tube to side of the cabinet. ???

There is a large damper diode string which is part of the high level PDM. Both ends of the diode string need to be attached the modulator plate/HV and someone before me had pulled the strap off the near end of the damper diode string. They had also removed and tossed out the 2 damper diodes under the modulator tube. Replaced the missing strap and had the problem repaired and transmitter making making full power inside of 10 minutes.
Last edited by CRippel on Mon Aug 21, 2017 8:10 pm, edited 1 time in total.

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RGORJANCE
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Re: Gates MW-5 Problem

Post by RGORJANCE » Thu Aug 10, 2017 2:44 pm

Can you tell us what plate voltage and plate current are running as the overloads occur, and what they are with the rig running low power?

Several possibilities pop into my mind.....damper diode breaking down - possible bad cap/s in the output network - modulator tube shorting out. Been MANY years since I worked on an MW-5.

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RGORJANCE
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Re: Gates MW-5 Problem

Post by RGORJANCE » Thu Aug 10, 2017 2:48 pm

I should have read your post a bit closer.......disregard my thoughts, the trouble probably isn't where I was thinking. I will keep scratching my head for other ideas.

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Shane
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Re: Gates MW-5 Problem

Post by Shane » Thu Aug 10, 2017 8:51 pm

The HV is close to spec but the rig is only running 500 watts. What would the HV be if you could run full power? Methinks it would be less, maybe a lot less, but you can't get there to see it.

I didn't see anything in there about a blocking cap.
Mike Shane, CBRE
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RGORJANCE
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Re: Gates MW-5 Problem

Post by RGORJANCE » Fri Aug 11, 2017 6:24 am

Here are some typical numbers from an MW-5A factory test data sheet. The A version included the ME-1 modulation enhancer. Xmttr color is black and blue. Earlier MW-5 was a beige grey, and did not have the ME-1. Otherwise, readings for the two should be near the same----they will vary a bit from unit to unit/frequency.

CARRIER-NO MODULATION
OP FREQ 1590KHZ
OUTPUT IMP 50 OHM
EP-5,375
IP-1.17
GRID I-233
TPO-5,600 WATTS
REFL-0
PA EFF - 89%

SUPPLY E 14.4 KV
SUPPLY I 0.43 AMPS
RF DRIVE E 54 VOLTS
RF DRIVE I 8 AMPS
IPA I 1.02 AMPS
MOD SCRN V 400 VOLTS
MOD SCRN I 0.41 AMPS
AUX DRIVE I 4.5 AMPS
MOD BIAS E (-)145 VOLTS
AUDIO DRV V 100 VOLTS
AUDIO DRV I 137 AMPS

XFMR taps set for 230VAC source

I hope these readings will help diagnose the problem.

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RGORJANCE
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Re: Gates MW-5 Problem

Post by RGORJANCE » Fri Aug 11, 2017 6:34 am

I should have included lower power readings.

At 1,000 watts - these readings are with 100% mod:

IPA I 0.52
EP V 2,575
GRID I 245 ma

2,500 WATTS TPO

IPA 1 .78
EP V 3,700
GRID I 240 ma

Just spotted a note that says grid current must be 225 ma, minimum for power level.

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CRippel
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Re: Gates MW-5 Problem

Post by CRippel » Tue Aug 15, 2017 7:13 am

Yep, the HV would be lower were the MW-5 make power. Blocking cap replaced with a new one.

In thinking about it (more), it almost seems to me (by virtue of the difference in the exhaust air temps in the PA, Modulator 140 degrees, PA nearly ambient) that the PA is not amplifying. Is it possible the power being displayed is "spill through," for lack of a better term) from the modulator. I mean, jeeze, the PA is dirt simple in that transmitter. Put HV on the plate, heat the filaments (which are heating) add drive to the grid and away it goes.

BTW, the driver stage will make up to 260 MA. Also:

IPA I 0.52
EP V 2,575
GRID I 245 ma

Can make the IPA current and Grid current. Not the HV. Can get to maybe... 800V or so before overload. No sign or anything arcing over or breaking down. Each cap tested with an old fashion crank-generator megger which puts out about 350 V. Now, 350 is not 800 but it should give a clue, yes ? :?:

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RGORJANCE
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Re: Gates MW-5 Problem

Post by RGORJANCE » Tue Aug 15, 2017 7:31 am

It has been about 25 years since I worked on an MW-5 so I am really rusty. Only thoughts are possibly the damper diode breaking down as the voltage comes up.

I seem to recall replacing a coil that might have been part of the PDM filtering but can't recall the symptoms. I had one other instance that sounds similar that turned out to be a flakey buss fuse on either the PDM board, or audio board.

Other than these, I am outta ideas.

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