MW10B-A little help?

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FirebirdTN
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MW10B-A little help?

Post by FirebirdTN » Wed Apr 05, 2017 3:25 pm

I know..its older than dirt. You know how it goes...shoestring and bubblegum budget...

Anyway, station went off-air. Upon arrival here is what I found:

No faults. PA volts pinned. PA current low. Power out low. Audio driver and aux driver readings screwed up (don't have notes in front of me for exact values). All other multi meter readings normal (Screen, IPA, Supply, RF Driver, etc).

I know there is a "feedback" from the PA to the PDM board I believe it is, so after readjusting the power pots this is what I have now:

All multi meter readings pretty much normal. Low PA current. Low power out. I can get a KW out of it before things start to go south.

Here is something that really stood out to me:

When adjusting the power pots to increase power (starting from zero watts out), plate current goes up, but Grid current goes down. It gets to a point where grid current is 0, and that is about all she wrote. Any further attempt and increasing power only increases PA voltage. Power out doesn't change.

I am just not intimately familiar with this transmitter. If anyone can at least give me a clue I would greatly appreciate it!

Tubes are 2 1/2 years old.

Thanks,

-Alan

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Shane
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Re: MW10B-A little help?

Post by Shane » Wed Apr 05, 2017 10:28 pm

What's the tube complement?
Mike Shane, CBRE
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FirebirdTN
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Re: MW10B-A little help?

Post by FirebirdTN » Thu Apr 06, 2017 6:55 am

4cx15000a modulator
3cx15000h3 PA

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Dale H. Cook
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Re: MW10B-A little help?

Post by Dale H. Cook » Thu Apr 06, 2017 10:30 am

What does the PDM waveform look like? It is one of the first things to check in a PDM/PWM transmitter as the duty cycle must be correct for proper transmitter operation.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
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FirebirdTN
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Re: MW10B-A little help?

Post by FirebirdTN » Thu Apr 06, 2017 11:51 am

I am about 99% sure its just the tubes. Don't I feel stoooopid.

I am a tad younger than most engineers I know, so I lack the "tube" experience. All my electronics training/schooling was solid state.

I normally change the tubes in this transmitter about every 2 years. I don't just change them on a time line; I normally wait until positive modulation starts falling off, which is normally pretty gradually. I have noticed in the last couple months it has been falling a little bit, but nothing to write home about. I have never seen a set of tubes fall flat on their face so quickly.

Anyway, I went back out there and ran it at 2kw. As I started to increase the power, positive modulation fell flat, and the grid current went down. When I hit that point, I reached down and cranked the filament control to max (over volted them). Power came up, grid current came up, and my positive modulation came back as well.

Thanks and sorry for the time waster.

-Alan

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Re: MW10B-A little help?

Post by RodeoJack » Thu Apr 06, 2017 3:22 pm

Might be the tubes.

On the other hand, it seems odd that you'd go from 10kW to less than 2 without noticing it on the way. That's a sudden drop to lay at the feet of an aging tube.

This might seem simplistic, but considering you're getting such a dramatic change by raising the filament voltage... are you sure there's not a problem there?

Numerous times, I've read posts from other engineers, advising resistance to blame sudden problems on what seems the most obvious... the tube(s). Thinking back over 50 years of working with various rigs, I've had shorts crop up in a very few power tubes, but overall, they've been far more reliable than one might expect.

FirebirdTN
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Re: MW10B-A little help?

Post by FirebirdTN » Thu Apr 06, 2017 3:59 pm

I would agree. Another tip that I picked up from somewhere (can't remember now), was it is almost NEVER the tubes. That is also why I was second guessing myself. Even though not long ago I had a filament open up in a 6 month old tube in a BE FM25T LOL.

But in this case...oh, how do I say this in a politically correct fashion:

There are no "daily log readings" to track the recent history of the transmitter performance. Last time I was at the site was a month ago, and things were fine then. As I said I had been noticing some loss of positive modulation the last couple months, but nothing earth shattering.

But I agree, that is a short period of time for it to fall flat like that.

-Alan

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Dale H. Cook
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Re: MW10B-A little help?

Post by Dale H. Cook » Fri Apr 07, 2017 11:16 am

FirebirdTN wrote:
Thu Apr 06, 2017 3:59 pm
Another tip that I picked up from somewhere (can't remember now), was it is almost NEVER the tubes.
That is why I asked "What does the PDM waveform look like?" It is something that you can easily 'scope, and I have sometimes seen PDM problems blamed on tubes by insufficiently experienced engineers.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
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davedybas
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Re: MW10B-A little help?

Post by davedybas » Fri Apr 07, 2017 4:13 pm

I've worked on MW-5 in the past and ran across similar symptoms when a some resistors in the DC supply opened up. IIRC these were power resistors that were located near the front of the rig and were inside on the swing open door. It's been many years since, so I may be imagining things :roll:

Good Luck !

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Shane
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Re: MW10B-A little help?

Post by Shane » Sat Apr 08, 2017 6:41 pm

That could just be it...the resistors, not that you're imagining things, Dave!
Mike Shane, CBRE
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RGORJANCE
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Re: MW10B-A little help?

Post by RGORJANCE » Sun Apr 09, 2017 6:24 am

I am putting my two cents in here, and probably oing out on a limb at the same time.

"Back in the day" I had an MW-5 go down. Don't recall exact symptoms, but might have been similar.

It acted like the solid state driver had failed. No stock on the shelf in Quinky at the time. Traced things out and decided to use a "remembered trick" with a clip lead to bring up drive thru pdm. This was after thorough check of fuse continuity.. Placed clip lead in the "proper places" and heard a slight "POP". Blew fuse as I forgot they changed the connection sequence. Replaced fuse and rig came to life at full power.

What had happened was that the fuse had developed a resistive path connection in the end cap that would show continuity and voltage, but would not pass enough current to light up the solid state driver module. Second time I got caught in that same trap. First one was in a fan circuit in a car. Duhhhhh!

Fossil

FirebirdTN
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Re: MW10B-A little help?

Post by FirebirdTN » Tue Apr 11, 2017 3:06 am

Just thought I would post an update:

It WAS just the tubes. They came in yesterday, and it tuned right up. Thanks again to everyone who chimed in though. There may come a time when I get stuck again.

This thing has lived a hard life. And it does seem to break down I swear once a month. I have been pretty lucky so far in I have been able to tackle most of its issues, but every once in a while I end up scratching my head...

Thanks again to all,

-Alan

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Deep Thought
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Re: MW10B-A little help?

Post by Deep Thought » Tue Apr 11, 2017 7:58 am

I know this isn't really on topic, but maybe it will help you or someone else later...

A client replaced a MW-10 with a Nautel XR-12 awhile ago thinking that it would pay for itself in about 5-6 years with savings on the electric bill. Between the electrical savings, the elimination of tube replacement costs, the lower heat load in the transmitter building and almost $2000/mo in repair costs, it ended up being about three.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

FirebirdTN
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Re: MW10B-A little help?

Post by FirebirdTN » Wed Apr 12, 2017 6:52 am

Deep Thought-

I ABSOLUTELY WHOLE-HEARTEDLY AGREE!!!

I don't do a whole lot of contract work. But most of the ones I do work for are very penny-wise, but dollar foolish. They would rather spend 1k a month maintaining something, than spend 10k all at once to replace it with something more reliable, even when you present them with the logic.

The other issue at this particular location is I don't think a new solid state transmitter would survive the environment this old MW10 operates in. Yes, the facilities are in that bad a state of dis-repair. I have voiced my opinion about it, but I guess they will just keep doing what they are doing until the towers fall over or building collapses, then they will deal with it, or just close shop.

-Alan

vacuum tube
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Re: MW10B-A little help?

Post by vacuum tube » Wed Apr 12, 2017 7:09 am

The old broadcasters rule of thumb "we're going to save $1000 even if it costs $10000"

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