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Author Topic: Just fried my AL-80B  (Read 16536 times)
AE5X
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« on: February 03, 2015, 05:27:31 PM »

Oops...

I had it loaded up properly on 160m. After an operating session, I came back & forgot to turn RF drive down on K3 - drove the amp with 100 watts rather than the normal 60.

Now have no output and input SWR is infinite. I guess I need a new tube, right? And what else may have been damaged?

Thanks,

John AE5X
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KK5DR
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« Reply #1 on: February 03, 2015, 05:44:59 PM »

Not a foregone conclusion.
Does the filament still light up? If yes, then there is hope.
3-500z tubes are very robust and can handle high drive levels for long periods.
It more likely that you burned a band switch either on the input, or output, or both.
That would explain the high input SWR and no output.
The high input SWR is causing your exciter to cut back into protection mode. With little or no drive signal you won't see any output signal.
When you key the amp, does the plate current rise to about 150mA? If you have the EBS turned on, turn it off. Try it again. If you have a rise in plate current, it means the amp is still operational.
Do you have plate voltage? If yes, then there is still a good chance the amp is ok, just in need of a little repair.
Email me privately, and I'll try to help you more.
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W9OY
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« Reply #2 on: February 03, 2015, 05:47:21 PM »

Anything between the input connector and fil choke like the relay or input filter band switch for example.  Does the relay click?  Do you have HV?.  It's pretty hard to blowup a 3-500.

73  W9OY
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N3QE
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« Reply #3 on: February 04, 2015, 06:47:38 AM »

Very unlikely you damaged the 3-500Z. I'm not going to say it's impossible to melt a 3-500Z grid but they are very very robust.

Make sure you have HV. Try other bands. Make sure bandswitch knob setscrew hasn't gotten loose. Check the relays.

Do you have a GOP-100 or other "grid protection mod" in your AL-80B? If so, it may be as simple as resetting it.
« Last Edit: February 04, 2015, 07:07:46 AM by N3QE » Logged
N3DT
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Posts: 1792




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« Reply #4 on: February 04, 2015, 07:26:03 AM »

That's what I was going to say if you have the GOP mod just go to Standby and back to Operate and it clears the GOP. If not something may have been overloaded. I've hit mine with 100 W a few times and nothing happens other than the grid current goes way up but I've got the GOP so it usually goes off but not always.
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AE5X
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« Reply #5 on: February 04, 2015, 07:35:31 AM »

Ok, I've poked around a bit, slept on it and followed the tips suggested previously. I think the tube is ok - filament lights up, HV is normal.

The band switches appear ok as well with no sign of arcing anywhere in the cabinet.

I think the problem lies in the internal QSK card and that the amp is never internally switching to transmit. Receive on the exciter is fine, which would not be the case if I had an infinite SWR as shows when I attempt to transmit. The fact that I have the QSK mod makes it difficult to verify any of this since the relay is out of the picture.

One more relevant thing that I disregarded at the time:
I was on 160m when this happened and it was raining outside - had been all day. To operate on 160m, I use a knife switch to change my 10-80m dipole into a 160m T (using the feedline as the vertical portion).
I sent my callsign and noticed that an external wattmeter (with 'Peak-Hold' function set to Hold) initially read 600 watts at the beginning of each letter and then had decayed to about 200 watts by the end of each letter of my callsign. The AL-80B's built-in meter did not do this and indicated 600 watts the entire time. This led me to believe that the external wattmeter was being influenced externally by RF feedback and was not showing a true indication to what was going on.

I now think the wet external knife switch may have been arcing and that by operating this way for 5 minutes or so, I destroyed something on the QSK card. Unfortunately, the installation instructions for that card do not include a schematic.

John AE5X

(KK5DR - I may take you up on that offer pending what I decide to replace)
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W8JX
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« Reply #6 on: February 04, 2015, 08:26:22 AM »

Very unlikely you damaged the 3-500Z. I'm not going to say it's impossible to melt a 3-500Z grid but they are very very robust.

I disagree. With full 100 watt drive you would exceed tubes rating a good bit and failure from overheating is likely. While a 3-500 is a study amp tube, especially in parallel with another one, it is not properly cooled in AL80 to run at or beyond max rating and live a long life. I would not look for gremlins in circuitry that likely does not exist. Replace tube and learn from mistake. Myself I run 50 watts bare foot as that is my amps drive requirement (sometimes 60 watts) so I do not forget to change drive level when using amp. the 3 db difference is not a deal killer 99+% of time and when I/you go to 1000 watts with amp you get a 12db+ gain over barefoot at 50 watts. 
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Ham since 1969....  Old School 20wpm REAL Extra Class..
KM1H
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« Reply #7 on: February 04, 2015, 09:00:06 AM »

Yep, Id put my money on the QSK board and the likelyhood of the tube still good is close to 100%. Its simple enough to bypass the QSK if the relay is still in place and do a quick test.

Carl
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W8GP
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« Reply #8 on: February 04, 2015, 09:28:52 AM »

Driving your amp with 100 watts, while not necessary, should not hurt it at all. As others have stated, it is something in the input network, the infinite input SWR should be the clue. However, if you want to believe that the tube is bad, I'll be happy to give you $10 for the old one! Grin
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W8JX
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« Reply #9 on: February 04, 2015, 09:36:00 AM »

Driving your amp with 100 watts, while not necessary, should not hurt it at all. As others have stated, it is something in the input network, the infinite input SWR should be the clue. However, if you want to believe that the tube is bad, I'll be happy to give you $10 for the old one! Grin

It can if you overload tube. While 100 watts into a pair of 3-500's is no concern but into one tube if your ducks are not all in a row you can damage tube.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
W8GP
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« Reply #10 on: February 04, 2015, 09:52:39 AM »

OK 'JX, you win....
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K4RVN
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« Reply #11 on: February 04, 2015, 10:01:56 AM »

The first thing I would do is switch to another band on a dummy load if you have one. Use about 40 watts drive and see what you get. If you don't have a dummy load use the antenna on another band. I have forgotten to run my drive down after running barefoot several times on my AL 80B putting full 100 watts drive on it but on SSB mode. No damage done there. You may have blown the 160 meter padder cap(s) if you had arching on the knife switch. Going to another band would eliminate that possibility I think. Check it out in the SSB mode to see if your tube is OK.

Frank
« Last Edit: February 04, 2015, 10:06:10 AM by K4RVN » Logged
KM3F
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Posts: 910




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« Reply #12 on: February 04, 2015, 10:07:49 AM »

I run 95 in at all times to my AL80B so I can use ALC to cut back a small amount to insure no overdrive can take place.
There is no issue with a 3-500 running 100w in on SB.
My tube is 5 years old.
It gets beat much worse running AM.
The amplifier was purchased used with a build date in the late 90s.
If the bias cutoff feature is still active in an AL80B, you will not see any plate current on keyup with no drive.
Good luck.
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AE5X
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« Reply #13 on: February 04, 2015, 10:11:01 AM »

Thanks guys for the input on this. I did switch to several other bands and have no output on any of them. And I have accidently driven the amp with 100 watts before with no negative results, though not on 160m with a unique antenna situation.

I know Ameritron/MFJ get berated a lot but I want to report that I had a fairly long and detailed conversation with a tech there this morning who asked a lot of precise questions on what other symptoms I was having. Very friendly, professional and unhurried. He's leaning toward the QSK board as being the culprit. I now have to decide whether to order $80 worth of PIN diodes (for a "maybe" fix) or the complete $300 QSK board (for a "probably" fix).

I'll update the forum with what I do and the results so it may help someone else down the road.

John AE5X
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W8JI
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« Reply #14 on: February 04, 2015, 10:17:19 AM »

It is extremely UNlikely you damaged the 3-500. The chance of damaging the tube from what you did is so remote it isn't even worth entertaining, unless you physically melted a hole in the glass or anode from turning the anode white hot, and that takes time to do.

It would not have damaged the input circuit, either.

It might damage the tank through arcing if it was not loaded heavy enough, but you should have heard that.

If it has a QSK switch in it, and if you let the SWR get real high at high power, you can easily pop some PIN diodes. If some PIN diodes are bad it can do exactly what you describe. The root cause is SWR, and not drive levels.

By the way, with proper loading you can hammer that amp with 100 watts all day and all night and not hurt the tube or anything else. I certainly wouldn't run it that way, but someone could and they would get away with it. Mis-tuning or a high SWR load are the worrisome things.

You can easily bypass the QSK board, so it should be a 100% fix.

The most likely diodes are the receive diodes and the TX output diodes.
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