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Author Topic: al-811h load control fully clockwise on 80m  (Read 12674 times)
W3RSW
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Posts: 606




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« Reply #30 on: May 15, 2016, 07:25:08 AM »

Ok Lou, did that, makes sense. Be interesting to see what's found.
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Rick, W3RSW
WA1RNE
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Posts: 1010




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« Reply #31 on: May 15, 2016, 08:15:58 AM »

Trust me, I have used a dummy load. This isn't my first rodeo. But for what it's worth, I'm using an external antenna tuner, so the amp doesn't see the antenna anyway. Is sees the 50 ohm load of the tuner. The main reason for tuning into a dummy load, is: A) You don't have a tuner; and B) You don't piss off everyone on the frequency who might be trying to use it. But again, there is not difference at all.

 I read this post to mean that you've used a dummy load before and didn't need to bother because your confident about the load the external tuner presents to the amplifier.

 Now that you've clarified that you have used a dummy load, you are getting close to 500 watts output and grid current is near the maximum rating for the tubes, the problem - as W1QJ and others have suggested - is more than likely to be that the front panel loading knob is installed with its pointer set to "10" with the capacitor at maximum capacitance.

Verifying that must be your next step.

Aside from that, the lack of additional loading range may be due to a bad loading pad capacitor and/or a switch contact that is open, either because it overheated, oxidized or has a mechanical issue and needs to be repaired.  Again, you need to get inside and verify these things.

Of course, going inside means SAFETY FIRST - unplug the amp from the mains and make sure the HV supply capacitors are discharged.

The good news is, these are all fairly simple issues to repair and you should have a normally running amplifier in the near future.

 ...WA1RNE
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WY7CHY
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« Reply #32 on: May 15, 2016, 08:36:34 AM »

I have checked the knob and meshing. They are fine. And as mentioned previously, all other bands load exactly where they should. The one thing I haven't done is to desolder the 3 caps on the band switch to check each to see if their capacitance is good or not. I'll probably get to that next. Thanks.
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Born Wild - Raised Proud: 73
Cheyenne, Wyoming
WA1RNE
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Posts: 1010




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« Reply #33 on: May 16, 2016, 10:15:30 AM »

Quote
The one thing I haven't done is to desolder the 3 caps on the band switch to check each to see if their capacitance is good or not. I'll probably get to that next. Thanks.

 Before unsoldering those capacitors unnecessarily, I'd suggest checking continuity between the amplifier Output connection at the 160 meter inductor and each capacitor lead first as the switch may be open.

 ...WA1RNE
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WY7CHY
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« Reply #34 on: May 16, 2016, 12:59:24 PM »

Not sure I follow. The bandswitch works on all bands. Even the 80m. The problem isn't that it won't load or tune 80m. Just that it requires full load (Least capacitance scale of 10). At that point, I can get about 475-500 watts CW with about 40-50 watts input. That's normal; but to get more input power in (60-65 watts) to get the rated 600 watts CW, the grid current would go too high. Normally, I would increase the load control higher to reduce grid current; but being it's at 10 already and physically can't go any further, that's not an option.

If the bandswitch was open, I think there'd be a problem operating on that band.

Please let me know if I'm misunderstanding what you are saying. Thanks. Mike.
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Born Wild - Raised Proud: 73
Cheyenne, Wyoming
W3RSW
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« Reply #35 on: May 16, 2016, 01:35:22 PM »

If the three fixed load caps are marked with values, check to see if they are in correct order before unsolderimg anything. If 500pf is switched in where 360pf is expected that would explain a lot. Seeing the QC problems at the new Ameritron, won't surprise me what's found in assembly for a series of rigs before factory caught it.

Fwiw, several others have noted somewhat lower output on 80 as the nature of the amp.
Can you get a plate current dip on 4.0 mHz but not on 3.5 before full load knob rotation?

Unless new under warrenty, I know what I'd experiment with.  Also check to see if your 80 meter coil tap matches pix or others' same model.  A few online pix have sufficient resolution to count the turns.  A kind soul may count them in a better working rig for you.
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Rick, W3RSW
WY7CHY
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Posts: 953




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« Reply #36 on: May 16, 2016, 01:53:10 PM »

I'll definitely try and take a look. It's a little tight for viewing. I do know that this amp is one of the "Older" ones. You can tell because it doesn't have the newer style Parasitic Suppressor on it. It's the old resistor style wire wound soldered to the star connector. Not sure if the caps in the "Wrong Order" would have any affect on the other bands. The other bands are proper load and plate; as suggested in the owner's manual. They're perfect. Just the 80m. It's possible one of the capacitors is bad. I'll have to look at it. Thanks. Mike.
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Born Wild - Raised Proud: 73
Cheyenne, Wyoming
WB2WIK
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« Reply #37 on: May 16, 2016, 03:27:46 PM »

"Cutting the green wire" can't have any impact on 80m or 160m.

All it does is lift the ground connection that shorts out the input circuit on 10/12m, opening that circuit so it can work properly.

There's no connection of any kind to the other bands.
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WY7CHY
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« Reply #38 on: May 17, 2016, 07:02:44 AM »

That's what I figured. The only thing I can figure, is that it might be one of the caps on the switch
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Born Wild - Raised Proud: 73
Cheyenne, Wyoming
W3RSW
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Posts: 606




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« Reply #39 on: May 17, 2016, 07:23:14 AM »

I can see a cap failing open but not with an increase in value without totally shorting. Your maxing out the load control all way to right indicates too much total capacitance without out any from the variable in line, not too little on 80.

All three padders totaling 1220pf are switched in on 160 meters so no op. diff. expected there.
Only one at 360pf is switched in on 40 meters, so the question is 80 meters.  According to the schematic that position should total 860pf. (500 and 360). Maybe too much.  Hey, you might even find the 500 is actually an 800, stock room error into the parts bin...  Yeah for 20 of the builds before someone caught it.  Wink

I'm wondering if C24 at the 160 tap should be the 500pf and the 360's reserved for 80 and 40 meter switch positions.

Coil taps also as mentioned to be looked at.  For the 80 meter tap I count about 11 turns back from the 40 meter end of the coil.  Who knows with Ameritron. Other errors abound in the schematic. No DC at one of the filaments, etc.

Coil /cap boards common on parts list for rhe three tube vs. four tube version?
Pi and Pi / L nets not super critical but for a 25 to 33% different high impedance load? ...another Ameritron approximation?  Lots of stuff to check with others' build years.
« Last Edit: May 17, 2016, 08:06:02 AM by W3RSW » Logged

Rick, W3RSW
W3RSW
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Posts: 606




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« Reply #40 on: May 17, 2016, 08:18:12 AM »

Continuing , I notice that earlier builds seem to have brown micas for the padders, later builds the yellow differing type, ceramic? Current handling probably marginal. I'd substitute two parallel half values for each switch position, but particularly on 160 and 80 even if I found all working ok.

A lot of padding design is simply to make sure the tune and load controls are somewhat in the middle of the rotation range but with the rotation slightly increasing in intuitive order with reasonable efficiency.   You'd think efficiency would come first. Luckily there's agreement for the most part.  (LC)^1/2 rules.  That's why the sum of all three padder caps is not as critical on 160 as two of them on 80.
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Rick, W3RSW
WA1RNE
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Posts: 1010




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« Reply #41 on: May 17, 2016, 10:35:32 AM »

Quote
Not sure I follow. The bandswitch works on all bands. Even the 80m. The problem isn't that it won't load or tune 80m.

 Yes, but band switch wafer S1B also switches the 3 padder capacitors in and out of the circuit depending on the band - and you seem to be lacking a significant amount of load range on 80 meters - the load control should not be set near full clockwise (10) with a 50 ohm resistive load.

 All of the padders have one wire grounded:  the other lead gets switched in depending on the band, putting the capacitors directly across the output. Going by the schematic, it looks like three are switched into the circuit on 160 and two for 80 meters.

 So if you attach one lead of an ohmmeter to any connection you can find on the output tank coil and the other on the ungrounded side of each capacitor, you can confirm whether each of the capacitors are being switched into the circuit depending on the band switch position. You may find that one of the two needed for 80 isn't being connected which would account for your loading difficulty. You could still have a bad capacitor but if that's the case, you'll typically see physical signs of failure from overheating.

Make sense?

 By the way, you never mentioned if you are able to load the amplifier on 160 into a dummy load. If you have a similar problem or worse, the padders and/or the switch become prime suspects.


 ...WA1RNE
« Last Edit: May 17, 2016, 10:40:26 AM by WA1RNE » Logged
KJ4QXC
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Posts: 1




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« Reply #42 on: November 16, 2016, 12:50:46 PM »

Anyone find a fix for this I have a amp with to much loading capacitance only on 80
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W7JHN
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Posts: 56




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« Reply #43 on: November 25, 2016, 10:15:06 PM »

FWIW I have an 811H and just checked the settings for 80 meters. The load control is at about 5.4 with the plate set at about 5.1. I drive it with 60-65 watts and get 500+ out.

Using a watt meter I tune for max power out. Only look at amp meters to make sure I don't exceed max grid or plate currents.

The load control will not go past 10 on the scale.
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