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Author Topic: FL-2100B Plate Current Drifting  (Read 2515 times)
M1BJR
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Posts: 13




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« on: January 13, 2013, 02:47:43 PM »

Hi, I acquired one of these as a budget intrduction into valve amplifiers.

I have read most all there is to read, and I might make a few small mods as many recommend.
I know all the shortcomings, and the design flaws but for the lunch money it cost it will do fine for now (even if it becomes a parts bin later)
But before I go ahead 'improving' I need to fix an odd problem I'm having with the tuning....

After tuning up (on any band) I see decent output (600w on old Taylor tubes) and all works as expected until I re-transmit again a while later.
For max output ~ 600w on 10m, I'm drawing 0.5A at 2400V plate (drops to 2kV on load) which is about right I guess.

But - when I key down again, say 20 seconds later, the plate current is a lot higher and the output is way down. Some 300w out and over 600mA plate.
The current falls slowly back to the original tuned value over a few seconds as power again rises to normal.
If I call full power within a few seconds it delivers okay.
I don't seem to have any weird oscillations going on but have noted a slight AC hum on the carrier at full bore.

So it seems heat related.
Could anyone suggest what is happening so I can learn how to fix it please?

Thanks in advance,
Steve M1BJR
« Last Edit: January 13, 2013, 02:50:29 PM by M1BJR » Logged
AD4U
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Posts: 2152




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« Reply #1 on: January 13, 2013, 03:24:48 PM »

Your initial 2400 HV and 500 mA plate current and about 600 watts is exactly what you should be seeing.

I have no idea what the problem may be.  Since this amp is "new" to you and you apparently do not know its use history and since it is 30++ years old I have a few suggestions.

First look the amp over very closely top and bottom for burned components, expecially resistors and especially around the bases of the tubes.  Check the wiring to make sure everything is where it is supposed to be.  You do not know the history of this amp.  Clean all switches etc with DeOxit and exercise them.  Clean all relay contacts.  Check the parasitic suppressors on the anode of each tube to see if the resistors are burned, bubbled, or open.  If possible try another set of tubes.  My guess is this is where you will find the problem.  If nothing else helps replace the HV filter caps and while in there replace the HV diodes.

Obviously I am just shooting in the dark, but if it were mine, this is where I would start.

Dick  AD4U
« Last Edit: January 13, 2013, 04:17:02 PM by AD4U » Logged
KA5N
Member

Posts: 4380




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« Reply #2 on: January 13, 2013, 04:11:27 PM »

You're drawing too much current and not putting out enough signal.  Look at the tubes
when you have this condition and note the color of the anodes.  If slightly red, probably
OK if cherry red then look for grid bias problem (could be bad components).  If you
have another set of tubes, try swapping them out.  Reduce driving power and see if
problem goes away.  Look for overheated parts and resistors that have changed
values.
Good Luck
Allen KA5N
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M1BJR
Member

Posts: 13




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« Reply #3 on: January 13, 2013, 04:45:56 PM »

Thanks Guys....

I will add, after a little cooling period and keying again the high plate current can be brought down with a small adjust to the plate tune knob.
Otherwise it will drift back down 'into tune' ina  few seconds as it warms again.

Its like the capacitance has shifted quite a lot with the heat. Basically you need to keep chasing the plate tune....

Noted earlier that peak output does not coincide with minimum plate current either.
Its peaking output with a bit less capacitance than it should and so drawing an extra 0.1mA or more.

Internals look (to the eye) in good shape for 30yrs, nothing burned or obviously overheated, but I haven't gotten around to measuring stuff yet.

Am enjoying learning all over again.
I was never much with electronics to begin with but this is reminding me what the hobby is about after a ten year absence Smiley

Steve, M1BJR

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KD0REQ
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Posts: 849




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« Reply #4 on: January 14, 2013, 03:02:26 PM »

consider a read on the W8JI website... http://www.w8ji.com/fl2100_problems.htm  and http://www.w8ji.com/hf_stability.htm .  verify the present amp against the schematics, check your ground paths for really good ground, and then consider modding and testing after each change.
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G3RZP
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Posts: 4360




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« Reply #5 on: January 15, 2013, 05:55:20 AM »

Might be worth looking at the coupling capacitor between the anodes and the pi network. If that drifts with heat - which it may well - the effective tuning capaciy will change.
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KE3WD
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Posts: 5694




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« Reply #6 on: January 15, 2013, 06:27:43 AM »

Might also happen from loose or dirty tube socket connects. 

As said already, "could be anything" and basic troubleshooting of amps and the proper safety techniques to protect yourself while doing so are in order. 


73
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K4DPK
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Posts: 1077


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« Reply #7 on: January 18, 2013, 11:12:08 PM »

To me, it sounds almost certainly like a capacitor heating up and changing value.

Since the change can be corrected by a small variation in plate tank tuning, it will probably either be the plate blocking capacitor (as Pete mentioned) or one of the fixed padding capacitors to ground across one of the pi-net variables.

Phil C. Sr.
k4dpk
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KC4MOP
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Posts: 729




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« Reply #8 on: January 19, 2013, 03:56:33 AM »

To me, it sounds almost certainly like a capacitor heating up and changing value.

Since the change can be corrected by a small variation in plate tank tuning, it will probably either be the plate blocking capacitor (as Pete mentioned) or one of the fixed padding capacitors to ground across one of the pi-net variables.

Phil C. Sr.
k4dpk
Definitely could be defective caps in the tank circuit heating up and changing value. The Ten Tec Titan amplifier was plagued with that problem on 160M. The caps were the mica type mounted deep inside the amp.
Fred
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M1BJR
Member

Posts: 13




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« Reply #9 on: January 21, 2013, 03:12:46 PM »

Thanks for the ongoing input guys.

I have located a much higher kV, but new, 1000pF doorknob cap for the plate coupling.
All of the caps look in good order, no bubbling or cracks even viewed with a torch.
But it must be something simple like this, so will try this one first and then the other two to ground.

I have replaced the PSU caps whilst waiting for these bits from Russia, and the AC hum is reduced and 2.4kV is still stable.

The parasitic resistors look like new but I may as well chop them out. They say 2Watt but they look way to fat for that?
Hard to find data for resistor voltage ratings - are they just large becuase of this factor, or simply as they are so old ?

The PA guy has a mod to disconnect the front panel V/C and using some RG213 take the ground directly to near the tube base.
IS this worth doing as I do notice some instability on 10m when tuning at full power too>?
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G3RZP
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Posts: 4360




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« Reply #10 on: January 21, 2013, 11:02:03 PM »


>The PA guy has a mod to disconnect the front panel V/C and using some RG213 take the ground directly to near the tube base.<

Sorry, I don't understand this. Can you explain in more detail? Plus exactly what do you mean by 'V/C'?

If the parasitic resistors are OK on value, I'd say leave them alone, although my experience with 572Bs is that they can be a bit of a bear to cure parasitics on, especially on 10m. Not an ideal valve, being rather too physically long in the internal leads.
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M1BJR
Member

Posts: 13




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« Reply #11 on: January 26, 2013, 02:52:02 PM »

http://pa0fri.home.xs4all.nl/Lineairs/FL2100Z/fl2100zeng.htm

Variable Capacitor.

Anyhow, a rainy-day project now.

This Acom1000 is just TOO good Smiley
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M1BJR
Member

Posts: 13




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« Reply #12 on: March 08, 2013, 12:26:54 PM »

Phil & Pete - thanks.

I replaced the plate blocking cap (1000pF doorknob 3kV) with a NOS Russian one rated 6kV and its working just great.
Everything is much more stable, not so much weird SWR swings during tuning either. 10m will still break into oscillation if you are not careful,
but that only seems to happen with 100w drive - its quite possible its the Flex TRX having a fit actually due to feedback so needs more exploration.

I have a spare 1000pF 6kV new doorknob if anyone needs one, PM me or my email is on QRZ dotcom as I had to buy a pair from Ukraine.
The threads are both metric but a little bigger, so I brazed a couple of stanless screws back to back to make up the shorter distance and adapt threads.

Happy days!
« Last Edit: March 08, 2013, 12:29:46 PM by M1BJR » Logged
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