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Author Topic: Ameritron Al-811 problem  (Read 8411 times)
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Jerry-n5ugw
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« on: September 10, 2008, 03:52:26 PM »

Hope someone hs an idea where to look, I'm all out. Simptoms are on 80 &160 with 25w AM carrier input I can only get 50w output. All other bands 25 in 120w out in AM. What I've done. Replaced input coupling caps and coils. adjusted coild for lowest SWR input loading. Amp did the same with old parts. Load setting is at lowest or 1 not where the book recomends. It did work and then one day it just started doing this. I have looked and looked for a band switch issue and can not find one at all. The only difference in the band position on these two bands is there are caps coupled to grond on each of these positions. Anyone have any ideas on what to measure and where to look?? Thanks
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WQ9E
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« Reply #1 on: September 10, 2008, 05:19:18 PM »

Jerry,

If the bandswitch contacts themselves are OK then I would be very suspicious of C-24 and/or C-25.  These may not have held up to the continuous current of high level AM.  From the parts list these do not appear to be larger "doorknob" style caps and if they are the smaller dipped micas or postage stamp size their current carrying capacity is pretty limited and they heat up quickly and may have been permanently damaged.

If you have any type of capacitor checker (some DMM's include this function) I would check them.  The schematic I downloaded wasn't that great so it wasn't clear to me when these fixed caps come into play.  On 160 they are both in parallel but I am not sure whether only one is used on 80 or if they are also paralleled on this band.  If 80 and 160 both don't work and only one is used on 80 then start by checking this capacitor since if bad it would affect both bands.  On the other hand if the first one comes into play on 40 and they are both in parallel for 80 and 160 then if it works OK on 40 then only the second one would be suspect.  It will be easy to tell which ones are used where either with a good schematic or by examining the operation as you turn the switch.

If you have an MFJ or similar analyzer you can (make sure the power is disconnected and the supply is discharged) disconnect the tube plate caps and tie a resistor that represents the operating load impedance of the 811A's from a plate cap to ground.  Then connect your analyzer to the output (you will have to manually close the T/R relay contacts) and it should indicate a 50 ohm match with the bandswitch, plate, and load controls adjusted to their normal settings.  The advantage of using your analyzer is that you can temporarily substitute either a small mica or ceramic disc cap of the proper value for C24 and/or C25 to test if they are the problem.  They don't need any sort of high current/voltage rating for this test since you are using the low level signal from your analyzer.

Rodger WQ9E
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Rodger WQ9E
WQ9E
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« Reply #2 on: September 10, 2008, 06:15:34 PM »

As a footnote, if you were doing this test with an analyzer on the higher bands then the internal tube capacitance would be important and you would want to leave all of the tube caps connected.  But on 160 and 80 the additional capacitance provided by the tubes is not really significant compared to the capacitance provided by the tank circuit components and unavoidable stray capacitance and isolating the tubes from the circuit removes an additional factor which could muddy your results.  On the higher bands if the tubes are suspect you could still disconnect the plate caps and use a small capacitor (equal to the combined capacitance presented by the paralleled tubes) in parallel with the resistor.  In Jerry's case his problem appears to be only on the lower bands so I suggested a simple approach.
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Rodger WQ9E
Jerry-n5ugw
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« Reply #3 on: September 11, 2008, 06:13:01 AM »

Thanks for this input... C24 and 25 are only 1000v, not very heafty for this application. I got to looking real close at this amp and it is not designed for AM use. small gaps and short air variables, very cheep tank coil. I am concidering replacing all of this. The box is big enough to do almst anything in it. Power supply is scrottley enough for maybe 150w carier. Now if the confounded tuned input had some gutts it would be better.
I need to find a supplier of coils and air variables and I might make a turbo Al-811 with 572b's.. Anyone have any spare parts they want to part with... 
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Jerry-n5ugw
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« Reply #4 on: September 11, 2008, 03:20:46 PM »

Ok here's the deal, It most likely is the c24,25 caps on the band switch. I think it's time to modify the tank circuit to increase heat margin. So her goes. It has #16 in the coils and week plastic seporators. already showing signs of melting from heat.  I would think #12 on solid form the same diameter, but at what length? I could use a coil from a bigger service with all taps for 16-10m. Bump up the coupling caps to 2k instead of 1k and look at the band switch as well. Then i'd  drive the audio chain into the patch input of the 902dm, I just might be heard and cleanly at that.
Where would one find all of these components. Sold the jumk box wen I moved.
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