[Amps] Amp failure advice please.

[R.Measures]

On Nov 15, 2004, at 3:31 AM, <g0jhc at blueyonder.co.uk> wrote:

>  have an amp problem anyone any advice what to do next please?
>
> Completed it last week,  tuned up and worked great to begin with, but 
> after a little “soak testing” something has gone wrong, I have lost my 
> output!
>
> The amplifier still gives an indication on both the PSU meters and amp 
> meters that it is drawing current. As an example plate voltage no load 
> is still 4500v, plate voltage at ZSAC (350mA) 4200V, plate voltage at 
> 2kw 4000v, and plate current 1 amp.
>
> As I said ZSAC is still there at 350ma when I go to TX ( no drive)
>
> Heater volts remain fine.
>
>
> 	The tube still blows off warm air.
> 	
> 	Everything is there to indicate that RF is being produced but no 
> longer finding its way to the output, as my meters no longer show any 
> grid current, very little power output or reflected power.
> 	
> 	When I adjust the load ( a 10kv vacuum variable) and plate capacitors 
> ( a flapper) little if anything happens, although I can sometimes 
> “peak” it to 250w before it goes back to zero. None of the trips, grid 
> or plate kick in just the tune trip! I can not then just reduce the 
> drive and bring it back up to the failure point, I have to re-tune to 
> a completely different set of c1-c2 values.

Neil -- What is the measured withstanding potential of the 
vacuum-variable capacitor?

> 	
> 	If I vary the power from my drive rig, I vary the plate voltage and 
> plate current to the same levels as when I had output.
> 	
> 	I can still tune for a 1.1 match doing the standard cold test with my 
> analyser and load resistor. When it was working I had only minimum 
> adjustments to make the first time I powered up, suggesting my 
> spreadsheet analysis is close.
> 	
> 	I have removed the anode choke and checked it, nothing arcing or 
> burnt out there, the  decoupling cap which brings the HV into the RF 
> cabinet seems to cold test ok for SC/OC.
> 	
> 	I have removed the 3 200pf HT57 blocking caps, and each test cold as 
> you would expect, reading 200pf on my capacitance meter.
> 	
> 	I have removed the Z50 on the output as a last resort.
> 	
> 	
> 	All tests conducted directly from the rf output socket, removing the 
> directional coupler or relays from the system and into a dummy load.
> 	
> 	
> 	What would be my next move to finding the problem. Could it still be 
> the blocking caps?

DC-blocker caps fail short-circuited.

> There is no obvious arcing visible.
> 	
> 	Unfortunately I don’t have any spare vacuum caps or blocking caps to 
> substitute and they are expensive items, and don’t want expenditure if 
> its not the problem.
> 	
> 	How would I progress with any fault analyses under these 
> circumstances please?
> 	
> 	Hope the experts can advise.
> 	
> 	
> 	
> 	73 Neil G0JHC

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org