[AMRadio] Re: AMRadio Digest, Vol 37, Issue 45


John Lyles jtml at losalamos.com
Mon Feb 26 12:28:26 EST 2007


Steve
So what you are saying is that the total voltage across the 4 K resistor load was 5 x 200 or 1 kV rms audio? I assume you drove it midband (1 Khz or 400 Hz etc). If you have a good HV scope probe, you should look at the plates of the modulator 833s and see that both have substantial swing, before you blame that iron. Although, I don't know what the standard readings are for modulator current for each 833, you are getting 200 mA per tube metered DC current. 

Also, are you certain that the levels through the 807 stage are correct for high level tone? 

You've probably been there already, but don't these rigs have a switchable attenuator in the input somewhere, for when they go to nighttime 250 watt power level? Is it possible that this is in the wrong mode?  

I had an RCA transformer that i gave away two years ago. Let me know if you need something like that, because I might find it again from the guy who took it away. 

73
John 
K5PRO


> 
> Message: 10
> Date: Sun, 25 Feb 2007 22:42:56 EST
> From: SBJohnston at aol.com
> Subject: Re: [AMRadio] Low modulation Gates BC-1T
> To: amradio at mailman.qth.net
> 
> Deep in the bowels of my junk I found some resistors for a test of the whole 
> modulation section of the Gates BC-1T.  I put five large-ish (each like a roll 
> of quarters, maybe 50 watts?) 800-ohm wirewounds in series across the 
> secondary of the modulation transformer.  Thaty would seem a good load: 4000 ohms.  I 
> put an RMS voltmeter across one of the resistors in the stack.
> 
> Applied tone to the input... enough drive to produce 200 mA of combined 
> modulator current produced about 100 volts across the one resistor.  400 mA got me 
> about 200 volts.  And the tube plates were medium-glowing at 0.2 A (what I'd 
> consider normal for 833 modulators) and quite brightly glowing (more than I'd 
> like to see on an 833) at 0.4 A.
> 
> Didn't run it too long, but after I shut it down the resistors were hardly 
> warm at all.  
> 
> Steve WD8DAS
> 
> 

> Hate to say it, Steve, but I think your modulation transformer is fried.
> 
> Had a similar thing happen with a Ranger I mod xfrmr... plenty of plate 
> current on the 1614's... excessive, in fact.  Just no, or very low 
> modulation.  Turned out be a blown mod tranny.
> 
> Good Luck!
> 
> --
> 73
> -W5OMR (home for 2 days)




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