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[Amps] ALPHA 77

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Subject: [Amps] ALPHA 77
From: (2)
Date: Sun, 4 Aug 2002 05:28:58 -0700
>Hi Bob: I do not have a specific idea about where your problem lies but
>thought you, or others, might be interested in my experience in fixing a
>77DX which was inop when I got it. 
>  1. The chain drive for the input and output bandswitches was not
>indexed properly. On some bands the input switch was between contacts
>when the output sw was indexed. Loosening the sprockets , resetting
>them, and the idler tension adjustment fixed that. 
>  2. The input reed switch was cracked and worked intermittently. I
>replaced that.
>  3. The HV bridge rectifier blew each time I turned it on. This was the
>tough one to find. The bleeder resistors are under the chassis in the
>rear. The pos HV lead was routed near the ground end of the resistors
>and was arcing to the resistor terminal. 
>  4. I seemed to be able to get proper output only when the grid current
>meter read about 100ma. This really worried me so I put a post on Amps
>about it. Several people told me that the 500ma f.s. meter was actually
>set at the factory to be 250ma f.s. Apparently the deflection when
>tuning on a 500ma scale is too small to see. 

Why would one need to tune for grid-current ?  Minimal grid current does 
not give a correct tune.  Maximal grid-current does not give a correct 
tune.  A correct tune is max out with the drive needed to produce 1A of 
anode current per 8877 -- which coincidentally results in c. 100mA 
grid-current per grid, which could be seen on a 0-500 scale.  However, 
there is no need to measure  grid-current on a cathode-driven amp.  
Grid-driven, yes.   


-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,  

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