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[Amps] ALPHA 77

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Subject: [Amps] ALPHA 77
From: (
Date: Sat, 3 Aug 2002 11:13:45 EDT

I have just acquired the two 8877 Alpha 77 SX.  It worked the first time I 
tried it (output to dummy load easily over 1.5 KW with less than 100 watts 
input) (made several contacts on 40 M with good signal reports [amplifier on 
and off]).  The next time I turned the amplifier on I could get no output.  

I've checked the following:

- No crispy critters / all components seem normal; 
- No bangs, ticks or pops during first trial; 
- HV OK, no idling plate current... there was idling plate current when I 
first checked it out; 
- continuity check of HV circuit to plate from HV power supply OK; 
- continuity check of cathode circuit to power supply OK, however have 
observed what appears to be changes to the electronic bias board... zener 
diode - D206 replaced with 4 diode rectifiers; 
- continuity check of filaments of 8877s with an analog ohm meter - both OK;
- filament voltage measures 5.0 VAC; 
- input and output relays function properly; 
- when first checking out the amplifier, input SWR as read on the exciter was 
low and within limits... now it's high; 
- operating bias voltage measured between cathode to ground appears to be OK 
- 8.1 to 8.2 volts.  When amplifier is first turned on it measures approx 50 
VDC, when timer period (approx 1.5 minutes) complete, the bias drops to 8.1 
to 8.2 VDC... makes no difference if I key the exciter, apply drive, or 
not... stays the same;  
- when trying to test amplifier, I get some motion of grid current and no 
plate current or output.
- When first operating, the amplifier it seemed to be putting out too much 
heat even when not transmitting, which leads me to believe there is an 
electronic problem. 

Anybody have any recommendations?  Please reply direct.


Woodbridge, VA

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