[Amps] ALPHA 77

2 2@vc.net
Sat, 3 Aug 2002 12:13:35 -0700

>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; 

This is pretty common if the ZSAC isn't right with the zener, or if a 
mysterious burst in cathode current caused the orig zener to short.  //  
There are some trade-offs with the RF-actuated electronic bias scheme 
since the bias changes to not-linear mode within words that contain 
soft-syllables.  This creates a spit-spit sound on adjacent frequencies 
and gives the Tx audio a raspy sound.  The fix is to sync the bias with 
the current in the 77's T/R relays, and Let the transciever control the 
amplifier relays -- i.e., the old-fashioned way.  

>- 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 
>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.

I would measure full scale current meter indication with a rheostat, LV 
supply, and a DMM.  If the meters do not have back-to-back 3A diodes 
across the movement, it might be a good idea to add them.

>- 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. 
I would cold-check the tubes for excessive current leakage due to 
gold-sputtering from the grid's gold plating.  With 8kV plus or minus, 
the current leakage shoud be equal and under 10uA.  I see gold sputtering 
in roughly half the kaput 8877s I test/autopsy.  [see Fig. 24 on my Web 

>Anybody have any recommendations?  Please reply direct.
Since other 77 owners are likely to have - or have had - similar 
problems, it may be helpful to report on this project to AMPS group. 

cheers, Bob

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