>AMPS
>
>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
>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.
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
site]
>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,
www.vcnet.com/measures.
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