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[Amps] ALPHA 77

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Subject: [Amps] ALPHA 77
From: (dan hearn)
Date: Sat, 03 Aug 2002 04:59:30 -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. What a surprise, and the
manual doesn't say a word about that.
  5. The 1000pf blocking and bypass CRL doorknobs showed radial carbon
tracks on the ends so I replaced them although they seemed to not be
shorted and worked OK. 

73 and GL, Dan, N5AR wrote:
> 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.
> Thanks,
> Bob
> Woodbridge, VA
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> Amps mailing list

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