[AMPS] Alpha 77DX Bias Switching

[Richard Byrd]

Usually the 'popcorn' sound can be fixed by allowing a tiny bit of quiescent
(sp?)current to flow when the amplifier is keyed in SSB with no drive from
exiter.
The problem is caused by too much cutoff bias.  That circuit is a tri-level
bias.
58 volts to cut the tube off, around 20 volts keyed with no RF coming in,
and the operating bias which is supplied exclusively by a 8.2 volt zener.

Depending on the vintage, your amplifier may have a pot to adjust the threshold.
The large (CB-2) PC board has one or two blue mini pots.  One is for ALC
threshold which controls the sensitivity of the LED on the front panel.  The
other, if it has a second one, controls this troublesome threshold.  Key the
amplifier but apply no drive and adjust this pot for a slight indication on
the PA current meter.
If the board has only one pot, you can add a 2.5k mini pot or play around
with some fixed values around 1k.

Remember, lethal voltages are present with the top off.  Before you fire it
up with the hood up make sure to defeat the big white shorting bar on the
power supply.  Defeat the interlock switch at the rear by pulling it's
plunger outward...away from the radio.

Schematics from December 1979 show two added components at the Bias
transistor but they may or may not have any impact on the popcorn sound.
This is the transistor at the top of the PC board near the center.  It has a
heat sink.  On the back side of the board there should be a .01 disc
capacitor from the transistor's base to ground.  Also added was a 1N4004
diode.  It's cathode is connected to the transistor's collector.  The other
end is to ground.  Later production models used a MJE 802.  Older rigs may
have a different number transistor.  It may be wise to upgrade the
transistor but verify the pinout is the same.

The more idle current you allow, the less popcorn effect you'll make.

Dick Byrd

At 12:34 PM 3/13/97 PST, you wrote:
>Hello ampers,
>
>Has anyone had any experience with the problem of excessive Electronic
>Bias Switching speed in the 77 amplifiers, causing distortion and the
>recognizable "crackling" sound that results?
>
>>From my experience with a home brew EBS, an integrating circuit across
>the switch eliminated the problem and yet it was fast enough for QSK. 
>My supposition is that ETO felt the high-speed switching was necessary
>for QSK operation but the results, in some that I've heard, are rather
>poor.  In one amplifier test, with the EBS shorted out, the quality was
>perfect.
>
>Are there any mods for the apparent problem?
>
>73,
>
>Marv, W6FR
>
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