I note that the light on the CW/Low button came on when I pressed that
button. Also, one of the meters is lit. If I am reading things right,
all bulbs are 28 VDC, so I expect that the 24 volt supply is OK.
I looked over the original Operating and Technical Manual, and there is
a handwritten note on Fig 21, the CB2 board, that this unit has the
CB-2A board. There are other cryptic notes, but there is no specific
change to the schematic. I have no idea where to find a CB-2A schematic.
I will see if I can contact my friend with the test bench and a 24-volt
On 02/19/2011 02:29 PM, Paul Christensen wrote:
> The problem appears to be isolated to the three-minute warm-up timer.
> You're seeing 2600V which is correct while the three-minute timer is
> First, measure the +24 VDC supply voltage. It should be fine since
> you don't seem to be blowing the step-start fuse. That would render
> the amp completely dead after the step-start resistors finally blew
> and threw shrapnel all over the inside of the amp.
> The timer circuit is on circuit board CB-2 and is very simple in
> operation. Upon sensing +24VDC, C207 (22 uF) begins charging through
> R222 (10 meg). This in turn activates SCR Q207, which simultaneously
> switches a more robust SCR Q208. Q208 fire and pulls in relay K202.
> Once K202's contacts close, +24VDC is supplied to the "READY" lamp
> (and other amp circuits) and the amp is ready to make power.
> CB-2 unplugs from an edge-card connector so you can safely do some
> troubleshooting outside of the amp -- If you have an adjustable +24VDC
> bench supply, use it for testing. Otherwise, be extremely careful
> when testing components in the amp. CB-2 is not far from the HV
> contact points.
> As W8JI once said, he's never had to troubleshoot an amp with AC power
> applied. He's right. It may take longer, but it's a much safer way
> to diagnose a root problem. If you've ever diagnosed an amp problem
> by yourself, now is not the time top learn on your own. Get on-site
> help if you cannot work on CB-2 outside the amp.
> Paul, W9AC.
> ----- Original Message ----- From: "Jan Ditzian" <email@example.com>
> To: <Amps@contesting.com>
> Sent: Friday, February 18, 2011 10:59 AM
> Subject: [Amps] Alpha 77 start up
>> I just received an Alpha 77 Dx, modified as a two-holer, as a gift (I
>> have very understanding friends). However, it came with a problem--it
>> does not work. The startup light does not come on. The previous owner
>> sent a board back to Alpha to fix this problem, and their paperwork
>> shows that they repaired it and then tested it in an Alpha 77Sx, which
>> is equivalent to the two-hole modified Dx, and it worked. However,
>> after putting the board into this rig, the problem remains. Since I am
>> new to this amp, and did not have it integrated into my station, I did
>> the following checks:
>> I set up the amp with no exciter, but with power, and the fan comes on
>> and the voltage goes to about 2600 volts. I followed the regular startup
>> procedure and, sure enough, the light does not come on after six minutes
>> (it is supposed to take about 3minutes). I then put a cable into the
>> relay jack on the back, repeated the above process, went ahead and put
>> the amp into CW/low power mode after a six-minute wait, and then shorted
>> the relay cable. I did not hear a relay click--nothing happened.
>> Can someone help me to isolate the problem with this amp? Until this
>> happened, the previous owner (the donor of the amp) was using it without
>> a problem. There was no cataclysmic event that preceded the failure to
>> start up.
>> I can imagine problems like bad contacts on the board connector, or
>> perhaps the failure of another relay somewhere.
>> Thank you,
>> Jan, KX2A
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