Jan,
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
activating.
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" <ditzian@windstream.net>
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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