The 1994 vintage Alpha 87A that I repaired for a friend was picked up by him on
January 9.
The amplifier, when tested on my antennas, made 1.5KW output on every band
using about 60 watts drive with no faults or problems of any kind.
Besides correcting a few issues following a blower change, the main problems
preventing the Alpha from working were in the wiring harness. The first issue
was an open wire on the Rbias monitor circuit which resulted in a hard fault 1
when the operate switch was pushed. This wire runs from the TR output module
over to the microprocessor board. Not only was this wire open, one side of
the wire was at chassis potential.
The second harness issue was an open wire between the TR output module and the
tube deck compartment in the operate relay circuit. The input and output relays
are in series, fed from a 34VDC driver on the microprocessor board. The open
wire was in the circuit between the negative side of the input relay coil and
the positive side of the output relay coil, preventing the relays from closing.
The harness failures must be related to the blower change that my friend
performed on his amplifier, but close inspection failed to reveal any wires
that were pinched. The AC input module, if one is not careful, could pinch
the wire harness but these were clear. The harness makes a 90 degree
transition behind the AC module into the Output TR module and below it, where
the harness runs over to the tube deck enclosure. The wires then make another
90 degree bend and enter the tube deck enclosure.
There is an opportunity for the blower housing to pinch the wire harness
beneath the Output TR module, but these wires were in the clear. At the
opening where the wires enter the Output TR module, the sharp edges are bushed
by plastic inserts, which were in place. Where the wires enter the tube deck
enclosure, the sharp edges of the opening are bushed by foam rubber pads, which
were in place.
After failing to find any obvious problem with cut wires or harness pinching,
I consulted with the amplifier owner. The options were to replace the open
wires and hope for the best or to partially unravel the harness and pull the
affected wires, so we could observe what happened to them.
If the owner didn't pinch the wires during the blower installation, the problem
could have been caused by simply moving the wires around. Both the Output TR
module and the tube deck enclosure must be moved for the blower install.
In the end, the owner elected not to risk further damage to the wiring harness
by partially unraveling it.
I would like to know exactly what happened to the wires in the harness but
agreed with the owner we did risk additional problems by moving the wires
around. These are very stiff, stranded wires.
It is possible the wires were pinched at one time and this problem was
corrected before I got the amp.
I will likely see this amp again, but hopefully the situation will remain
stable as long as the harness is left alone.
73 Charlie N8RR
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