Specifications

The PIN diode T/R system of the 89 was upgraded from the system used in the 86. I am told
that an 86 can be upgraded to become equal to an 89 for the sum of $1500 in parts. The
primary parts of this upgrade are the T/R board, and digital control board.
Recently, when the price of EIMAC 3CX800A7 tubes and power transformers had risen to
the point that they bit into the profits on the 89, Alpha/Crosslink Inc. discontinued it.
Currently, the model 99 has taken it's place, but now uses the Svetlana 4CX800's.
Busted 86
How I came to own an 86 is a twist of fate. A friend asked if I could fix a broken 86 for him,
he planned to purchase one at a low "busted" price. I told him most likely yes, so he sent the
unit to me for evaluation and repairs.
After a short while, I knew what the problems were, but I wished to do much more extensive
repairs an modifications on it, which he may not want to pay for, so instead, I purchased it
for the same amount he had paid for it, plus shipping. Poof! I have an 86, broken, but I have
one. I thought this to be good, since working examples had been selling for about $2500-
2800 at the time. The 86 sold new for about $3995 in 1989.
The "post-mortem" evaluation on the unit revealed that the RF output PIN diodes were
completely destroyed, vaporized by something, by what, is unknown. In addition, one tube
was dead, the other weakened, but usable, so a new set of tubes would be needed.
The AC primary "step-start" system was also faulty, causing damage to the resistors in that
circuit.
There were a few other deficiencies, that I will expand on later.
PIN diode problems
The most common problem in the 86 is the failure of the PIN diode T/R system.
The problem is rooted in the devices and the SWR protection system. The PIN diodes in the
RF output line are rated at about 5 watts dissipation each without a heat-sink. The two
diodes are heat-sink mounted to handle about 10 watts each, two devices in this
configuration can handle about 20 watts reflected RF power. The forward RF power
capacity of this diode configuration is about 2500 watts. The problem happens when
reflected RF power exceeds the dissipation rating of the devices. Here is where the SWR
protection system comes into play. The SWR protection system is factory set to take the
amp off-line when the SWR hits 2.1:1, which at 1500 watts forward, is 150+ watts reflected.
You could see why the diodes would be vaporized in this condition. Under certain
conditions, on the lower frequency bands even a moderate SWR can create very high RF
voltages, which can damage the output PIN diodes.
To prevent this type of damage to an unmodified 86, the SWR reflected RF power sensor
level should be lowered to an SWR of about 1.5:1, by adjusting RP-1 on the control board.
These controls can be found inside the cabinet cover on the left hand side of the front panel.
A vertical series of blue trimmer pots are for various settings of the control board, RP-1 is
among them.
The receive PIN diodes can be damaged also, by what is known as "hot-switching". Hot-
switching is when the amplifier is still in a receive condition when the exciter returns to