By Matt Erickson KK5DR Copyright © 2004 M.A. Erickson, KK5DR. All rights reserved.
Introduction Many hams have never heard of this particular model of ETO Alpha amplifier. A few have heard of it, but have never seen one, even fewer own one. I happen to be one of those very few who owns an 86. I'll elaborate on how I came to acquire it later. Only 274 of these jewels were built between December 1987 and October 1989. The last 74 units being what is known as "export" models, meaning they covered 1.8-30MHz continuously. The unit I have falls into this final group.
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.
transmit, or vise-versa. The RX PIN diodes are much smaller, and have a greatly lower power dissipation rating, so are easy to damage, with even a few hundred milli-watts of RF power. If the amplifier is in a receive condition when the exciter goes into a transmit condition, the RF can enter the RX PIN diodes, and burn them out, but there is an RX fuse in-line with these diodes. If the fuse is burnt out, the exciter will not have good RX. Replacing the fuse will usually fix the problem.
If you wish to do this mod, you will have to consult others that have done it, or figure it out for yourself. The mod is not difficult for a technician with good radio knowledge & experience in amplifiers. If you can't do the mod, contact someone to do it for you, but don't expect the work to be FREE! The parts you will need are a pair of Jennings RJ-1A vacuum relays or equal type, for about $45 each. You need hardware to mount the relays too.
Plate choke In fig.1, the letter "C" points out the plate choke. The letter "B" points to an open-frame relay that switches in or out a capacitor which alters the series resonance of the plate choke on a given segment of frequencies. I feel that this method, although it does work, is not a very elegant way to achieve the goal. The relay has 2.5kV across the contacts, and could arcover very easily, which could cause catastrophic damage, under certain conditions.
The yellow line connecting at the top of the network comes from the HV power supply. The bottom of the network connects to the base of the plate choke. The top end of the plate choke connects to the tube side of the plate DC decoupling capacitor bracket marked letter "E". At this point you will notice that the plate decoupling caps are NOT original, and your right. The original caps can be seen in Fig.1, letter "D".
from ever happening. The letter "A" points to the RF input coupling capacitors, which are a pair of 0.02µF @ 1kV disk caps. A single 0.01µF @ 2.5kV silvered-mica cap would be a good substitute. The SM cap being a more temperature stable type, and able to carry more RF current without heating up. The letter "D" points to the filament line. I feel that a choke is needed, even with tubes that are "indirectly-heated" cathodes.
I took some time to trace and re-designate the proper part numbers on the schematic, this should help me allot, later. Over-drive There is an LED indicator on the front panel that displays when the grid current is being driven into over-drive. The level of "trip" on the display is set at the factory for 120mA of grid current. This level is "hard-wired" into the bar-graph display of the grid current, it can't be changed without moving the sense line from one point on the driver IC to another point.
Fig.5 In Fig.5, the interior of the power supply side of the amp is shown, with the power transformer removed. The letter "E" points to the "step-start" fuses. The letter "B" shows K12, and "C" points to K-11. K-11 is the primary contactor, which engages when the "ON" switch is pushed. K-12 is the "step-start" relay, which closes after a short time-delay, after K11 closes. K-12 shorts the step-start resistors out of the circuit, when closed.
I have suffered a catastrophic loss of an amplifier due to lack of surge suppressors in the past so, I believe in them, and use them extensively. I installed a set of 130Vac @ 340 Joule MOV's, pointed out in Fig.5 by the letter "A". There is one per primary line, grounded at the mount center. The MOV's are connected to line 1 & 2 between primary fuses F-1 & 2 ("F" in Fig.5) and K-11. Should an AC line voltage surge happen, the MOV's will shunt the current to ground.
In Fig.6, Letter "A" points to the 28Vdc regulator, which was an LM-317T, good for about 1.5A. @ 32Vdc. The schematic calls for an LM-350T, rated 3.0A. @ 32Vdc. Since this regulator provides all the DC current for all the controls, displays, and relays, the circuit could easily exceed 1.5A. I replaced it with the proper LM-350T. There is another pair of LM-317T on the control board which reduces the voltage to + & 5Vdc for the digital IC's on that board. It is fed from the LVPS 28Vdc regulator. Fig.
Now, we move to the blower, which is rated at about 40CFM under normal conditions, with the "aux" fan installed, the flow is upped to about 50CFM, well above the CCS rating of the tubes which is 39CFM at rated plate dissipation. A common problem with the blower design has been excessive noise build up, most notably in older Alpha amps such as the 76 series and the aging 86.
In Fig.8 you see an example of the panel rocker switches. After I examined the switch, I found that the blade of a small screw-driver could be inserted on each side of the switch, which opens the switch plastic frame enough to allow the rocker to be removed, exposing the internal parts of the switch. The red dots show the position of the center pivots, care should be taken not to push the blade in at these points. The blade should be placed at points "A & B".
modifications. The pot is placed in series with the ALC line to the ALC jack, seen just below the ALC pot. I can't stress this topic to strongly you MUST use ALC with this amp! Not using the ALC WILL result in damage to the tubes. Many operators don't use ALC feed-back with their amps, saying that they can control RF output with the controls on the radio itself, but this does not protect against "accidental" over-drive, high SWR, etc.
Any legal-limit amplifier should always be operated on 240Vac mains. Here is a list of the transformer voltage & current outputs of the secondary: 13.5Vac @ 4A. for the tube filaments. 7Vac for the 5Vdc regulated power supply. 50Vac for the 28Vdc regulated power supply. 1250 & 1785Vac for the High voltage power supply. Hipersil® transformers were patented by Westinghouse Co. back in the 1930's, designed by none-other than Nikola Tesla himself. The patent expired many years ago.
RF section Fig. 10 Fig.11 Fig.10 is the RF "tank" circuit for RF output. "D & C" points to the moving parts that should be periodically treated with Pro-Gold®. The stock plate choke is pointed by "A", and the HV "glitch" resistor is "B". Fig.11 shows the "tuned input", consisting of silvered-mica capacitors, and toroid coils. The band-switch of the input network should be treated with Pro-Gold® also.
I do not plan to even attempt the conversion, as I have a lifetime supply of EIMAC 3CX800A7's, plus I don't think it can be done in a way that would be of sound engineering design and good reliability. Acquiring a good "spare" set of triodes would be a good idea for the owner of a working 86, with good full output. The very long lifespan of these tubes would essentially last a lifetime, or enhance a resale.
