I was floating along on a cloud after the successful repair of the Fault 1 problem (see Post below) but 12 days later, when attempting to power on the Alpha 87a, I got a "soft fault" designated as a No. 20 Fault Code. The manual describes this as "Filament current greater than 3.5 amp. Possible shorted filament." Bummer! Did that mean I was going to have to replace one or both of the 3CX800A7's in the amp? Eimac versions of those are now selling for $1,050 EACH but Alpha has just introduced their own line of tubes and I could get those for $400 each. Still, that's a BIG expense.
|Underneath View of Alpha 87a Tube Compartment|
Now that I was ready to begin this new repair of the 87a I downloaded the PDF version of the manual from the Alpha web site to make it easier to read (on the computer instead of on paper.) I then pulled the 75 pound amp off the shelf after disconnecting the cables including the AC. Once again I needed to remove the "30" screws holding the cover to the chassis as well as four more screws that hold an access cover below the tube compartment. But, I've been down this road many times before so it's getting to be old hat.
The high voltage crowbar activates when the cover is removed and should eliminate any residual high voltage within the amplifier but I also short the high voltage to ground in the area of the tube plates or the plate RF choke with an insulated screwdriver as an extra precaution as described in the manual.
|Large Red Connectors on Control Board Need to be Reseated|
Taking the photo at the upper left proved very useful to me. When I cleaned and attempted to re-install one of the 7 connectors in the tube compartment, I was having difficulty getting it to fit on the male pins. Then I noticed there were TWO male connectors on the board and I did not know which one was the correct one to accept the female connector. Looking at the photo clearly showed me there was an extra (unconnected) male connector. Also, and the reason I was having trouble plugging in the connector in the first place, the second connector is installed upside down (in relation to the correct one) and has more pins. The correct one has the two center pin holes plugged so it cannot be plugged into the wrong connector. Nice design by Alpha!
As an extra precaution, I put paper towels behind each connector while spraying it to make sure no "overspray" landed on any other components. Alpha did not mention this but I decided to take this step purely as a precaution.
Once all the connectors were re-installed, it was time to put the "34" screws back into the covers and reconnect all cables (after hoisting the 75-pound amp back onto the shelf.) I also took this opportunity to connect the Palstar AT-Auto tuner back into the station. That is another complex piece of equipment that required me to review the manual and make a few changes to the default settings for my particular station. One of those was to run the Keying Relay Input line for the Alpha 87a through the QRO Keyline circuit of the Palstar tuner. This will inhibit the keying of the Alpha 87a while the Palstar is tuning. Just one more piece of protection to keep me from hurting myself (or the Alpha) when (not IF) I make a mistake!
|N8LP LP-100 Wattmenter Showing 1496 watts Output|
This entire operation took about 3 hours with about 2 of those spent on the actual dis-assembly/repair/reassembly of the Alpha 87a. I am REALLY thankful I did not have to purchase new tubes and the repair only cost $15 for the Deoxit plus a trip to Cross Lanes. I'd say that was more than reasonable!
One final thing I did was to connect the computer to the Alpha 87a and check the actual filament current. Alpha suggests you check it several times so I made 15 checks of the filament current over a 5-minute period and saw values ranging from 2.70 amps to 3.01 amps. Since the Fault Limit is 4.0 amps, I think the actual filament current is A-OK and the Fault 20 errors I received were erroneous and due completely to corrosion on one or more of the connectors in the amplifier. Case closed!