Notes on Arizona Alpha 76 Linear
History
This linear bought by CCC from N7BG ($1200), who obtained it from a K8MFO estate sale. W8TK picked it up in PHX on 22 October 2003 for testing. It was then driven to K8ND by W8TK. We believe the original owner was Gerry Crosby, WB8YUO.
January 2004
K8ND is testing and evaluating the linear to determine what maintenance is needed. Click here for the test report on the amp and here and here for the test report on spare 8874 tubes.
June 2004
Linear tests good. K8ND passed it to W0CG on I-71 10 June 2004 for transport to Curacao.
July 2004
Used amp at Station # 1 in IARU contest -- performed perfectly.
October 2004
Will not turn on. Severe buzzing sound. W0NB and K8ND observed severe arcing in K1. Brought to U.S. by W0CG in white case shipped to WB9Z by Cindy and then brought empty to Curacao by WB9Z. Shipped to Alpha for repair 16 December 2004.
March 2005
Amp was repaired in Colorado -- new K1 relay and precautionary replacement of all six electrolytics ($150). Total tab, including shipping, was $650. Was shipped from Alpha to WB9Z, who brought it to Curacao 1 March 2005. On 2 March 2005 we replaced the power transformer and checked tube seating and powered up. Step start fuse blew instantly (I thought it was a panel lamp) and the panel lamps did not come on. HV indication was normal. After about six seconds there was smoke and frying sound. Turns out R1 (step start resistor) burned up as well as the wiring to that resistor. Acrid small and black residue on the caps and PS board above. Electrolytics test OK. Will begin troubleshooting as soon as we know that the repair warranty, if any, will not be invalidated by our troubleshooting. On 3 March removed a tube from this amp and put it in the Alpha 76 no tune. Works well in that amp. Also on 3 March found a flat washer rattling around in the case of the Arizona linear.
March 19, 2005
After much troubleshooting found I had crunched a purple wire in the transformer bracket, dead-shorting the 40 VAC secondary to the chassis. This explains the failure, blowing the step start fuse and then burning up the step-start resistor. I did a temporary repair, putting in a scab resistor until the "real" parts can be obtained. Ordered several parts from Alpha to be shipped to K9SG.
Installed a new 8874 to replace the one taken out for the no-tune Alpha. Made some QSOs -- full output.
Need to do: replace R1, replace my tape with heat shrink, put on missing color caps on lamps and reverse them -- they are presently backwards. I installed two 18 ohm 2 watt resistors in parallel as a temp replacement for the 10 ohm 5 watt step start resistor that burned up....
Troubleshooting Notes: It's necessary to remove the chassis HV short spring to get out the power supply board. Remove two Phillips head screws at top of the big standoffs and the board will come out easily.
Transformer
Wires: 120 V primaries are blk/yel
1,3 and red/violet 5,4
28 volt
secondary is violet/violet/grey 7, 8, 9
HV
secondary is red/grn/blu
13, 12, 15
filament
secondary is grey/yellow 1,2
November 2008
Put amp at Station 4 after Titan I failure prior to CQWW CW. Amp turned on with a pop and spark. Troubleshooting showed the 1 1/4 amp 3AG step start fuse had blown. Cleaned inside and tested all obvious things. Then initially it would come on and then turn itself off after a minute or two. After repeated attempts it stayed on and tested good - 1200 watts output on 40 CW. Next morning it blew the step start fuse again. In closet waiting further investigation.
4 March 2009
Comes up and stays on reliably. Produces up to about 450 watts of output, but when pushed higher instantly turns itself off.
14 March 2009
Further troubleshooting shows that with the mode switch in "CW," it is possible to tune the amp to 1000 watts out with no problems. But in the "SSB" position, which is a higher plate voltage, the amp trips off at about 500 watts. This implies a possibly bad tube. Will swap out all tubes and see what happens. Swapped in three N4RV tubes, but amp turns itself off after about 5 seconds. One of them is probably bad. Installed three totally new 8874s. Amp stayed on OK. Will season these tubes for about four hours before applying RF. After seasoning, with new tubes, amp still tripped off at high loads. Next, put the old tubes back in (they are good), put the new tubes back in inventory, and try to disable the plate overcurrent circuit. Disabled the plate overcurrent circuit by unsoldering the Pin 3 wire from the board, but amp still tripped out when in SSB position. This blew a primary fuse. On the next attempt after blowing a primary fuse the amp blew R111 (25 ohm, 25 watt) in the HV stack and blew a primary fuse. Obviously the plate overcurrent circuit was doing its job, so I put that back into service by resoldering the wire. Now suspect an electrolytic or diode. Swapped in a known good power supply from the WI9WI amp, but linear tripped out exactly as before at about 450 watts output in the CW position.
Disposition 15 March: Put in the original power supply. Order a replacement for R111 25 ohm 25 watt screw-terminal resistor. Set amp aside and confer with the guys what to do? Options are 1. Ship it to Alpha for repair, 2) Leave it here as a parts spare for the other Alpha 76, 3) Leave it here and wait until I have an expert with a lot of time to work on it.
22 October 2009
K6AM repaired
amp by replacing R111 25 ohm 25 watt wirewound. This had crept up in value and it was causing
the plate overcurrent circuit to get exaggerated voltage. Also replaced one bad
8874. Makes 1200 watts easily. Operated for three hours with no problems.
5 March
2018
Seasoned tubes for eight hours. Amp appears very normal i.e. in CW position 35 watts drive = 700 watts with 20 ma grid current. 40 watts of drive gives 750 watts. In SSB position 35 watts drive = 900+ watts, grid current 40 ma. KB7Q