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Subject: [AMPS] ACOM Amps
From: (2)
Date: Sat, 7 Jul 2001 03:24:22 -0700
>Sorry I didn't get to replying sooner.
/\  Your e.mailer app. does not indicate the correct number of 
attribution marks on quotes. 

>/\  ¿Does the vacuum relay operate from a speedup circuit using a coil PS
>of 3 - 5 x the rated coil V, current limited by a series-R.?
>I think (strictly from memory) that the coil voltage is 48V. I think there's
>a series limiting resistor, but I can't remember (and there are so many
>screws in the cabinet that I'm not going in the amp unless I have to!)
48v is probably right.  Dick didn't seem to understand that 90% 
over-voltage is not enough to make the 0.3H coil act fast enough for 
modern radios.  200% over-v is about the lower limit.  
>/\  ¿Do you have a kaput 3CX800A7 from the Alpha 87A?
>No, the tube was still under warranty and I had to send it back to get a
>replacement. But the story is kind of interesting:
/\  Indeed.  I was surprised by Dick admitting that he had seen hundreds 
of tubes with gold-sputtering, yet he insisted that Alpha amplifiers 
never ever had one parasitic osc.  As I understand it, Dick lost a 
lucrative contract with General Electric's medical div. because of an 
8877 instability problem. 

>I got the tube to replace one of the 87A's original 3CX800A7s that had been
>causing "low filament current" faults shortly after warmup. The new tube
>operated about 100 hours before failing. When installed, it had a somewhat
>higher number of new tube "thumps" than I've seen in the past, 

/\  I don't subscribe to the tube barnacle and vanishing-gas sea stories. 
 I have never seen gas vanish with use and I have never found a barnacle 
during an autopsy.  

>but then
>settled down. At about 80 hours or so, the amp inexplicably blew the two 20A
>fuses at power up. It operated fine for several thousand QSOs in the next
>contest, then started getting PIN diode voltage faults. I had seen that once
>before and had replaced a couple of PIN diodes. I had a couple left and
>started to do the troubleshooting procedure that pinpoints (no pun intended)
>the bad diode(s). 

/\  chortle

>Then I started getting several other fault types
>indicating problems with the low voltage power supply. At one point, I saw a
>small puff of smoke coming from the LV power supply just before the fault
>To make a very long story short, I tore the amp apart and tested both the HV
>and LV supplies. In the latter case, I desoldered dozens of components to
>check them out. I couldn't find anything wrong with either supply. When I
>got the amp back together, it blew the 1A fuse in the step-start circuit. At
>that point I realized there had to be a short somewhere and started checking
>in the tube compartment. When I found nothing obvious there, I finally
>realized that it could be a tube arc. Sure enough, when I removed the new
>tube, the amp came up fine. My theory is that air had been (very) slowly
>leaking into the tube, resulting in more and more severe arcs, until it was
>so bad that even the low plate voltage during the step start caused an arc
>which blew the step start fuse. Any other ideas out there?

/\  Marginally stable amplifier tubes with gold plated grids tend to 
evaporate gold from the grid.  Eventually, the gold meltballs can arc 
across the anode-grid insulator.  This malady can only be found with a 
hi-pot breakdown tester.  However, since the presence of gold meltballs 
on the cathode reduces emission (and gain), the tube tends to become more 
stable (and harder to drive).  
>I still had the old tube that had caused the low filament current trips, so
>I put that one in and -- lo and behold -- the amp came up and operated
>perfectly. I ran it through the WPX CW contest, making thousands of QSOs,
>with no problems at all. I have a couple of theories of why the old tube
>works now. One is that during troubleshooting I happened to replace the op
>amp on the LV power supply board that measures filament current. 

It's puzzling that Dick Ehrhorn chose to monitor heater current.  

>Maybe it
>was bad. Another theory is that when I removed the tubes when I replaced the
>PIN diodes, some dust or gunk got down in the tube socket, compromising the
>connection with a filament pin.
/\  Not likely

>One more interesting note: when packing up the bad tube for return to Alpha
>(then to Eimac), I stupidly rested the tube on its side. It rolled off the
>table onto the floor, denting the outer ring and cooling fins. I figured
>once Eimac saw that, they'd refuse the warranty claim. I told Alpha about
>it, and said I'd be willing to swear in writing that the tube had been
>faulty before the accident. Scott Ehrhorn at Alpha said he'd appeal to Eimac
>for me, and they ended up replacing the tube. Good service from both
Curiously, Eimac's Warranty Dept does not test for gold leakage.  

thanks, Dick

-  R. L. Measures, 805.386.3734,AG6K,  

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