I know Don resolved his problem -- a bad wattmeter diode -- but wanted to a
couple of comments to Eric's description of the L1 problem.
- My L1 was arcing not only to the PCB, but also to the mounting screw Eric
describes below. When I first inspected the T/R board, before Brad suggested
checking L1, I didn't notice the carbon deposits on that screw. If I had, I
would have realized something was going on with L1. So, one thing I'd recommend
to 87A owners is to open the T/R box, which isn't difficult, and check the
screw and the area near L1 for burned spots. You may be able to tilt L1 a
little to see if there's any damage to the insulating pad and/or windings. If
there's any sign the pad is disintegrating, replace it.
- Once I removed L1 and the disintegrating insulating pad underneath it, I
found a burned area with a deep divot in the PCB.
73, Dick WC1M
-----Original Message-----
From: k2cb@comcast.net [mailto:k2cb@comcast.net]
Sent: Friday, December 9, 2016 1:01 PM
To: Dick Green WC1M <wc1m73@gmail.com>
Cc: ve6jy 1 <ve6jy.1@gmail.com>; Amps@contesting.com
Subject: Re: [Amps] Alpha fault 17
Good points, Dick. I seem to recall a similar thought when I first inspected
the board. At first glance, I thought there was no issue there. Only once I
removed the coil and scraped away the foam did I see the damage (carbon) on the
circuit board itself. It seems to want to arc near the one end to the left of
the amp, where there is a mounting screw close by, and the associated ground
plane. I also had damage to the winding wire itself, about 1/4" from the board.
The end of the wire near the board was brittle, and broke as I handled it. I
was able to repair the winding lead without having to rewind the entire coil,
but if it was any further into the torroid, I would have had to rewind the
entire coil.
While I don't recommend it, you could defeat the safety interlocks, ramp the
amp up in rf power, and closely watch the area to see if it arcs when the fault
occurs.
A pen cap slid over the HV shorting terminal works well as a temporary defeat.
Eric
K2CB
----- Original Message -----
From: "Dick Green WC1M" <wc1m73@gmail.com>
To: "K2CB" <k2cb@comcast.net>, "ve6jy 1" <ve6jy.1@gmail.com>
Cc: Amps@contesting.com
Sent: Friday, December 9, 2016 12:48:08 PM
Subject: RE: [Amps] Alpha fault 17
Some additional points:
- You may not be able to see evidence of arcing until you remove the choke.
- You may need to replace (rewind) the coil. Just count the turns and use
the same gauge/type of wire
- When my choke coil started arcing, I didn't get fault 17. I got faults 8
and 9 (and 1, I think).
- This was evidence that the arcing damaged one of the transmit PIN diodes,
which I had to replace.
Don't use the amp until you attend to the choke coil. You don't want to risk
damage to the PIN diodes, which are expensive.
73, Dick WC1M
-----Original Message-----
From: K2CB [mailto:k2cb@comcast.net]
Sent: Friday, December 9, 2016 12:54 AM
To: ve6jy.1@gmail.com
Cc: Amps@contesting.com
Subject: Re: [Amps] Alpha fault 17
Do a search of the 87A Yahoo group regarding the large L1 choke coil on the
RF output wattmeter / pin diode / bypass relay module near the rear antenna
jack. Don't quote me on the exact part designation, as I am not at my desk
top at the moment, but it is the large green one on that board.
Remove the four screws and shield cover. You will see a large green wire
wound choke coil. Alpha placed a piece of black foam between the coil and
circuit board as an insulator.
This material is breaking down over time, and causing the coil to arc over
to the ground plane of the circuit board. It will get worse over time, and
the fault 17 will occur with less and less rf output power as time goes on.
Remove the choke from the board, remove all the old foam pad, clean off any
carbon on the circuit board from the arcing , and install a replacement
piece of insulation material. I used some thin black plastic. Then
reinstall the choke coil.
If you must remove the entire circuit board, be especially careful with the
pin diode mounting and associated insulators. They are somewhat fragile.
If you like, I can send you some pictures I took of the last 87A I repaired
with the same problem. The photos are on my desktop. I can send them
tomorrow when I get back to my office.
FYI - Credit is due to Brad from Alpha (or whatever their name is this
year!) for originally suggesting this as a possible Fault 17 cause.
Eric
K2CB
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