[AMPS] One more failure mode
km1h@juno.com
km1h@juno.com
Tue, 13 Apr 1999 11:16:46 -0400
On Mon, 12 Apr 1999 12:38:30 -0600 "John T. M. Lyles" <jtml@lanl.gov>
writes:
>
>I will add one more failure mode to the SB220 bandswitch which I
>haven't
>seen mentioned. You all beat around the other observations, and they
>make
>sense to me too. I picked up my 220 in 1993 at a hamfest locally for
>a
>song, practically. It seems that it had an arc somewhere. When I
>rebuilt
>it, and did modifications, I noted that the bandswitch had an arced
>contact. When I looked closely I found that the long posts which hold
>that
>switch together, and align the various wafers to exactly the same
>switching
>points, were loose. At the 10 meter end, the thing was misaligned
>enough
>that an output Pi contact was barely touching it's wiping fingers. And
>it
>was burnt enough to be intermittent. When operated on 10, it would
>spit
>sparks there, so I didn't try more than a few seconds. I cleaned the
>contacts with a relay burnisher, those fine abrasive strips, and
>realigned
>and tightened the standoffs. It fixed the problem, and no problem has
>been
>noted there, subsequently.
That had been mentioned by someone before John but tnx for bringing it up
again. That would be a definite #5 to the list.
Please add a #5a also: Be sure all mounting hardware is installed in the
proper order. See the manual pictorial.
While removing switches from 220's I have noted lock washers and
phenolic insulators, upon occasion, installed in a rather haphazard
manner. This results in misalignment of the output wafer.
I subscribe to KM1Hs theory here, that
>cheap
>bandswitches will bite you eventually. Although I don't know what
>Heath
>engineers paid for that switch, I was not impressed with how easy it
>was to
>misalign the wafer and twist the indexing around. I wonder if some of
>the
>bandswitch problems can be this?
VERY possible and probable. Several switches that I am looking at right
now show the condition you describe.
Another question. What is the RF current derating by frequency when a
switch is factory specified at 115VAC 60Hz ?
>
>Cockpit error is certainly plausable, I haven't done it myself, but
>would
>imagine that in haste, when a rare DXer pops up, and one is tuning
>the
>beast, it could happen. Rotating the Bandswitch while hot would be
>hard on
>that switch. As for the open contacts arcing due to a parasite, it is
>plausable too;
I still cant buy into that theory. A true parasite due to internal tube
resonances should not make it that far into the pi-network.
However, a secondary resonance formed within the tank coil/switch due to
the shorted turn effect is plausible. They appear most frequently when
the switch is in the 40M position and are not limited to the SB-220
either.
I believe that I proved that several times by installing a 10-12pf 1KV
cap directly across the 20 and 40M contacts. This procedure is nothing
new; I first became aware of it in 1963 when National Radio used it in
the NCL-2000.
73 Carl KM1H
but these pure mechanical malfunctions are much more
>probable, and easy to reproduce.
>
>John
>K5PRO
>
>
>>I first worked on an SB220 as a Heathkit tech in the mid 70's.
>>Since then I've owned, serviced, modified or for some reason had
>>my hands inside 20 or 30 SB220s (not the 200 Rich has tested).
>>In the 220s Ive worked on with bandswitch damage that damage
>>can be directly attributed to the operator rotating the band switch
>>instead of the load control while tuning up. The physical layout
>>of the tune, load and bandswitch controls makes it easy to turn
>>the wrong knob and cause the "parasitic" damage.
>>73, Bob, W5AH
>
>>------------------------------
>>
>>Date: Sat, 10 Apr 1999 09:53:10 -0400
>>From: "Dick Green" <dick.green@valley.net>
>>Subject: Re: [AMPS] Parasitic suppressors/another question,
>>Yes! It's about time somebody brought that up. I can't count the
>times I've
>>reached for the wrong knob on that amp. Luckily, I've never turned it
>far
>>enough to break contact, but I can see how that could happen easily
>in the
>>heat of a contest. There really is not that much difference in
>mechanical
>>resistance between the Load and Band knobs, so you don't get much
>physical
>>feedback before it's too late.
>>
>>Seems to me that we have reasonable alternate explanations for all of
>the
>>symptoms attributed to parasitics in the SB-220:
>>
>>1. Burned-out band switches caused by accidentally turning the band
>switch
>>while tuning.
>>2. Cooked suppressor resistors caused by pushing the amp to the max
>(1200W
>>CW @SSB position).
>>3. Arcing at the Tune capacitor by over rotation of the Load
>capacitor (easy
>>to do.)
>>Hmmm. Operator error in all three cases. What a surprise. Are there
>any
>>others we can add to the list?
>>Thanks for your post, Bob.
>>73, Dick, WC1M
>>
>>--------------------------
>
>>>In a real amp that is 100% stable to start with, the switch failures
>are
>>>primarily due to the mfg trying to save a few $$ and using a poor
>>>initial choice .
>>>Before the doubters wind up in a snit I suggest that they take a
>real
>>>look at the specs of the commonly used switches and try to
>understand
>>>exactly what they are spending money on.
>>>
>>>73 Carl KM1H
>>>
>
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