>
>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 have never seen a rotary switch that operated correctly with loose
screws.
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. 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?
>
>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; 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
>
>>------------------------------
>>
To: <amps@contesting.com>
>>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
>>>
>
>--
>FAQ on WWW: http://www.contesting.com/ampfaq.html
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>Administrative requests: amps-REQUEST@contesting.com
>Problems: owner-amps@contesting.com
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>
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
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