[AMPS] One more failure mode

[John T. M. Lyles]

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. 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

>------------------------------
>
>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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