** GLOBAR-KANTHAL's Type SP resistors are superior to garden-variety
wirewound resistors for glitch service. (phone order # 716 286 7610)
>While I was in the process of installing a new bandswitch
>in our club's Alpha 78, I noticed that the plate overcurrent
>shunt resistor R111 was cracked into two pieces, and
>that the G-10 PWB underneath this resistor was burnt
>pretty badly. It looks as if the resistor was running too
>hot which in turned slowly cooked all the epoxy resin in
>the G-10 laminate. When I scrapped away the soot, all
>that remained was the criss-cross pattern of the fiberglass
>layup weave.
>
>In any case, I don't think this was a "big-bang" kind of
>failure.
** Amplifiers with tubes that utilize tubes with gold-plated grids
usually don't go bang because a parasite that might induce a stentorian
HV-chassis discharge in air in TL-922s, SB-220s, et cetera is typically
replaced by a discharge inside the envelope through the gold meltballs
that stuck to the anode-grid insulator during the parasitic oscillation.
[ref: http://www.somis.org/8877.gs2.JPEG ]
>The resistor was not blackened at all. Instead
>the resistor's ceramic form material is "crazed"
>indicating that was overheated.
** Crazing is typical of a sudden overload.
>Presumeably the hot
>resistor proximity baked the resin in the adjacent laminate
>material over time. The resistor is a 25 ohm x 25 watt
>unit, which in my mind is too small. This resistor carries
>the full supply current of the amplifier which is probably
>close to 1.4 amps when running full bore!!
>
>Anyone run into this problem before with Alpha 78?
>By the way, this might be related to the problem that
>Jorge, EA2LU is having with his Alpha 76.
>
>73 de Mike, W4EF
>
>
>
>
>
>
>
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