>
>A friend brought over a Heathkit SB 220 for repair. I thought
>I'd tell you what I found.
>
>I removed the chassis from the case to first find the ALC
>electrolytic capacitor strewn about the bottom in confetti and
>foil.
There is no such capacitor in the ALC circuitry -- in the 110VDC supply,
yes.
> One of the grid chokes appeared to have been hot.
>
>I replaced the capacitor and the entire alc terminal strip
>parts assembly, then spent some time slowly reforming
>the HV capacitors back up. The disabled and retired
>owner has little or no money for new capacitors.
>
>So I bring the unit up in slowly on a home brew fully metered
>special variac unit with a dmm on the alc supply to view the
>proper operation of the new terminal strip parts. Near 80%
>of normal line voltage, the ALC 120 volt supply went up to
>above 350 VDC, very strange indeed. At that point I thought
>about a transformer break down. After much work to isolate
>hi-pot and load test the filament transformer, it was deemed
>healthy and returned to the regular amplifier wire harness.
>
>Parasitics and or a tube problem might be the next thing to
>investigate.
>
Parasitics that are intermittent are not easy to investigate.
>The unit fired up without tubes showed no problems at the
>alc point, then each tube was tried in individually at both
>locations. Again, no abnormalities observed.
Be sure to remove the wire between the bias contacts on the relay and the
110VDC power supply. If you don't do this, a shorted tube can easily
destroy the filament transformer.
- Rich..., 805.386.3734, www.vcnet.com/measures.
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