Hi Mike,
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On Sun, Jul 21, 2024 at 08:19 PM, Mike M wrote:
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Comparing your schematic to Steve's I see a few differences:
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- The secondary of T2 is directly connected to ground in Steve's and through capacitors in yours.
That's how it was
- 1.2k to ground at the output.
2.7K ? I don't know why it was there, but I double-checked when I saw it, and it was definitely present.
- As noted, 41pF instead of 82pF across loop.
The input transformer was actually 3 + 3 turns centre tapped, but the tap wasn't connected to anything.
- As noted, R4 is 500k instead of 1k
That is a mistake, the part was damaged during the reverse engineering process of boiling the assembly to remove the epoxy. I couldn't check with a multimeter as it had snapped in half, and the coloured bands were unclear. I assume it was actually a much lower value, and 1K or 500R is probably more likely.
Static bleed, as the loop is "floating" above ground. The input transformer centre tap was not grounded or connected to anything else.
That's how it was
- I assume R1 is to bleed of static charge. What is R6 for?
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I have no idea why it was there. As the unit was hand made on stripboard, it may have been meant to be connected somewhere else in the circuit. Or maybe it's serving some other purpose during assembly and test ?
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- Are all of the differences from tracing a different unit or are some things your own changes?
I produced the circuit diagram directly from what I saw and measured in a faulty unit I had been given. I carefully picked the resin away from the circuit board after boiling it for an hour to soften the material.
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The transformers were the most difficult part to understand, as the binocular cores were full of resin. I had to cut the windings and count the number of wire ends to determine the number of turns. I measured the resistance and reactance curves of the ferrite material using a VNA, and compared it with test windings on same sized cores using different materials. It was similar to 73 mix, but a closer fit to N30. The wire numbering in my diagram is incorrect, as there are two number 4's.
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It could be that my unit was an earlier version, in comparison to the ones that others have taken apart, due to the "kitchen table" construction, slight differences and errors have probably crept into each build, and the design may also have "evolved" slightly over time.
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I will revise the diagram and repost it to correct the mistakes.
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Regards,
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Martin
