Correct me if I'm wrong, but the Boonton 42B/92B chopper is electro-mechanical, no chopper lamps are mentioned in the manual, not sure if you meant something else using lamps (such as HP stuff, which this definitely isn't). The failure method is going to be chopper switch contact wear isn't it or the coil going open circuit?
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Also the cable is described in a manual as a "special low-noise design", some pictures of the cable here;
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David
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On Sat, Oct 5, 2024 at 10:37 PM, Michael A. Terrell wrote:
I currently have very poor vision, so I missed tat part. I've had eye problems my whole life, but I now have cataracts and hav? been unable to have the surgery. These units have fa outlived their design life and the chopper is always suspect because the lamps weren't rated for a very long useful life. I would add a sub board with the modern circuit, rather than mess with reusig badlt aged lamps., but that choice is yours. I repaired equipment for? daily use in production, along with my opersoal shop.? so the equipment had to meet all specifications at outr in house Metrology lab. I prefer to repair a fault, just once. My work centered on testing and calibrating boards made in house for our products, but I ofteny worked with the lab to find was to repair equipment that there were few or no spare parts.
On Sat, Oct 5, 2024 at 3:30?PM Bill Berzinskas via <bberzinskas=
[email protected]> wrote:
@Michael T -
I mentioned that the recorder port is presenting 13.5v always, so there may be something there to look at.? ?If i remove either the chopper or shaping amp boards, it returns to zero.??
This is why it leads me to believe the digital part is ok, its just passing around signals from further up in the chain, i assume.? ? Will dig into it further, still digesting the schematic and manual, tracing through what i can when i have time.? ?
I do think the chopper is passing through to the board and into the circuit but i'm not seeing anything coming OUT of it into the shaping amp, might be the opamp on the output side, tbd.? ?
Either way, I'll cross post to the other group if I get a chance today.??
@Curt, mines the opposite and pined on the high stop.? ?One thing that's driving me mad, the only subsection of the block diagram besides the actual board labels is the lone word "chopper".? I'd love to understand more specifically WHERE the attenuator, 2nd amplifier, sync detector actually reside.? ??
On Sat, Oct 5, 2024 at 3:21?PM Curt Helstrom via <cahelstrom=
[email protected]> wrote:
I have repaired two 92B's with this technique.? It appears that as the choppers age, their timing effects the meter's operation (both of mine
showed meter pined to the negative stop).? What I did was to swap the cathodes of CR207 and CR213, that is I connected CR207 cathode to
the original connection point of the CR213 cathode and vis versa, connected the cathode of CR213 to the original connection point of CR207.
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This changes the timing of when the synchronous detector measures the signals coming from the chopper through the amplifiers, and compensates
for the delays caused by an aging chopper.
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As mentioned, I've repaired two 92B's with this technique, both meeting all specifications after this modification.
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Hope this helps in your efforts to repair your meter.
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factory