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SC502 Trace Problem
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Greetings!
I have been working on an SC502 that I suspect has been out of service for some time. I got all the power supplies working, about 10 faulty components. Then I found a bad trimmer pot that I replaced which got the trace back. Then, a tantalum ruptured while I was looking at it. Its been running now for 24 hours displaying a sine wave from my FG. However, the trace is confined to the bottom of the screen. When I try to position the trace above mid screen the trace clips at exactly the mid point. I am looking for advice from the group as to what this problem might be and a plan of attack. Thanks! Barry |
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Half of the vertical amplifier is effectively dead.? The causes could be anything from a broken wire to a dead transistor.
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1. examine the wires to the CRT, they should both be connected (can happen!)? Voltage at the CRT pins should be equal with the trace centered, and should both change with the position control. 2. trace back (or forward if you want), to each collector.? The amplifier is a single input (current) with an emitter coupled phase inverter.? That gives you an opposite phase on the "upper" and "lower" transistors.? The amplifier is then pretty much identical to the deflection plates.? Each "lower" transistor has a corresponding "upper" transistor.? changing the position control should give you a corresponding change on both "upper" and "lower" parts of the amplifier.? You could also use a nice sine wave as well if you want. If the parts are socketed, and identical, you can exchange between the two signal paths and see if the fault goes with the part. Checking power supply voltages to each part of the amplifier is a good idea right after seeing if the wires to the CRT are intact. Harvey On 3/1/2024 10:38 AM, Barry Breaux wrote:
Greetings! |
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Harvey,
Thanks for your quick response! 4This morning I decided to remove the CRT. Just after it cleared the chassis I noticed that the Upper / lower deflector plates were not connected. At the time I didn't know if I had pulled them out or it they were not connected to begin with. My guess is that someone worked on this scope, gave up and didn't take the time to reconnect the upper and lower plate wires. I asked the question a bit to soon. Shows that I don't have experience with CRTs. I was successfull at reinstalling the crt and indeed it is functioning. Thanks for your response. I am sure it will help someone else someday! Regards, Barry |
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That's a very common event.? Glad you got it fixed.
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Harvey On 3/1/2024 12:32 PM, Barry Breaux wrote:
Harvey, |
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