Bun's Lab – Telegram
Bun's Lab
@BunsLab
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102 videos
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Electronics projects, vintæg computing, programming and repairs. A minimalist blog of sorts.
@BunsGarden @BunsNook
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Bun's Lab
147 subscribers
Bun's Lab
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It's pretty dead
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Bun's Lab
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Bun's Lab
The main oscillator is not running. Shown here is the range selection and integration capacitors
147 views
Bun's Lab
philips--pm-5133--service-user--ID8639.pdf
27.2 MB
Service manual
133 views
Bun's Lab
Theory of operation and a simplified schematic of the main oscillator
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Bun's Lab
p70.png
10.9 MB
Relevant schematic parts in full
p67.png
16.5 MB
148 views
Bun's Lab
The transistor pair 423 of the comparator is realized as a CA3183E transistor array in a PDIP package.
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Bun's Lab
Cutting the gate of the BF245 JFET out of circuit and feeding the blue signal into it. Red is the final output, I can control the amplitude and waveform just fine. Green and yellow are the bases of the two comparator transistors 423.
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Bun's Lab
Blue is still the input to the gate of the JFET, red the final output. Green the base of the second transistor of the comparator pair, yellow the signal at the junction of the emitter of 365 and collector of 366. According to the schematic the amplitude there should be 5V. Blue cuts out for anything below 5.2V however. Curious. No idea if I'm onto something.
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Bun's Lab
Adjusting pots 830 and 837 of the comparator for 50% duty cycle and stable triggering for 5V signal amplitude
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Bun's Lab
Since I know that the green section works, I cut out the burst control by lifting one side of the diode there and checked those four transistors manually
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Bun's Lab
One of them is iffy. A BF450
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Bun's Lab
I only have BF440 on hand. Seems similar enough to serve as an impromptu replacement.
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Bun's Lab
For some odd reason my -20V rail suddenly dropped to -13.5V. I couldn't find the culprit by looking for a heat source
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Bun's Lab
The PCB has those solder bridges on the voltage rails. -20V gets split into A, B and C. With the bridges you can cut out rails
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Bun's Lab
The fault is definitely on rail B. My solution was to feed this rail not from the internal supply but from my lab supply, to then pump more current into it and to hopefully find the culprit through its thermal emissions. Can't find it tho :/
195 viewsedited  
Bun's Lab
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The thing does oscillate now. This is constant frequency mode, not the sweep. So something is really wrong in the control section
205 viewsedited