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HP200CD Ocillator Jan 30/2025
HP200 CD is one of earier version wein bridge audio/supersonic oscillator. It's sit on top a shelf at working area in radio museam. I never trying to fix a Tube equipment, but i guess they all had failed capacitor.
The tuning dial's shalft had been seized up for long time, i guess. Even with a big piler, it won't turn even a bit. I tried hardly use piler, soak it with oil, it wont' make things even a little better. The experieces guy in measum told me we should use gas torch heating the shaft, and then use hammer shock it.I sound like would be useful.
we heat it use weller soldering gun, not a gas torch. weller gun is not powerful as the gas torch, but it works. while it stat to turn, even though i had use the biggest piller make it, this still make a big step.
Then i start oil the shaft in back and forth, seems like the oil wont' go inside the shaft. i finnaly got a piece of tissue soked in oil, then put the tissue inside a suitable little plastic cap, then put the cap on the shaft, the heading home, let time fix it.
Be carefull while you trying to lose the screw, a better way is use a electric screw driver. use the most weakest gear, then it will impact the screw instead use the maxium torque.
I don't have my screw driver ready, so do that like a impact driver.
btw, the HP200CD acctually use a bigger screw, that acctually easy to get it loose, because the screw is strong enough to withstand much more higer torque.
when come the electronic restoration part, it's easier than fix the mechanical one. checking the power supply first! and i Immediately identified the L6 6H inductor was open inside， when i remove that big ballast inductor， there was obviously someone trying to fix the open circuts, tear few turns from the frame, but the enameled wire brokne deeply in the coil, the we located another one as replacement. I'm stuning while see dozens of shelves filled with enormous parts and devices.
It's obviously no doubt that the main filter capacitor is dead, and the rectified tube has leaked， replace both of them revover the HP 200CD! There are some twists and turns to fixing it, ofcause. I fogot connect the Neg side suppy, drive me crazy! when i find this, i then wonder how the 'GND' works anyway.
It turns out that the GND is just divided by a R-C charge redistribution, which depnds on, the "GND",accually a 'middle' voltage, not exactly the half the voltage diffrence, only provided very tiny charge to bias the Tube. this trick works only for tube amplifer, won't work for BJT and IC.
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