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You must be talking about the 7704A which is different from the 7704. The power supply needs the display unit to have minimum load. I usually measure what I can at the horizontal input connector.
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Jerry Massengale
-----Original Message----- From: raymonddompfrank To: TekScopes Sent: Mon, May 21, 2012 8:20 pm Subject: [TekScopes] 7704 power supply
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I am a newbie Tek collector, having used my own 465 until some 20 years ago. I am reasonably knowledgeable in electronics.
The other day, I purchased a 7704 after only giving it a very quick test. I agree that wasn't very clever. Unfortunately I noticed some brightness and timebase issues and I decided to have a look inside the instrument. That wasn't very clever either... I checked the +54.2 +- 0.25V without removing the power supply. It turned out OK (54.4). I decided to check some other voltages on the top regulator board and decided to remove the display unit (D7704) to reach in. I realized that many switching power supplies don't like very light loads but decided it was easier and the location of the description of how to remove the D7704 in the maintenance manual was such that I concluded it was safe. Also, I haven't found any warning re. this procedure. Anyway, since I removed the D7704 and switched the PS on, there's a hissing sound from it and from what I measured, all preregulated voltages are much lower than normal: +54V now is 30V, -54V is -30V, 7V is 2V etc. Also, the front lights remain off (not surprising). I'm anxious leaving the PS on for more than a few seconds. My Question: Did I indeed blow up the PS and if so, is there a component that usually blows up and is the repair doable or even easy? There's no nasty burning smell or anything. Or is this just a coincidence, and have I just witnessed another electrolytic capacitor dying? Thanks for any ideas! Raymond |
