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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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Tektronix D755 oscilloscope excessively bright at switch on
Inherited this antique as a DOA, so thought I'd try to fix it for some fun.
Immediate obvious problem was that the electrolytics in the PSU had failed, so on general principle I replaced every electrolytic in the machine, and now it powers on and I've got the vertical amp calibrated against local references (no they aren't certified, but have been checked against kit that *is* in calibration), and the sweep timings appear to be pretty darn close anyway, so I've not dinked with the sweep unit. +24V, -24V and +105V are rock stable within seconds of power up. One remaining problem though, when I power on the scope, the scan line is excessively bright even with intensity turned right down and this takes about 20 minutes to settle down to a reasonable level (at which point I need to turn intensity up to about 50%). Where should I be looking for the culprit - my initial thoughts are pointing me to the HT/EHT section or possibly the "bright-up" circuit. Any suggestions on the likely causes of the problem would be helpful. Could it be the ceramic caps in the HT supply, or possibly the switch on Thermistor (TH301)? How best to check? Thanks -- Dave Partridge (remove xwy from replyto email address) |
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