Re: 7B80 sweeping off the screen


Roger Evans
 

You can operate the 7B80 in any of the three slots of the 7603 and it will drive the vertical deflection if you put it in a vertical slot. So if you insert only the 7B80 and take all side panels off both the mainframe and the plugin you have some options for probing but you will need to run in auto trigger mode to have the time base sweeping with no trigger. If you don't have a second scope but do have an analogue multimeter you can run on the slowest possible sweep speed and the voltages should largely follow what is in the manual but obviously on a slower timescale.

If you probe test point TP345 (waveform 4) you can confirm that the sweep generator is producing the correct sweep voltage range and polarity. Measuring the waveform at the collectors of Q448 and Q458 (waveforms 8 and 9) you should see if either half of the paraphase amplifier is causing the problem. Then you need to follow the signal through Q428 etc to track down the culprit. If the transistors are socketed ( mine aren't) I would just begin by testing Q424, Q428, Q434, Q438.

Regards,

Roger

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