waveform at 555 + HV variance - amplifone monitor

Hi all, Just a quick sanity check: Should the waveform from the 555 be a true square wave or do they vary a lot? I hooked up a dummy load (60w light bulb) across pins 1 and 9 in place of the HV transformer and connected a 'scope (10x probe, compensated) to pin 1 (ie the light bulb) and what I saw was a slow ramp up to the top of the wave (more like quarter of a sine wave) then a sharp cut off (more like what I expected), the frequency is still around 20kHz. I figure the data may be skewed because of where I am measuring it (or an effect of the lightbulb) and maybe I have to measure directly from pin 3 of the timer IC? Secondly (quite possibly related),  I went ahead anyway and installed a cinelabs type flyback but I can hear occasional arcing and I think it's somewhere actually on the HV board. When this happens my HV probe shows a brief but significant increase in HV from, say 19.5 normally up to 25kV+.  I can't decide if the arcing if the cause of the HV increase or if some other part of the circuit is playing up and therefore ramping up the HV and the arcing is just a side effect. So far all the electrolytics have been changed, +- 24v regs, 50 ohm resistors, 1n4001 diodes and the CR3 was missing so I used a sub. Old parts: the BU406D has not been replaced -tested okay as far as one can tell from the FAQ, and I haven't replaced the 555 timer. The output of the =- 24 regs is stable but a little out of spec on one rail like +2 volts or something with the lightbulb attached and I put this down to the 50 resistors. So logically speaking I think I shall replace the 555 timer, BU406D, and perhaps the frequency adjustment pot next. Thanks in advance. Karl.
---------------------------------------------------------------------------
** Unsubscribe, subscribe, or view the archives at http://www.vectorlist.org
** Please direct other questions, comments, or problems to chris@westnet.com
Received on Mon Aug 13 06:29:37 2012