Tektronix 547 Lack of Sweep or Intensity control - Page 1
Forum section: electronics / repair

author: gordo70
author_rank: Newbie
timestamp: April 15, 2025, 04:02:32 pm
content: My ancient Tektronix 547 oscilloscope with 1A1 plugin was working fine until a few weeks ago when the display suddenly went intermittent and bright, and an arcing sound came from the area of the crt high voltage circuit. No arcing could be seen anywhere, but the sound definitely came from the vicinity of the high voltage assembly. At this point, the Intensity control had no effect, and the intemittent sweeps were very bright. The sweeps only occurred when the arcing sounds were heard. Otherwise, a bright spot stayed on the left side of the display. The focus control also had only a slight effect.Suspecting the flyback transformer T801, I sourced an assembly from another 547 that included the transformer and the voltage multiplier comprised of the 5642 diodes. Symptoms remained the same, unfortunately. Replacing one of the regulator tubes in the 350V supply (6CW5) with a brand new one strangely made the arcing go away, but still no display. In fact, no more intermittent sweeps, only the ultra bright spot at the left hand side of the display. Currently, I have no way of measuring the high voltages from this assembly (10kV), so I’m not sure if they’re within spec. I also changed out the four 0.01uF caps in the Intensity circuit in case they were arcing, but no improvement.Scoping the output of the B Sweep Generator section (at tube V91), there was no sign of a sweep waveform (consistent with the stationary bright dot on the left side of the display). I’ve tested every tube in the scope, except for the 6080s. None gave the impression that they would be show stoppers. I also scoped all B+ and B- rails. All were rock steady except for the +350V line, which had some ripple. Even after changing out the 6CW5, it was still there.It’s obviously generating high voltage as evidenced by the bright spot on the crt. I can’t understand the relationship between the inability to control Intensity and Focus and the lack of a sweep signal. It doesn’t matter whether I’m in A, B or ALT timebases. Could the crt have an internal fault?Any advice or best guesses as to where I should look next would be much appreciated. Thanks in advance.

author: Dave Wise
author_rank: Regular Contributor
timestamp: April 15, 2025, 11:10:18 pm
content: For the snap, I would suspect the ceramic strips in the high voltage compartment. Unfortunately, a transient of that size may have propagated far and wide leaving behind secondary damage. Check transistors.The groups.io "tekscopes" group is the highest concentration of Tektronix expertise, you should join.HTH,Dave Wise

author: gordo70
author_rank: Newbie
timestamp: April 16, 2025, 06:15:36 pm
content: Thanks Dave. Will look at transistor health next as well as the groups.io.

author: MathWizard
author_rank: Super Contributor
timestamp: April 20, 2025, 04:40:51 am
content: All that old Tektronix gear is beautiful. Hear's what is says on the Tek-wikiQuote

author: Dave Wise
author_rank: Regular Contributor
timestamp: April 20, 2025, 05:11:33 pm
content: For the runaway intensity, look at Q373 which handles CRT unblanking on Sweep A, and Q173 and Q184 for CRT unblanking on Sweep B.They are 151-0063-00 Germanium PNP 2N2207. I have spares, or if you increase the speedup cap, you can use KSA992, BC556, or others; ask if you want details.