Tektronix SC504 limited x deflection trace visibility - Page 1
Forum section: electronics / repair

author: Doug Brewster
author_rank: Contributor
timestamp: July 16, 2018, 08:32:03 pm
content: This unit does not display a full width trace unless the intensity is turned up to an unacceptable level. Any suggestions as to where to look?

author: Chris56000
author_rank: Frequent Contributor
timestamp: July 19, 2018, 10:46:13 am
content: Hi!Do you have another oscilloscope available?You'll need one to diagnose this fault as the triggering circuitry may not be set-up or working correctly!Download the SC504 Manual from TekWiki W140, BAMA Archive or Kevin Chant, etc., (it is a good PDF and all complete!) and first of all, go thro' the basic operating procedure as given in Pages 12-18 just in case it's some perculiarity of front-panel switch settings etc., if you are completely happy the trace doesn't respond correctly to the basic operating procedures in pages 12-18 of this manual, you will need another oscilloscope and DMM to investigate the triggering/free-run circuits and the horizontal amplifier.As with all faulty Tek equipment, the internal power supply voltages MUST all be correct, both for voltage and ripple BEFORE any detailed fault-finding can be carried out on the remainder of the circuits! (Diagrams 8, 9 and 10)If you have, or can get access to another oscilloscope, begin by referring to the wave-forms given on Page 79 of make sure all of these are present and correct.If you find anything that doesn't look right, take a picture of the faulty waveform (a phone camera will do) and post it - one of us will be on hand to assist!The relavant circuit-diagrams you need to study are:-a) Diagram 4, TRIGGERING, on page 91;b) Diagram 5, SWEEP GATING, on page 94;c) Diagram 6, SWEEP GENERATOR, on page 97,d) Diagram 7, HORIZONTAL AMPLIFIER & OUTPUT BOARD, on page 99.If you feel this is too difficult or you don't have enough equipment, and you're not too far from Birmingham/Midlands (your name shows as a UK poster) please PM me and I'll get back in touch by PM with a view to assisting personally!)Chris Williams

author: floobydust
author_rank: Super Contributor
timestamp: July 19, 2018, 02:31:55 pm
content: CRT electrostatic deflection sensitivity cm/V should not change with beam intensity, from a review of the basic equations.Horiz. width changing with beam current... I can't see a correlation, as the power supply voltages are supposed to be regulated and independent.Only the +70V rail is common or there are still troubles in the HV power supply with OP's repair.

author: Chris56000
author_rank: Frequent Contributor
timestamp: July 19, 2018, 06:24:59 pm
content: Hi!The fault is nothing to do with the deflection-sensitivity of the CRT, however, faulty power-supply smoothing and/or decoupling may be causing the time-base saw-tooth and the intensity modulation to interact!Another possibility is the sweep gate not being held on enough for a long enough period of time to un-blank the CRT over the full length of scan, hence my advice to the OP to check the power-supply rails, firstly to rule out the fault in the first paragraph, and a careful check of the wave-forms on Page 79 will identify un-blanking faults in the second paragraph!Chris Williams

author: Doug Brewster
author_rank: Contributor
timestamp: July 19, 2018, 07:43:48 pm
content: Thanks for the responses. First off, the SC504 with the trace width problem seems to have righted itself - temporarily? We'll see, can't really look for a problem which isn't there.Floobydust makes reference to an HV issue that may be still causing issues: I think you may be looking at another thread I have going that deals with a failed HV multiplier which is in another SC504 that I have on the go.Yes I do have another scope (actually yet another SC504 and a couple of 475s) and a BAMA manual so if the problem recurs I'll be using your responses to investigate further.I'm about to update my HV multiplier thread with a couple of pics and progress so far...