BWD 509 oscilloscope weak trace intensity - Page 1
Forum section: electronics / repair

author: ROAMER
author_rank: Newbie
timestamp: November 09, 2016, 09:37:59 am
content: Hi all!!!I have BWD509b also that had trace issues that I repaired (well nearly there in reality). I also have dim trace problem. In normal daylight it's not even visible. Voltages that I have are as follows:Cathose, pin 3: -1440 volts (intensity on max)Focus, pin 4 - 1010 volts (best focus setting)Astig, pin 11: +114 voltsGrid, pin 2: -1440 voltsSo voltages seem good enough. Does anybody have any idea what might it be? I'm knowleageable in electronics, it's just this is my first time coming across cro tube, so I don't know what to expect.Many thanks for your replies!

author: IconicPCB
author_rank: Super Contributor
timestamp: November 10, 2016, 09:41:40 pm
content: http://www.thevalvepage.com/teletech/crt_rej/crt_rej.htm

author: ROAMER
author_rank: Newbie
timestamp: November 12, 2016, 08:40:19 am
content: Quote from: IconicPCB on November 10, 2016, 09:41:40 pmThanks for this article. Will try it! Do you know if anyone had real sucess with these methods?Regards,

author: oldway
author_rank: Super Contributor
timestamp: November 12, 2016, 09:59:49 am
content: Don't try it if you are not 100% sure that your crt is weak !You will risk to ruin your crt.I repaired a lot of analog oscilloscopes and I never found a weak crt.It happened often with color tv crt's (red electron gun weak) because cathode current was relatively high but that is not the case with analog oscilloscope.You must measure the voltage between cathode and G1 (whenelt) and compare it with data sheet of your crt.But be careful because cathode is at high voltage.What about heating voltage ? Is it correct ?Heating current is right or not ?

author: ROAMER
author_rank: Newbie
timestamp: November 12, 2016, 01:41:36 pm
content: Quote from: oldway on November 12, 2016, 09:59:49 amHi, thanks for you reply. Can you explain to me what whenelt is? My crt tube is 130 BE B31 and I can't find any data for it.I know electronics, but not CRTs to that extent where I can reckognize all the terminals just by looking at the drawing and what they are usually marked (like G1 K1 K2 etc). I can see cathode, mesh and deflecting plates but don't know what voltages should be at every point.I have listed in original post what I have measured so far. Heating voltage is 6.4 volts, but I haven't checked the current. Will do and report.I'm comfortable with high voltages. I have 3500V probe that I'm using, so no problem thereThanks for your help again !!

author: oldway
author_rank: Super Contributor
timestamp: November 12, 2016, 03:50:10 pm
content: Wehnelt (not whenelt, sorry) is the first grid (G1) in crt's.https://en.wikipedia.org/wiki/Wehnelt_cylinderIf you did measure cathode and grid 1 voltages with enough precision and you are sure they are the same voltages of -1440V, you don't need to measure the cathode/grid voltage.I do this to be sure.You could eventually try to short circuit grid and cathode (pins 2 and 3), but beam must be out focus to not risk to burn the phosphor.Have you a bright trace ?What brand is this crt ? It is difficult to find informations about this crt...130BRB31 is most common.

author: ROAMER
author_rank: Newbie
timestamp: November 12, 2016, 04:22:32 pm
content: Quote from: oldway on November 12, 2016, 03:50:10 pmHi,I measured but I'll measure again just to be sure. What do I expect when I short pins 2 and 3?Brand is BWD, it's local electronic manufacturing company that was based in Melbourne years ago.

author: ROAMER
author_rank: Newbie
timestamp: November 12, 2016, 04:31:48 pm
content: I have attached a schematic for the Oscilloscope where crt is visible and pin numbers that I measured voltages on. If someone could name these pins for me and voltages should be expected I'd be very thankfull!!!

author: oldway
author_rank: Super Contributor
timestamp: November 12, 2016, 06:42:20 pm
content: I doubt very much that BWD was making its own crt's ....BWD is the brand of the oscilloscope, probably not the brand of crt.Are you sure that the beam is not out of screen ?What you see is perhaps only a ghost image, not the beam.Did you measure the Y and X plates voltages ?

author: ROAMER
author_rank: Newbie
timestamp: November 12, 2016, 07:52:12 pm
content: Quote from: oldway on November 12, 2016, 06:42:20 pmI doubt very much that BWD was making its own crt's ....BWD is the brand of the oscilloscope, probably not the brand of crt. - Yeah, BWD is the brand of the oscilloscope, I will have a look on the crt if there is any brands stamped.Are you sure that the beam is not out of screen ? - pretty sure it is on the screen, when I go to XY mode, I can see the dot that is pretty bright. But when I switch over to tracing, intensity is just not there. I'd say 30-40% compared to normal scope intensity.What you see is perhaps only a ghost image, not the beam. - Doubtful, when I have a look at my XY voltages, it puts the beam right at the middle of the screen, just where it is.Did you measure the Y and X plates voltages ? - I measured and they are moving withing limits what they should be. For X envelope is 180 volts approx. and when the dot is in the middle they measure 90. I forgot what figures were for Y but pretty much correct.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: November 12, 2016, 08:05:34 pm
content: If you haven't said it already, check all resistors around the CRT. Top to bottom in the schematic, so R43 to R41 at the bottom and make sure that they are in spec. Do discharge all HV caps before you measure. Speaking of measuring, a HV high impedance meteris a must for voltage readings in the CRT circuit.I see other references to the 130BEB31 sub or cross being a C529P31B

author: oldway
author_rank: Super Contributor
timestamp: November 12, 2016, 10:59:30 pm
content: QuoteThat's a very important information, it means that there is nothing wrong with the crt.Your trouble is with blanking circuit, not with crt.

author: ROAMER
author_rank: Newbie
timestamp: November 15, 2016, 08:57:44 pm
content: Hi All!!! I'm away from the oscilloscope for a few days but I'll report back as soon I do bit more investigation. Thank you all for your help so far!!!

author: ROAMER
author_rank: Newbie
timestamp: November 22, 2016, 08:16:50 am
content: Bit of an update. Tried shorting pins 2 and 3, cathode and grid, no change in trace intensity. heater measured 6.77 volts, but I didn't have time to measure current. What sort of current draw should I expect?Thanks!!!