VU Data PS935 intensity issue - Page 1
Forum section: electronics / testgear

author: xwarp
author_rank: Frequent Contributor
timestamp: May 15, 2014, 12:45:10 am
content: Got this little scope for free and pretty impressive little portable.35mhz two channel jobber.I initially found a few bad tantalums in it and replaced all those.Trace looks good with the exception of lack of control over the intensity.Problem is the trace is too bright and the intensity adjustment has very little control over it albeit it does dim very slightly.Any thoughts would be appreciated.Will be adding snapshots of the high voltage schematic shortly.I've attached the service manual for the scope. The page in question is 183

author: tautech
author_rank: Super Contributor
timestamp: May 15, 2014, 01:53:32 am
content: Firstly is the 1000V test point at R13 correct?

author: xwarp
author_rank: Frequent Contributor
timestamp: May 15, 2014, 01:54:42 am
content: EDITED:I can't find the full size page that I had with all the voltage and signals written on it, so I measured across the R13 again and the Triplett 630 is showing about 920.Will find the hard copy manual because I did write a lot of the readings in it.

author: tautech
author_rank: Super Contributor
timestamp: May 15, 2014, 02:08:53 am
content: Does the Focus appear to work correctly? If it does de-focus the trace to prevent CRT damage.Have you checked the intensity pot (all resistance combinations) and R54/55?

author: xwarp
author_rank: Frequent Contributor
timestamp: May 15, 2014, 02:14:38 am
content: Focus and astig seem to function properly.I keep it pretty blurred when I am running it for measurements.And in regards to R13:I can't find the full size page that I had with all the voltage and signals written on it, so I measured across the R13 again and the Triplett 630 is showing about 920.EDIT TO ADD:While measuring across R13, pot R16 can be adjusted so that fully ccw puts 1000vdc across R13.Adjusting R16 fully cw drops the R13 to about 600 volts. At that point, the trace does dim, but then continues to dim until it disappears and there is still no apparent adjustment with visible intensity although rotating the intensity control ccw and cw shows no more than a 20vdc drop in the value of of the voltage across R13.The approx 20vdc change occurs with the intensity pot regardless of the value across R13.

author: tautech
author_rank: Super Contributor
timestamp: May 15, 2014, 08:14:52 am
content: Here is a link to a good Tek document in another scope repair thread. Posted bysoldo63.Go to P42 in your viewer. Typical HV problems.https://www.eevblog.com/forum/testgear/ma-4049-oscilloscope-repair/msg427677/#msg427677

author: xwarp
author_rank: Frequent Contributor
timestamp: May 15, 2014, 08:37:11 am
content: I'll take a look at the voltages across CR6 and 7 tomorrow.Appears that the grid bias should always be about 60vdc lower than the cathode.

author: xwarp
author_rank: Frequent Contributor
timestamp: May 15, 2014, 10:48:49 pm
content: Progress update 5/15Made some measurements across CR6 and CR7 and found that the voltage on the cathode of CR7 was -550 volts.Replaced CR6, CR7, and CR9 and now have what appears to be full control of the intensity as normal.Next up is the calibration voltages as listed on the attached page.Test point 2 for -22 vdc is good.Now unable to adjust R47 for +25 at TP1. The max value on that one is 7 vdc.Moving down to -1000 vdc, R16 adjustment for that works and the reading at C10 is -1000 with the intensity control working properly with less than a 10vdc drop from dim to bright.Wonder if I should go ahead and replace the rest of the diodes on the board as I have plenty of them. (1n4148)Edit to add:I think that there is a typo in the cal, it shows to adj R47 for +25vdc at test point 1, but I think that should be TP2 because when measuring that point with the intensity fully clockwise, R47 can be adjusted for +25vdc.