Tektronix 468 schematics - Page 1
Forum section: electronics / testgear

author: KJDS
author_rank: Super Contributor
timestamp: May 25, 2014, 12:19:52 pm
content: I've got a Tek 468 that is all working well apart from the intensity control will only vary the display from too bright to even brighter than that. I've looked through the service manual from ko4bb but it doesn't have schematics. Probing round the board near the intensity control pot shows things look ok there, but the block diagram also show signals coming from several other places that could be causing issues and a schematic would speed up fault finding, so does anyone have one or know where I can find one?

author: Lunasix
author_rank: Regular Contributor
timestamp: May 25, 2014, 12:40:38 pm
content: I've had similar problem on a Tek 465. It was a simple small diode which was good with multimeter but had leakage or avalanche breakdown under real conditions. Not so easy to find, need schematics, second oscilloscope and thinking ! Don't remenber where it was, I don't have anymore the scope and schematics.

author: v1t0r
author_rank: Contributor
timestamp: May 25, 2014, 01:05:01 pm
content: HIYou Can get the full service manual in Art Manuals.

author: PaulAm
author_rank: Frequent Contributor
timestamp: May 25, 2014, 01:44:03 pm
content: The 468 has a 2 volume service manual, you'll need both volumes (volume 2 has the schematics). There are free copies floating around, but the quality is not the best. Artek sells high quality scans and if you actually need to read the schematic, that would be a good choice.

author: fluxcapacitor
author_rank: Frequent Contributor
timestamp: May 25, 2014, 01:45:33 pm
content: You can get the 2 volume version from elektrotanya ,and a smaller pdf download from KO4BBhttp://elektrotanya.com/tektronix_468_volume1.pdf/download.htmlhttp://elektrotanya.com/tektronix_468_volume2.pdf/download.htmlhttp://www.ko4bb.com/manuals/index.php?dir=Tektronix/Tektronix_-_468_Digital_Storage_Oscilloscope

author: iDevice
author_rank: Regular Contributor
timestamp: May 25, 2014, 06:05:46 pm
content: I would start by checking CRT Bias adjustment.

author: oldway
author_rank: Super Contributor
timestamp: May 25, 2014, 06:11:20 pm
content: Always starting by checking all the power supplies.

author: Sarasir
author_rank: Regular Contributor
timestamp: May 25, 2014, 06:34:13 pm
content: Hi,I was service engineer with Tek and remember this problem. there is a capacitive coupled transfer of the intensity signal from normal potential level up to cathod/grid level or better to say down as I remember the cathod of the CRT had negative HV applied whereas the anode was positive. I have the manuals here I think and if you got stuck can give you some further ideas. Its basically - out of my memory many years ago - two HV ceramic capacitors and diodes around - some kind of pump circuit. Chech the diodes first and then the Cs for small leakage. not to be tested with 1.5V powered multimeter but better with much higher V. Several 100 Volts would be perfect.Best wishes

author: Sarasir
author_rank: Regular Contributor
timestamp: May 25, 2014, 06:37:22 pm
content: .....want to add that the compnents mentioned above are located under the HV shield at the rear side of the main PCB....

author: TerraHertz
author_rank: Super Contributor
timestamp: May 26, 2014, 02:00:59 am
content: Quote from: Sarasir on May 25, 2014, 06:34:13 pmHeh. You know what's going to happen now, right?You're going to be inundated with requests for advice.I think I will be in that queue. I have several faulty Tek 7000 series scopes put aside. Some not looked at yet, others with problems that beat me (in the time I had for them.)

author: Sarasir
author_rank: Regular Contributor
timestamp: May 26, 2014, 04:10:07 pm
content: well, basically no problem if you do not expect me solving all probs at once...I was also servicing the 7000 series so start with the first one. What I know I will contribute.

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2014, 02:50:19 am
content: It is usually easier and safer to just change the high voltage capacitors and the diodes in the DC restorer if a problem is suspected there. As Sarasir points out, a high voltage is needed to test them adequately for leakage and I figure if I had to pull the parts to test them, I might as well install new ones anyway unless they are expensive or difficult to find.Before going that far unless there was an obvious problem like a low voltage short in one of the diodes or capacitors, I would certainly test the power supply voltages for level and ripple and do the z-axis calibration procedures including the grid bias adjustment. Then I would take a look at the output of the z-axis amplifier to verify that it is responding correctly.When working with the z-axis circuits or if there is a problem in that area, it is a good idea to defocus the CRT to prevent damage to the phosphor.