Tektronix 7904, no horizontal trace on B input.  - Page 1
Forum section: electronics / repair

author: Hermansson Amplification
author_rank: Newbie
timestamp: November 16, 2019, 01:23:45 am
content: First post on this forum! I mainly work with tube guitar amps but always end up at this forum when I search for info regarding repairs of my electronic measurement equipment so it’s about time I join you!I just bought a Tektronix 7904 scope and realized that something is wrong with horizontal input B.Horizontal input A works fine but I got no trace on input B. If I press“Beam finder” slightly I see dots for each inputs that are activated in the vertical slots and if I press the “Beam finder” harder I see bigger dots. I only see the dots when I press “Beam finder” otherwise the screen is blank when horizontal input B is activated.Any suggestions where to start? I’ve measured the voltages on the Z axis-PCB but nothing alarming there. Since the horizontal Input A works something might be bad on the “Trigger B”-PCB? Or where should I start?Thanks,Henric

author: Hermansson Amplification
author_rank: Newbie
timestamp: November 16, 2019, 01:26:37 am
content: B trigger schematic.

author: Hermansson Amplification
author_rank: Newbie
timestamp: November 16, 2019, 01:27:43 am
content: B trigger PCB.

author: MadTux
author_rank: Frequent Contributor
timestamp: November 16, 2019, 01:40:59 am
content: Any other timebase except that 7B92A available, to exclude a problem on timebase? (I don't particular like 7B53A and 7B92A, too much things got stuffed into those)Does it get trigger, if set into normal mode, with signals present on amplifiers? Green trigger LED should light up then.

author: David Hess
author_rank: Super Contributor
timestamp: November 16, 2019, 03:36:49 am
content: When the timebase is set to AUTO mode, then it will generate a sweep when no valid trigger is present so the trigger is not the problem.With the 7B92A in slot B and the beam finder pressed, does the 7B92A's position control move the traces horizontally?Another test you can do is to place a vertical plug-in into the B horizontal slot. This configures the oscilloscope for X-Y mode and the position controls of the plug-ins can then move the beam along both axis.

author: med6753
author_rank: Super Contributor
timestamp: November 16, 2019, 03:51:10 pm
content: I tried simulating your issue with my 7904 but as you notice it is working and I have traces. However...if you notice my time base readout is in the upper right, not the lower right as yours. That could be a clue. My time bases do not have delayed sweep (7B80 and 7B70). Your time base has delayed sweep. Are you sure it's set up for "A" time base and not "B"? Also, try changing the Triggering mode from "P-P Auto" to "Auto" and see what happens.

author: rge59
author_rank: Newbie
timestamp: November 16, 2019, 05:19:26 pm
content: I have just checked on my 7904 with a 7B92A timebase. When the 7B92A is displaying A sweep only the readout is in the lower position, but when displaying A and B sweeps together the A speed is in the upper position and B speed in the lower position.Your 7904 looks very clean but if it has been idle for some time the vertical and horizontal mode switches (A/left, alt, chop, B/right) can get to be intermittent and improve greatly with a large number (ie multiple 10s) of cycles of the buttons. Certainly do this before randomly spraying switch cleaner.Regards,Roger

author: David Hess
author_rank: Super Contributor
timestamp: November 16, 2019, 08:19:07 pm
content: Quote from: med6753 on November 16, 2019, 03:51:10 pmThe dual timebases all have Auto in place of P-P Auto and their readout positions are different from the single timebases and not even consistent between different models. One of the reasons I prefer the single timebases is because of their support of P-P Auto which is a feature I sorely miss on modern oscilloscopes.The 7B92 and 7B92A were mostly intended for use in the fast 3 bay rack mount 7000 mainframes but of course are very useful when you want to use one of the instrumentation plug-ins like the frequency counters in a horizontal slot of the 4 bay mainframes.

author: Hermansson Amplification
author_rank: Newbie
timestamp: November 16, 2019, 09:57:28 pm
content: Thank you all for your replies!I believe this scope has been stored for many years.Found a label in it said it was inspected in 1972 so it’s an early unit.I’ve been exercising the switches for a while without any progress.Here’s a video showing the problem:https://youtu.be/KuPenOSkyJg

author: Alfons
author_rank: Regular Contributor
timestamp: November 17, 2019, 07:52:29 am
content: I would check if there is not a contact problem, so clean all contacts of the slots. I had the same problem with a 7603 whose contacts were dirty.

author: med6753
author_rank: Super Contributor
timestamp: November 17, 2019, 11:48:57 am
content: First, if you don't have a manual you can download one from here:http://w140.com/tekwiki/wiki/7904It sounds like you'll have to start trouble shooting from the back plane. Luckily it's the outside slot so access, while not great, is easier. I assume you visually checked the plug-in side for any damage. Before going any further I would check the back side. The PSU can be moved back slightly without disconnecting any connectors or wiring. Pull the 4 screws securing it to the rear panel and pull it back, making sure not to snag any wiring or connectors. Do a visual check of the back plane. Now while not difficult if necessary the PSU can be removed completely. Just be sure to discharge the HV lead to ground when you pull the connector.If a visual check doesn't reveal anything I really can't offer much as I've not seen this issue myself. The area that you'll need to concentrate is the A2 Main Interface Board and go from there.Hope this helps.

author: perdrix
author_rank: Frequent Contributor
timestamp: November 18, 2019, 04:06:00 am
content: The "goto" place for assistance with Tektronix test gear ishttps://groups.io/g/TekScopes/Not a GUI based forum like this, but lots of very knowledgeable people.David

author: Kaveh
author_rank: Newbie
timestamp: December 10, 2020, 08:38:06 am
content: Hello,I recently purchased a Tektronix 7904. The device was probably lying in the attic unused for a long time. After I opened the device, I noticed that the board for the horizontal deflection was quite dirty (see the picture). First I cleaned the circuit board. Now the device can be switched on. The power supply seems to be ok. The vertical deflection works. but not the horizontal. The two vertical channels are visible on the screen when I take the horizontal circuit board out of operation and when I reconnect it, I don't see anything on the screen. I measured the voltages for the horizontal board, they are all there. I suspect that a short circuit occurs in the circuit board as soon as I connect it to the CRT. Perhaps someone here also had this problem with 7904. I measured the output transistors, they are all ok.you have an idea what the fault could be.Thank you

author: Kaveh
author_rank: Newbie
timestamp: December 10, 2020, 08:45:38 am
content: after the clean up

author: perdrix
author_rank: Frequent Contributor
timestamp: December 10, 2020, 11:17:32 am
content: Don't post the same problem on multiple threads

author: David Hess
author_rank: Super Contributor
timestamp: December 10, 2020, 06:21:40 pm
content: Quote from: Kaveh on December 10, 2020, 08:38:06 amOne of my 7834 oscilloscopes had the same problem. The positioning of the horizontal CRT amplifier board means that it catches anything which falls through the perforated vent at the top-back-left of the oscilloscope's enclosure.I agree with perdrix; start a new discussion thread for your 7904 problem.

author: Kaveh
author_rank: Newbie
timestamp: December 11, 2020, 01:53:09 pm
content: Sorry, it was a mistake that my post appeared here.