Tektronix 7613 display and power supply problems - Page 1
Forum section: electronics / repair

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: December 03, 2023, 02:33:06 am
content: I recently bought a Tektronix 7613 from a local electronics recycling center, and it doesn't work, as one might just expect.It appears that the 7B53A sweep module is creating sweep voltages, however, probing the horizontal deflection plates using differential inputs, as in putting channel two to invert and use the add function on my tek 2230, the waveform looks garbage and filled with noise.The voltage on the 130v supply is low at 119, and the intensity doesn't go up. The trace is very dim. Using my cheap multimeter I read 0.008 volts AC ripple on the +5v line. I will use my HP 427A once I find it to make sure that's true.Applying a signal to the vertical amplifier produces some odd results with the signal amplitude varying with vertical position. Amplitude is low at the center, amplitude gets higher when the trace is offset.Measuring the anode voltage with a BK HV probe yields zero volts? I hear high voltage and the CRT is displaying something however my meter reads nothing. Perhaps my meter is broken.Disconnecting power to the horizontal amp board gives me an unfocused dot at the center of the screen.I have Sunday and today to convince my mom that spending $80 was not a waste and she doesn't need to kick my ass.Keep in mind I don't have any formal electronics education but can do basic troubleshooting and read schematics. 4 years self taught from the age of 10.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: December 03, 2023, 02:38:55 am
content: Using my HP analog meter I read 14 volts AC on the 130v line, 14v AC on +50v, and oh my god everything supply line was greater than 2v AC.Perhaps just a filter capacitor issue? I doubt it though...

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: December 03, 2023, 04:14:31 am
content: So it was a capacitor problem on the 130v rail. Adding a 1960s 150uF 150v sprauge capacitor in parallel gives me a nearly spot on 130v coming in at 129.2v.Trace also looks much better but still having focusing problems. This is without inserting a vertical plug in. Inserting a vertical plug in gives me garbage noise.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: December 03, 2023, 04:15:09 am
content: Garbage on the vertical

author: p.larner
author_rank: Frequent Contributor
timestamp: December 03, 2023, 04:28:27 am
content: check all the psu caps.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: December 03, 2023, 04:29:54 am
content: Quote from: p.larner on December 03, 2023, 04:28:27 amAll test fine except for the 130v filter one. It's leaking everywhere. I will need to clean it before it causes corrosion... I mean it already did but before it does more damage.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: December 03, 2023, 04:39:40 am
content: I fixed it! Sneaky bastard. Q1118 on the CRT circuit had failed short. Somehow after I replaced that one transistor in the CRT circuit the scope works fine now. But the readout is still distorted.I don't really understand how that tiny transistor in that specific part of the circuit messed up the horizontal... but I'm not going to complain as I have a mostly functional scope now.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: December 03, 2023, 05:04:39 am
content: aaaaaaaannnnndddd back to square number one.Here's what happened. I connected a 1000uF capacitor to the case of Q952 and ground. This created a spark and the thing broke again. I had made the mistake of assuming the case of Q952 at 22.3 volts DC was caused by AC ripple and it should be at 25v DC. Boy was I wrong. I should have checked the service manual and this transistor connects to 130v through a resistor!Now the -15v rail reads -21 volts and the trace is now a dot, with, or without any plugins.Edit: I think CR958 has failed? I think it's meant to make the final -15v regulationChecking else where the base of Q952 measures 0.5v instead of 0.11v.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: December 03, 2023, 06:05:25 am
content: Q956 has an emitter to collector short. No continuity at all from the base. I need to replace this transistor. Any recommendations? I know an exact replacement is best but I don't have any of these on hand now.It is a 2N5859I am thinking of using an S8050 as a drop-in replacement for now. The original is rated for 2A, but this new one is only for 1.5A, plus it has a max of a mere 25v.Edit: Also anyone think that the -21v on -15v rail has possibly damaged any plug-ins? The two are 7A26, and 7B53A.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: December 03, 2023, 05:53:40 pm
content: Replacing Q956 with an S8050, which is underrated compared to the 2N5859, the scope runs perfectly now, except of course the little TO-92 case transistor becomes very hot and unstable after running the scope for more than 20 seconds.But I see a trace, which is good!Now I have to wait a week for the replacement transistor to arrive.Edit: Does anyone know how to easily access the rectifier and filter board? It's buried under 5 PCBs and the CRT.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: December 03, 2023, 10:06:15 pm
content: The scope now works just fine. I glued a piece of scrap metal to the S8050, which does a good job as a heat sink. It is only slightly warm to the touch.The readout system still sucks though.I see the trace becoming unfocused at the ends of the trace. In the middle, the trace looks exceptionally sharp, but the ends look blurry. I followed the service manual for Z-axis and CRT adjustments but no further improvement with the unfocused ends.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: December 04, 2023, 07:38:18 pm
content: Scope is now fully operational with readout.J503 on the Horizintal amp board was not making a good connection with the center conductor in the coax cable. Fixing this solved the readout issue.Still having a great amount of trouble accessing the A13 rectifier board to permanently implement the capacitor fixes. Any tips on accessing it would be greatly appreciated.Also plastic bezel is missing on this scope so I was wondering if some one else had a 7613, or 7633 scope to take measurements of the bezel and post it here so I can 3D print a new one. Or maybe such a file already exists and I can not find it yet.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: September 01, 2024, 09:43:14 pm
content: And a new issue has arisen whilst trying to fix an old issue.The scope worked perfectly fine before, with only one caveat: The -15v, +15v, and +5v rails would drop out randomly and would require percussive maintenance to rectify the issue.I thought this was due to the TI socket that holds U973 was faulty and wasn't making a good connection anymore. So I replaced it with a machined IC socket.Now the +5v rail reads DOUBLE at 10.00v The rest of the voltage rails seem to be just fine.pin 4 of U973 reads +9.8vanode of CR981 reads +12.89vI hope I didn't damage U973 as I think that would be a difficult part to source.nearly 10 volts at the 5v rails indicates to me that Q988 is constantly on, or shorted across collector and emitter, bringing in the almost full 11.5v to the 5v line.Q985 tests fine with the diode tester. This means it might always be on, U973C may be the cause of that.U973C might always be on which could be caused by U973E not being on enough.Edit: Voltage at base of U973E is 4.7v, which means that the transistor should be conducting, and thus bring down the voltage at base of U973C. To me this is indicating a bad IC.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: September 01, 2024, 10:37:44 pm
content: Voltage at emitter of U973E is +8v. So if there's +4v at the base, no current flows through the base which means that the transistor will be off. That tracks, but why is there +8v at the emitter of U973E?Perhaps U973D is shorted and is giving full voltage from the 5v rail to the emitter, not allowing current to flow through the base of the E transistor.Anyway, I bodged a 500k pot from the collector of the E transistor to GND. Adjusting that I can adjust the 5v rail and the scope works perfectly fine.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: September 01, 2024, 11:04:59 pm
content: I think the D transistor was indeed shorted because I replaced the entire transistor array with discrete transistors and it works fine now. The 5v rail is spot on.Here's the transistor abomination if you want to see images.Found a suitable replacement. Turns out the transistor array was a standard type and not some super custom version. It's equivalent to the LM3086/LM3046

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: September 02, 2024, 06:46:53 am
content: Tested each transistor in the old transistor array. U973E gives me some funny-looking transistor curve traces.I have provided two photos. One of a good transistor in the array, and one of U973E. I think you can determine which one is which by yourself.The scale factors are the same in each photo.