Tektronix 475 short circuit - Page 1
Forum section: electronics / repair

author: Warsp
author_rank: Contributor
timestamp: September 11, 2023, 07:45:47 am
content: Hi all.Recently the +5v rail on my tektronix 475 randomly decided to short out after I changed out one of the bridge rectifiers and a couple capacitors. None of the repairs I made had anything to do with the +5v rail, and before I put on the vertical board everything appeared to work just fine. Once everything was reassembled, the regulated side of the +5v rail developed a dead short (>0.1 ohms). After poking at the scope for a bit, the short went away but it came back the next day. I have narrowed the location of the short to be on the main board, but I am pretty much lost from here. I have already replaced/tested C1458 and C1263 but they are not causing any problems. My scope also doesn't have CR1458, so there aren't any diode shorts as far as I am aware of. I have been searching through the schematic, but so far I haven't found anything else that could cause a dead short on the +5v rail. I also don't think it could be from messy soldering because the short is semi intermittent. Does anyone else have any ideas on what could be causing this short?Manual: (schematics start on p.223)https://w140.com/tekwiki/images/2/2f/070-1862-00.pdf

author: wn1fju
author_rank: Frequent Contributor
timestamp: September 11, 2023, 12:39:42 pm
content: Normally, power rail shorts are caused by faulty tantalum capacitors in the Tektronix 400 series scopes. And there are plenty of those hanging off the 5V rail to ground throughout the 475.If the short was permanent, not intermittent, I would start looking there. For intermittent shorts, I would hang an ohmmeter across the 5V test point to ground (with the scope powered off) and start wiggling boards, banging on boards, shaking wires, etc.

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 11, 2023, 01:53:30 pm
content: Inject 5V current-limited to 1A, pretty sure you'll find something heating up.

author: Warsp
author_rank: Contributor
timestamp: September 11, 2023, 03:10:42 pm
content: I did try that but my power supply current limited at 0.1V and nothing much was heating up with such little power going through. Could I possibly put an incandescent bulb in series with the ground test point and push 1A through the power rail that way?

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 11, 2023, 03:36:40 pm
content: Remove every board and connection from the PSU.Don't power the scope PSU! Only test with DMM. Luckly the short will be gone. Otherwise it's in the PSU!Then connect each board, each connector, only one at a time. When the short appears, follow that connector/board.Use thick wires in the bench PSU output, or the power will be lost in them.If at 1A you find nothing, increase to 2, 3, 4, 5A...You want to avoid burning any track, so give 1 minute between each step, so the shorted part has time to heat up.

author: Gyro
author_rank: Super Contributor
timestamp: September 11, 2023, 05:57:47 pm
content: I would hold the current limit down to no more than 1A (from an external PSU as DavidAlfa says), but then probe around between the Ground - 5V traces with a dmm set on its mV range and narrow down to the lowest reading you can find (don't forget 5V to chassis!). It's important that you don't share any common connecting lead or clip between the external PSU and the dmm, otherwise the voltage drop will swamp your readings. Try to apply as little meter probe pressure as possible to avoid making the fault go away again before you locate it.

author: WimWalther
author_rank: Regular Contributor
timestamp: September 12, 2023, 12:13:34 am
content: WN1FJU is right. If you want an older Tek 400 series to run reliably, just shotgun all of the tantalum caps. I wouldn't even bother t-shooting it unless that job has been done..