Tek 577 - a bag of troubles bringing it back to life - Page 1
Forum section: electronics / repair

author: ~
author_rank: Newbie
timestamp: February 02, 2025, 01:35:55 pm
content: Hi,I've scored a nice Tek 577 D1 with unbroken L/R switch on 177 - it had a few other scratches and scuffs and a few broken knobs. I've clean it up as best I could and went to town on it. I've checked the PSU first and quickly found out that I have no -12V. I replaced faulty opamp (U205) and -12V came right back. Shortly after that I heard a loud pop with a very distinctive smell that told me that there was atleast one and that it decided to release it's magical smoke.No worries though, I replaced it (temporarily with a electrolytic cap) and then I messed up - royally. When checking the voltages on the PSU, I accidentaly shorted the input of -12V regulator to the common - which effectively sent roughly +25V to the -12V rail!What an idiot...That mistake took out majority of the opamps - I replace both in the 177 and 8 on the main board. I also replaced Q436, which turned from NPN into a diode.However I still have an issue that I cannot figure out: The horizontal & vertical offset pots are not working - or rather the beam is stuck in the lower half of the screen and when I turnverticaloffset pot, the beam moves horizontally (in "jumps"). Horizontal offset does nothing at all. Plus the NPN transistors Q1124 and Q1126 on the vertical and horizontal amplifier board are getting quite hot.The Q1124 & Q1126 was probably not caused by the short - it was the problem I was trying to figure out in the first place before being interrupted by the tantalum detonator.Does anyone have an advice where to look? Any advice would be greatly appreciated

author: mawyatt
author_rank: Super Contributor
timestamp: February 03, 2025, 02:45:28 pm
content: Getting +25V on the -12V Rail likely forward biased the substrate PN junction on any IC that's on the -12V Rail, and same for any Transistors or FETs (EC or SD PN junctions).This might destroy or seriously degrade any IC or Transistor/FET connected to the -12V. Also this condition would reverse bias all the Electrolytic/Tantalum decoupling capacitors on the -12V Rail and even back to the Power Supply/Regulators. Troubleshooting might be difficult and maybe consider replacing all these Capacitors, ICs and Transistors/FETs.BTW do all the supply voltages agree (voltage ripple) with the repair manual?Here's a link that might help.https://www.eevblog.com/forum/repair/old-tek-577-return-to-life-story/msg4479919/#msg4479919If you can't find the various adapters for the 177, heres a link that might be useful.https://www.eevblog.com/forum/projects/smd-test-fixture-for-the-tektronix-576-curve-tracer/msg4553639/#msg4553639Good luck, hope you can restore the Tek 577.Best

author: ~
author_rank: Newbie
timestamp: February 03, 2025, 05:41:58 pm
content: Yeah - there was a *lot* of swearing when my probe slippedSuch a dumb and preventable mistake, too!I replaced all the opamps so far + that one NPN. Luckily there is only one tantalum on each rail - plus the -12V is mainly used "just" for the opamps.Voltages are OK now, well within specs - I did not check/tune the CRT one as I don't have a voltmeter that could do -+2kVWill have to get a HV probe for that.I also *think* the main issue (the one with the unresponsive postition knobs) is caused by the opamps I had to replace. I misread the datasheet (N5556 vs N5558) and I put a wrong opamps in there by mistake so there is no wonder it did not work. I only have dual opamps with different pinout. I'm now waiting for some OPA277s to arrive as a N5556 replacementMaybe these will solve the overheating CRT driving transistors - will have to see though. I got the 577 as a restoration/repair learning experience and it definitely does deliver in that department.Thanks for the 177 adapter link, I love your idea of putting the clamps for the SMD devices!