Tektronix 485 sync jitter - Page 1
Forum section: electronics / repair

author: thehobe1
author_rank: Newbie
timestamp: March 20, 2021, 05:13:07 pm
content: After using my Tek 485 for a few months on my bench, the scope started clicking and would not turn on, shutting down the switching power supply in a "safe" mode. I used the Tektronix recommended procedure of measuring the internal power supply combination test point and found that the scope was shutting down due to the +5V supply measuring +5.2V! I found that the LM741 Op Amp controlling the supply had somehow got damaged. After I replaced the LM741, the 5V supply came up properly and the scope no longer was protecting the switching power supply and turning off and on. The traces are now working but now the +9V power supply is riding high at around 10.2V! Because there is a lot of internal power supply "sharing", I have loaded the 9V supply with around 200mA and have found that the 9V power supply is in regulation. I cannot find where the phantom current is propping up the 9V power supply but most everything is working properly. The other, and only other problem I have is a pretty bad jitter on the sync giving rise to around 0.2uS(1 horizontal division on the 0.2usec/div H range) on the higher frequency calibration signal and about the same 1 horizontal division even on the low frequency calibration signal! I had to replace the time base switching relays when I originally received the 485 with some Teledyne TO5 relays because the Tektronix originals were missing. The sync problem has been there since I have had the scope but I do not know if the Tektronix relays are very special. Any help is appreciated. I am actually a retired Analog IC designer but have not done much test equipment repairing. That is a different talent.

author: tggzzz
author_rank: Super Contributor
timestamp: March 21, 2021, 10:59:10 am
content: I'm skeptical that it explains your symptoms, but the 485 has several 15V tants on its 13V PSU rail. See other posts for more detail.

author: David Hess
author_rank: Super Contributor
timestamp: March 21, 2021, 01:33:33 pm
content: Before looking for leakage from a higher voltage rail, I would check the pass transistor and operational amplifier for the +9 volt regulator.It is not unheard of for old ICs and transistors to fail without being abused because of contamination inside of or through the package.There is something else to check. Are the 741 operational amplifiers socketted? I do not know if the 485 used them but Tektronix used those unreliable edge wipe sockets from Texas Instruments.

author: thehobe1
author_rank: Newbie
timestamp: March 22, 2021, 05:04:59 pm
content: Actually, I have checked almost all of the tantalums in the 485 and have found none that were bad/shorted. That was the first thing I checked. The 13V supply is also fine. Before fixing the 5V rail by replacing the op amp, the 9V rail seemed fine!

author: thehobe1
author_rank: Newbie
timestamp: March 22, 2021, 05:11:41 pm
content: I did try replacing (and actually checking ) the 9v regulating Op Amp to no avail. I just received a new batch of 741's so I will recheck for stability and accuracy of the Op Amp. I also checked the NPN transistor that the Op Amp is driving for leakage, breakdown, and current gain and it all checked out fine. I think the jitter is related to either the tunnel diodes or the horizontal ramp reset system since it seems to be directly proportional to the H time base setting, yielding around 10% or 1 division regardless of signal being synchronized.

author: David Hess
author_rank: Super Contributor
timestamp: March 22, 2021, 08:18:10 pm
content: Quote from: thehobe1 on March 22, 2021, 05:11:41 pmThe tunnel diode trigger circuits may simply need to be recalibrated, but I would try to track down the power supply problem first.