Help with Tektronix 545A Troubleshooting - Page 1
Forum section: electronics / repair

author: TekHead
author_rank: Newbie
timestamp: August 22, 2021, 06:41:12 am
content: Hello, I currently just got my new pet project a tektronix 545A that I bought off of eBay. The good news is that the scope turned on and no smoke, the scope is responsive (The neon lights turn on when I play with the knobs ect) and the scope looks good (Visual check). The bad news is that there is no trace on the CRT. The CRT works at first it was just a smug on the screen but then I replaced two failed tubes and Now there is nothing but the "out of range" warnings are on. When I unplug the CA Plug in I get spike on the centre and the Horizontal "out of range" warning goes out. I think my issue is that the scope works but the trace is entirely out of range. I have no idea why. (The following is an overview of diagnosis that I have already done)The first thing I did was play with the front panel, this proved that the "logic" of the scope worked as described. nothing helped with adjusting the horizontal or vertical of the trace. I did an in depth visual and voltage check, the voltage was ok. two tubes had failed and I replaced them, there was no shorts. next I played with every adjustment pot that I could find, Nothing helped, Lastly I unplugged the CA Plugin and this proved an spike that I could move a bit with the Horizontal position knob I believe that all the tubes work and the CA plugin seems to be fine (all the tubes are silver on top and the heaters work also none are cracked).If anyone can help then that would be appreciated, I will try my best to answer every question/provide a photos and try any suggestions, I'm new to electronics of the era so be kind =), also yes I read the Tektronix manual for this scope. Thanks in advance.

author: sean0118
author_rank: Frequent Contributor
timestamp: August 22, 2021, 07:45:42 am
content: I'm not familiar with this scope at all.But I wonder if you have ripple on the supply rails? Or did you already check for that? Without the CA module I would expect you to get a horizontal line, but ripple on the supply rail might be sweeping the trace up and down.It looks like you might not have a horizontal sweep either, I think you should be able to see it on the 'Sawtooth A' binding post on the front panel. If you can see the sweep there I would use another scope to probe the connections to the horizontal deflection plates of the CRT, there also shouldn't be a large DC offset between them (as far as I know).Have a look at pg 5-11 of the545A Instruction Manualfor more info about troubleshooting the horizontal sweep.

author: bob91343
author_rank: Super Contributor
timestamp: August 22, 2021, 05:08:00 pm
content: Well yes the power supply voltages need to be right. Use the procedure in the manual to verify and repair if needed.Once that's done, the horizontal should be addressed by circuit tracing. In AUTO mode the free running sweep should appear. If it's in the circuit but not on the screen, address the horizontal amplifier. If not in the circuit, that's the place to look. Voltages are on the diagram. With zero differential signal to the horizontal deflection plates the trace should be in the center. But it needs to sweep.

author: TekHead
author_rank: Newbie
timestamp: August 22, 2021, 06:35:25 pm
content: I have good news, I was able to get an picture on the scope and the timebase works (Timebase A at least), The picture is just an smear on the screen that fades in and out based on the time/cm of timebase A (Timebase b has no effect but I can move the smear around with the horizontal position knob) it seems to be fading at the same rate as one of the neon bulbs.I did some more diagnostics and Checked and as far as I can tell all the appropriate voltages are present, I checked for ripple with an multimeter as I'm not able to isolate my other scope from ground and don't want to short it out. The bad news is that I think there is ripple, as there is ac that come and goes on the rails. (I have no idea when it comes to power supply)I'm going to press on with with diagnosing the CRT and the amplifier, and trying to get an sweep along with diagnosing the power supply I just wanted to give an update.

author: bob91343
author_rank: Super Contributor
timestamp: August 22, 2021, 07:27:33 pm
content: I don't understand why you can't measure the ripple with the other scope. Everything is referenced to the chassis. You can't measure ripple with a multimeter, or at least you can't decide where it's coming from.If you have excessive ripple it will just make the trace fuzzy or unstable but all should work.

author: sean0118
author_rank: Frequent Contributor
timestamp: August 22, 2021, 11:54:39 pm
content: Well it's good there are some signs of life. But it might be difficult to troubleshoot further without using the other scope.You don't need to isolate either scope from earth, and it's safer not too in my opinion. Just check that the chassis of the 545A has a good earth connection, then you can be sure it's at the same potential as the gnd connection on the probe of your other scope. As long as you leave the probe gnd clipped to the chassis you won't blow it up. Be careful not to exceed the max voltage either, don't go probing the high voltage of the CRT with either your scope or multimeter.If you want to be extra safe you can connect the probe with the 545A turned off and unplugged.The reason I mentioned power supply ripple is I just fixed aTek 7603with a similar problem, one filter capacitor and regulator had failed so bad that it had 20V of ripple on a 15V rail. It averaged out to about 15V so it looked okay with a multimeter. With ripple that large it can steer the beam off the edge of the CRT.It's a good sign you can steer it with the horizontal position knob. I'm not sure if that means you have a horizontal sweep though, someone else might be able to confirm. Would be good to check if you are getting that saw tooth I mentioned earlier.Look on the bright side, if you blow your second scope up you have a reason to buy a third one.

author: richnormand
author_rank: Unknown
timestamp: August 23, 2021, 03:34:55 am
content: Quote from: TekHead on August 22, 2021, 06:41:12 amI see you are in Ontario, I am located in Ottawa. I have a functioning 545A and several plug-in units. I think I still have most (have to dig up the old stuff in storage) of the schematics and user manuals for it. I can send you comparative voltage readings and photos if you need. Just be aware of solder use on the ceramic rails points. As well I have a tube tester if you need some checked.It is a great heat source in winter...These were built to last...

author: WattsThat
author_rank: Frequent Contributor
timestamp: August 23, 2021, 04:03:46 am
content: QuoteWhat richnormond was pointing out was that the ceramic terminals require silver bearing solder for any resoldering or repairs. Tek shipped those scopes with a hank of the correct stuff wound in a figure eight on two posts somewhere inside the unit, forget if it was on the chassis or a cover.

author: jh15
author_rank: Frequent Contributor
timestamp: August 23, 2021, 02:54:55 pm
content: I have had the use of a 535A since the 70's and owned one since around '81. I had a 545 for awhile and remember the gazillion rubes in the distributed amplifier. I also have a decades old Hickok 545A clone that was being tossed from a defense plant. It was de-tubed except for the CRT, and I hope the HV tubes. This was well before the tube craze.I have used the 535A for neginners to show concepts of delayed sweep, etc, besides, it can't be damaged easily.This scope came long before "hot chassis" and SMPS was even a thought. Most everything is referenced to chassis ground, the frame. Very safe to work on if you don't put fingers or tongue on the power rails. Sray away from HV tests for now, as it obviously works.Glad for this post, as I haven't fired up my boat anchors for awhile as I like to do. I loved my 475 transistor curve tracer, and it needs a wake up run.I should just part out this 'historic' Hickok clone as it needs so many tubes. If it was a 535, I'd swap in my 535 tubes for testing, but don't have the dist amp tubes.In my test equipment repair years, nothing beats good old industrial gear with all the documentation available to the end user. Especially the circuit descriptions. Is when I first heard of the 'Miller effect" when working on a 535A decades ago.DO check All power supply test points for ripple with a scope as per manual. Don't adjust the 150v pot if it is in spec, as all other supplies use it as a reference, and you'd have to tweak everything else afterwards.Don't trust the old handles. Also it could overheat per manual if run with covers off, but I never worried about it. Pretty sure they mean when put back in servlce.Call the electric company for special water heater or electric heat rates