Tektronix 2235 no baseline trace, troubleshooting - Page 1
Forum section: electronics / repair

author: PEvans
author_rank: Newbie
timestamp: September 09, 2023, 07:58:36 pm
content: I do not have tons of experience with oscilloscopes. I have had this Tek 2235 for a couple of years and it did work for the couple of things I used it for.A house move later and two winters in my unheated shop, I tried it out. Initially I got two flickering base traces and some flashing of the light in the A TRIGGER section, but it went away before I could really see it.Now, when I turn it on I get no base trace. A stationary, centered dot is visible if I press BEAM FIND, and the the vertical and horizontal position both work.I went through the Tek "Troubleshooting Your Scope" to the "Troubleshooting the Timebase" and found this:- in XY mode the dot is visible without BEAM FINDER- if I press A trigger SGL SWP and rotate the A trigger LEVEL all the way clockwise, the READY light lights. If I rotate the A trigger LEVEL counter-clockwise a dot goes across the screen and the READY light goes out. Rotating the LEVEL back counter clockwise does NOT light the TRIG'D light- if I press A trigger PP AUTO in (to the TV LINE position?) I get repeating dots going across- if I connect a 10X probe and press A trigger NORM and move the LEVEL around I get a dot going across and the TRIG'D light flashes each time.I'm not quite sure about the A trigger buttons. SGL SWP does not stay in. PP AUTO and NORM both stay in, and I have to push one of the other buttons to get either to come back out.I will attach an image of the front panel as it was set when I did the "Troubleshooting the Timebase" steps.Thank you in advance for any suggestions.

author: MathWizard
author_rank: Super Contributor
timestamp: September 10, 2023, 07:51:15 pm
content: Do you have another scope ? IDK how much digital stuff these have, like microprocessors and ADC/DAC's, but I'd try to trace a signal from the input. And also work back form some output drive transistors or op-amps. Some of that might have pretty high voltage, I'd skip most of the HV stuff, till last, if nothing else was wrong.IDK what's in the troubleshooting flow chart, but the schematic's are usually full of test point voltages.Then there's the power rails, have you checked any of that yet ?Maybe some little part fried when you powered it up. Hopefully not some rare chip. These most be pretty old, I bet there's lots of old old caps in it. Did Tektronix use any carbon resistors back then ??

author: Paceguy
author_rank: Regular Contributor
timestamp: September 10, 2023, 08:34:35 pm
content: I had a similar problem with one of my 2235's. It was a bad electro cap. Check them all with a ESR tester.

author: floobydust
author_rank: Super Contributor
timestamp: September 10, 2023, 08:53:59 pm
content: What does it do on Line trigger? I think that forces triggering and Normal mode too.Otherwise, it can be a problem with the trigger circuitry OR a noisy rail in the power supply.For the age, I would first replace the power supply electrolytic capacitors sight unseen. They don't last 40+ years here. Spreadsheet with a list of the caps and sizes to replace including Rifa fire bomb capacitors is here:https://www.eevblog.com/forum/repair/tektronix-an-usm-4882235-high-8-6-v-ripple/msg2539461/#msg2539461

author: bdunham7
author_rank: Super Contributor
timestamp: September 10, 2023, 08:55:24 pm
content: Your scope appears to be upside down. I'm not sure what you mean by 'base trace', but if you are looking for a horizontal line without any input, you'll need to set it up for that.Push the A TRIGGER P-P AUTO button in and set the horizontal sweep to something reasonable like 1ms/div or faster. I've edited your photo with a red arrow pointing to this setting. You should get a line then, but you may still have to adjust the intensity as well as the vertical and horizontal position knobs.

author: jdragoset
author_rank: Frequent Contributor
timestamp: September 10, 2023, 09:23:07 pm
content: Connect a probe on channel 1 ( .1 V setting) to probe adjust and try to trigger (Norm in, Ch 1 int, Ch1 source,AC coupling, adjust level) on the 1k Hz square wave (1 mS per cycle).Disassembly requires T-7 thru T-10 Torx drivers.

author: PEvans
author_rank: Newbie
timestamp: September 11, 2023, 12:41:28 am
content: Thank you for the replies.@bdunham7, help me with the A TRIGGER P-P AUTO button. This was one of my questions. To get the baseline trace the OM says the A trigger mode should be P-P AUTO. Is that with the button pressed in, as you suggest?Set up as @bdunham7 says I do get a baseline trace.Set up as @jdragoset suggests I do get the PROBE ADJUST square wave,Also, thank you for the reminder on the Torx drivers.

author: bdunham7
author_rank: Super Contributor
timestamp: September 11, 2023, 12:55:09 am
content: Quote from: PEvans on September 11, 2023, 12:41:28 amYes, button in is the P-P AUTO mode. This is a particularly useful feature on Tek scopes because unlike most other AUTO modes on scopes, in addition to just supplying random triggers, it tries to adjust the thresholds to trigger on whatever the input is. This is the default mode for the scope, you only need to use another mode in specific circumstances, usually when you have used P-P AUTO to dial in everything else first.

author: PEvans
author_rank: Newbie
timestamp: September 11, 2023, 01:47:27 pm
content: Quote from: floobydust on September 10, 2023, 08:53:59 pmWhich caps on that list would you call "power supply electrolytics" and "Rifa firebombs?"

author: PEvans
author_rank: Newbie
timestamp: September 11, 2023, 01:57:34 pm
content: Quote from: bdunham7 on September 11, 2023, 12:55:09 amSo are the typical non-single sweep A TRIGGER modes P-P AUTO/TV LINE (button in) or NORM (button in), with a variant of TV FIELD (both buttons in)? That is, there is no mode with both buttons out?