Weird trigger problem with Tek 2225 scope - Page 1
Forum section: electronics / beginners

author: MichaelDing313
author_rank: Newbie
timestamp: December 22, 2012, 04:41:19 am
content: I just got a tektronix 2225 oscilloscope from my school a while ago, had nothing with it except a manual for the Tek 2205. Just got the probes for it recently, it's a cheap X1/X10 passive from ebay. Hooked it up to the probe adjust signal and the triggering is very weird. It will only trigger if i set the vertical scale to less than 10mV peak to peak (10mV with 1X probe, or .1V with the X10 pulled out on the vertical knob.) so it mean that I can't get a look at the entire waveform with one channel because it's so zoomed in. It's common cross both of the channels, and I could see the waveform fine if i hook up both the channels to probe adjust and trigger off one that have it's vertical scale set below 10mV p-p. Do anyone else have this problem or know how to fix it? It's a beautiful scope and I hate to see it just sitting on a shelf because it's broken.

author: w2aew
author_rank: Super Contributor
timestamp: December 22, 2012, 05:44:33 am
content: Please describe the position of each of the switches and knobs in the trigger section of the scope. It may help to debug the problem, or to see if it is just a setup issue.

author: firewalker
author_rank: Super Contributor
timestamp: December 22, 2012, 06:31:19 am
content: Post a highres picture of the cro front panel.Alexander.

author: MichaelDing313
author_rank: Newbie
timestamp: December 22, 2012, 10:14:16 am
content: Thank you very much for the reply, here's the pictures of my setup.Triggering properly, off channel 2, set to 10mV/divLost trigger, exact same setup, channel 2 set to 20mV/divAlso very weird shape while beam find is held down, it gets worse when channel 1 vertical scale is set lower.Big closeup of the front panel(pictures are too big to be hotlinked)Edit: Both probes are on 1X setting.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 22, 2012, 11:06:48 am
content: What happens when you try to trigger from the external trigger input? (move the probe from ch2 to external and set the source switches both to ext)

author: grumpydoc
author_rank: Super Contributor
timestamp: December 22, 2012, 01:54:45 pm
content: There's an operator's manual on K04BB (Google is your friend). There is a better copy available from Tek as well - you have to register on the site but the manuals are free. The service manual is a bit harder to find :/I'd echo the comment about trying the Ext trigger input and add that you ought to see if you can get trigger to work in "Normal" mode.What effect (if any) does adjusting the level control have?It's possible something is attenuating the signal in the trigger path - I'd start with a little contact cleaner on the various trigger mode switches.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 22, 2012, 02:56:31 pm
content: Quote from: grumpydoc on December 22, 2012, 01:54:45 pmI think it is justnotattenuating, as it fails when there is more input signal. My idea is that the AGC (automatic gain control) of the trigger amplifier is not doing its job correctly.About the service manual, you can download it here:http://www.alternatezone.com/electronics/files/2225-SM.pdfDescription of the trigger system starts at page 3-7, diagram at 9-25.

author: MichaelDing313
author_rank: Newbie
timestamp: December 22, 2012, 06:49:19 pm
content: Thanks for all the suggestions. I've had the manual as well as the service manual for the scope(got it from alt zone, thanks for the link tho), and i have opened the scope up to to test the switches and pots for trigger level and hold off, they seem to be fine Ill try have a closer look at that. External trigger does not work at all. Triggering on normal have the same problem as auto p-p, only trigger when vertical is set to 10mV div or less, trigger level do sometimes affect the trigger when it's just set to 10mV and it will lost trigger if i reduce the trigger lever past some point. Also the trim pot for adjusting the trigger sensitivity also have an effect on the trigger when I change it while i still have a trigger, I currently have it set to the highest sensitivity possible (as far as that pot would go). So how can i test if the AGC is functional and can i some how repair it if it's not?

author: MichaelDing313
author_rank: Newbie
timestamp: December 22, 2012, 06:58:52 pm
content: Since the triggrting problem is common cross all the inputs (both channel and ext), ill try hunt down the circuit that all of them share. The only thing that completely confuses me is that the vertical scale affect the triggering, shouldn't they be completely different sections of the scope?

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 22, 2012, 07:25:18 pm
content: Not sure what I was thinking but forget the "it fails when there is more input signal" comment, it looks like it is just very insensitive.The vertical scale setting attenuates the signal, the trigger signal is picked up after that. See the block diagram on page 9-6 of the service manual, the trigger signal is picked up from block 2 -> block 3, the vertical scale setting is block 1. Apparently the trigger circuit is so insensitive that it only works if you overload it by setting the input at its most sensitive setting. Since that is something you can't do with the external trigger circuit it fails to trigger on that at all. Since the trigger has a problem on all inputs, you would think that it has to be somewhere in the circuit common to all inputs and not the CH1, CH2 or external trigger amplifier, but looking at the diagram the output of these amplifiers are in parallel, so in theory one of them could be broken and shorting the output for all 3.Time to measure the voltages given in the trigger diagram I think.

author: MichaelDing313
author_rank: Newbie
timestamp: December 22, 2012, 08:55:14 pm
content: Thanks a lot of the advice, i'll check the voltage test points once i get time to do so.