Oscilloscope Troubleshooting - Page 1
Forum section: electronics / beginners

author: Legion
author_rank: Frequent Contributor
timestamp: February 09, 2014, 10:19:50 pm
content: I picked up a Tektronix 468 the other day. Since I'm new to electronics I couldn't really test the scope in the store. Since I got it home I've been reading the manual and trying stuff out. Unfortunately it seems like it may have some problems, not sure if they're easy to fix or not.First was the calibration. It's supposed to be a 300mV, 30mA, 1kHz square wave. But I get about 255mV - 260mV instead. I think it may be because the calibrator loop is corroded. My DMM shows the loop has 1.5 ohms resistance, which at 30mA is about 45mV, so maybe that's why it's coming up short?Next were the digital storage and measurement features. You have to get out of pure analog non-storage mode in order to use features like the voltage and time cursors. When I turn on digital storage mode the signal gets very noisy and the controls/read outs for the digital features are intermittent with LEDs flashing on and off and the 7 segment display going on and off. Sometimes I can get the LEDs and display to come back if switch between non-storage and normal mode a few times. I've highlighted the relevant intermittent controls in green in the picture.I'm hoping it's easy to fix or the features aren't that useful cause I just spent $350 on it and there's no refunds.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: February 10, 2014, 12:10:57 am
content: Teks are usually pretty sound. I would clean all the controls and switches first, then see if the calibration procedure needs to be done.

author: c4757p
author_rank: Super Contributor
timestamp: February 10, 2014, 12:14:09 am
content: Quote from: Legion on February 09, 2014, 10:19:50 pmThe only thing that has to be true about that waveform is that it has nice, flat tops and fast edges. It's just for calibrating the probe compensation capacitor. The amplitude is usually off by a bit.

author: Legion
author_rank: Frequent Contributor
timestamp: February 10, 2014, 04:18:22 pm
content: Looks like the time/div is off by about 50% as well.Is it possible to calibrate a scope through the front panel or can you only check values? Do I need to open up the scope and refer to the service manual for calibration?

author: grumpydoc
author_rank: Super Contributor
timestamp: February 10, 2014, 07:21:49 pm
content: QuoteNot seen a Tek 'scope with settings that far off due just to having drifted with age.As ever, start with the basics - check PSU voltages and ripple. It's likely to need new caps which has been discussed several times on eevblog. The Yahoo groups are excellent sources of information as well.The 468 is basically a 465 with a bolt on DMM and digital storage, I have one in my (large) "needs fixing" pile.

author: calin
author_rank: Regular Contributor
timestamp: February 10, 2014, 09:25:28 pm
content: I am working to fix a 475A (similar beast) - gotten quite far as I had to literally rebuild the power supplies.So, check the power supply voltages (especially the 50V one), check all beefy smoothing capacitors careful these are big and need lot of heat to desolder) and tantalum capacitors. Also look at diodes in the power supply section - I found quite a few that were blown. I've been reading around, there is a guy that used to work for Tek services and he was saying than many tant caps were mounted the wrong way around.Wish you luck .. and patience. Your "patient" sounds less sick than mine tough - at least you have a trace. I am waiting for a HV probe I got on ebay to check HV supply for mine - no trace yet even that I have the low voltage supplies all spot on.

author: w2aew
author_rank: Super Contributor
timestamp: February 11, 2014, 01:03:40 am
content: Quote from: Legion on February 10, 2014, 04:18:22 pmI've never seen horizontal cal off that much. Sounds like someone pulled off the time/div knob and reassembled it in the wrong spot. Dial it all the way counterclockwise to XY and see if you get a 'dot' when the pointer lines up with XY.