Another Tek 475 Fault (yet!) - Page 1
Forum section: electronics / repair

author: Yago
author_rank: Frequent Contributor
timestamp: December 18, 2014, 04:23:14 pm
content: Sorry if this is seen as clutter, two threads for the same (similar?) scope.I thought if I hijacked the other things would only get confusing.Anyhoo...Had this scope for a while, but I haven't used it until I wanted to check a function generator.After an hour or so the screen went crazy, like the input was being dragged around by low freq signal.I measured PSU before and after fault condition:The -15 is a little low at 14.779More tellingly the +5 is at 4.985v when running, but drops to 4.135Vdc during fault and I was able to measure 1.0768VAC on there too.I read a few of the threads, and the bridge and filter cap are a big suspect in this fault, with a bridge fault producing weird things on the CRT.Here's a pic of the screen when faulty (apologies if I cock the size of the pic up):I have no other scope to check things with, and the option forward seems to remove the bridge and test it.The 5v bridge is right down against the board (unlike the others, lay on side), very difficult to get to.I am tempted to cut it out if possible and replace, and whilst there change the cap too.In that respect, would this bridge and cap be suitable?:http://uk.rs-online.com/web/p/products/7328257/http://uk.rs-online.com/web/p/products/8116169/Finally, please note I have not worked on scope before, so all warnings are greatly appreciated.Thanks.

author: oldway
author_rank: Super Contributor
timestamp: December 18, 2014, 06:45:27 pm
content: When you check a power supply, you have to measure also the ripple on ac range of your multimeter.You have to check ALL the voltages of the power supply, not only the 5V and the 15V.If you have high ripple on the 50V, you will have also high ripple on 5V rail.Measure also the voltages and ripples before the serie's regulator on all the rectifiers.

author: Yago
author_rank: Frequent Contributor
timestamp: December 18, 2014, 07:00:13 pm
content: Sorry should have posted that!The rails were:+110: 111.73Vdc:0.003Vac+50 : 50.06Vdc : 0.0015Vac+15 : 15.36Vdc : 0.003VAac+5 : 4.985Vdc : 0.003Vac (this changed under fault to 4.136Vdc : 1.0768Vac)-8 : -8.006Vdc : 0.003Vac-15 : -14.779Vdc : 0.4661Vac (didn't change under fault)

author: oldway
author_rank: Super Contributor
timestamp: December 18, 2014, 07:11:12 pm
content: -15 : -14.779Vdc :0.4661Vactoo high. Check voltage and ripple on C1472. (3000µF)Also replace C1418 (10µF)For the 5V, measure voltage and ripple on C1452 (5000µF) (NB: if ripple is too high : 100Hz ripple, that's a faulty capacitor, if ripple is 50Hz, that's a faulty bridge rectifier.)

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: December 18, 2014, 07:25:15 pm
content: It kind of looks like the MIX button is depressed, and "NORM" mode triggering. Does it only do it in those modes, or in "A", and "CH1" trigger source as well? All sweep speeds, all input signal frequencies, and amplitudes, and what about a free run un-triggered, but slowed with VAR sweep knob? What about free run with no Vert input/triggering? Can you see any of that added jittery signal in the various sweep speeds then?

author: Yago
author_rank: Frequent Contributor
timestamp: December 18, 2014, 08:24:32 pm
content: I will check those for ripple and freq, thanks again Oldway:)Will try and also include more function test too Sound, and include any telling pics, cheers.I am pretty sure that this scope hasn't free run in the time I have seen it on (about 90 minutes!).

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: December 18, 2014, 09:00:22 pm
content: Just another remote possibilty here... by chance do you hear any slight arcing sounds, or a faint smell of ozone? That also reminds me of time I have seen that cause a distorted look to the waveform such as that. Will be curious to know if you see any of that disturbance without an input signal in AUTO free run.

author: Yago
author_rank: Frequent Contributor
timestamp: December 22, 2014, 11:17:58 am
content: Well I had another look and things are not so good.This time I could get not trace at all, nothing from beam finder (could see the screen flicker as the scope is turned on).The 5 volt line was behaving, but the -15 was at -21volt.I had to stop looking (life's responsibilities calling), but I did notice quite a lot of rework has been done to the scope in the past. IE components tacked ontop of the remaining legs from old cut-out components, pads missing (including at least one from a big PSU res cap).Short story is I think that this could be a reasonable scope with time, care and work.As my only scope though, I feel it may not be reliable enough.In short I am getting tempted by a Rigol, and will repair the Tek as I get time.Thanks for helping Old and Sound, I'll bump the thread in the future when I return to the repair (time permitting!!).

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: December 22, 2014, 03:23:07 pm
content: When you jump back in it, take a look at these components in the 15 V reg ckt. they would be most likely, but if not, you'll need to troubleshoot more. That dirty signal you had on the screen might have been a short starting to breakdown one of those parts. Hopefully your +15 circuit throughout the scope is still good!!!

author: Yago
author_rank: Frequent Contributor
timestamp: December 23, 2014, 12:58:13 pm
content: Thanks Sound!No faint smell of ozone, just a waft of a Hamlet moment!Seems to make sense to play it "safe" with a new Rigol 1054z, safe being in quotes for the unknown quantities that are down that path!The Tek, I'll either return to, or donate to someone here that has the experience or time and energy to burn!