Tektronix 475A issue  - Page 1
Forum section: electronics / repair

author: Kidon
author_rank: Regular Contributor
timestamp: July 06, 2020, 01:52:27 pm
content: So I was working peacefully and I had my 475A on for several hours. All of a sudden it went dead.I checked the mains fuse, and it was blown. I replaced it with a 800mA instead of a 750mA, as this is all I had available.I figured something was wrong, fuses blow for a reason, right. Anyway, I turned it back on, and the trace was pretty weird (cf. Picture).It took me a few seconds to take a picture, and I switched it off.Anyone has seen this symptom already?

author: PaulAm
author_rank: Frequent Contributor
timestamp: July 06, 2020, 02:04:03 pm
content: Check the power supplies and the ripple. I've seen that weird retrace happen when one of the filter caps go open.

author: Kidon
author_rank: Regular Contributor
timestamp: July 06, 2020, 02:24:05 pm
content: Quote from: PaulAm on July 06, 2020, 02:04:03 pmYeah, probably. I just didn’t feel very comfortable leaving it on like that while I check it. I figure it could damage more internal parts. At least not until I replace it with the proper fuse....

author: PaulAm
author_rank: Frequent Contributor
timestamp: July 06, 2020, 03:52:36 pm
content: Well, you have to test it to find out what's wrong.As a general rule, excessive ripple on these scopes does not cause collateral damage. Set up the probes, turn it on and look, turn it off if you're anxious. Just probe the LV power supply test points on the main board with another scope. Ripple should be below 10mV, it will be obvious if something's wrong.

author: Kidon
author_rank: Regular Contributor
timestamp: July 06, 2020, 04:47:04 pm
content: Quote from: PaulAm on July 06, 2020, 03:52:36 pmYou’re right, I know. But it makes me nervous...The funny this is that, when it happened, I was actually using this scope to check stuff on a 466 as per the calibration procedures. But I do have other scopes. I’m actually tempted to just shotgun all the PS caps and replace them. I figure that if one’s gone bad, the others won’t be far behind...I’ll probably finish the calibration on the 466 before I tackle this one though.You guys seem quite sure it’s one or more filter caps. You have seen that before, correct.These are things that I’m not used to. But I learning to identify and fix these issues (the hard way, I guess...)

author: Gyro
author_rank: Super Contributor
timestamp: July 06, 2020, 05:01:58 pm
content: My guess would be a shorted diode in one of the bridge rectifiers. This would cause both high ripple and excessive current draw. If so, it should be quick to find just using a DMM without risking powering it up again - you don't want to kill the mains transformer.

author: Kidon
author_rank: Regular Contributor
timestamp: July 06, 2020, 05:36:58 pm
content: Quote from: Gyro on July 06, 2020, 05:01:58 pmYeah, that sounds like a good/safe starting point.I remember watching a YouTube video about 6 months ago, where the guy had a bridge rectifier in a 475A that checked good as well, but it was bad, and temperature sensitive. I’ll post it if I find it. I don’t remember if he talked about the symptoms though.

author: Kidon
author_rank: Regular Contributor
timestamp: July 06, 2020, 05:38:52 pm
content: I think this is the one:https://youtu.be/F460UG1vfNs

author: 0culus
author_rank: Super Contributor
timestamp: July 07, 2020, 03:35:33 am
content: Quote from: Gyro on July 06, 2020, 05:01:58 pmDefinitely a good thing to rule out before running it too much. I had a pair of dead short diodes in my 585A's "low" voltage power supply that were causing instant blown fuses on power on (before the relays enable B+). Replaced them and the scope fired right up. You definitely do not want to stress the mains transformer any more than you have to.

author: Kidon
author_rank: Regular Contributor
timestamp: July 07, 2020, 12:51:33 pm
content: I see no reference in the manual “Replaceable Electrical Parts” section to tantalum capacitors. Do they reference them under “ELECTLT”?? Hmmmm.

author: Kidon
author_rank: Regular Contributor
timestamp: July 07, 2020, 07:09:30 pm
content: Update:I haven’t done anything to it since the problem happened yesterday. I decided to carefully check voltages today,But the mains fuse blew right away when I turned it on... It probably has a short to ground somewhere... Hmmmm.... apart from checking the bridge rectifiers, and visually check for burn components or traces, I’m trying to figure out how I’m gonna go about this...

author: Gyro
author_rank: Super Contributor
timestamp: July 07, 2020, 07:43:55 pm
content: Quote from: Kidon on July 07, 2020, 07:09:30 pmDo those first and hopefully you won't have to think about it further. If the fault is blowing the mains fuse then it is very close to the mains transformer (and will probablybecomethe transformer if you're not careful).I really do caution you about powering up again and blowing the fuse - that won't end well.

author: Kidon
author_rank: Regular Contributor
timestamp: July 07, 2020, 08:17:12 pm
content: I only tried to power it up today because it did power up yesterday after it blew the fuse. That’s when I took the picture of the trace, then I turned it off immediately. I sure won’t do it again....Besides checking the rectifiers and visual inspection, I don’t know how to go about this... But as you said, hopefully I won’t have to worry about it...

author: Gyro
author_rank: Super Contributor
timestamp: July 07, 2020, 08:41:35 pm
content: Sure, I realise that you didn't intend to run it to fuse-blowing point. When you get this sort of fault though, the best methodology is to seek out the low impedance path in a powered off state, avoiding the risk of all of the primary power going into a single overloaded secondary and causing local overheating and shorted turns.Hopefully the fault location will quickly become obvious with resistance / diode tests and you can breath easier.

author: Kidon
author_rank: Regular Contributor
timestamp: July 07, 2020, 09:24:17 pm
content: What’s funny is that “fuse-blowing” point didn’t happen yesterday when I powered it on after changing the fuse - 800mA instead of 750mA as these are not available in Europe. The norm standard goes from 500mA, 600mA and 800mA - but happened 1/4 of a millisecond today after powering it on. I barely had the chance to see Volt/Div lights turn on, and it went dead. Weird... Maybe a clue there. Temperature related? Naaah. It had been running for a few hours straight when it blew the fuse the first time, and I took that picture about 5mins later.... it was a fairly hot day as too. Hmmm...

author: 0culus
author_rank: Super Contributor
timestamp: July 07, 2020, 09:31:33 pm
content: That's basically exactly what my 585A was doing. It would try to come on, then pop the fuse. Definitely check the bridge rectifier diodes with a multimeter. The two bad ones I had were dead short when measured in circuit and the one that didn't disintegrate when desoldered was confirmed to be just a wire measured out of circuit.Another thing...what is the condition of the fuse when it pops? Is it just that the conductor has broken or is it blackened and burned inside. This can give you a clue about how bad of a short you have.

author: Kidon
author_rank: Regular Contributor
timestamp: July 07, 2020, 10:35:16 pm
content: I’ll check that tomorrow. It’s 00:30am here.Will report back.

author: Kidon
author_rank: Regular Contributor
timestamp: July 07, 2020, 10:47:19 pm
content: Quote from: 0culus on July 07, 2020, 09:31:33 pmI just hope the transformer didn’t take a hit. That would really suck. Hope it can handle it.I also thought the tantalums could be an issue. They’re notorious for shorting out for no reason in these 400 series scopes. It’s the first time in my life I have to fix an oscilloscope. I actually just got a 465 that power up and all, but no trace. Probably one of the rails... That’ll be the next.... I mostly work on guitar tube amps and pedals. I’m not used to doing these things. Trying to learn though.

author: med6753
author_rank: Super Contributor
timestamp: July 08, 2020, 03:50:08 am
content: On the A9 Interface Board (bottom large board) there are test points for the PSU voltages. +110V, +50V, +15V, -15V, -8V, and +5V. Check each for short to ground. If you find one shorted you can be 95% certain it's a shorted tantalum capacitor. The manual does outline how to isolate the boards to track down which is shorted.

author: Kidon
author_rank: Regular Contributor
timestamp: July 08, 2020, 10:01:01 pm
content: Very good idea. I should’ve thought of that.Thanx.Will do.

author: Kidon
author_rank: Regular Contributor
timestamp: July 08, 2020, 10:03:29 pm
content: Quote from: 0culus on July 07, 2020, 09:31:33 pmHard to tell cause the one I had is filled with something...But you can definitely see the burn trace on it.See picts.

author: Gyro
author_rank: Super Contributor
timestamp: January 28, 2023, 09:24:56 pm
content: A quick search brought up this thread in which I appear to have posted 3 replies, offering sage advice on protecting the mains transformer (I don't even remember writing them). I'm resurecting it to document my own carelessness and stupidity... and not following my own advice!I was using my trusty Tek475A a couple of days ago to monitor a DDS sig gen and thought I would let it stretch its legs for a while as it hasn't had much exercise recently. After an hour or so, I left the room for a few minutes and came back to a completely dead scope. Initial checks showed the 750mA input fuse blown, but not in a dramatic style.Mistake #1: Having checked that the fuse for the EHT supply was intact, I assumed that the input fuse must have aged and replaced it (luckily there was a nice packet of original BUSS fuses still in the pouch). This immediately blew, in the same non-dramatic fashion. Unnecessary stress on the transformer.Mistake #2: I checked for shorts on the DC rails to rule out Tants andthoughtI had checked all of the bridge rectfiers. I also checked the two primary windings for equal resistance. By this point I was beginning to suspect the mains transformer. This time I slowly powered it up on the variac so that I could monitor the DC rails coming up. Before I had finished though, a nasty varnish smell alerted me to the heating up mains transformer. Damn! By this time I was seriously worrying that the transformer was fried. The only way to know for sure would be to disconnect the secondaries. Unfortunatly these are inaccessible without removing the trigger board. Time to sleep on it.Mistake #3: After pondering overnight, I resolved to systematically re-check everything. This time I discovered that the bridge rectifier, CR1412, supplying the Unreg +50V, 110V, and Reg +50V was showing a low resistance fault between all four legs (how did I miss that?!). Ok, faulty bridge rectifier. As it was flush to the board, I crushed it and unsoldered the pins (to get to the underside of the PSU/Horizontal board you need to remove the vertical amp board and trigger board, plus knobs, heatsinked transistors etc.) and patched on a replacement.Without checking, I again started to bring the scope up on the variac. Oh no, varnish smell again!Getting it rightat last: Checking carefully around CR1412, again what appeared to be a 4 wire low resistance short. The Unreg +50V was still showing no short (after initial cap charging), confusing. After looking carefully at the schematic I realised that only shorted C1412 or C1414 could cause the symptoms, sure enough, C1412, arrowed in Red showed a dead short! After solder sucking the anode pin and isolating it with slivers of wooden coffee stirrer the short on the rectifier had dissapeared. Bodging on a 100uF 200V electrolytic rewarded me with a fully working scope with clean trace and all supply rails spot-on.What had confused me so much was the configuration of the rectifier. The -ve output is grounded and the unreg +50V is taken from the transformer centre tap, with the +110V taken from the +ve. The capacitor short, together with the secondary winding resistance and grounded -ve pin made it look like a dead bridge rectifier. I've run the scope for several hours now and the transformer seems to have survived the abuse. Needless to say, the trigger board is coming off and all reservoir caps replaced before it goes back in its case!Learnings:1. Checkeverythingpossible at least twice before attempting to power up anything after a blown fuse.2. Bringing up things on Variacscanbe a really bad idea!3. Ageing electrolytics can fail short as well as drying out (even though I've owned the scope for over 40 years and it has low hours).4. Learn to recognise your own declining faculties as you age and compensate by slowing down and allowing more thinking time.5. Follow your own advice...evenif you can't remember giving it!