EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: MrSlack on December 31, 2015, 04:23:54 pm
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This was in another thread. I thought I'd write down my progress on this for the sake of someone who may stumble across the same issue.
So I fired up my Tek 465B and nothing happened. So I bought another slightly less sick one to see what was up with it.
The issue manifested itself as no sweep. I found a broken resistor in the sweep generator after carefully probing around to see where the signal path stopped. It had cracked. This was replaced with a metal film one I had lying around. I didn't take any photos of that initially.
When this powered up I noticed there was a ton of ripple on the display and the triggering wasn't working properly. It would randomly trigger and occasionally just stop working all together. I noticed this was showing roughly the same behavior as the line trigger option so I poked around at the rails and discovered some ripple on the 55v rail. A quick inspection of the board and there was a blown Tantalum capacitor.
In circuit:
(http://i.imgur.com/J6WUMsP.jpg)
Extracted:
(http://i.imgur.com/wuAcVqI.jpg)
Exploded tantalum capacitor took the solder mask of the board. I cleaned it out carefully and replaced with a 4.7uF 100v alu electrolytic unit from RS. Total cost £1.87 for 5x. I had to use two as I crimped the leads slightly too short on the first one (doh). This was a case of desolder the first one, solder wick the holes out, solder in a new one without melting the RF cables in close proximity.
Result just on the calibrator (no sig gen in the kitchen :))
(http://i.imgur.com/LyXHukt.jpg)
So now it's working reasonably well. The beam return blanking isn't working properly now (shows a light trace) and the intensity and focus is jippy but I'll sort that out in the next few days.
Then I'll fix the other one which has a noisy channel A and some ripple problems as well.
Edit: fix a duff link
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Beam blanking failure was a shorted transistor. Poked a bc337 in the relevant hole while I wait for a proper replacement and it seems happy. Noticed that quite a few transistors run very hot in the 465 so have ordered some TO92 heatsinks as well for any replaced/weak ones.
Fixed intensity problem. Out of tolerance resistor in the intensity control circuit.
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Beam blanking failure was a shorted transistor. Poked a bc337 in the relevant hole while I wait for a proper replacement and it seems happy. Noticed that quite a few transistors run very hot in the 465 so have ordered some TO92 heatsinks as well for any replaced/weak ones.
Fixed intensity problem. Out of tolerance resistor in the intensity control circuit.
That is so common in CRO's.
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Tell me about it. I yanked about 20 of the old carbon composition resistors that were in my old Tek 453 (before the cat pissed in it and finished it off) and most of them were way out. Surprised it still worked.
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Well done! Mr Slack.
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Working on the other unit now...
This one has noise, beam blanking issues and dodgy attenuator as well as intensity problems. Power rails are all fine, including ripple. This has much newer shrink-wrapped capacitors compared to the plain metal ones in the older unit. Doesn't look like they've been replaced. Still waiting for cheap "Transistor Tester" from ebay to arrive so I can check the ESR (gave up on building an ESR meter).
There was a lose screw on the A attenuator that if you pressed on it cleared up some of the noise. I removed the entire attenuator and cleaned the cam contacts (I hate these things!) carefully with IPA and some torn off A4 paper, then reassembled it. Screwing that down hard seems to have solved some of the trace noise issues. Amazing how simple things can affect circuits.
I then went through the Z amp circuit again and checked the resistors after the last one's problems and found one out of tolerance. I bought a pile of good quality stock resistors from Farnell a couple of days back so this got a nice Welwyn 0.5W one in it. No more cheap nasty no brand ones any more.
After this I spent the entire evening, much to the annoyance of my wife, removing, testing and reinserting every transistor in the unit. Found a way off beta device in the Z amp. This was replaced with a BC556. Display is much sharper than it was after the resocketing process. However the intensity pot is completely knackered. New pot ordered.
Also ordered some decent allen keys from RS as the cheap ones I've got feel like they are going to completely muller the grub screws on the knobs on these.
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Just wipe a little Watney's Red Barrel on them as you tighten, should muller them up for good. :)
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Haha. That would indeed :)
More progress...
Two hours dismantling and restoring the attenuators, with the aid of some Greene King IPA and isopropyl IPA being careful to not get them confused, and some sheets of lint free cloth and we have no more noise. Intensity pot half replaced.
My soldering iron power supply blew up (damn you crappy Aoyue junk) half way through the pot replacement. New Weller WTCP51 kit arriving Monday.
Old scopes are expensive!
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Ok so ESR checked on all the caps on both scopes. They're all knackered including one of the new Vishay ones I bought for scope A. Currently pricing up replacements versus buying a Rigol DS1054Z and stripping these for breadboarding parts...
Interestingly I found that the power supply ripple I measured on scope A with scope B wasn't actually an issue with scope A but an issue with scope B's power supply. It didn't go away when I shorted the probe. Tried another probe, still happened. Stuck a 50 ohm terminator on the scope, still happened. Used scope A to measure the ripple on scope B; bingo, massive ripple on the 55v.
Sigh. On the positive note, they are both stuffed with nice pots and transistors!
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So another couple of hours down the line and a dig in the junk box and I managed to remove the old caps and then tack solder some that are close in value I had lying around from a surplus lot I got. They have good ESR measurement. Problems all gone away! £30 of capacitors ordered from RS.
Also found another duff resistor in the HV circuit which has made the display much sharper.
I will not be defeated!
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Ok so ESR checked on all the caps on both scopes. They're all knackered including one of the new Vishay ones I bought for scope A. Currently pricing up replacements versus buying a Rigol DS1054Z and stripping these for breadboarding parts...
That would be a crime punishable by ....... :scared:
There'd be numerous members happy to have a go at them if you don't.
So another couple of hours down the line and a dig in the junk box and I managed to remove the old caps and then tack solder some that are close in value I had lying around from a surplus lot I got. They have good ESR measurement. Problems all gone away! £30 of capacitors ordered from RS.
Also found another duff resistor in the HV circuit which has made the display much sharper.
I will not be defeated!
That's more like it. :-+
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Hahaha :)
I regularly get periods of: "f*** why am I doing this? I spent all day with my face in a pile of broken and unhealthy software puked out by the lowest bidder that I need to piece together into a cohesive whole, and now you dare to hurt me this evening you f*** b***** thing. All I wanted to do is check the clipping on this CE amp. Die! I'm going to eviscerate you with my soldering iron!".
Learning to walk away for a few minutes is valuable :)
Apart from when the cat pissed in my Tek 453. The cat went on Gumtree. For a period I wished it had pissed on the anode cap while it was on.
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Ok busy day today. Scope A has had the cams cleaned properly with IPA, replacement intensity pot, CRT grid bias pot wiggled around a bit (it was crudded up), as much recalibration as I can do without a time-mark-generator and new power supply smoothing capacitors installed which was rather difficult. They are nowhere near the size of the original ones so I had to do a couple of frigs I'm not very proud of involving heat shrink to protect the slightly trailing leads, plastikard for spacing and cable ties but it's safe, tidy and it works well now. Back into service.
(http://i.imgur.com/gC9Uqh7.jpg)
(yes I know that's a crap sine wave - my func gen is a bodge with a couple of tl072s and a naff diode shaper)
Now on to scope B...and scope C is on the watch list...
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My soldering iron power supply blew up (damn you crappy Aoyue junk) half way through the pot replacement. New Weller WTCP51 kit arriving Monday.
Old scopes are expensive!
You didn't want to get a Hakko FX888D that many people have been raving about?
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Nice progress. :)
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My soldering iron power supply blew up (damn you crappy Aoyue junk) half way through the pot replacement. New Weller WTCP51 kit arriving Monday.
Old scopes are expensive!
You didn't want to get a Hakko FX888D that many people have been raving about?
Nope. Weller stuff is worth the investment if you ask me. I had a TCP iron and PS2D supply from 1990 to 2007. Sold it to pay the bills during a bad patch. The Aoyue was a temporary purchase until I had some cash to replace it with another Weller TCP.
Hakko doesn't get a look in. I spent up to 4 hours a day with that TCP iron for 5 years and it just ate up anything thrown at it. Not only that I can replace stuff 26 years later with the same parts delivered next day.
New baby: http://uk.rs-online.com/web/p/soldering-stations/3320357/ (http://uk.rs-online.com/web/p/soldering-stations/3320357/)
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My soldering iron power supply blew up (damn you crappy Aoyue junk) half way through the pot replacement. New Weller WTCP51 kit arriving Monday.
Old scopes are expensive!
You didn't want to get a Hakko FX888D that many people have been raving about?
Nope. Weller stuff is worth the investment if you ask me. I had a TCP iron and PS2D supply from 1990 to 2007. Sold it to pay the bills during a bad patch. The Aoyue was a temporary purchase until I had some cash to replace it with another Weller TCP.
Hakko doesn't get a look in. I spent up to 4 hours a day with that TCP iron for 5 years and it just ate up anything thrown at it. Not only that I can replace stuff 26 years later with the same parts delivered next day.
New baby: http://uk.rs-online.com/web/p/soldering-stations/3320357/ (http://uk.rs-online.com/web/p/soldering-stations/3320357/)
I have been using Weller stations since 1975 or thereabouts.
You are hard pressed to find something as reliable for the money.
I am currently using the same station, it gets the job done and has only failed me once since 1995.
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I think I still have a pile of Weller stations, and parts somewhere... I wonder if I should try to revive any, or just bury in recycle???
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EBay them - worth a small fortune.
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I'm interested in buying a decent weller station, as long as it's got all the parts and is adjustable.
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I return with another repair story.
So I now have another problem on the main scope. This nearly made me fish out for a new one, but no I'm not being defeated again! On the good scope, channel 1 decided to merrily snuff it. I suspect this was due to transients as I was probing a switch mode I was designing. It may have had a peak of 500v or more dumped into the front end. You live and learn...
After a couple of hours of poking around with the parts mule scope I determined that U1790 was duff on the vertical amp board. I removed a good one from the parts mule scope and replaced it. A complete pain to desolder and replace that was as you have to dismantle half the of the scope to get it out. However, success and there was a very noisy trace. After some poking with attenuator settings and finding nothing at all, I noticed that if you flick the 10x attenuator module it would start or stop being noisy. I swapped 10x attenuators between channels and the same thing happened with the other channel, so duff module. I popped the lid on the module and the thick film resistor has bubbled slightly. FAIL. Swapped a 10x module from the mule scope and frigged the compensation capacitors to get a clean trace and all is good.
Some attenuator porn - the 10x module is the dead one:
(http://i.imgur.com/WzwKFf3.jpg)
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I removed a good one from the parts mule scope and replaced it. A complete pain to desolder and replace that was as you have to dismantle half the of the scope to get it out.
Yes, I was very unpleasantly surprised by the 465 in that respect. The HP1740a (same vintage) and 485 (later vintage) are much better.
Rather than desolder the 465's delay line (FFS!) etc etc, I replaced an ali electrolytic by clipping the leads near the body and soldering the replacement to the legs. Used forceps as a heat shunt to ensure the ex-leads didn't drop out.
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I've been after an HP 1740a for a bit. Nice industrial design.
I've done worse things in repairs that I'm really not proud of.
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I've been after an HP 1740a for a bit. Nice industrial design.
Maybe I'll get around to selling mine sometime, now that I've got a 485. But yes, it is nice.
The only really bad misfeature I've found is that when the risetime starts getting worse it is necessary to fondle and massage the delay line (yes really!). Plus the design of their timebase switch, which is guaranteed to abrade PCB tracks (but is easily repairable); teh Tek switches are better there.
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I spent half the day cleaning Tek cam switches so meh ;)
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Thanks to another forum member, I've obtained a third unit! This one is slightly more unhealthy than the other two by the looks. The front panel is absolutely immaculate as it has been stored with the front protective case on. However the inside is not so good. Rather gunked up with various hairy things living in i.
After a quick earth check to make sure it wasn't going to kill me and a cursory inspection of its guts looking for burns and burst caps, a quick power up and drop into X-Y mode shows an absolutely huge shift horizontally. Step one as always is to check the power rails. First with a meter, then check any suspicious ones with a scope.
The meter check showed all the rails being within tolerance apart from the 110v rail which was down at 103v. So a quick poke with another unit:
(http://i.imgur.com/5dDA3hB.jpg)
Hmm that's not right. Quick poke of the unregulated line and it looks just as bad.
(http://i.imgur.com/eirvFRp.png)
C4439, C4429 look to be the suspects here as there is precisely no smoothing on the unregulated line.
C4429 is specified as 1200uF 100v electrolytic. Replacing with 1200uF Nichicon 160v RS part 846-6694. £2.89 ex.
C4439 is specified as 550uF 100v electrolytic. Replacing with 560uF Rubycon 100v RS part 749-7235. £2.31 ex.
Ordered.
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Had an hour spare today so I did a front panel clean and massive knob swapping and refurb session on the best operating unit, the objective being one mint scope, one second best and one parts mule. Some of the knobs had broken entirely on the original unit and some of the grub screws were completely corroded away. Despite this, I didn't have to drill any out or resort to violence. A slightly larger Wera spiky Torx jammed in the hole on the grub worked nicely. The vernier dial on the delay timebase was a dick to get on and off. The top has a grub screw and the base has a collet ring with two holes - pliers worked in the end in the holes.
If anyone is interested, IPA (RS 567-890) and Plenty tissues for cleaning. No issues with legends being dissolved.
(http://i.imgur.com/Ppqo7yM.jpg)
(http://i.imgur.com/UJTeesl.jpg)
(http://i.imgur.com/ZX1yMJp.jpg)
Scrubs up well for a 35 year old piece of kit!
Edit: I just noticed I missed the scale illumination knob....
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Nice MrSlack. I have two 465B's awaiting similar treatment. If I could just find the spare time....
Any idea how to get those coupling and source toggle plastics better looking, they seem to have developed a whitish chalky surface finish with age?
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All mine have that problem. I tried giving them a wipe with water only and nothing happened. Then I cautiously applied some IPA on a tissue. This worked fine for a bit and they look a lot better but the finish seems to go chalky again pretty quick.
I'm not too bothered as the plastics aren't compromised by this by the looks. Will just have to live with it.
Since last post I've replaced the mains lead and position controls on another unit. Mains lead isn't trivial and requires some soldering.