Help with HP 1740A CRO: smells, buzzes, and has a potentially leaky *thing*? - Page 1
Forum section: electronics / repair

author: ogulpal
author_rank: Contributor
timestamp: December 24, 2023, 12:31:29 pm
content: I just got my first CRO ever. But the seller didn't pack it properly and so the "cal" buttons for chan A & chan B got banged and bent. I've mostly unbent them by pushing sideways the other way but it's not perfect. What's the best way to undo this damage? The buttons still work, they're just off axis.Another thing is that when I turn it on, it makes a buzzing sound. It's not the 15kHz hum that CRT TVs used to do, it's lower frequency than that. The display lights up and shows a flat line. I can't test it because I have no probes yet, but it turns on and powers the CRT. Is this normal, or is something dying in there and I should investigate?Also, the CRO reeks of a WD40-like smell. I can't find any evidence of WD40 spraying (was looking for greasy streaks anywhere inside or outside), but when I open it the smells comes out. It's really powerful and bothersome. I tried smelling around, but it's just the general area that smell. Is it how old CROs smell because something inside is off-gasing? Is it a failed/failing component? Or is there a spot that people typically spray with lubricant that I should find and clean to remove the smell?Finally, I'm attaching a picture of a suspicious component that looks like a capacitor that has leaked. But the component doesn't look anything like a capacitor to me. It's stamped I901-0638, but I couldn't find anything on the Internet with this reference. What is it, should I replace it, and how to find an equivalent? On the schematics, it's on board A16, low voltage power supply, component CR4. It looks like a bridge rectifier, but why would it leak? There is no electrolyte in these as far as I can tell (with my very limited knowledge). I attached the schematics and highlighted CR4.--edit: I found what I think is CR4 from board A16 in the components list, it just calls it a "diode full wave bridge 100V 5A". When I search for this on eBay, I can't find one that has this long, rectangular shape like on the board...

author: tggzzz
author_rank: Super Contributor
timestamp: December 24, 2023, 01:00:15 pm
content: Welcome to the forum. It is a pleasure to see decent photos, investigation, and thought.I hope you complained to the vendor and reclaimed money. The 1740 is, when working, quite a nice scope. You don't mention whether traces were visible before it was shipped. I hope the CRT hasn't been damaged. If traces are/were visible that is unlikely, but it is possible for an internal electrode to be damaged causing a bent or wonky trace.A few pointers which might guide your investigation.Old scopes shouldn't smell.If someone has used WD40, it is likely to be around the timebase and attenuator switches. WD40 shouldn't be necessary, and if it affecting the scope's operation should be removed. Those switches are best cleaned with IPA.Other failed components can smell. The infamous ones are RIFA "delayed action smoke generators". I can't see any on the circuit diagram, but they might be lurking inside the mains connector P1.There is a weakness in one of the 1740's bridge rectifiers. I don't remember which one, but Dave made a video about it; his videos are usually well constructed unlike most on yootoob.Typical components that fail are electrolytics that dry out, have a high ESR, and can leak corrosive chemicals. Tantalum beads also fail and become short circuit. Either of those could damage a bridge rectifier.My suggestion: measure the power rail voltages and ripple; where necessary replace old electrolytic capacitors. After all the power rails are good, proceed with other faultfinding.

author: wasedadoc
author_rank: Super Contributor
timestamp: December 24, 2023, 01:09:17 pm
content: 1. You do not need probes to do a cursory check. A single piece of wire from the CAL terminal in the bottom left corner to the centre of the BNC socket for CHA should give you a square wave. Then move the wire to CHB.2. I'm not convinced that rectifier is an original. But they don't smell unless running so extremely hot that the plastic does. They don't leak anything even if faulty.

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 24, 2023, 01:18:25 pm
content: The discoloration can be caused by the rectifier getting hot, or maybe a clumsy repair job.

author: factory
author_rank: Super Contributor
timestamp: December 24, 2023, 01:47:21 pm
content: If it hasn't been done already, resolder the connector next to the rectifiers, they can get badly dry jointed, the poor solder connections become higher resistance and the resultant heat burns up the PCB & connector.https://www.eevblog.com/forum/testgear/hp-1740a-power-supply-burn-up/msg2637378/#msg2637378David

author: Zenith
author_rank: Frequent Contributor
timestamp: December 24, 2023, 02:26:05 pm
content: How did the seller describe this item?Old scopes that haven't been used for a long time may smell slightly musty for a minute or two, as the dust heats up. It shouldn't last long. If you believe WD40 or similar has been used, wash it off with pure IPA. If you have to lubricate controls on electronic equipment, spray some PTFE dry lubricant into the cap that comes with the can and use a small paint brush and apply it exactly where it's needed.Straightening bent shafts is a pain. Tweak them carefully with a pair of pliers until they are right, but obviously there's a risk of doing more damage. Don't expect to achieve perfection. The better solution is to replace the control, but that may not be possible.The first step with a piece of equipment like this is to look it over and check for anything obviously missing, burned, damaged, disconnected or bodged.The next step is to check that all power supply rails are within spec for voltage level and ripple. If they are not, tests on other parts of the circuit are based on a false assumption.Bridge rectifiers can fail, but it's uncommon. Sometimes there are bad soldered joints, especially if they'd been made to carry excess current and overheat. Electrolytic capacitors often fail with age. I agree with ArdWar, the discolouration looks more like heat damage than something which has leaked. I suppose there could be something on the other side of the board that's overheated.I suspect that electrolytic may have failed short circuit, but it could be something downstream, in the regulated part of the power supply.

author: ogulpal
author_rank: Contributor
timestamp: December 24, 2023, 02:29:43 pm
content: Quote from: wasedadoc on December 24, 2023, 01:09:17 pmThanks for the tip regarding testing without probes. It looks like the scope works as chan A and chan B both show the test square wave. I don't know how to use an oscilloscope much yet, but I was able to set time and amplitude (is it the right word? Volts/div) so that a square wave shows for both channels. That's a relief as I wasn't sure it worked until now!Quote from: tggzzz on December 24, 2023, 01:00:15 pmThank you, I did put some time and thought into it before posting hereI got the CRO for rather cheap and because it was damaged in transit I got a 50% refund from the seller. All in all, I paid 20.- for it shipped (domestic), without probes and without the manual.Quote from: wasedadoc on December 24, 2023, 01:09:17 pmQuote from: factory on December 24, 2023, 01:47:21 pmI wonder if the CR4 is original too. It looks different from the other 5, but matches the shape on the board schematics and also what it looks like inDave's video. I don't know if it's been replaced, maybe the dark spot is flux that was left behind, or heating from the loose joints?The board does look a little cooked like in the 3rd and 4th attachments here:https://www.eevblog.com/forum/testgear/hp-1740a-power-supply-burn-up/msg2637378/#msg2637378.I'm worried about shocking myself from the CRT so I haven't taken the connector out... I've never actually discharged a CRT, I've just seen many people do it on video by using a screwdriver with a lead to the chassis and sliding the blade under the "suction cup" on the CRT, loud BANG, discharged. Is that the right way? Is that the only thing I should do (unplugging from the wall) to safely poke around inside?

author: ogulpal
author_rank: Contributor
timestamp: December 24, 2023, 03:02:17 pm
content: The seller didn't say much, just showed a picture of it with two flat traces on the display. I don't know anything about the scope's past other than that it last serviced by HP on 30-10-1978!I'll check the joints and resolder them if needed.

author: Zenith
author_rank: Frequent Contributor
timestamp: December 24, 2023, 03:40:47 pm
content: It isn't always obvious that you have a bad joint. They can look good, but not make a good connection until the instrument warms up, or be good when cold but not when warm, or be affected by the slightest shock, such as moving the instrument.I suggest you remove the solder from all of the connections to that rectifier bridge, check with a magnifying glass to make sure there's nothing off, such as a broken circuit track, then resolder them.

author: tggzzz
author_rank: Super Contributor
timestamp: December 24, 2023, 05:40:09 pm
content: Quote from: wasedadoc on December 24, 2023, 01:09:17 pmI've see unpleasantly hot bridge rectifiers in Tel 475 and 2901. There was little difference after replacing them with Schottky diode variants.If those are the traces on your scope, then it is worth putting some effort into getting it working.

author: tggzzz
author_rank: Super Contributor
timestamp: December 24, 2023, 05:53:48 pm
content: One design flaw in the 1740 is in the strange timebase switch. Seehttps://www.eevblog.com/forum/testgear/hp-1741a-oscilloscope-restoration-project/msg980550/#msg980550Another weird flaw is that occasionally you have to fondle and massage (literally!) the delay line.https://www.eevblog.com/forum/testgear/cheap-tek-500mhz-passive-probes-on-ebay-can-they-be-used-on-any-scope/msg1030288/#msg1030288Be aware it is easy to overload the 50ohm termination resistor; just make sure you normally operate in 1Mohm mode.

author: ogulpal
author_rank: Contributor
timestamp: December 31, 2023, 08:11:07 am
content: For now, I have looked at the low voltage power supply (board A16) and found some suspicious joints on the rectifier bridges and the sockets (like in EEVBLOG #803 and 804). I've redone them properly.I'm waiting for the probes now to poke around and see if anything else is off.Thanks to everyone in this thread for the help!