BWD881 scope failure - Page 1
Forum section: electronics / projects

author: moffy
author_rank: Super Contributor
timestamp: April 20, 2023, 06:12:21 am
content: I was doing some low level noise measurements and decided to use my old analogue BWD881, 4 channel, differential input, 20MHz scope, built here in Australia in the mid 80's. I hadn't turned it on in ten years and as you can imagine when I did there was a loud pop as the dual fuses blew and that slight but familiar smell of burnt plastic. Perhaps someone should turn it into a cologne for EE's. I was fortunate to have the repair manual, but when I found it, it was for a completely different scope, the BWD540. So much for my observational skills over the years. I did manage to find a complete manual online and I've included the schematic for the PSU.When I opened the scope I was surprised how clean and new it looked. I was expecting bulging caps and dust but it was very clean, nice build quality also. I focused on the PSU, no noticeable shorts, but saw C803 with a pin hole in its case (it is connected to pin 14 of the TL494) and C806, ceramic,(between pin 15 and 16 of the TL494), it looked like it had been rubbed against a grater.From the schematic it has an unusual flyback configuration that feeds 32V to a self oscillating pair of 2N3055 transistors and transformer. This supply seems to have slowed edges for low noise and powers the rest of the scope. Since there were no shorts detected I decided to hook up a lab PSU set to 32V to check the rest of the circuitry. Sure enough the scope powered right up, which was very satisfying. I am thinking of just building my own 32V supply rather than attempting a repair of the unusual flyback. It only requires 40W so I am thinking just a small transformer, rectifier and regulator, I don't want to use a switching supply because of noise. Any way what would you do, fix the flyback or replace it, and with what?

author: tautech
author_rank: Super Contributor
timestamp: April 21, 2023, 08:40:09 am
content: Well darn it, another BWD881 !I had one years back acquired to fix with similar issues however IIRC nothing blown but a shorted Tant paired with an electrolytic as remote decoupling on one of the power rails some distance from the rail source.That was enough to drag the rail down and overload the 32V from the prereg and trip the SCR crowbar blowing fuses.About that time I starting using a DBT which saved a pile of fuses while you sorted the thing out.Nice little scope if you can get her up and running which as a starting point I would carefully study TL494 implementation in its datasheet. You might even find their Typical Application schematic closely mirrors how it's used in the 881.

author: moffy
author_rank: Super Contributor
timestamp: April 21, 2023, 10:55:26 am
content: Yeah, I think the crowbar circuit triggered blowing the fuses, it also looks like the flyback IRF830, main switching FET, is shorted. I understand the reason for the flyback configuration, but I dislike that a failure will attempt to dump the full mains across the 32V rail, apart from the crowbar protection. I prefer a less drastic failure mode if possible, hence my idea of a separate 32V supply, but space is an issue. Thank you for your advice, I'll look a bit closer at the application circuits.

author: tautech
author_rank: Super Contributor
timestamp: April 22, 2023, 10:20:12 am
content: Looked in my archives and here are the 881 docs I have. Hope they can help.

author: moffy
author_rank: Super Contributor
timestamp: April 22, 2023, 11:55:17 am
content: Thanks very much, your copies of the schematics are clearer and easier to read than the ones I downloaded.

author: moffy
author_rank: Super Contributor
timestamp: April 23, 2023, 08:15:01 am
content: This is what I am thinking of replacing the flyback converter with, just a linear (48V-36V) to 32V series regulator. I haven't worked out yet how to disable the input power on short circuit etc., so if any suggestions let me know. The new circuitry is to the left of P1 and P2, to the right is the existing PSU(see BWD881 PSU schematic, bottom). I can then use either 36V battery or an external supply, will have to consider ground in such a case.

author: tautech
author_rank: Super Contributor
timestamp: April 23, 2023, 09:09:22 am
content: You'll need an additional transformer, correct ?Don't forget to use the line feed for line triggering.Not sure I'd do that but instead shotgun most of the componentry in the prereg to get that working.

author: moffy
author_rank: Super Contributor
timestamp: April 23, 2023, 09:22:59 am
content: I haven't quite found room for a transformer yet, so I am thinking of running an input socket for the DC 48V-36V. But if I do find a place then I can just hook it up to the regulator, which I have found space for. Forgot about the mains trigger circuit, I could hook the mains lead up just to run that.

author: moffy
author_rank: Super Contributor
timestamp: April 23, 2023, 10:38:21 pm
content: I have added current limiting to the regulator, about 2A, Q3 and R6, a small but important improvement.

author: moffy
author_rank: Super Contributor
timestamp: April 26, 2023, 04:21:22 am
content: Just waiting on a couple of components before I construct my 32V regulator, also added a fuse, flyback diode for the inductor and a protection zener for the TL431. I am also going to fix the BWD881 mains supply, but am finding significant discrepancies between the schematic and the board I have. It looks like they have gone through a number of revisions with this supply. I'll probably need to draw the differences, but at least it's relatively small circuit and a single sided board.

author: tautech
author_rank: Super Contributor
timestamp: April 26, 2023, 08:45:50 am
content: Another BWD I worked on was835Athat reminded me I'd seen that PSU elsewhere too which after downloading from Kevin Chant's great BWD site looks very similar and might be the slightly different version that could match yours.Look at P132The link won't Copy/Paste properly so get it from here:https://www.kevinchant.com/bwd.html

author: moffy
author_rank: Super Contributor
timestamp: April 26, 2023, 10:08:28 am
content: Thanks for the recommendation, I'll check it out.

author: moffy
author_rank: Super Contributor
timestamp: April 26, 2023, 10:24:24 am
content: Well it's by far the clearest schematic but almost identical to the BWD881 drawing, except it uses a TL594 instead of the TL494. I might check out a couple of the other manuals.

author: tautech
author_rank: Super Contributor
timestamp: April 26, 2023, 10:36:18 am
content: Quote from: moffy on April 26, 2023, 10:24:24 amMany are just ordinary linear CRO supplies but if you spend enough time looking you might find what you seek.Good luck.

author: moffy
author_rank: Super Contributor
timestamp: April 29, 2023, 05:39:37 am
content: I have managed to trace out the relevant parts of the failed BWD881 mains PSU, which had quite a few variations from the BWD manual. They have made the mains PSU to work over a 100V to 380V DC rectified range. They included a TIP50 (Q801 in schematic included in post) as a startup regulator to get rid of the normal start up resistors. It's a nice idea as it saves the constant load of these resistors during operation, but failure of the TIP50, which is what happened provides a low impedance path to the 32V supply and crowbar circuit across the mains. I think I'll just replace the TIP50 with a 100k/4W resistor, should provide enough current for start up, and provide one less low impedance failure path.

author: tautech
author_rank: Super Contributor
timestamp: April 29, 2023, 07:18:23 am
content: Following with interest.Do like Altuim layouts.When is the magic smoke test forecast to happen ?

author: moffy
author_rank: Super Contributor
timestamp: April 29, 2023, 08:02:48 am
content: Sometime soon, need to get the resistors, but also need to test some more components to make sure I've got all the baddies. I'll start by putting 12V across the TL494 to make sure it is operational. I'll also make my alternate supply, got the parts and have laid out the PCB, just need to print, etch and drill. The layout, I must admit, took a little while as the original schematic is a little confusing, but then they had to fit much more on a single sheet, much harder to do.

author: tautech
author_rank: Super Contributor
timestamp: April 29, 2023, 08:10:16 am
content: Yep externally powering TL494 is very wise and then scope its outputbeforeapplying any mains.You did a quite good layout as they are sometimes tricky to get looking logical.The only thing I might add is to adhere more to the left to right rules for input > output, meaning the 32V rail Net label would be better on the right to match the original.

author: moffy
author_rank: Super Contributor
timestamp: April 29, 2023, 08:16:13 am
content: Quote from: tautech on April 29, 2023, 08:10:16 amGood point, I'll move it across.

author: moffy
author_rank: Super Contributor
timestamp: April 29, 2023, 09:00:45 am
content: Here is the left to right corrected as well as the transformer.

author: moffy
author_rank: Super Contributor
timestamp: April 30, 2023, 06:36:18 am
content: Well, I put 12V across the TL494 and it oscillated and produced full width pulses to drive the IRF830. Looks like that is working.

author: moffy
author_rank: Super Contributor
timestamp: May 02, 2023, 12:58:25 am
content: I have completed the new regulator, and have done some preliminary testing and it seems to work as expected. For 32V out the minimum in is about 35.5V, and for a 300ma load I was unable to measure the voltage change, less than 10mv. The TIP122 doesn't have a heatsink fitted yet, and the fuse holder is not available yet, so I put a link in instead.I have the parts to modify the original supply of the BWD881, I think I'll knock up a 100V supply, have an old Marantz transformer, and use that to test the original supply.

author: tautech
author_rank: Super Contributor
timestamp: May 02, 2023, 02:12:44 am
content: No DBT ?

author: moffy
author_rank: Super Contributor
timestamp: May 02, 2023, 02:21:09 am
content: Quote from: tautech on May 02, 2023, 02:12:44 amNever heard the term "DBT" before, what I would like though is a variac, but I don't have one of those.P.S. Dielectric Breakdown Tester? Best I can come up with.

author: tautech
author_rank: Super Contributor
timestamp: May 02, 2023, 06:25:01 am
content: Quote from: moffy on May 02, 2023, 02:21:09 amDim Bulb Tester.

author: moffy
author_rank: Super Contributor
timestamp: May 02, 2023, 08:41:04 am
content: Quote from: tautech on May 02, 2023, 06:25:01 amThen I need to go out and buy an incandescent bulb, what wattage would you recommend? Certainly cheaper than a variac.

author: tautech
author_rank: Super Contributor
timestamp: May 02, 2023, 09:09:28 am
content: Typically you want a least a couple to cover most needs, 60 and 100W and maybe 15 and 40W too.Shit simple to make if you have a junk pile like I do.Just apply some thought to the current draw from the DUT to help you select the best one to suits your needs and recommend you experiment with it on a few subjects to know how it should behave with a working device before using it in anger.Should it shine the lamp bright and it not quickly dim, well....Houston we have a problem !Pic grabbed from an old thread of the one I knocked together:Post:https://www.eevblog.com/forum/repair/suggested-for-a-sticky-part-one-comments-or-additions-please/msg470686/#msg470686

author: moffy
author_rank: Super Contributor
timestamp: May 04, 2023, 07:12:09 am
content: Project for the day, as well as hanging out the washing, a DBT. Incandescents are a bit hard to find, so I had to go with both a bayonet and edison to cover the bulbs I found. Wooden box from the Reject Shop and the dual power point is just what I had. The fittings are wired in parallel, but I'll need to get some screws to fit them. Looking forward to trying it out.

author: moffy
author_rank: Super Contributor
timestamp: May 07, 2023, 11:37:44 pm
content: I have finished my DBT, but still cannot test the BWD881 as I chose the wrong values for the mains startup resistors. I chose the values based upon the original scope schematic which showed two 47k resistors in series. After fitting my 4x100k/1W to make a 100k/4W resistor, I decided to test how much current was needed for startup. Seems it needs around 5ma, so the current resistor value will not work. Have ordered some metal film 100k/2W, rated for 500V and when they arrive I will give the new value 50k/4W a try.I have also thought of a way of lowering the dropout voltage of the new mains supply by about a volt, which might be handy. The LED drops about 1.8V which when put on the positive side increases the dropout voltage by that amount. But if I put the LED current source on the negative side and use a current mirror on the positive side I gain almost a volt in lowered drop out voltage for the regulator.P.S. The screw type bulb isn't tight, only the bayonet bulb is currently in series.

author: tautech
author_rank: Super Contributor
timestamp: May 08, 2023, 12:24:37 am
content: If using a 2 lamp DBT, bulbs need be in parallel but in series on the supply rail.Connect them in series to the mains and it becomes obvious why.

author: moffy
author_rank: Super Contributor
timestamp: May 08, 2023, 12:37:06 am
content: They are in parallel, otherwise you need both connected to get any voltage.As mentioned previously I had to go for two styles of fittings, bayonet and screw, because of the limited choices in filament bulbs. But it doesn't hurt to be reminded.

author: moffy
author_rank: Super Contributor
timestamp: May 11, 2023, 06:03:14 am
content: I modified my new regulator to use an Infra Red LED for LD1 and changed R1 to 100R, didn't have a 120R at hand. The supply now has a dropout voltage of 2.2V@300ma, so for 32V out it needs 34.2V in, which is reasonable for an NPN darlington.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 11, 2023, 01:39:35 pm
content: I once had an 400MHz scope (I think Philips PM3295A) with worn out and defective SMPS power supplies, so I replaced them with two small toroidal transformers. After my "repair" the scope traces were very wobbly. The likely cause was that even the toroidal transformers had too much magnetic leakage, and this interfered with the CRT. A short while later something else in the scope gave out, and it was a too complex beast to attempt to repair (even a copy of the service manual was like EUR140 because it was so thick) and I ended up scrapping it.

author: moffy
author_rank: Super Contributor
timestamp: May 11, 2023, 10:49:37 pm
content: Quote from: Doctorandus_P on May 11, 2023, 01:39:35 pmThat is sad, and a 400MHz scope to boot. I have noticed that in the BWD881 all the power supply and mains are behind a full length steel panel, for the reasons you stated to keep the wobblys out of the traces. Your sharing your experience reinforces my need to keep any PSU mods behind that panel, thanks.

author: moffy
author_rank: Super Contributor
timestamp: May 12, 2023, 05:45:32 am
content: Finally received the 100k/2W resistors today, fitted two to make a 50k/4W startup resistor to the BWD881 PSU. Tried out my DBT(Dim Bulb Tester) for the first time in series with the BWD881. I disconnected the rest of the oscilloscope and put a dummy 300ma load across the 32V main supply, measured 32.8V. As you can see from the photo it is working. The switch on of the PSU was intermittent with the 54W bulb, but was reliable with the 100W bulb, the 54W must drop too much voltage for a reliable start up. A nice day and beautiful weather as well for winter.

author: tautech
author_rank: Super Contributor
timestamp: May 12, 2023, 06:42:27 am
content: Yes you must make a best guess as to what wattage bulbs to use although a proper power up requires more than the basic DBT test.Typically if the DBT glows bright right from power ON and doesn't then dim then something's not right.Normally I'll just use mine to check it dims after transformer magnifying current or SMPS cap charging and if all is well connect the DUT directly to mains.Earlier I did suggest you get a feel for how various working appliances light the DBT so to get an idea of what to expect.Anyways, good progress.

author: moffy
author_rank: Super Contributor
timestamp: May 12, 2023, 07:13:13 am
content: Thanks for the advice, every piece of test gear even as simple as the DBT needs a little experience, mine is just beginning.

author: moffy
author_rank: Super Contributor
timestamp: May 13, 2023, 12:55:33 am
content: It lives! The scope is working but the startup resistor value seems low as it will only start on every second switch on, first must charge the caps. I get it now, the startup supply takes a while to get up to voltage because of the large caps on the 32V supply.This means the startup resistors need to supply power for a while before the flyback can supply power to the TL494. Might need to go back to the original design, I'll try an extra 100k/2W in parallel first.

author: tautech
author_rank: Super Contributor
timestamp: May 13, 2023, 01:01:18 am
content: Quote from: moffy on May 13, 2023, 12:55:33 amNice but is the DBT tester still in series with the mains and if so you need feed the scope directly from mains and check startup then.

author: moffy
author_rank: Super Contributor
timestamp: May 13, 2023, 01:28:13 am
content: I tried it direct from the mains, same issue. I am getting a little suspicious that there is still a gremlin lurking in the flyback supply.

author: moffy
author_rank: Super Contributor
timestamp: May 14, 2023, 01:34:03 am
content: The scope runs well off my home made 32V regulator, which is fed from a mains powered 36V PSU I purchased from Digikey. The main job of the 32V regulator, is to drop the 36V and reduce the switching noise from the mains switchmode PSU. Seems to work well, no noticeable increase of noise on the traces.

author: tautech
author_rank: Super Contributor
timestamp: May 14, 2023, 01:45:12 am
content: Powering ON reliably now ?

author: moffy
author_rank: Super Contributor
timestamp: May 14, 2023, 02:26:31 am
content: Quote from: tautech on May 14, 2023, 01:45:12 amOnly with the external supply, the internal flyback makes me nervous.

author: tautech
author_rank: Super Contributor
timestamp: May 14, 2023, 03:16:56 am
content: Quote from: moffy on May 14, 2023, 02:26:31 amMaybe some study of the power up behavior of TL494 can point to why it's acting up.

author: moffy
author_rank: Super Contributor
timestamp: May 14, 2023, 05:44:28 am
content: Quote from: tautech on May 14, 2023, 03:16:56 amIt's not really the TL494 that is the problem. The flyback has an UVLO(under voltage lockout) circuit made of a couple of transistors and a zener. When the voltage reaches a threshold it turns the supply to the TL494 on, and the TL494 needs to run off the stored cap voltage until the flyback supply winding gets up to voltage and starts feeding the TL494. But the flyback supply has large resonances on the windings caused by the 200uH load inductors and couple of thousands of microfarads load capacitance. So this flyback supply looks a little unstable, and looks like it could easily overshoot sometimes and trigger the crowbar. You can't snub it either as the frequency of the resonancy is about 4 times the switching frequency, they are almost running the flyback in a semi resonant mode. The whole startup of the flyback is quite slow, no doubt due to the large external caps it has to charge. The UVLO circuit can also get stuck sometimes where it is partially turned on, it doesn't have a very sharp transition. That could be solved by more current on startup, go back to the TIP50, but that then provides a low impedance path for failure across the mains. The use of an external supply and my regulator bypasses all these issues and I think would be safer.

author: tautech
author_rank: Super Contributor
timestamp: May 14, 2023, 05:55:31 am
content: Could those big caps be tired with high ESR ? Did you test them ?

author: moffy
author_rank: Super Contributor
timestamp: May 14, 2023, 06:00:48 am
content: Quote from: tautech on May 14, 2023, 05:55:31 amThat is a possibility.

author: tautech
author_rank: Super Contributor
timestamp: May 14, 2023, 07:25:26 am
content: Quote from: moffy on May 14, 2023, 06:00:48 amNow kick me if you like as when thinking about SMPS over dinner I'd forgotten to mention I've often found the SMPS Vcc rail caps to be sick which is why you need to double kick it to start. Pins 12 Vcc+ and Vcc- 1,7 and 13.You should carefully check those associated with the flyback supply for TL494 and the divider chain from HV DC.330uF C802? and even the DC restorer diode should be checked in case one or both are leaky.10uF C804 should go too as seen too many of these low value caps associated with SMPS give trouble.Edit to add TL494 datasheet link:https://www.ti.com/lit/ds/symlink/tl494.pdf

author: moffy
author_rank: Super Contributor
timestamp: May 14, 2023, 08:42:32 am
content: I really appreciate your advice. I changed C802 for a 470u, just in case. The diode tested fine, but haven't checked the tantalums, but I am really leaning to completely bypassing the flyback as there would be no extra cost and very little effort to achieve what I consider a more reliable solution. It would save time, effort, cost and provide a better result.

author: tautech
author_rank: Super Contributor
timestamp: May 14, 2023, 09:51:49 am
content: Sure but you sacrifice Line coupling.Let's look harder at the TL494 supply.Startup current is via R801, 802 and the 5V6 vener referenced Q803 and where we should focus study as it seems the run flyback is doing its job. Although it's arguable startup current path includes R805 and R808 also.All those components I would check especially if the resistors are old carbon types.I've seen HV dropper resistors drift or go open resulting in no SMPS start and the Vcc rail caps so leaky or out of spec they also inhibit startup.We know once started and providing 32V it's stable but getting reliable startup seems the only issue but really how many start related components are there to check or even shotgun the lot with new components.....but it's on your bench not mine.

author: moffy
author_rank: Super Contributor
timestamp: May 14, 2023, 12:12:32 pm
content: Quote from: tautech on May 14, 2023, 09:51:49 amI will still include line coupling, the mains filter fuses, switch etc. will feed into the new 240/36V SMPS and I'll just disconnect the flyback from the mains. R801 and R802 have been replaced with 100k/2W resistors now in parallel, they provide the startup current, the circuit I am working on, redrawn, and posted previously is quite dissimilar to the standard schematic. The TIP50 and 15V zener were removed as they failed, that is the low impedance path to the crowbar, the resistor in series with the TIP50 also failed as a low value. The UVLO circuit works fine i.e. the 5v6 zener etc. but doesn't have a very sharp transition, so it is possible for it to partially turn on and just stay there. This is because I am using startup resistors rather than the TIP50 which has its own issues i.e. a low impedance failure mode across the mains. I could try continuing to fix the present flyback, or fit what I think is a safer solution which I have already built and tested. It's an old scope with issues, some of the switches, so I think I'll choose what I think is the better and less costly solution. If I went back to the flyback as the supply I think I would be wincing every time I turned it on, I wouldn't be very confident in it. That is a real problem as a designer, you always want to improve the circuit not just fix it.