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Repair of a BWD 820 Oscilloscope

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Spuriosity

New Member
I recently managed to acquire a broken BWD 820 CRO for $20 and was planning on fixing it. According to the person I bought it from, it was working fine until the fuse blew in it - now, the fuse blows whenever you replace it and try to turn it back on. Does anyone with experience repairing CROs know a) what the cause of this could be?

UPDATE:
Replacing the quick-blow fuse with a slow-blow type fixed the initial issue. Luckily, there are more issues to deal with!

Once the scope cathode had been allowed to warm up for a while (~30 seconds), with an audible 50Hz hum (presumably from the transformer), I could see a single trace present on the screen. However, it looked very unusual - rather than a thin, green line, faint spots of green were visible. Trying to adjust the position of the trace has no measurable effect. The only controls that I could successfully manipulate were focus and intensity.
Worrying that leaving the system energised could cause further damage, I unplugged the unit at the wall.

Schematic and service guide:
http://www.eserviceinfo.com/downloadsm/67446/BWD_820.html

(I've worked with high voltage before by the way, but this is my first time repairing a CRO)
 

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spec

Well-Known Member
Most Helpful Member
Hello Spuriosity,

Nice to see another Australian on ETO. Which part are your from?

You are the third person from down under that has reported a faulty scope (BW521) http://www.electro-tech-online.com/...-oscilloscope-no-traces-hissing-noise.147684/

Yes, I have had a lot of experience with designing, building and repairing oscilloscopes and there are a load of electronics experts on ETO who will also probably help.

For reference/discussion below is the circuit diagram for the CR820 power supplies.

spec

2016_06_11_ETO_BW828_SCOPE_POWER_SUPPLIES.png
 
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spec

Well-Known Member
Most Helpful Member
U401 and U402 are three terminal +15V and -15V regulators with current and temperature overload protection. The current is limited to around 1.2A. Because of this, unless the regulators are faulty, it would seem that the fault is to the left of the regulators unless the excess current is being taken by R172 (56K) (17V AC), which is unlikely.

An immediate suspect in any power supply is that the reservoir capacitors, in this case C401 and C402, have become leaky or short circuit.


spec
 
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spec

Well-Known Member
Most Helpful Member
You say that the scope is blowing the fuse. By that I assume you mean F401 (100mA for 235V, 200mA for 117V). Have you checked that the transformer is wired for your mains supply which I think is 240V in Australia. Have you checked that the correct value fuse is fitted. I would recommend that a slow-blow type is used. It strikes me that 100mA and 200mA seems like a very low current value.

Just a few general questions:
(1) Do you have have any experience of working on electronic equipment?
(2) Do you have any tools: soldering iron wire cutters, pliers?
(3) Do you have a multimeter?

spec
 

Spuriosity

New Member
Thanks for the detailed reply!
I have little experience working on factory-built electronics, however, I have significant experience in constructing circuits that deal with high voltage.
Sorry for being so nonspecific - you are correct in assuming I mean F401 (i.e. the fuse accessible from the outside of the case).
I picked up the CRO in Australia off a guy that was using it here, so I'm 99% sure it's wired for 240V @50Hz.
I have a reasonably full kit - temp. controlled soldering iron, DMM (only voltage/current/resistance) diagonal cutters, solder etc., and I have a friend with a working CRO that I can use to attempt to diagnose the fault.
I thought it would be prudent to post here before embarking on such a dangerous enterprise without consulting someone that knew what they were doing.
 

spec

Well-Known Member
Most Helpful Member
No sweat

That all sounds good. You obviously have the skills and equipment to fault-find on your scope. The big secret for safety is to unplug the scope and make any measurement connections necessary and then turn the scope on and observe the measurement and then turn it off. On no account touch the scope while it is plugged into the mains. This is a bit laborious but it is the best way at first.

Can you carry out these operations:
(1) Make sure the scope is unplugged. Check that the transformer is wired for 240V by looking at the wiring in the manual.
(2) Ensure the scope is unplugged. Fit a good fuse (of any value)
(3) Ensure the scope is unplugged. With the scope mains switch turned off measure the resistance between live and neutral and between live and earth.
(4) Ensure the scope is unplugged. Turn the scope switch on and note the resistance between live and neutral and between live and earth.
(5) With the scope unplugged from the mains and turned off measure the resistance across C401
(6) With the scope unplugged from the mains and turned off measure the resistance across C402

spec
 

Spuriosity

New Member
No connection between live and neutral, or live and earth with everything switched off.
when switched on, no resistance between live and neutral, no connection between either and earth.
Resistance across both capacitors starts off at zero, then gradually increases to infinity. No visible signs of damage.

Testing all 5 diodes with a diode tester show all 5 are good.

What the hell?
 
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Spuriosity

New Member
Not sure what this site's rules on necro-posting are, but I'm just under 6 months so I hope it's alright.
Anyway, I've finally diagnosed (part of) the fault - the fuse installed was a quick-blow type, presumably being tripped by the initial rush of current to charge the power supply capacitors. I've since installed a slow-blow type, and the unit (amazingly) switches on!
Now onto the deeper issues.
Once the scope cathode had been allowed to warm up for a while (~30 seconds), with an audible 50Hz hum (presumably from the transformer), I could see a single trace present on the screen. However, it looked very unusual - rather than a thin, green line, faint spots of green were visible. Trying to adjust the position of the trace has no measurable effect. The only controls that I could successfully manipulate were focus and intensity.
Worrying that leaving the system energised could cause further damage, I unplugged the unit at the wall.

Any advice on what measurements to take?
 

spec

Well-Known Member
Most Helpful Member
No probs about necro posting as far as I know.

The buzzing transformer and other faults do indeed sound like they would damage the scope further, so I would avoid turning the scope on again until we can have a think about the best way forward.:)

spec
 

JimB

Super Moderator
Most Helpful Member
Not sure what this site's rules on necro-posting are, but I'm just under 6 months so I hope it's alright.
It is your thread, you are just continuing the discussion after a bit of a break.
No problem.

JimB
 

cowboybob

Well-Known Member
Most Helpful Member
Classic issue with old CROs is dust, especially around the High V anode area of the CRT. This can easily draw off power and create the symptoms you describe.

This area is also very dangerous, in that upwards of 24kV can be present.

So take care!

With the unit OFF, but plugged into the mains (and grounded). very carefully inspect (but don't touch) the HV area of the CRT (including the cable from the flyback transformer and its environs) for dust and possible arcing paths. Using a keyboard type "duster" (pressurized air), thoroughly clear those areas.

Try powering up again and see if anything improves (even if only slightly).

Again, be very, very careful.
 
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