Oscilloscope CRT problem

[Toasty]

Re: Oscilloscope CRT problem

I'll state the obvious - Google then eBay. You may end up finding a unit with a problem electronically (storage function) that has a pristine tube. I see them going from ~$50 and up.

[lawmann]

Re: Oscilloscope CRT problem

Hi, I'm new here. I really enjoyed reading this thread, lots of obvious experience and insight here. I've recently got back into electronic repair and you all have really got the brain cells working again.

I have a Hitachi V-650 (which is my current shop workhorse), a Tektronics 468/DM44, and a HP 1741A. The Tek 468 and HP 1741A are my current projects. The HP has significant burn in of the CRT and I need a new 5083-5070 CRT...anyone know a source for one? Or know of a suitable substitute?

Thanks again for the thread!

[etnietering]

Re: Oscilloscope CRT problem

Sorry it took me so long to get back to this: Mine didn't come with a manual. I used the same online one that stj posted a link to. Also, I found a schematic online here that is to a very similar scope, and as far as the electronics go they seemed identical. Good luck!

[stj]

Re: Oscilloscope CRT problem

this is a good start.

[electrofe]

Re: Oscilloscope CRT problem

Hello:
I`m needed 2120 Service Manual, help me please. Thank you
Oscar

[Monolith222]

Re: Oscilloscope CRT problem

Too bad the thread has gone asleep, it is very interesting.

Did the BK2120b eventually function again?

I have the same Oscilloscope that has developed a Horizontal Control problem.
I also have the schematic, but really need some help defining the problem spot.

Monolith222

[barry wilkins]

Re: Oscilloscope CRT problem

If you dont have a HV meter then it would be difficult to measure -12KV .
I have a probe from my TV days ,an earth clip and an insulated probe with a meter ,it reads up to 30KV .Also have a HV probe to be used with a standard Fluke DVM .
And I have 10 to 1 divider I made to measure up to 10KV with a dvm ,to measure PMT supplies.
When I worked for Marconi ,we had HV dividers .You can buy HV resistors from Maplin
So it would not be difficult to make a quick divider 1M +47M+47M would be close to 100 to 1. As long as you are careful with unsulation.

Barry Wilkins

[ratdude747]

Re: Oscilloscope CRT problem

i meant it as a joke.

[Toasty]

Re: Oscilloscope CRT problem

>>Oh man I get it (2) 25V caps are in series. Are you fricking kidding me?<<
&
>>The problem here is if one cap is slightly leaky it will transfer it's voltage to the other cap which could exceed its voltage destroying it in a cascading fashion. This is why this setup annoys me.<<

hmmmm... not really. Look at the ground connection. One cap filters the +12v & +5v, the other the -12v. The center tap on that winding is ground too, so only about 15v presented to each cap.

Toast

[Toasty]

Re: Oscilloscope CRT problem

>>im thinking of perhaps doing one of these:<<

By hijacking this thread?

Krankshaft is known to digress...

Best to start a thread on what you'd like to do. It's easier to keep things clear for everyone who is contributing to the threads' subject matter and for the OP to follow.

It's also polite.

Toast

[i4004]

Re: Oscilloscope CRT problem

^^
i thought about something simillar before i got scope.

but all i've found was a way to mod the zx spectrum to do scope.
imagine what sort of scope would that be.....hihi...
even worse than that one on telly!

[ratdude747]

Re: Oscilloscope CRT problem

im thinking of perhaps doing one of these:

[Krankshaft]

Re: Oscilloscope CRT problem

Dang it I'm highjacking the thread sorry about that.

In summary check the 140V source (C904), +12V (C906), -12V (C908), and +5V (C907) right on the output filters after the regulators. Measure right across the output caps they're easy enough to find.

Are they all normal?

[i4004]

Re: Oscilloscope CRT problem

>That BK Precision scope looks an awful lot like the one CRT model they still sell at Fry's for $389 or so.

yeah, it seems they're stil making 2120b model.
http://www.bkprecision.com/products/...s/?model=2120B


>Does "It lets you see what the electricity is doing" sound like an OK answer?

quite right.
you can also do "how electricity looks inside the wire"..hehe...

scope-clock
http://www.dutchtronix.com/ScopeClock.htm
it uses a freakin mcu to put clock on scope display...
talking about idle mind and devil's playground...hehe...

another one
http://www.franktechniek.nl/Kits/osc...scopeclock.htm

>It also has automatic screen repositioning so you don't need to worry about burnin.

clock should be on all the time, so worn cathode is thing to worry...hehe...
i wonder how many days could it do....hm...
i wonder can that tube do 3k hours...

[Krankshaft]

Re: Oscilloscope CRT problem

Yeah the circuit is anything but simple.

It's pretty advanced actually with an OSD to configure the clock and how it looks.

http://www.dutchtronix.com/ScopeCloc...1-Standard.htm

It consists of a Clock IC PCF8563 a Digital / Analog Converter AD7302, and a microcontroller to tie it all together so yeah this advanced version is a bit more complex. It even has a battery to keep the time just like a computer.

It also has automatic screen repositioning so you don't need to worry about burnin.

If it looks cool then it's probably complicated .

[KeriJane]

Re: Oscilloscope CRT problem

Hi Krankshaft

That's a really neat trick... to put a clock on the scope.

That sounds like a kinda complicated "circuit" to me.
Wouldn't it involve making the scope trace out a raster pattern?
Is some kind of video IC involved?


Tried to build a digital clock project once that was supposed to go bing-bong-bing-bong. But it didn't.
Keri

[Krankshaft]

Re: Oscilloscope CRT problem

The problem here is if one cap is slightly leaky it will transfer it's voltage to the other cap which could exceed its voltage destroying it in a cascading fashion. This is why this setup annoys me.

Also on a related note there is a POT for HV adjust (VR963) on the scope by messing with POTs randomly you may have inadvertently compenated for a sagging supply voltage from the main transformer.

[Krankshaft]

Re: Oscilloscope CRT problem

Ok let me break down the PSU circuit for you the main transformer not the HV one.

First since you have no ESR meter I recommend you replace ALL electrolytic caps in the unit but for the time being just try to replace the ones I circled in red.

Now onto the circuit breakdown off the unregulated 140 volt winding (used by the CRT) it's bridge rectified and filtered by C901. Its regulated by a discretely built voltage regulator comprising Q901 and U904 this produces a regulated 140 volts.

The +12V, +5V, and -12V supplies are fed off the same winding and rectifier.

The transformers 32 volts output is rectified and filtered by C902 and C903.

The +12V source is further filtered by C906 after the regulator. The -12V source is filtered by C908 and the +5V source by C907.

They use IC based regulators here since they're not in weird voltages like the 140V source.

A thing that gets me though is that the winding should be putting out 32 volts.

At center tap according to the schematic you have 16 and 16. To use a bridge rectifier you need to use both ends of the secondary winding to use the center tap you would go with a full wave 2 diode rectifier.

Looking at the input cap it's only rated for 25 volts when unregulated the transformer can put of 32 volts !

Oh man I get it (2) 25V caps are in series. Are you fricking kidding me?

This is just sloppy as hell. 2 caps in series are the sum of their working voltages so 25+25= 50V rating.

[Krankshaft]

Re: Oscilloscope CRT problem

An oscilloscope CRT is just a graph illustrating voltage in the x-axis and time in the y-axis. It graphically represents voltage over time.

I think that's a simple enough answer.

But you can also drive the X and Y deflection amps independently and do some pretty cool things like the oscilloscope clock.

http://www.coolcircuit.com/gadgets/w...cope-clock.jpg

That's not my scope in the pic but I have the same exact model and have built this circuit.

[KeriJane]

Re: Oscilloscope CRT problem

Hi Etnietering

Cool! you got an actual working scope AND an education about what makes it go.

That's almost like the really old ads in my Dad's really old electronics magazines.... "Build your own Oscilloscope and Earn while you Learn!"
I always wanted to get one of those but they were before my time.

That BK Precision scope looks an awful lot like the one CRT model they still sell at Fry's for $389 or so.


Astounded at the talent here,
Keri

PS. Someone once asked me what a scope does at Fry's once. He caught me setting a slow 2-channel sweep on it and wanted to know what those lines were.
Does "It lets you see what the electricity is doing" sound like an OK answer?