Oscilloscope CRT problem

[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

[ratdude747]

Re: Oscilloscope CRT problem

i meant it as a joke.

[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

[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

[electrofe]

Re: Oscilloscope CRT problem

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

[stj]

Re: Oscilloscope CRT problem

this is a good start.

[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!

[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!

[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.

[LightySA]

Re: Oscilloscope CRT problem

Hi All

I'm new to the forum and wonder if you could please help.

I have the same oscilloscope although re-branded as "Toptronic" that is dead, with very similar faults. (mine is the 20mhz version)

I had used the scope quite a bit, it then went on the blink and I managed to get it working by replacing U950 (741) and the D880 for the HV transformer. it worked for a while before dying again. This time after replace the same components it hasn't helped. I've also noticed that it started to arc on the board near the output from the HV transformer, I cleaned it up and sealed the board.

After changing all the components I find that the D880 (HV transformer driver) gets extremely hot, to a point where it actually melts the insulating washer. I assume that the D880 is not oscillolating?

The CRT has a very slight orange glow in a dark room, but I think this could be the heater? as it even glows with the D880 (for the HV transformer) removed off the board.

Any Help would be greatly appreciated! as I don't have money for another scope.

I've also tried to download the schematic from the previous posts, but comes up with an error. I've managed to find the user manual, but it only has a block diagram at the end.

Thanks in advance!

Justin

[Agent24]

Re: Oscilloscope CRT problem

It might help if you can post some photos of yours in case there is something you have missed or such. Hopefully someone else can help you as I know nothing of this!

Might help too if you start your own thread, with a better title and include photos. That way it is likely to get more attention.

[LightySA]

Re: Oscilloscope CRT problem

Thanks Agent24

Thought I would just use the same thread as alot of the info in this thread is relevent as its the same "design" and problem. If I don't get any response I'll take your advice and start a new thread!

Thanks again

[LightySA]

Re: Oscilloscope CRT problem

Ok, so it appear the transformer is toast!

So now to try have have it re-wound.

[monpirri]

Re: Oscilloscope CRT problem

Well i had the same problem with a 2120 ,now is a door stopper.
The HV output transformer (CVT T502 :063-016-9-001) got shorted and then it lost its 2kv volts.
Re-wound didn't work as secondary has a very special isolated wire very hard to find .

I sent an email to B&K and they don't give a supplier nor replacement.

Guess the best choice is to find a non working one as a source of parts .

[LightySA]

Re: Oscilloscope CRT problem

Hi monpirri

Must be a common thing with these units then.

Could you elaberate a bit more on your attempt to rewind it! as this is what I was planning on doing!

As it appear the transformer only supply the HV to the rest of the circuit and in not cotrolled by any sort of time from the rest of the system, could one not use a small flyback tranformer from a small 5" B&W TV or something? it could be simply driven from a seperate driver? I'm desperate

Would you perhaps have the complete high resolution schematic? I've tried EVERYWHERE to find it?

Thanks

[budm]

Re: Oscilloscope CRT problem

The SCH that MONPIRRI shows is a simple Self Oscillating with the OP Amp driving it to regulate the 2kvdc output, the rectifier section is the Voltage Doubler circuits, so the output voltage from the secondary may be about 700VAC, once it goes through the Voltage doubler circuits and charge up the filter cap to the peak voltage, you will get around 2KVDC output, I estimate the output current may be up to 5ma. You can use the flyback transfromer but you need to regulate the output using the circuits to do that.

[tom66]

Re: Oscilloscope CRT problem

Heh, B&K seems to be a cheap re-branding company, judging from the build quality and use of cheap caps.

[LightySA]

Re: Oscilloscope CRT problem

here is the schematic of the exact power unit. I would imagine the same would apply for the "voltage doubler"?

So if I turned a new transformer I would need to calculate its output to be about 700V, perhaps 800V for safety, as the voltage reg system should drop it if required?

Is a TV flyback not an option?

[Agent24]

Re: Oscilloscope CRT problem

A TV Flyback would be far too high voltage wouldn't it? That's even if you got an old one without the tripler inbuilt.

[budm]

Re: Oscilloscope CRT problem

You just have to drop the driving voltage on the primary winding.