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National VP-5220A Repair Help Please.
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PostPosted: Fri Sep 26, 2008 9:51 am    Post subject: National VP-5220A Repair Help Please. Reply with quote

I've just recently been given a National VP-5220A oscilloscope. I was awfully excited as I've always felt the need for one but never stood the chance of affording one. I can't get it working though. I was told by the man who gave it to me that it had worked when he first turned it on then went off. I have found that when I switch it on I get a trace jump across the screen (sometimes wiggle across depending on settings) then I get nothing. The power light is on and the Scale illumination control works. I was wondering if anyone could give me some advice on getting it working. I'm willing to read and put effort in. I don't have a manual or anything like that for it.

Here's what I have done so far...

I opened it up and checked for obvious burnt out parts or loose wires etc.

I then checked the fuse? (silly really considering the light etc was working ) Then broke the fuse holder by over tightening the screw.

The above point meant today I opened it up and had to dismantle it. Wasn't sure about discharging caps so I did it anyway (through a 1k 5watt resistor) as some had 3kV on them so wasn't going to take any chances. After that I could poke about and replaced the fuse holder and remade a couple of dubious connections. I also cleaned up a little of the dust and used switch cleaner on all the pots and switches I could get to.

After all that It's back to where it was except now the trace jumps right across the screen but only once. So barely any difference to before; Probably none (just my wishful thinking!).

In case it helps the scope is a CRO and is twin channel and 20MHz.

Any ideas? I hope there aren't too many questions. I'm just keen to learn about scopes which will hopefully result in getting this thing working. Thank you for taking your time to read this.
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PostPosted: Fri Sep 26, 2008 11:24 pm    Post subject: Reply with quote

There really are so many variables to even attempt to guess at the problem... the guy who gave it to you knows what a oscilloscope is?

I asked this because I was given one and the guy couldn't get a picture on .... A HP 150Mhz digital storage scope and it worked OK - his words not mine "I tried plugging the TV aerial and just gets a wiggly line", I assumed he thought it was some super dooper TV receiver for DXing.

Maybe you ought to try applying a signal, a sine wave say to see what happens, check and see if it has a 'beam finder' or auto blanking etc... any faults like this could well be a power supply problem, a blanking problem or some tube biasing component, best bet is to try and get a manual.

I keep waiting for him to come back with a spectrum analyzer - no such luck Very Happy ah well, maybe tomorrow.
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Joined: 30 Oct 2007
Posts: 85
Location: Nr. Manchester (UK)

PostPosted: Fri Sep 26, 2008 11:27 pm    Post subject: Reply with quote

Sorry dunno whats happening tonight I keep logging out,

Dave (signed in as guest above)
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PostPosted: Sat Sep 27, 2008 9:20 pm    Post subject: Reply with quote

Thank you very much for your reply.

The man who gave me it definitely knows what and oscilloscope is so I shouldn't have to worry about that one. I'll try applying a signal but I'll be surprised if I get anything. Like I say when I turn it on the beam does track across the screen once sometimes but then disappears. How it tracks depends on the time base and trigger settings etc. I have a feeling the main power is O.K. as the power LED comes on.

I have a feeling the H.V. power supply isn't working properly because I don't think there is any H.V. stored on the caps when I've opened it up. I tried shorting a 3KV one instead of the usual discharge through resistor routine to see if I got a spark or anything but absolutely nothing. I don't have a volt meter that will measure high enough voltage to confirm this. The H.V. supply section isn't that big so I'm considering checking various components. The only suspicious looking component is a large value and high power resistor that appears to have a big crack in it. I plan to test this.

Any suggestions for random tests I could do on components in the H.V. section with my 600V volt meter? Transistors etc? (Ideally while they are in circuit.) Just as an important side note these tests must be unplugged because I don't feel I have adequate knowledge or confidence to safely perform live testing.

Come hell or high water I seem unable to obtain a manual for the product. The kind person who gave it to me is hoping to find the service manual he has somewhere, at some point. I just don't know when and if he will find it.

Thank you again.
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Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Mon Sep 29, 2008 7:06 am    Post subject: Reply with quote

If the oscilloscope is from the 70's or even older, there is a possibility that the HV rectifiers are not silicon diodes but so-called dry rectifiers or metal rectifiers (they consist of a pile of thin metal shims that are chemically treated on one side). Those rectifiers are known for aging and sooner or later going bad. If you need to replace them, the HV rectifiers used in microwave ovens will probably be useful.

Also, check if there is a short or high leakage in any of the high voltage capacitors, be particularly suspicious to electrolytics and paper capacitors.

In order to measure higher voltages than your meter can show, you could make a high voltage probe by connecting a suitable resistor in series with it. For example, if the meter's highest voltage range goes to 600 V and you want to measure up to 3000 V (five time the meter's range), then connect a series resistor of four times the meters internal resistance (probably 10 Mohm, you will anyway find information about it in the meter's manual). So, if your meter has 10 Mohm internal resistance, then you need to connect 40 Mohms in front of it. Make sure that the resistors have enough voltage rating.
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PostPosted: Mon Sep 29, 2008 6:08 pm    Post subject: Reply with quote

Thank you for your advice. I have reformed a lot of the solder joints on the HV board to check there were no dry joints. I also removed random caps and checked them for shorts and that they are their correct value. I've just got to work my way through the scope. I think I may have to try replacing suspect components. I'm just checking and checking for bad parts really. Thing is there are hundreds and hundreds or parts and there could be possibly just one fault. Rolling Eyes

I did check out that suspect resistor and it's all good. There are these two funny little glass components that I don't know what they are on the HV board. They look light tiny valves or light bulbs. Just a bit bigger than an LED but smaller than a fairy light. They just look fragile and a suspect to go wrong.

Anyhow I'll keep posting here and I'm open to any ideas, advice and comments. Thanks to everyone who's helped me so far!
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PostPosted: Thu Oct 02, 2008 10:11 pm    Post subject: Reply with quote

I've still not had any luck. Have not been able to try replacing HV caps as I don't have any spares. I did go to Maplin today and get some electrolytic caps so have been replacing the large electrolytics in the power supply. I am still not getting anything. This is really driving me mad now. The trace isn't even flashing across the screen at all when I turn it on now. How would I go about checking the CRT to see if it's still got any life left in it? I'm running out of ideas now. When is it worth giving up? Sad Are there places that one can take an oscciloscope to get it fixed at a small price or is it a very expensive job. ( I guess a lot of it depends on what has broken).
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Joined: 26 Apr 2008
Posts: 45
Location: Kenya

PostPosted: Sat Oct 04, 2008 12:21 pm    Post subject: Reply with quote

Sounds strange to me that initially you could get a sweep across the screen either horizontally or vertically.
That sweep appears only once subject to the feed probe and the settings on the sweep rate, pulse level, volts - micro or mili, trigger level, ac, dc etc.
My scope displays nothing on screen with no probe connected, but unless i peak up illumination I wouldnt know its on till i have a feed, even by thumbing the probe. But without probe connected nothing displays.
What am assuming is your scope could have been functional at first instance but incorrect adjustments on face, or no probe or probe faulty.
Anyway keep trying you might be ok eventually.
confirm that the crt has light at base on elements.
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PostPosted: Sat Oct 04, 2008 9:47 pm    Post subject: Reply with quote

I did try connecting a probe and shorting it. Was that not adequate? Oh no! I hope I haven't killed it. I'm trying not to do any work with the equipment live and the chassis open but I'm O.K. leaving the cover off just to look for light at the base of the CRT. I can do that from a distance. Laughing Am I just looking for a glow just the same as a valve in a vale amp?

Thanks for your comments. I feel I'm running out of ideas to try and it's kind of disheartening that I may have made the situation worse. It would be way easier if I actually had a schematic.
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PostPosted: Sun Oct 05, 2008 11:18 am    Post subject: Reply with quote

Here's the score at the moment. I measured the +12 and -12 volt points leaving the power supply board and going to the board called main board. They were both reading +12V and -12V as they should. This board called main board is quite small and appears to have two identical sections on part of it. From this there are two wires leaving two PCB pads. One called +V and the other -V. These go to the CRT. One wire is purple and the other is white. One goes to pin 7 and the other pin 9. These wires are considerably narrower than the others. On this board as I said above there appear to be two identical sets of components. Part of each set has two 3W resistors (Big blue things with no stripes) I think they said they were about 220ohms. Here's the deal though. The ones associated with the +ve wire going to the CRT get very hot but the negative ones remain cold. I'm gathering the ones associated with the -ve bit are broken because they are heatsunk so I'm gathering they are designed to run hot. The base of the CRT does not glow. Are these two smaller wires the heater?

Does any of the above information sound familiar with a particular problem. I'm beginning to think that there was never anything wrong with the section I've been playing about with for weeks. I did resolder a joint or two on the smaller "Main board" as two (power transistor/Voltage regulator look alike) had come loose.

Just another point for the last week there has been this big hunk of tape that was wrapped around the CRT that had been hanging off and sliding up and down the neck of the CRT. I only realised today that it has two wires attached and is linked to the beam rotation section of one of the boards. I reattached this with tape (hopefully correctly).

Any ideas given the new information?
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