Issue with LBO-501 Oscilloscope

Thread Starter

GeorgeTR

Joined Feb 3, 2018
35
A few months ago, I bought an Leader LBO-501 oscilloscope at a surplus store. It worked when I bought it, but it needed quite a bit of calibration and cleaning. After a lot of work, I got it looking and working almost as good as new.

However, recently, while I was using it, for no apparent reason, it stopped working. Upon opening it up, I saw that the main fuse was blown. This was a 1 amp time delay fuse. I went to my local auto parts store, and all they had were fast acting fuses, but I bought some anyway. When I put a new fuse in, it instantly blew when I turned the oscilloscope on. So I bypassed the fuse holder with a piece of wire, and used a clamp digital ammeter to measure the current flowing through it. When I turned the oscilloscope on (very carefully), I saw that there was a fairly constant 5 amps going through the wire. Looking at the front of the oscilloscope, it worked, but only sort of. A dot appeared on the front of the CRT, but it was a ghostly dot, not a sharp one, and it soon flew off to the left rather than forming into a horizontal line like it should have.

I have attached the schematic in two versions, one as it originally was, and another copy I have that has notes from the previous owner, some indicating changes he or she made. It would be great if someone could either let me know what they think the problem is, or let me know what my next steps should be for diagnosing the problem. I would be happy to try to provide any information people might like about the oscilloscope, or try to take any readings that might help diagnose the problem. I have access to another oscilloscope, so I would be happy to use that if people think that would be helpful.

Also, please don't tell me that there are oscilloscopes that are much newer and have many more features available for reasonable prices, and I should just get one of those. I am fully aware of this; the main reason I like this oscilloscope and want to try to fix it is because I like the way it looks, and I already put in a fair bit of time restoring it.

Thanks,
GeorgeTRpano 1 small.jpg
pano 2 small.jpg
 

Thread Starter

GeorgeTR

Joined Feb 3, 2018
35
I'm not sure how you intended for me to check for shorts, but I looked around visually, and I could not see anything that might be causing a short. The difficult thing about this is that there isn't just one power supply circuit, there are many, any one of which might be causing the problem. I decided to start at the transformer, and see what circuit coming off the transformer was drawing so much current, so that I could narrow down the problem. I used my clamp multimeter to test the current being drawn by each circuit, but it soon seemed like this was not a good method, since the transformer being so close by made the multimeter not give accurate readings. Even when there wasn't a wire within the clamp, the multimeter claimed current was flowing through.

So I changed my approach, and disconnected wires one by one, seeing if the current would drop to a normal value. But while I was doing this, I smelled a strange smell, so I quickly disconnected the power. I was concerned that I had damaged the transformer, so I disconnected all of the load wires from the transformer, and reapplied power. Thankfully, the transformer seemed fine, and only drew a minimal amount of current. I still am not 100% sure it works correctly, but hopefully it's fine.

Then I decided to measure the resistance with my multimeter of each the circuits I had just disconnected. One of them had a resistance of only 1.4 ohms, and I thought I might be on to something, but then I realized that that circuit powered the heaters for several of the tubes, so that actually seems reasonable. I ended up testing all the circuits, and the only ones with low resistances were ones that powered tube heaters, so I did not find anything I would consider a short. However, I'm not sure this method would necessarily detect the kind of short AlbertHall was referring to.

All in all, the oscilloscope is a mess, completely taken apart, and even putting it back together to the state it was previously in would be quite the task. I really don't know how to procede from here, so any help would be much appreciated.
 

AlbertHall

Joined Jun 4, 2014
10,522
In the diagram, the mains transformer is shown on the right hand side half way down. Immediately to the right of transformer there are two black boxes around groups of components and within these boxes there are 12 diodes. Connect your multimeter on a low ohms range across each of these diodes for shorts (except D112 and D116 - these will noy cause your problem).

[EDIT] " Immediately to the right of transformer..." => Immediately to the LEFT of transformer...
 
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Thread Starter

GeorgeTR

Joined Feb 3, 2018
35
Thanks very much for the help! I tested all of the diodes, and none of them seemed to have a short, but I did notice something peculiar when I examined the T-257B board, which is the high voltage rectifier. R122 through R124 were missing, and someone had replaced D104 and D105 with a single diode, and D106 and D107 also with a single diode. These diodes did not have a short, but I decided to also test all of the diodes with the diode mode on my multimeter. All the other diodes gave reasonable readings of about 0.7V, but these two diodes gave no readings at all. I even soldered wires onto one of them to make sure I had a good connection, and I still got no reading.

Could this be causing the problem? It seems peculiar that both the diodes would be exhibiting this problem, and also that appearing to not let current through would cause too much current to flow through. But maybe they're performing differently under high voltage, I don't know, and maybe one failing caused the other to fail. And since this is a place where the circuit was obviously modified, it seems like a probable location for something to have gone wrong.

If this is the problem, how should I go about replacing the diodes? Should I return things to the way they were in the original circuit, or should I just replace the two existing diodes? And in either case, what specs should my replacement diodes have?
 

AlbertHall

Joined Jun 4, 2014
10,522
They will presumably be high voltage diodes and inside may be multiple diodes in series so they wouldn't read on your meter as it won't proide enough voltage to turn them on. I doubt they are causing your excess current problem.
Next, check the capacitors that all those diodes feed for shorts.
 

Thread Starter

GeorgeTR

Joined Feb 3, 2018
35
It turns out that when I was testing the diodes, one of them had a particularly low voltage when I tested it in diode mode on my multimeter. I hadn't gotten any reading on the ohms setting though, so I assumed the reading was due to some surrounding circuitry. But I decided to double check, and low and behold, it was hard to get a good connection, but when I did, I got a reading of about 25ohms, so definitely not good. I removed it from the PCB and got the same result, so definitely no external circuitry was the problem (and looking at the schematic, there wouldn't have been any anyway).

The diode is either D108 or D109, I don't know which (I don't think it really matters). The other diode of the pair seems like it was replaced, so it makes sense that this would be the issue.

What specs should the replacement diode be? Would a 1N4004, since I have some on hand, work?

Also, I don't think of diodes as components that often fail, but in this case one has. How likely would this be to happen again? Would it be worth my time to replace all of the rectifier diodes?
 

Thread Starter

GeorgeTR

Joined Feb 3, 2018
35
The diode in question says its a Sanyo DS-16A. The only reference I could find to this is this page, which seems to be an ad for an equivalent diode. However, it seems to give conflicting information as to whether the diode is for 1A or 2.5A. Any ideas?
 

Thread Starter

GeorgeTR

Joined Feb 3, 2018
35
Great, thanks! I presume the 1N4004 should work fine then. Or would that be cutting things close on the voltage? The 1N4004 is rated for 400V, and it seems like in this place in the circuit it would be experiencing 300V.

Also, any thoughts on replacing the rest of the diodes?
 

SamR

Joined Mar 19, 2019
2,753
What I saw was 1300V repetitive peak and 1600V single peak for the BY 133 which is the "ersatztyp" which I am translating as "suitable substitute". The 1N400X series tops out at 1KV. This is a HI-Voltage diode.
 

Thread Starter

GeorgeTR

Joined Feb 3, 2018
35
That may be so, but I was looking at the schematic, and the actual voltage that the diode will expereince when it is installed. From that perspective, it doesn't seem like such a high voltage diode is necessary. Pins 9 and 10 on the transformer are each labled at 260V, which is under the 280V RMS rating for the 1N4004. I would think this is maybe cutting things too close, though, so a 1N4005 would be better. But if you think a higher voltage diode would be best just to be sure, that is certainly something I could do.
 

AlbertHall

Joined Jun 4, 2014
10,522
According to the diagram these diodes produce a 300V DC output, so the reverse voltage would be 600V. A 1N4004 won't cut it. 1N4006 should be OK, but I would go for a 1N4007 to leave plenty of margin.
 

Thread Starter

GeorgeTR

Joined Feb 3, 2018
35
Great, thanks! What about replacing the other diodes? Do you think it would be a good idea to replace D110, D111, D114, and D115 while I have it taken apart, or do you think this was a one-off fluke that isn't likely to happen again?
 

CROSSBOLT

Joined Dec 9, 2008
21
George, you may want to check antiqueradios.com or some such if you have further questions. Not that you received bad advice here, quite the contrary but those guys over there RESTORE practically anything electronic/electrical in wood/metal/plastic boxes or on planks (Atwater-Kent radios) as well as test equipment of all types! They have guys that restore oscilloscopes just to watch sine waves!
 
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