Aaron oscilloscope 35MHz BS635

[Gadgetman]

Hello All,

I am a complete novice to the world of electronics but in my long life (over 50 years) I learned quite a few completely new (to me) things in very short time, so I decided to face the music.
In the last few weeks, I watched hours of videos and instructions on YouTube (that's how I migrated here) including full lectures by professor Ravel Ammerman from Department of Electrical Engineering and Computer Science at Colorado School of Mines - Introduction to Electrical Circuits (highly recommended). I fully understand that it will never substitute for a proper formal education and/or years of experience and practice, but you have to start somewhere. My knowledge grew dramatically, however, I realise that I still have quite a few miles to cover.

Recently, I bought very cheaply Aaron BS635 2-channel dual trace oscilloscope (made in Japan) with the original manual including all block schematics, board overlays and full schematics as well.

It doesn't work but second-hand analogue oscilloscopes come very few and far between in my part of the world and whatever occasionally is available - I can't afford so I thought - since I paid only $25 for it - I could take a risk and attempt to fix it.

The seller told me that when he bought it he thought it was working because when he powered it up, the cathode screen came up but then the fuse blew and everything shut down. Since then he didn't touch it for the apparently long time.

On the surface, it looked to me like some sort of short (blown fuse).

The fuse was replaced, I turned it on and "everything" works... except the screen doesn't come on. All the diodes light up at the appropriate moments (even the triggering diode when I manipulated the triggering knob), all the switches and potentiometers seem to work smoothly. Of course, I checked the screen dimming and position, horizontal and vertical divisions, magnification, etc. Seems to be dead in the water. While it may be some trivial electrical problem inside, it also could be the cathode tube itself. I didn't have the time yet to inspect it thoroughly. I plan to methodically go section-by-section, starting from the power supply, each rail and the cathode tube circuit. Yes, I am aware of a high voltage. If worst comes to worst and everything else fails, I may try to replace the cathode tube. My problem is that I may not find easily the replacement tube - I couldn't find virtually anything about this oscilloscope on the internet. Could a tube from a different "scope" be suitable? Any advice would be much appreciated.

Also, should any of you need the schematics/manual for this particular model, I could scan it and post it here. Especially, that it is so hard to find anything on that piece anywhere...

[ebastler]

I'm afraid a remote diagnosis will be challenging, and repairing a CRT scope is not the best beginner project. But I think it is not too likely that you will have to replace the actual CRT. The problem might rather be in one of its voltage supplies, or maybe the deflection amplifiers. (Just guessing...)

Anyway, one bit of information that may be helpful: It seems that the Aaron scopes are rebadged, and the original manufacturer was Hung Chang in Korea. Their model designation was Hung Chang OS 635, and there were related models with different bandwidths which are probably very similar inside (OS 620, OS 650). Maybe you can find some repair hints for these online?

[Gadgetman]

Thank you for your prompt reply.

Yes, I realise that a CRT scope repair is not an ideal starting point for a newbie, but I am not afraid of the challenges And in case I fry it up really badly, I loose $25 I paid for it. It could be a small price to learn something useful. I will try not to kill myself in the process

By the way, it's only one of my "starting" projects I have also on board an "universal" desktop power supply based on a rewound MOT (slightly modified/extended version of a PS shown by Schematix on

 https://youtu.be/m-y0Cjx7KeE

I have MOT halfway done (removed the secondary coils and cleaned the core ready to wind the new wire) and just waiting for the ordered parts to ship from overseas.

I plan to expand his design to include 3 regulators that can be variable or pre-set for 3.5, 5 and 12 volts respectively plus a 4th that will be variable or pre-set for charging 18v battery. I know that it's probably an overkill to have 4 sources when 1 variable could do but I want to have a possibility to connect several circuits simultaneously, each at a different setting. I am just lazy and don't want to keep re-connecting my circuits and changing the settings on a single supply.

Also, thank you for pointing me in the direction of Hung Chang. I will certainly explore this. I have noticed before a reference to Hung Chang in respect to Aaron BS601 but somehow the possibility of the different models being connected to them in the same way escaped me.

[tautech]

OP, got your PM yesterday and done some hunting for you.
Partial manual.....with the bits you need anyway:
https://elektrotanya.com/hung-chang_protek_os-635_2ch_35mhz_oscilloscope.pdf/download.html

Wait for "processing" to change to "get manual".
~22 Mb

P18 shows the PSU schematic and the HV bits for the CRT which I suspect in your case will be the problem.
There's a LV feed (fused) to the HV oscillator circuitry. These can give problems and without HV you'll have no display.
Concentrate your efforts on the main PSU and HV PSU for now.

EDIT
Actually the manual does look mostly complete, the operation section is somehow at the end and the beginning of the document. 

[Gadgetman]

Hi  Tautech, thank you so very much for your response.

Perhaps I wasn't clear on the part that I do have a full manual with all the schematics included, so I feel sorry for your time and effort spent on it, but I am very grateful for it anyway. It shows that there are people on this forum who genuinely want to help. It's great!

As to your other comments, they are very valuable and for sure I will concentrate on these areas.

Thanks again

[tautech]

Hi  Tautech, thank you so very much for your response.

Perhaps I wasn't clear on the part that I do have a full manual with all the schematics included,

You were plainly clear but for those of us playing along at home it's always best to provide a link so we're all on the same page. 

so I feel sorry for your time and effort spent on it, but I am very grateful for it anyway. It shows that there are people on this forum who genuinely want to help. It's great!

Took but seconds to find with ebastler's clue. 

As to your other comments, they are very valuable and for sure I will concentrate on these areas.

You'll likely find this useful too:
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

Understanding the basic building blocks of a CRO is important and knowing how the failure of any one is presented as a symptom. Do some googling and come back when you think you have identified crook components for one of us to help you move forward.

[Gadgetman]

Hi Tautech,

You'll likely find this useful too:
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

I downloaded this PDF Troubleshooting Manual and at the first glance, it will be extremally useful. Thank you so much!
I will read through it later, but I already found some very valuable things in it (e.g. Troubleshooting Power Supply chart).

I will keep you posted

And BTW, thanks for the reply to my private message

[tautech]

Yep, the PSU is where you need focus your efforts first.
The fixed Vreg voltages should all be checked as it might serve as some indicator or an overloaded rail elsewhere.
There's a 150V and a +38V adjustment that need be checked.
Check the 0.5A fuse that's in the unreg supply to the primary of the HV oscillator transformer and naybe check the draw across it is not more than a couple hundy mA.

Then:
Points of failure that I've experienced in CRO's
The oscillator (Colpitts I think) not working.
Thermistor open (S1)
High value resistors open or drifted values. eg. R31,32
Buggered E caps....anywhere.
Buggered HV caps.
Dicky pots.

Then you must have a DMM with at least a 1500V range to look at the secondary side of the HV, forget all about checking voltages for PDA, 4.5kV. 
Take note the HV is minus 1.5kV so connect leads accordingly if you're using a moving coil meter like an Avo with their HV ranges.


Oh, and before you do anything be sure to mark any preset you might adjust.

[Gadgetman]

Hi tautech,

An update for my struggle with the oscilloscope

I replaced ALL the electrolytic caps throughout the oscilloscope (what the heck - they were only 30 or so ).

I managed to connect everything correctly (or at least as it was before I took it apart) with the help of the photos I was taking before each step of the way. I also labelled all the cables with the source and destination connectors numbers for a future reference.

While working on the Power Supply board, I also discovered an internal fuse, which was blown. Of course, I replaced it as well.

When I powered up the oscilloscope for the first time after assembling it, the CRT came alive!

But there was a lot of sparks going between the 4.5kV CRT cable mount (solder) on a high voltage PCB and the internal high voltage enclosure (grounded). There is a distance of about 8-10mm between the PCB and the enclosure.

Since there were already some parts missing when I bought it (e.g. the high voltage enclosure lid, as well as some plastic shields here and there - I found one loose inside not in its place and I can't find where it belongs), I thought of simply shielding/insulating between the PCB and enclosure.

This helped - at least to eliminate sparks going there. But they started to form between the legs of the high voltage ceramic (metal film?) caps! The legs were originally insulated in most parts by some sleeves, but there were tiny exposed lengths just at the PCB connections. Using insulating liquid ("liquid insulating tape"), I made sure to cover these tiny bits. This helped too.

But... then the sparks started to fly between the CRT 4.5kV cable itself (through its insulation!!!) and.. the tops of the caps!

That was the moment when the internal (power unit) fuse decided it had enough.

I took out the PCB again, replaced the fuse and started to examine the PCB under the magnifying glass.

The first thing I noticed, the CRT cable insulation had several micro cracks (age), so I covered all of it in several layers of "liquid tape".

Then I noticed that one of three Neon Indicator Bulbs had one leg missing (poor thing).

Here starts my problem. The original neon bulbs (N1, N2 and N3 on the schematic) are type NE38B. These are small "pigtail" tubes, about 5mm long (picture below). They are connected in series. The black top on the neon bulb (on the photo below) is a little bit of the "liquid tape" (by accident) but is irrelevant to the problem.

Unfortunately, I can't find anywhere not only any replacement but even any data about them. Apparently, they don't exist. I also don't have any clue at what voltage they are rated, as the schematic doesn't mention any voltage on the rail they are on. The cap next to them is rated at 500v, so my guess is they should be similar. The highest rating I found for the available new ones is 250v. I don't know virtually anything about the neon bulbs (just read the basics today). The fact that they are connected in series means that they add their value, but to what voltage? This is a high voltage circuit and surrounding caps are rated for between 500V to 2kV. Two series resistors in parallel to the neons are 22M? each. This is heavy stuff and without any hint to the rail voltage, I can only guess the kind of magnitude there.

I could perhaps try to analyse the circuit and attempt to calculate voltage at that point, but with my VERY limited knowledge and experience, I would not trust myself.

I might resort to redrawing the circuit in the software (at least from the known voltage points - or maybe the whole Power Block Unit) and spice it at this rail. Sounds like fun

Perhaps this could be a reason for the sparking (too high voltage because of the faulty neon bulb). If they are in a series and one doesn't work (open circuit) none of them works. They theoretically don't exist in the circuit. So, Heavens know what voltage was shooting through that CRT wire and I only hope I didn't fry the tube itself. On the other hand, it looks like this problem was present for some time (certainly since I bought the oscilloscope) and CRT survived (it lit up several times) during my tests (until the last one when the fuse and perhaps something else might have died), so there still might be a hope...

I haven't tested the actual voltage because I have no meter able to test 4.5kV... or even 1.5kV.

If anybody could help me more with any comments/ideas, I would be extremally grateful!

[tautech]

Neons in this application are series'ed to gain the voltage rating required.
They commonly are always 90V devices.
Jaycar has them:
https://www.jaycar.co.nz/search?text=neon&CSRFToken=45c32a94-4968-403e-b874-a231a3ffb7af

The PDA voltage could be too high because the EHT output at pin 7 of the transformer is too high.
But do not test it there (yes you need check and maybe adjust it), check for minus 1.5kV anywhere after the DC restorer diode (D6 ?) and series resistor (R27) and to the cathode connection on the CRT.
Adjust to spec with VR7 (I think/suspect) on the LV side of the EHT SMPS. Check the SM for procedure.

Hunt for an old AVO Mk7 or 8 with a 2.5 or 3kV range, they can often be picked up for just a few $.
Remember to set/connect your leads to measure a negative voltage.

Lastly, do NOT try to measure the +4.5 kV PDA voltage, we know it works.