Author Topic: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]  (Read 26165 times)

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Offline Stonent

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My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« on: August 04, 2013, 06:16:40 am »
This is my first scope and I picked it up for about $50 total. The picture in the auction below shows a trace but when I got it, I'm not having so much luck.



When I turn it on I get a red power light and it is otherwise perfectly silent, no sound, no HV hiss. I've got pretty good hearing though. I can normally here the high pitched sound of a muted TV from another room.

I turned the intensity up all the way and still nothing. I fiddled with the knobs and switches and when I was about give up, I turned it off and the screen did a quick green flash.

I turned it back on and got a trace that traveled about 2 divisions, made a slight pulse of about 2 divisions tall, then another that was 1 division tall and then one that was about .5 divisions tall and then blank. This all happened in about 1/5 of a second.

I turned it on and off 3 or 4 times and could occasionally get the blip to repeat.

I fiddled with the knobs and switches again and now I can occasionally get 1 dot to appear for about 1/4 a second stationary, then travels about 2 divisions to the right in about 1/2 a second then disappears.

I'm not familiar enough with scopes to know if this is indicative of a specific failure or something else.

I took the lid off and found a 4 pin cable going to the main board that looked about half cocked. I pressed it down but it didn't change anything.

I have the manual with schematic if that helps.

https://docs.google.com/file/d/0B3nYGMOtVmrVdEVSd1U4dlRxMjQ/edit?usp=sharing

I'm going to try to make a video once I can get enough light. These 60W equivalent CCFL lights in my apartment aren't very good for filming.

http://www.ebay.com/itm/Tenma-oscilloscope-/300937689970
« Last Edit: August 10, 2013, 05:14:10 pm by Stonent »
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Online edavid

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Re: My $50 scope seems DOA Tenma 72-720
« Reply #1 on: August 04, 2013, 06:23:14 am »
Did you try "LINE" sync?
 

Offline Stonent

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Re: My $50 scope seems DOA Tenma 72-720
« Reply #2 on: August 04, 2013, 06:39:29 am »
I was filming a video with my phone just now. I'm uploading it to youtube and will post in a few minutes.  If I flip between AC and TV I can get the dot to crawl again pretty much consistently.
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Offline Stonent

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Re: My $50 scope seems DOA Tenma 72-720
« Reply #3 on: August 04, 2013, 06:42:05 am »
I don't have any probes yet so I can't do a whole lot.  I'm going to try a needle in the center BNC and use an alligator clip over to the calibration connector.


« Last Edit: August 04, 2013, 06:48:03 am by Stonent »
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Offline Stonent

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Re: My $50 scope seems DOA Tenma 72-720
« Reply #4 on: August 04, 2013, 07:07:59 am »
Ok I think I've got it working now. The position was all the way to the left and I had to mess with the trigger. So now I can see a pattern like:

_--_--_--_--_--_

when I connect to a test point hooray!
« Last Edit: August 04, 2013, 09:41:54 am by Stonent »
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Offline Stonent

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Re: My $50 scope seems DOA Tenma 72-720 [Working now!]
« Reply #5 on: August 04, 2013, 09:43:05 am »
Just found out that this unit was also sold under the Elenco name.  I sort of like the panel coloration better, it really makes it easier to understand for a novice like me. The main thing I see different is the metal overlay and a toggle switch vs a push button on mine.

Elenco MO-1251



« Last Edit: August 06, 2013, 10:41:09 am by Stonent »
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Offline Stonent

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Re: My $50 scope seems DOA Tenma 72-720 [Working now!]
« Reply #6 on: August 06, 2013, 10:00:42 am »
I found yet another rebadge, a JDR model 2000


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Offline Stonent

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Re: My $50 scope seems DOA Tenma 72-720 [Working now!]
« Reply #7 on: August 06, 2013, 10:32:17 am »
Not quite Won Hung Lo, but apparently also Hung Chang OS-620.

http://www.radiomuseum.org/r/hungchang_oscilloscope_os_620_depos.html


« Last Edit: August 06, 2013, 10:40:35 am by Stonent »
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Offline dfmischler

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Re: My $50 scope seems DOA Tenma 72-720 [Working now!]
« Reply #8 on: August 06, 2013, 10:57:23 am »
So have you popped it open yet to see if you can tell who really made it?
 

Offline Stonent

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Re: My $50 scope seems DOA Tenma 72-720 [Working now!]
« Reply #9 on: August 06, 2013, 11:02:40 am »
Neotronics OS-620

]http://www.youtube.com/watch?v=0Vc2L59ziEY

Apparently used by the University of Sydney

http://www.ee.usyd.edu.au/tutorials_online/topics/labintro/cro.html
« Last Edit: August 06, 2013, 11:07:12 am by Stonent »
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Offline Stonent

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Re: My $50 scope seems DOA Tenma 72-720 [Working now!]
« Reply #10 on: August 06, 2013, 11:04:31 am »
So have you popped it open yet to see if you can tell who really made it?

I briefly had it open when I thought it wasn't working. But didn't want to disturb it too much in case I needed to send it back.

I do plan on taking it apart again tonight and poking around inside. I'm guessing it is Korean in origin because Hung Chang was a Korean company from what I could gather on the RadioMuseum website.

But yes I'm very interested in finding more on this.
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Offline Stonent

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Re: My $50 scope seems DOA Tenma 72-720 [Working now!]
« Reply #11 on: August 06, 2013, 03:36:45 pm »
I was surprised how much this thing looks like a cheap stereo inside.  All single sided through-hole. The chips were pretty much all TI, Motorola, RCA and some National Semiconductor.

The only identifying thing inside was the word OAK on several places underneath the CRT.  I remember Oak Technologies used to be a video card company but their logo wasn't like this one.

The big caps were labeled Samyung. And on the 2 low voltage boards all of the TO-220 parts were mounted inside a plastic plug, similar to the cables.

http://www.flickr.com/photos/29444141@N02/


0805132353 by Stonent, on Flickr


0805132353a by Stonent, on Flickr


0805132353b by Stonent, on Flickr


0805132353c by Stonent, on Flickr


0805132354 by Stonent, on Flickr


0805132354a by Stonent, on Flickr


0805132354b by Stonent, on Flickr


0805132354c by Stonent, on Flickr


0805132357 by Stonent, on Flickr


0805132357a by Stonent, on Flickr


0805132357b by Stonent, on Flickr


0805132357c by Stonent, on Flickr

« Last Edit: August 06, 2013, 03:46:11 pm by Stonent »
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Offline dfmischler

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #12 on: August 06, 2013, 04:03:05 pm »
Wow, look at all those resistor-shaped jumpers.  The only date code I could almost read on the ICs looked like it said 1988.

Nothing on the silkscreen except component labels?
 

Offline c4757p

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #13 on: August 06, 2013, 04:07:21 pm »
Huh. One of the cheapest-looking scopes I've seen the guts of.
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Online PedroDaGr8

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #14 on: August 06, 2013, 04:21:05 pm »
Wow, look at all those resistor-shaped jumpers.  The only date code I could almost read on the ICs looked like it said 1988.

Nothing on the silkscreen except component labels?

I have a Tenma 72-730 which looks very similar inside (the circuit boards are definitely different though) that being said they are identical in quality (or lack thereof). These things would be simple to repair because there isn't a custom IC anywhere that I can see. It's 100% discrete parts and easy to find DIPs.

My best guess on the manufacturer is GoldStar (now known as LG). This is hinted at by the Made In Korea but it also shares some design features with GoldStar oscilloscopes. Namely the look and shape of the knobs, some models have the same knobs but just in different locations (note this is easy because most knobs are attached to the board with flying leads).
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Offline Stonent

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #15 on: August 06, 2013, 04:22:20 pm »
Wow, look at all those resistor-shaped jumpers.  The only date code I could almost read on the ICs looked like it said 1988.

Nothing on the silkscreen except component labels?

The back of the unit has a Z axis input, IEC plug, 4 way voltage thing like most scopes seem to have and says "Made in Korea" and 08 89.

But I did find one chip inside that had 1389 under the part number so I guess the cases were labeled before the boards went in.

Yeah, it honestly reminds me of the inside of an old Radio Shack boom box I had as a kid.

There is a shielded are which I didn't get a picture of underneath the tube that was loaded with bright blue MOVs or maybe capacitors.
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Offline c4757p

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #16 on: August 06, 2013, 04:25:59 pm »
Hmm.... I see the possible GoldStar connection. The GoldStar I had was made a lot better, though. Still single-sided PCB and whatnot, but it actually looked pretty well organized.
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Offline Stonent

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #17 on: August 06, 2013, 04:27:39 pm »
Wow, look at all those resistor-shaped jumpers.  The only date code I could almost read on the ICs looked like it said 1988.

Nothing on the silkscreen except component labels?

I have a Tenma 72-730 which looks very similar inside (the circuit boards are definitely different though) that being said they are identical in quality (or lack thereof). These things would be simple to repair because there isn't a custom IC anywhere that I can see. It's 100% discrete parts and easy to find DIPs.

My best guess on the manufacturer is GoldStar (now known as LG). This is hinted at by the Made In Korea but it also shares some design features with GoldStar oscilloscopes. Namely the look and shape of the knobs, some models have the same knobs but just in different locations (note this is easy because most knobs are attached to the board with flying leads).


I did heavily scrutinize at least a dozen Goldstar scopes for that very reason. So many similar design elements. Goldstar too I think had an OS-620 model but it looked nothing externally like mine. Perhaps still the same guts, just different front layout.

Aside from the switches yes, I'd say every part inside was purely off the shelf.  All the TI and Moto stuff was 74 and 4000 series.  I didn't bother to look up the RCA chip near the front but I'd be willing to guess it was an amplifier.

I found another one mentioned a Kiotto OS-620.
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Online PedroDaGr8

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #18 on: August 06, 2013, 04:36:52 pm »
Wow, look at all those resistor-shaped jumpers.  The only date code I could almost read on the ICs looked like it said 1988.

Nothing on the silkscreen except component labels?

I have a Tenma 72-730 which looks very similar inside (the circuit boards are definitely different though) that being said they are identical in quality (or lack thereof). These things would be simple to repair because there isn't a custom IC anywhere that I can see. It's 100% discrete parts and easy to find DIPs.

My best guess on the manufacturer is GoldStar (now known as LG). This is hinted at by the Made In Korea but it also shares some design features with GoldStar oscilloscopes. Namely the look and shape of the knobs, some models have the same knobs but just in different locations (note this is easy because most knobs are attached to the board with flying leads).


I did heavily scrutinize at least a dozen Goldstar scopes for that very reason. So many similar design elements. Goldstar too I think had an OS-620 model but it looked nothing externally like mine. Perhaps still the same guts, just different front layout.

Aside from the switches yes, I'd say every part inside was purely off the shelf.  All the TI and Moto stuff was 74 and 4000 series.  I didn't bother to look up the RCA chip near the front but I'd be willing to guess it was an amplifier.

I found another one mentioned a Kiotto OS-620.

I'd lay odds that these are a modified Goldstar OS-7020A. Basically, reorganize the knobs a bit and add the component test feature.



For example move the display knobsfrom the bottom to the side (to make room for the component test).
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Online PedroDaGr8

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #19 on: August 06, 2013, 04:42:13 pm »
OMG this HAS to be the ugliest oscilloscope EVER!



Yum baby poop green
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Offline Stonent

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #20 on: August 06, 2013, 05:25:56 pm »


Any ideas on these grey and yellow parts?
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Offline dfmischler

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #22 on: August 06, 2013, 11:20:45 pm »
Any ideas on these grey and yellow parts?

I was guessing ugly capacitors, maybe tantalums.
 

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #23 on: August 07, 2013, 12:23:49 am »
Yeah my Tenma looks identical.

Those might be capacitors but I also noticed what looked like dipped inductors on my board (which were weirdly colored). I can't tell for sure from your pic though I'm leaning towards capacitors.

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Offline Stonent

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #24 on: August 07, 2013, 02:35:42 am »
I see you marked your knobs... I'm going to end up doing that too. I can't see which one is the pointer easily.
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Offline elliott

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #25 on: August 07, 2013, 11:20:14 am »
OMG this HAS to be the ugliest oscilloscope EVER!



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Offline mzacharias

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #26 on: August 07, 2013, 12:00:37 pm »
I had an older version B&K 2125 with component tester. Maybe similar late 80's to early 90's vintage. A real piece of Taiwanese crap. Built like a cheap boom box. I had bought it new at a time before eBay, and this was available from a local supplier.

Works OK at present, ( it's back in a corner of the shop) but in the past I have had to clean the controls several times, it developed a bad horizontal deflection transistor (open circuit emitter-base), a bad resistor in the focus circuit, and numerous bad solder connections, all in a scope that didn't really get used a terrible lot as these things go. The main board suffers from large areas of overheated components.

The Japanese (Kenwood) B&K's are much better.

I had a B&K 2190A (I think) basic 100mHz that was obviously a Goldstar and made in Korea. Decent basic performance, clean pacific rim type layout inside, no overheated components, and very reliable, but BORING. No fun to use; had a green trace, not the beautiful blue you get from a Tek with the blue filter.
 

Offline Stonent

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #27 on: August 07, 2013, 03:09:31 pm »
I think the black and red combination on scopes is just sexy.

I like the look of the newer black and red Uni-T scopes.
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Offline PA4TIM

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #28 on: August 07, 2013, 05:24:23 pm »
wow, that looks ugly and cheap from the inside. I scrapped a dead Heathkit from the 60's, that was tubes but the same type pcb.



as a contrast, the guts of a 1966 !! Tektronix. Thick goldplated pcbs. This is a 453, 50MHz scope, all transitors except two nuvistors
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Offline edy

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #29 on: August 10, 2013, 11:48:22 am »
OK, so I picked up one of these from Ebay as well, branded under the "JDR Model 2000" badge.

Looks like these are all the same:

Tenma 72-720
Elenco MO-1251
Hung Chang OS-620
Neotronics OS-620
JDR Model 2000

Looks like one of the cheapest CRO's around...Likely made by some OEM in Korea and bought by various small-time companies and rebranded to sell to Joe Public for cheap. There is a link to the original advertisement for JDR 2000 attached. Only $389.95 in 1987!!!!

So plugging in that value in http://www.dollartimes.com/calculators/inflation.htm revealed that "$389.95 in 1987 had the same buying power as $810.25 in 2013."  That would have been a cheap scope for that time, no? I'm sure it was built down to a price.

I found schematics here to download free PDF for the Hung Chang version:

http://elektrotanya.com/hung-chang_os-620_sch.pdf/download.html

So I just bought mine for $20 on Ebay... listing here:

eBay auction: #111134496460

Let's see how long this thing lasts.  ;) I've read on other forums that they weren't too reliable.
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Offline Stonent

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #30 on: August 10, 2013, 12:11:16 pm »
Finally someone bought one of the auctions I posted! I was really disappointed when those two Pintek scopes sold for $5.50 each to someone who didn't appear to be one of us.

Anyway mine seems to work ok. I got my probes in and the calibration signal seems to line up correctly with the screen.
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Offline edy

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #31 on: August 10, 2013, 12:33:14 pm »
I'll be getting it in a few weeks. I had it shipped to my parents in USA, but they are visiting me in a few weeks so they'll drive it up to Canada where I am.  Hopefully I won't need to fix anything and it will just work.

Dave was right, you can pick up a 20 Mhz scope for <$50, especially in the USA if you are willing to wait it out. And if you limit yourself to <$100 you can actually get even better deals on some good brand scopes. It'll be a good learning tool for any electronics student/hobbyist and it won't break the bank.

Yeah maybe they are limited these days, but a beginner like me would find it useful for viewing the characteristics of basic circuit designs/building blocks, testing components, etc. It is something fun to play with, either way.
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Offline Lightages

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #32 on: August 10, 2013, 01:05:34 pm »
I have one this scope in one of its incarnations. I have had mine since somewhere around 1992? It is still working well. Yes, it was one of the cheapest scopes available new at the time and it was my first scope.

I also have the full original manual, schematics, and calibration procedures. If anyone wants a copy I will scan and provide a link for download.
 

Offline Stonent

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #33 on: August 10, 2013, 01:33:23 pm »
I have one this scope in one of its incarnations. I have had mine since somewhere around 1992? It is still working well. Yes, it was one of the cheapest scopes available new at the time and it was my first scope.

I also have the full original manual, schematics, and calibration procedures. If anyone wants a copy I will scan and provide a link for download.

Well I've got the full Tenma manual. What brand was yours?
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Offline edy

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #34 on: August 10, 2013, 02:03:08 pm »
Thank you, a scan of the manual, schematics and calibration procedures would be amazing and much appreciated!!!

I have only been able to find schematics on the 35 Mhz version (JDR 3500 or Hung Chang OS-635), so I'm not sure if they are similar enough to the 20 Mhz version and it would be nice to know the parts in case something needs replacing.
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Offline Lightages

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #35 on: August 10, 2013, 02:06:43 pm »
A retailer in Canada was trying to sell this as their house brand "A-1 Electronics". They had removed the bezel with the original branding on it and copied the first page of the manual with their name on it. The original manufacturer is Hung Chang.
 

Offline Lightages

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #36 on: August 10, 2013, 02:07:22 pm »
Thank you, a scan of the manual, schematics and calibration procedures would be amazing and much appreciated!!!

I have only been able to find schematics on the 35 Mhz version (JDR 3500 or Hung Chang OS-635), so I'm not sure if they are similar enough to the 20 Mhz version and it would be nice to know the parts in case something needs replacing.

OK I will get on it over the next couple of days.
 

Offline Stonent

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #37 on: August 10, 2013, 02:22:22 pm »
Thank you, a scan of the manual, schematics and calibration procedures would be amazing and much appreciated!!!

I have only been able to find schematics on the 35 Mhz version (JDR 3500 or Hung Chang OS-635), so I'm not sure if they are similar enough to the 20 Mhz version and it would be nice to know the parts in case something needs replacing.

The manual is in the first post of this thread.
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Offline Lightages

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #38 on: August 10, 2013, 02:32:39 pm »
Yes that is right. I missed that for some reason. It is the same information that I have.
 

Offline Stonent

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Offline Stonent

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #40 on: August 10, 2013, 04:33:43 pm »
« Last Edit: August 11, 2013, 12:51:28 am by Stonent »
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Offline Stonent

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #41 on: August 10, 2013, 04:36:58 pm »
Even Tricky Dick got in on the action! Dick Smith Q-1240

« Last Edit: August 10, 2013, 04:39:14 pm by Stonent »
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Offline Stonent

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #42 on: August 10, 2013, 04:42:04 pm »
Wait, what's this from 2004?

http://www.electronicspoint.com/dick-smith-scope-t131532.html

Quote
I sent an email to "David L.Jones" and he kindly replied saying "
Hi Phil,
That is a spitting image of the (in)famous HungChang 20MHz scope. It is
probably the most re-badged scope in history! (it's an OK scope though!)
Sorry, can't remember off my head the exact HungChang model number or the
other brands it was re-badged under."

Hmm. Dave Jones... The name sounds familiar.

https://groups.google.com/forum/#!topic/aus.electronics/L64ccLS8phc

Even 2003...

Quote
Don't know that particular model (but most probably the typical 20MHz
cheapie), but Hung Chang are basically the cheapest nastiest scope you
can get. Haven't looked at them these days though, so don't know what
they are like now.
The same scope is (or was) rebadged under various brands.

Is it "descent" scope? - NO, Hung Chang's are bottom of the quality
scale.

But hey, it'll probably work fine, depending on what you want it
for...

Dave :)

https://groups.google.com/forum/#!searchin/aus.electronics/Hung$20Chang/aus.electronics/-QWidhOvlSk/51ZUmx56DgIJ

Quote from: David L. Jones
If memory serves me rightly, the Aron BS-601 is a basic 20MHz dual
trace unit, rebadged under half a dozen different names including Hung
Chang.
It's worth anywhere from $0 (some people give these away for free) to
maybe $150. $200 if it's as-new with probes maybe.

Dave :)



I just keep collecting these pictures!

Thanks Dave!



« Last Edit: August 10, 2013, 05:32:56 pm by Stonent »
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Offline amyk

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Re: My $50 scope Tenma 72-720 [Inside Photos!]
« Reply #43 on: August 10, 2013, 09:17:39 pm »
I have only been able to find schematics on the 35 Mhz version (JDR 3500 or Hung Chang OS-635), so I'm not sure if they are similar enough to the 20 Mhz version and it would be nice to know the parts in case something needs replacing.
Or "upgrading"? :D
 

Offline edy

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #44 on: August 10, 2013, 09:33:55 pm »
Wow, this is just hilarious yet terrifying!  :scared:

We are up to probably a dozen re-badges already....

Fortunately I bought a Hitachi 1565 at the same time, so it will be fun comparing the two.

I was hoping Dave could do a review and teardown of this Hung Chang scope. It has been a while since I heard him saying expletives (last time I think was during fake Apple charger teardown).
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Offline israel

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #45 on: August 10, 2013, 09:41:48 pm »
another one to the list

Oh my God! ... the clones attack   :) :)
 

Offline edy

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #46 on: August 10, 2013, 09:48:32 pm »
Don't forget Ramsey 2200... Also seen on ebay.

So here's the list so far (let's keep it going!):   :-DD

Tenma 72-720
Elenco MO-1251
Hung Chang OS-620
Neotronics OS-620
JDR Model 2000
A-1 Electronics
AW Sperry 620C
Tenma 72-320
Dick Smith Q-1240
Aron BS-601
Degem System 112
Ramsey Model 2200

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Offline Stonent

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #47 on: August 11, 2013, 12:49:08 am »
Don't forget Ramsey 2200... Also seen on ebay.

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Offline edy

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #48 on: August 11, 2013, 02:54:15 am »
You know, although it may be one of the cheapest scopes around, it may also be one of the easiest to service because the parts seem to be all standard off-the-shelf parts. Yes? No? Or am I just making excuses.  :-/O
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Offline Stonent

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #49 on: August 11, 2013, 03:37:44 am »
You know, although it may be one of the cheapest scopes around, it may also be one of the easiest to service because the parts seem to be all standard off-the-shelf parts. Yes? No? Or am I just making excuses.  :-/O

That's what I'm thinking at least. It's cheap but fixable.
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Offline edy

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #50 on: August 15, 2013, 09:19:34 am »
Confirmed from seller.... It's in the mail. Should receive my JDR 2000 in a little while. Can't wait to try it out and pit it against my Hitachi which I bought at the same time (the JDR was an "accidental" ebay win since I actually got the Hitachi first and nobody bid again on the JDR so I was stuck with it). But for $20 (plus $30 in shipping), I didn't cry over it too much.

Will post pictures and video once I get it... I know it is a cheap low quality scope, but will be interesting to see!
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Offline Stonent

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #51 on: August 15, 2013, 04:32:22 pm »
Confirmed from seller.... It's in the mail. Should receive my JDR 2000 in a little while. Can't wait to try it out and pit it against my Hitachi which I bought at the same time (the JDR was an "accidental" ebay win since I actually got the Hitachi first and nobody bid again on the JDR so I was stuck with it). But for $20 (plus $30 in shipping), I didn't cry over it too much.

Will post pictures and video once I get it... I know it is a cheap low quality scope, but will be interesting to see!

50$ scope club!  :-+
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Offline ddavidebor

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My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #52 on: August 15, 2013, 08:04:38 pm »
Not quite Won Hung Lo, but apparently also Hung Chang OS-620.

http://www.radiomuseum.org/r/hungchang_oscilloscope_os_620_depos.html


I've one!

Is the shietties shit i've ever seen. Looks like crap 80' consumer electronic inside
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Offline edy

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #53 on: August 15, 2013, 11:39:49 pm »
Still, if it works you can't complain too much. Even a cheap scope is an incredibly useful bit of kit, and I imagine you will keep it around for a long time even after you upgrade because sometimes you don't want to risk your expensive tools when checking some unknown device.

True, I'd much rather blow up the cheap scope while I'm learning. :-)

Another advantage is that with the schematics available and apparent use of cheap off the shelf components, one could possibly understand and fix it more easily. It is a basic design and being one-sided through hole you can follow all the traces and parts are large (none of that SMT stuff).
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Offline edy

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #54 on: August 26, 2013, 11:31:31 am »
Finally arrived, Here's my $20 ebay win... JDR 2000, 20mhz rebrand of the Hung Chang. Works like a charm! And it came in original box, with 2 sets of probes with spring-hook tip options, some fuses and paper manual!

One question... Why is the square ref wave look ok with 1x on both of my probes, but has a capacitance issue when set to 10x? I played around with the screw on the probe but nothing changes when it is set to 10x... It overshoots and then settles for each square... As soon as I go back to 1x on the probe it looks nice (like the attached photo).
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alm

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #55 on: August 26, 2013, 11:42:28 am »
See:

Your problem is probe compensation. The input of a probe is not just a 1 Mohm resistor, but there is some capacitance (in the order of 20 pF) in parallel. At higher frequencies (above a few tens of kHz or so), this capacitance becomes dominant. This does not matter for a 1x probe: it is 'just' a fancy specially engineered piece of screened coax. A 10x probe attenuates the signal by putting a 9 Mohm resistor in series, however. The capacitance in parallel with this 9 Mohm resistor has to match the capacitor in the scope (20 pF / 9 in my example). It becomes a capacitive divider at higher frequencies. Adjusting the cap matches the attenuation of this capacitive divider to the attenuation of the resistive part, so low and high frequencies are attenuated equally.

So turning the trimmer should allow you to switch the probe from undercompensated to overcompensated, with correctly compensated in between. Why doesn't this work?
- Maybe you're twisting the wrong trimmer. Some probes feature a LF and a HF compensation (ignore the HF compensation for now). Is there another trimmer in either the probe body and the part near the BNC connector?
- Probes are designed for a limited range of input capacitances. Maybe the input capacitance of your scope is outside this range? This is not likely unless the probe in designed for high frequencies well beyond 100 MHz, the probe is very old, or the scope has a capacitance much higher than 20 pF. You should be able to find the input capacitance of the scope either on the front panel or in the manual. You may be able to find the input capacitance of the probe in the datasheet if it's a brand name probe.
- The probe may also be broken.

What happens if you connect it to your other recently acquired scope and try to adjust it?
 

Offline edy

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #56 on: August 26, 2013, 12:23:11 pm »
Thanks for your advice. Ok, fortunately I got a couple probes thrown in with my Hitachi and when I set them to 10x and adjust the compensation I see the curvature flatten out on my square... On BOTH scopes it works perfectly, either 1x or 10x setting.

However, the probes thrown in with my JDR 2000 appear to be broken at 10x setting. The little screw turns and turns with no resistance. If I press and bend the screw at the appropriate tongue angle I see the compensation curvature vary... But it doesn't stick. So these are going to be good only at 1x setting.

Meanwhile, I got a surprising goodie with the JDR...

- Original manual (different from what I've seen online)
- Original invoice (purchase date, price)
- Lots of schematics, full parts list
- JDR Microdevices San Jose flyer Sept 1987

These are worth a scan and upload for you guys.. It is hilarious.

PS - The attached picture shows a probe that actually WORKS (one of the two that came with the Hitachi which both work at 10x)
« Last Edit: August 26, 2013, 12:29:50 pm by edy »
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Offline edy

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #57 on: August 26, 2013, 12:47:49 pm »
Here is the invoice and an exerpt from the manual (component tester). I thought that was interesting. It must work by doing some kind of sweep across the component and seeing the response. Looks like it is internally using the sweep to set X position while the component response becomes the Y. Otherwise you would have to use an external sawtooth to generate it. Is this correct?
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Offline edy

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #58 on: August 26, 2013, 12:55:12 pm »
Excerpt from the manual.... What's a "regurator" ?  ;)
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Offline edy

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #59 on: August 26, 2013, 01:09:23 pm »
One last set of images (divided over 2 posts)... A JDR microdevices Sept 1987 brochure. This was stuffed inside the JDR 2000 manual, next to the invoice.

Not bad for 1987... $350 for this scope. What was that in 1987 dollars converted to today?

Looks like JDR Micro and Dick Smith and scores of other electronics shops were able to import these and slap their name on it. Original box clearly says made in Korea. Wow it is 26 years old now.
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Offline edy

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #60 on: August 26, 2013, 01:13:42 pm »
Here's the second half of the brochure. Worth a blast from the past.
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alm

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #61 on: August 26, 2013, 01:32:16 pm »
Here is the invoice and an exerpt from the manual (component tester). I thought that was interesting. It must work by doing some kind of sweep across the component and seeing the response. Looks like it is internally using the sweep to set X position while the component response becomes the Y. Otherwise you would have to use an external sawtooth to generate it. Is this correct?
In generates a low frequency (usually mains frequency) sine that's fed through a resistor and the DUT. The scope plots the voltage against the current (voltage across the resistor). Search for octopus or Huntron tracker for similar equipment. Many Hameg scopes also feature a component tester.
 

Online PedroDaGr8

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Re: Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #62 on: August 26, 2013, 02:33:50 pm »

Not bad for 1987... $350 for this scope. What was that in 1987 dollars converted to today?

that's about$720 now.

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Offline Stonent

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #63 on: August 28, 2013, 09:37:55 am »
Notice the invoice made it sound like it was a warranty replacement for another scope.

My Tenma is the only one I've seen so far that doesn't have what appears to be a removable plate on top with the brand. 
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Offline edy

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #64 on: August 28, 2013, 02:35:48 pm »
Notice the invoice made it sound like it was a warranty replacement for another scope.

My Tenma is the only one I've seen so far that doesn't have what appears to be a removable plate on top with the brand. 

Yes I noticed that too. However, given that it was done in 1987 and I don't think these scopes were around for too many years, could be the first few models had problems and maybe died prematurely, and they needed to be replaced?

I had a look inside my "JDR 2000" and actually was surprised by how clean it looked! I wonder if they changed the internals along the way. My scope has only a few boards, few wires jumping around. I should post a picture so you can have a look. I somehow remember a photo previously in this thread showing a tangled mess, but I could be mistaken. It was actually fairly simple construction.

I attached a few photos here and in the next post (as file size exceeded).
« Last Edit: August 28, 2013, 03:52:53 pm by edy »
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Offline edy

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #65 on: August 28, 2013, 03:26:54 pm »
Ok, so here are the guts of my "JDR 2000". I thought it was cleaner, but it looks the same. I see some Toshiba, Intersil?, Motorola? and Texas Instruments branded stuff in there, and other companies but I can't find the IC parts online (seem to be too old). Here's a page with logos in case for manufacturer chip ID: http://www.classiccmp.org/rtellason/logos/semiconductorlogos.html
« Last Edit: August 28, 2013, 03:31:54 pm by edy »
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Offline Stonent

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #66 on: August 28, 2013, 03:50:12 pm »
Aside from the picture tube, you could build one of these at Radio Shack / Tandy :P

Now that I got a new phone I'll need to take some better pictures of the insides of mine.
« Last Edit: August 28, 2013, 03:52:07 pm by Stonent »
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Offline edy

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #67 on: August 29, 2013, 02:09:53 am »
Looks like someone is selling one in Canada (near Toronto)... Here is the listing:

http://toronto.kijiji.ca/c-buy-and-sell-electronics-20-meg-dual-channel-oscilloscope-W0QQAdIdZ517202894

They want $275 for it. Wow.  And the image (can't make out the branding):

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Offline ddavidebor

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #68 on: August 29, 2013, 06:43:04 am »
i'd like to pay like 8$ max only if the shipping is free and if 50% of the money go to some poor african kids
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Offline edy

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #69 on: August 31, 2013, 11:27:08 am »
Here is a "KB Electronics" branded version, being sold on Kijiji in Toronto, Canada for $200... Quick, somebody grab it before it gets sold!!!  :-DD 

http://ontario.kijiji.ca/c-buy-and-sell-electronics-KB-Electronics-Oscilloscope-with-Probes-Cash-or-Trades-W0QQAdIdZ516292376

KB Electronics Model 33330 Oscilloscope. I've attached an image.

Here's the tally so far:

Tenma 72-720
Elenco MO-1251
Hung Chang OS-620
Neotronics OS-620
JDR Model 2000
A-1 Electronics
AW Sperry 620C
Tenma 72-320
Dick Smith Q-1240
Aron BS-601
Degem System 112
Ramsey Model 2200
KB Electronics Model 33330
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Offline Stonent

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #70 on: September 02, 2013, 05:14:38 pm »
 :-+
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Offline Ahyuan

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Re: My $50 scope (Hung Chang) Tenma 72-720 [Inside Photos!]
« Reply #71 on: October 21, 2013, 11:25:14 am »
My physics teacher gave me one of these since it was broken and I fixed it!
An input filter capacitor failed short so the -9V rail was not working and another capacitor had caught on fire

The only capacitor in my very small parts bin happened to have the same value and I paralleled some resistors to get the 47 ohms and it works!

It's actually a very nice scope!

 


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