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).
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).
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?
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)
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?
Excerpt from the manual.... What's a "regurator" ?
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.
Here's the second half of the brochure. Worth a blast from the past.
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.
Not bad for 1987... $350 for this scope. What was that in 1987 dollars converted to today?
that's about$720 now.
Sent from my EVO using Tapatalk 4
Notice the invoice made it sound like it was a warranty replacement for another scope.
My Tenma is the only one I've seen so far that doesn't have what appears to be a removable plate on top with the brand.
Notice the invoice made it sound like it was a warranty replacement for another scope.
My Tenma is the only one I've seen so far that doesn't have what appears to be a removable plate on top with the brand.
Yes I noticed that too. However, given that it was done in 1987 and I don't think these scopes were around for too many years, could be the first few models had problems and maybe died prematurely, and they needed to be replaced?
I had a look inside my "JDR 2000" and actually was surprised by how clean it looked! I wonder if they changed the internals along the way. My scope has only a few boards, few wires jumping around. I should post a picture so you can have a look. I somehow remember a photo previously in this thread showing a tangled mess, but I could be mistaken. It was actually fairly simple construction.
I attached a few photos here and in the next post (as file size exceeded).
Ok, so here are the guts of my "JDR 2000". I thought it was cleaner, but it looks the same. I see some Toshiba, Intersil?, Motorola? and Texas Instruments branded stuff in there, and other companies but I can't find the IC parts online (seem to be too old). Here's a page with logos in case for manufacturer chip ID:
http://www.classiccmp.org/rtellason/logos/semiconductorlogos.html
Aside from the picture tube, you could build one of these at Radio Shack / Tandy
Now that I got a new phone I'll need to take some better pictures of the insides of mine.
i'd like to pay like 8$ max only if the shipping is free and if 50% of the money go to some poor african kids
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!
Add my Pantec Pan-8022 to the list. I would love to have a manual for this but I guess the Tenma 72-720 manual in the first post will suffice for now.