Recent Posts

1

Repair / Re: TDS744A: 1M input impedance measures: Ch3 = 160Kohm, Ch4 = 706KOhm

« Last post by John_ITIC on Today at 03:43:14 am »

I am suspecting that soldering the relays in may have heat damaged the three 1M voltage divider resistors. This post lead me to do some more experiments:
https://www.eevblog.com/forum/testgear/tektronix-tds520b-attenuator-swap/

Sometimes, input impedance of CH1 is 170ohms in 1Mohm mode and 38ohm in 50ohm mode as if a 170ohm resistor was in parallel with the input. Apart from this, attenuator behaves correctly.
The strange thing is that this fault disappears when I apply some DC voltage (12V for example): after that, the input impedance is back to normal (1Mohm or 50ohm) until powered off.

This lead me to try this experiment myself. When I apply some DC voltage to the 1M input (some 19V), I'm seeing that the input impedance rises several hundred Ohms. I can even go over 1MOhm. I then measure the input resistance with a couple of DMMs, one show around 1MOhm while the other show some 800 KOhm. The values drift towards etch other as I swap meters. It looks like the meter voltage is different and that the input resistive divider reacts to this change in meter current.

I'm seeing the same behavior on both Ch3 and Ch4 that I swapped out the relays on. Ch1 and Ch2 are stable across temperature and across multimeters (both show 998 KOhm input resistance). Ch1-2 do not get affected by applying a DC voltage.

So, has anyone seen similar issues where soldering iron heat has permanently affected the temperature coefficient of resistors in such a way?

Note: I'm also seeing on Ch3 and Ch4 that setting the coupling impedance to GND still measures 1.7MOhm on Ch3 and 17 MOhm on Ch4.
Both Ch1 and Ch2 measure infinite resistance when set to GND.

So, it seems there is some shunt resistance that is related after all. But where?

Thanks,
/John.

2

Projects, Designs, and Technical Stuff / Re: Simple illuminated time display that can be driven over USB?

« Last post by PCB.Wiz on Today at 03:40:36 am »

Is there a modest-sized (at least 3"×1" but not larger than 18"×5") OLED screen that I can control from a PC USB port from userland software, but not as an X/Wayland display? Or is there any four-digit seven-segment (with colon, please!) time display (preferably with 1" to 3" high digits and preferably white, blue, or at least not-red) that can be controlled from a PC USB port?

I recently went looking for a largish 7-segment display, with properly proportioned digits.
There are not many around.

This is nice and large, 30.5mm digits,  and will have very good viewing angles, but is glass and pins only, so you need to add a segment driver / board.
https://www.aliexpress.com/item/1005003087294038.html

Smaller 7-seg do exist, like this from Adafruit
https://www.adafruit.com/product/5581

and smaller ones from Aliexpress  10mm high digits.
https://www.aliexpress.com/item/1005004102385524.html
or
https://www.aliexpress.com/item/32924015493.html

or 15m high, but no colon
https://www.aliexpress.com/item/32944755027.html

I'm not adverse to adding a MCU to the mix and/or scavenging a display from some off-the-shelf clock. (I suppose that, with enough effort, I could coax an old smartphone into serving, but I'd prefer something a little more straight forward.)

The Pi-PICO may be the best building block with a OLED or TFT ?

eg This OLED is 77 x 19mm  active area, 256x64 pixels.
https://www.aliexpress.com/item/1005006051449911.html

3

Manufacturing & Assembly / Re: Green shmutz on trimpot screwheads

« Last post by floobydust on Today at 03:34:52 am »

It's an interesting puzzle. I suspect the PCB soldering/wash, something that attacks brass. The LED holder has crusties of some residue inside it, the gold jumper pins look damaged as well. I think the board got hit with some chemical that de-zincified the brass.
You can check the trimpot date/lot codes, but a blue one got it too as well as the grey ones.

4

EEVblog Specific / EEVblog 1611 - Top 5 Jellybean 7400 & 4000 Logic

« Last post by EEVblog on Today at 03:28:36 am »

The TOP 5 Jellybean 7440 and 4000 series logic chips.



Schmitt Trigger video:

00:00 - Top 5 TTL and CMOS Jellybean chips
00:37 - Different 7400 series family types
01:52 - What makes a Jellybean component
3:27 - CD4051  / CD4052 / CD4053 Analog Multiplexer Switch
17:42 - 74HC595 8 Bit Latched Shift Register
20:58 - Used in the MXO4 oscilloscope
24:09 - Driving LED's
26:12 - 74HC74 Dual D Flip Flop
27:00 - Used in the Rigol HDO1000 Oscilloscope
28:14 - Uses for a D Flip Flop
28:53 - Special Mention 74HC374/ 74HC574
30:35 - 74HC14 Hex Schmitt Trigger Inverter
35:18 - Last chip, what about NAND NOR or XOR?
36:06 - 74HC245

The jellybean Components Playlist: https://www.youtube.com/playlist?list=PLvOlSehNtuHv268f0mW5m1t_hq_RVGRSA

5

Test Equipment / Re: Rigol RSA3000N/RSA5000N VNA software needs some work

« Last post by EE-digger on Today at 03:21:54 am »

In keeping with the original title, I can vouch for the fact that in 2024, VNA software still needs work.

I'm in the process of returning a new RSA3015N after 2 intensive days of evaluation.  I won't go down the long list here but the final straw that broke the camel's back is that in S11, there are NO marker functions.  You can't mark a valley and  perform a bandwidth measurement, it just ain't there.

This is ridiculous that something like bandwidth of a filter (band or notch) or response of an antenna can't be measured.

This coupled with high noise below 300MHz or so in S11 are just a few of the reasons it's going back.  Rigol's support also shows that they are new to VNAs.  I will be kind and leave it at that.  Two lengthy sessions were a complete waste of time with lack of fundamentals.

Kinda sad because it seems to be a solid product, built "tighter" than Siglent, but Siglent has it all over them for speed, performance and software (with its own minor "niggles").  But taken further, if I had paid close to $10k for an RSA5065N, I would be totally pi$$ed that the product is in this shape.

added - for lower frequency S11, the Siglent SVA1032X (created from a younger sibling  ) cals to under -50dB or less from 100kHz to 3.2GHz.  If you're working in the low end, then calibrate in the low end.  This will allocated more points to the cal.  The same holds true for a log sweep.  Cal and test with 1601 points or more for enough data points at the low end of the log sweep.

disclaimer:  I have not received any compensation for any glowing comments on Siglent, but would sure like to    For the Rigol side, I would be glad to help with constructive input for development but at the prices being charged, the products should already be complete (more or less).

6

Test Equipment / Re: Adding Front Panel Connector to Agilent/Keysight DC Communication Source

« Last post by ZGoode on Today at 03:18:25 am »

That's good to know.  Although I do still want access to the sense lines, so I'm probably going to stick with my current solution.

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Projects, Designs, and Technical Stuff / Re: Need help with Repairing a Sony Digital Paper Ereader

« Last post by moffy on Today at 03:12:44 am »

Try different value resistors between the two white wires until you find something that clears the temperature error, you could start with 10k as suggested by Renate.
P.S. If you use, say a 50k pot you could adjust it to get the right resistance value.

Can you please translate this to a newbie? What type of device could I use to perform these tests.

Use something like this: https://www.jaycar.com.au/50k-ohm-linear-b-single-gang-24mm-potentiometer/p/RP3516
Connect the center pin to one of the outside ones, then wire the two connections to the two white wires, adjust the potentiometer until the error clears.

8

General Technical Chat / Re: Model Trains

« Last post by floobydust on Today at 03:03:07 am »

I wonder how much current the loco's use? Including the headlight and smoke generator.

O-scale train set I had, it was powered by a Louis Marx 1209 toy transformer. LV AC variac, no circuit breaker or fuse that I remember. It seems to have been a Marx train set, I'm not sure.
Made quite a few sparks on the tracks. Tried cleaning them with steel wool, uncle's idea. That lit up and started burning. Scared the shit out of me.
Then the figure-8 crossover shorted out intermittently and the track got red hot and smoked. The train set from hell lol. A manly model train, that's for sure.

Years later went N then HO and that was plastic baby in comparison.

9

Test Equipment / Re: SDS800X HD Wanted Features

« Last post by electronics hobbyist on Today at 02:58:51 am »

Yes, I will list a top x later, thank you for your suggestion.
When a suggestion is satisfied, the top x will be updated again.

10

Beginners / LM317 Based Digitally Controlled Power Supply

« Last post by EthannCraftt on Today at 02:57:43 am »

Hello. I'm looking to construct a digitally controlled power supply. I have a circuit drawn up with a PWM voltage control circuit of my own design, and the current control design that Dave drew on his PSU series hooked up to the input of the second LM317.

After being plugged into a circuit simulator, it seems to have really weird behavior even though both circuits work great on their own. Any help??? Schematic below.

(The +5v inputs are where the PWM will be inputted. An RC filter will be added in the final design. The 1ohm resistor to the far right will also be removed.)