Recent Posts

1

Metrology / Re: ESI RV622A Resistor Repair

« Last post by gmilliorn on Today at 10:30:28 pm »

I could not find out if the wires inside are Manganin/Constantin or just copper.  Since the point of a KVD is to eliminate
the effects of low-ohm switches, as I understand it, then I assume the wires are the same?  All the internal wires are
soldered to the switches.

Little info I could find on soldering manganin, other than a weird pair of youtube videos doing nothing.

In any event, I cleaned the area w/1200 grit sandpaper and cleaned again with IPA, and was able to solder it
relatively quick with no issues.  Either I was luckly or there's no issue soldering it.
3x cleans after soldering with IPA and it seems to be fine.

I can dial in any voltage within limits of my uncalibrated Fluke 8842, below reads as 1.0000V so I guess it is
off about 70mV.

2

Beginners / Re: 555 driver and Transformer questions

« Last post by DanMann on Today at 10:29:41 pm »

I am starting to understand why I have luck with other similar power supplies, but this type has been problematic.
A little more background on this unit, The transformer, capacitors, resistors needed to accomplish what is needed are all potted and not serviceable. The unit is meant to be portable and operates off of a 6 volt battery. So a bipolar bench supply is out of the equation.
Attached is a schematic that has been cleaned up removing the voltage divider, which should not effect this discussion.
The blue box is the potting and you can see there is no way to separate the ground of the primary wingding from the ground. The two capacitors in series I believe are just to filter.
The purple box is an external mechanical vibrator that works a little different than others I have played with on old car radios, portable two way radios. These units have center tap transformers.
There are electronic replacements out there, and I have bough or build many of them, and they work in other power supplies, but not this unit.
Trying various other mechanical vibrators I have found they get hot quickly. These are all 80 plus years old, inefficient, and many just no longer work. As mentioned the correct vibrators can be too slow or two fast and create voltage spikes.
this vibrator when energized takes the path of least resistance to the internal coil which pulls the read from the lower connection to the upper and puts power to the primary side of the coil. As this happens the current is limited by the internal resistor which releases the read where it begins the cycle all over again.
As I understand it the vibrator is doing nothing more than pulsing the DC to the primary.
This is where I got the idea of a 555 timer.

There may be high voltage feedback involved.

I am unclear what the .1uf capacitor is doing. is boosting the primary or vibrator coil? And to what end? or is it as Andy said "you could try a large 1000uF non-polarised capacitor* in series with your transformer winding.  This circuit topology is simple, but generally low power." and this helps with the saturation.
Or maybe saturation was part of the design. I say this as the  HV output is only a few micro-amps but it can zing you at the battery.
 

Looking into the H-bridge, would this even be possible as it would be reversing through the ground which might upset the voltage dividers (not shown)

3

RF, Microwave, Ham Radio / Re: Transistor tester

« Last post by graybeard on Today at 10:24:23 pm »

My $10 Chinese tester is doing an  acceptable job on Ge BJTs and diodes.   I have had these parts in my collection since I was in Jr. High school.

4

RF, Microwave, Ham Radio / Re: fake MPF102s from AliExpress

« Last post by TimFox on Today at 10:19:04 pm »

Have you tried Jameco (my usual source for obsolete semiconductors)?
https://www.jameco.com/z/MPF102-Major-Brands-Transistor-MPF102-JFET-N-Channel-TO-92_26403.html?CID=GOOG&gad_source=1&gclid=EAIaIQobChMI5tiM-fflhQMV_0f_AR3FaAF9EAQYASABEgKBevD_BwE

5

General Technical Chat / Re: Cable Management

« Last post by edpalmer42 on Today at 10:17:46 pm »

I came across an Ikea wire rack for holding stemware glasses at a bar.  I mounted it on the end of one of my shelving units and adjusted the width of the slot so that it holds a BNC connector while allowing the cable to slide in.  I slid vinyl tubing over the rails so that the cables didn't slide on the chrome rails.

I'm going to build another cable holder.  Take a flat piece of wood and mount dowels or threaded rods that are spaced to hold the connectors while allowing the cable to slide between the rods.  Put vinyl tubing over the rods and ensure that the tubing sticks out beyond the end of the rod so that if someone bumps into it they hit vinyl tubing rather than the wood or metal rod.

6

RF, Microwave, Ham Radio / Re: fake MPF102s from AliExpress

« Last post by Alex Nikitin on Today at 10:16:49 pm »

Just curious. Is there anything special about MPF102? Could J304 be a suitable replacement?

Cheers

Alex

7

Repair / Re: Mig welder wire feeder controller board not working

« Last post by .RC. on Today at 10:06:07 pm »

I have been doing the ear plugs on the wire for quite a few years now.   I did at first have those special foam things, but earplugs are much cheaper.  I do not so critical structural welds so I do put corrosion inhibitor on the wire as a roll may last quite some time.  I cannot say I have had porosity issues because of it. 

8

Test Equipment / Re: Siglent SDS3000X HD and upgraded SDS1000X HD

« Last post by Martin72 on Today at 10:04:48 pm »

I would rather enjoy the fact that there has never been so much “scope” for the money as there is now, instead of brooding about it.
Not so long ago, the 12-bit ADCs used were cited as one of the reasons why the 2000X HD was twice as expensive as the 2000X plus.
Now you can get them in a 500€ model.

9

Projects, Designs, and Technical Stuff / Re: Reactive Source Resonant Transformer

« Last post by Benta on Today at 09:55:16 pm »

Well..,
"This design posses (sic!) the potential to"... do something. Perhaps provide a therapy for AntiVaxxers? Or QAnon followers?

I dunno.

10

FPGA / Re: Analog video output with FPGA ?

« Last post by Jaunedeau on Today at 09:55:06 pm »

Since you mentioned using MCUs (I think?), note that generating RGB video at these resolutions would be plenty feasible with just a RP2040 and its PIO. But given its limited number of GPIOs, you'd probably have to limit the bit depth to something lower.

The PIO is plenty of fun and I'll try playing with PDM on the PIO, but with max overclocking and 4 bit depth per component, I would get only 3 samples per pixel with pwm and 160 px per line... still a bit low for my all grey / alternating black and white pixels problem. I even thought of a dual rp2040 solution with one acting as an SPI ADC, but the stm32g4 (170MHz, requires no external flash, $2.5 for one piece in 64pins package / 52 gpios and has an real datasheet ^^) seems a simpler solution if an R2R ladder + calibration can do the job.