Recent Posts

1

Computers / Re: Format a 256GB pendrive with FAT32 using a windows 10 pc

« Last post by Halcyon on Today at 10:33:29 pm »

Windows is a system of limitations.

And it's only getting worse.

But as DavidAlfa pointed out, the HP tool is a good one for Windows users. Used it many times when Windows format just fails.

2

Projects, Designs, and Technical Stuff / Re: more modern version of tl494?

« Last post by 2strokeforever on Today at 10:33:28 pm »

AFAIK, 598 is just 494 with totem pole outputs -- better for gate driving.  But you might want a proper driver on it, still, anyway; it's old bipolar stuff.

Tim

From the data sheets the rise and fall times were much better than the 494, some of my preferred gate drivers require a sharp edge to trigger on and this will help.

I never drive anything without a proper modern gate driver

3

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

« Last post by graybeard on Today at 10:31:26 pm »

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

Cheers

Alex

I could easily design using a J304.  However if the MPF102 parts were good, 10 for $1.79 was a good deal.   J304s are 10 for $23.50.  AliExpress gave me an immediate refund.

I may get some newer JFETs anyway because if I publish my designs I would like other people to be able to get the parts I design with if they want to build it.

4

Test Equipment / Re: Troubles with HP-8903B

« Last post by srb1954 on Today at 10:31:17 pm »

This week, I purchased an HP-8903B from a local gentleman.  I went to his home to view it and he mentioned that he serviced the unit by replacing the few capacitors in it and it was working "as it should" and within specifications.

So far, I have two problems with the HP-8903B.

PROBLEM 1 - Frequency Inaccuracy

Is it normal for the unit to not "hold" on a frequency and to have, what I consider, an abnormally high THD when simply using a loopback cable?

For example, if I turn off all filters and then just turn on a 1 kHz sine wave, the display will show a frequency from ~998 to ~1001.5.  No matter what frequency I put in to try to compensate for the +/- 3-ish Hz, I can rarely get it to stick exactly on 1 kHz.  Also, the longer I leave the HP powered on, the higher the frequency drifts.

The oscillator in the HP8903B is not synthesised so you can't expect perfect frequency accuracy. However, it shouldn't drift appreciably either as the the frequency is adjusted by the microcontroller using a "count and tune" method. It might be a good idea to follow through frequency monitoring and switching path from the oscillator to the microcontroller to ensure that the signal hasn't been lost along the way.

An unstable oscillator frequency could be contributing to the high distortion readings as the notch filter in the distortion measurement circuit will not be able follow the frequency variations and will be unable to notch out all the fundamental frequency. Check the oscillator O/P with a scope to see if there is any jitter in either the frequency or amplitude of the oscillator O/P.

PROBLEM 2 - THD %

When I use a loopback cable (regardless of the cable I try), I'm getting a "high" THD... AND the THD seems to also be dramatically affected by the amplitude.

Example #1:  All filters off. Frequency set to 1 kHz.  Amplitude set to 150 mA.  Distortion reads ~ 0.1240.  If I change the amplitude to 250 mA, the distortion drops to 0.77.  If I change the amplitude to 1V, the distortion drops all the way down to 0.022.

Example #2:  If I turn on the 30 kHz filter (1000 kHz tone @ 150 mA), the distortion goes from ~0.1240 down about half to 0.065. Choosing the 80 kHz filter results in a 0.09 distortion reading.

Am I expecting too much out of this machine or is something wrong with it?  The main reason I purchased this unit was to measure distortion so if that is going to be unreliable, it is not going to be the machine for me.

It should definitely do much better than what you are seeing. For a direct loop back connection you should be seeing distortion figures of 0.005% or better.

A useful diagnostic is to connect a scope to the monitor O/P on the rear panel to view the residual signal after the fundamental notch filter. The presence of any discernible fundamental frequency components indicates the notch circuit is not tuning correctly resulting in an inaccurate distortion reading.

The fact that introducing the 30kHz and 80kHz LP filters significantly reduces the distortion readings indicates that there is significant HF noise in the system. This could be due to a noisy front end on the HP8903B or interference from external devices. Turn off all computers, cell phones, WiFi, routers and LED lights in the vicinity to see if that makes a difference. Again, look at the HP8903B monitor O/P to determine the nature of the interference.

You should also try the 400Hz HP filter to see if there is any hum contaminating the measurements. This will also be visible on the monitor O/P. 

5

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.

6

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)

7

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.

8

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

9

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.

10

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