Test Equipment Anonymous (TEA) group therapy thread

[mnementh]

A few quick comments as I’m out the door to hopefully finish loading the trailer this AM...

@bean: Looking good on the woody! Hope it serves as nicely as you planned! 

@Kosmic: Thanks for the heads-up one that C14/NEMA-5 power adapter. Several times I’ve found myself wishing for just this adapter, in both genders.

@Brumby: That’s a great score on a Wanhao. Even tho I don’t like the “all angle-iron” construction typical of the brand, even I would jump on THAT deal.

@bean: MUAHAHAHAHAHA!!! (Just on GP)

@bd: ONE OF US! ONE OF US! ONE OF US!

mnem
*back out into the suck*

[bd139]

@mnem if it cheers you up I'm doing my effing tax return rather than playing with shit.

I'd rather have my stuff in a trailer an be buggered by satan himself than do this. I am also my own accountant which is a massive liability  . Had a tax audit a few years back and literally left a stain on the couch 

Edit: damn. So underpayment from 2017/2018 + CGT + CB repayment + disposals = royal buggering and no TE for the next 3 months at least

[xrunner]

Edit: damn. So underpayment from 2017/2018 + CGT + CB repayment + disposals = royal buggering and no TE for the next 3 months at least

Suggest avoiding this thread due to possible enticement to purchasing said TEA. 

[med6753]

Here's first pass schematic for the self-contained power supply I'll be building up for the 10MHz Reference. It will include a line filter as I discovered that this reference throws a lot of hash on the bus lines. I may also add a 0.1uf disc capacitor right at the feed thru to the reference assembly.

The balance of the parts to build this into a case should arrive tomorrow (Monday).

[bd139]

I'd knock that transformer voltage up a tiny bit. Dropout voltage of the 7912 is around 3V for it to be universally stable plus you need some headroom for the diode drops and the capacitor discharge troughs.

Also use an LM7812 and reference the positive output to ground. They are more stable than 7912 are. Also up that cap on the output of the 7912 if you use it. Negative regulators are always bastardy!

Edit: damn. So underpayment from 2017/2018 + CGT + CB repayment + disposals = royal buggering and no TE for the next 3 months at least

Suggest avoiding this thread due to possible enticement to purchasing said TEA. 

Yeahhhhh like that's going to happen. No one leaves TEA unless they are dead! Actually screw it the best way to do is to avoid playing by the rules where possible so rolls of twenties it is.

Can't wait until I retire. I'm going off the tax radar! Hamfests and ebay shell accounts.

[med6753]

I'd knock that transformer voltage up a tiny bit. Dropout voltage of the 7912 is around 3V for it to be universally stable plus you need some headroom for the diode drops and the capacitor discharge troughs.

Also use an LM7812 and reference the positive output to ground. They are more stable than 7912 are. Also up that cap on the output of the 7912 if you use it. Negative regulators are always bastardy!

That's the only transformer I have on hand right now. Yea, it might be on the hairy edge but I'll run with it and see what happens.

According to the spec sheet on the 7912 the minimum capacitance on the output is 1uf tant. I bumped it up to 2.2uf tant. If it goes ape shit I'll bump it up more. 

[xrunner]

Can't wait until I retire. I'm going off the tax radar! Hamfests and ebay shell accounts.

Did you ever get the replacement display for the 34401A, or did I miss it?

[0culus]

Any of you fine TEA folks happen to have a Tek P6042 you'd be willing to sell? I either need parts for mine (bad probe cable) or a working unit (mine becomes the parts mule, aside from the probe cable).

[bd139]

Can't wait until I retire. I'm going off the tax radar! Hamfests and ebay shell accounts.

Did you ever get the replacement display for the 34401A, or did I miss it?

Nope. Keysight called me, took card details and everything. Absolutely SFA turned up and they didn't even bill me. Been too busy meddling with 3DP to sort it . I'm actually killing the card I paid for it on next week as well as the interest free period is out so that's not going to happen by the looks.

[mnementh]

SOOOO missing my MetCal right now...

mnem
That is all.

[med6753]

When the 7904 was purchased and repaired this guy got unceremoniously booted off the scope cart and stashed under the bench like a neglected red headed step child. So I figured it was time to pull it out, dust off the cobwebs, and run it for a while. Seems to be no worse for the neglect. Powered right up and stable. But it will be going back under the bench.       

[Martin.M]

your guy have the option "1"  (no readout installed)

The typical university setup, option1, 2x 7A15 (manual says "low cost amplifier") , 7B53  (or the same as 5k, like a 5403, 5440)

[Martin.M]

Here's first pass schematic for the self-contained power supply I'll be building up for the 10MHz Reference. It will include a line filter as I discovered that this reference throws a lot of hash on the bus lines. I may also add a 0.1uf disc capacitor right at the feed thru to the reference assembly.

The balance of the parts to build this into a case should arrive tomorrow (Monday).

7912 is best spare for a missing SKTU 

[med6753]

your guy have the option "1"  (no readout installed)

The typical university setup, option1, 2x 7A15 (manual says "low cost amplifier") , 7B53  (or the same as 5k, like a 5403, 5440)

This scope is actually a military 7603N called an OS-245(P)/U. The vertical plug-ins are AM-6565/U (7A15) and the timebase is TD-1085/U (7B53). 

[Martin.M]

give him the fat logic analyzer setup with a display formatter2, that makes the readouts self 

[BU508A]

A few quick comments as I’m out the door to hopefully finish loading the trailer this AM...

@med6753:
A bd said, I would use a 7812 and just switch the outputs, declaring the +Uout as the new GND.
To reduce a bit the Uf of the rectifier diodes maybe you can use Schottky diodes. But, if you are doing this,
I recommend to place in parallel a 100pF capacitor on every rectifier diode. This will reduce a bit the sharp
edges of the dI/dt (Schottky diodes are faster than the usual 1N4007 or so).
Ah, and I'm always place as near as possible 2 100nF film capacitors to the regulator. One from input to GND, one from GND to output. Helps surpressing oscillation.

[Martin.M]

my favoite in homebrew is still the good old 723,
it is a poor noise regulator with current limiter. External Transistor is required for more then 150mA

[BU508A]

my favoite in homebrew is still the good old 723,
it is a poor noise regulator with current limiter. External Transistor is required for more then 150mA

BD139 is a big fan of them. He blows them up occasionly for no reason. 

[med6753]

Man oh man....all this hate for 7900 series regulators.  I've used them many times in circuits and never had any stability issues.  But it is absolutely important to have those 2 little tants mounted physically close to the input and output. But same also applies to the 7800 series too.   

[SeanB]

With the 79 series a little more output capacitance helps a lot, and here is where you use a plain old non low ESR electrolytic, in addition to the tantalum, around 220uF 25v works well, has enough capacitance to work, and a high enough ESR to damp oscillations nicely. Unlike the positive ones, the negative ones really want to be power oscillators, and need extra damping that you need to provide, though if you are using a low ESR capacitor just put in a series resistor, any value from 0.1R to 1R in series with the capacitor will do. The 100n ceramic caps right next to the pins are also good, you never know from batch to batch and manufacturer to manufacturer which ones are on the edge stability wise with them.

As to the 723, yes it will work, and you can get really low dropout with a PNP pass transistor, or even use it to make a really high power ideal shunt zener diode. I have even used them as voltage references, the internal one is quite reasonable for the age of the design, provided that you pay attention to power dissipation in the device.

[BU508A]

Just a quick update of my "measuring the leakage current of wet tantalum caps".
This is one of the Roederstein caps, 680µF / 50V date code 1972.
As you can see, the curve is not that smooth as on the other one, it is "sparkling".
Not very helpful if you want to measure the noise of a voltage source in the 200nV area.

[Martin.M]

about SKTU 

[bd139]

my favoite in homebrew is still the good old 723,
it is a poor noise regulator with current limiter. External Transistor is required for more then 150mA

BD139 is a big fan of them. He blows them up occasionly for no reason. 

[BU508A]

my favoite in homebrew is still the good old 723,
it is a poor noise regulator with current limiter. External Transistor is required for more then 150mA

BD139 is a big fan of them. He blows them up occasionly for no reason. 

*BWAHAHAHAHA* Always been a big fan of the Flying Circus.   

[factory]

The short appears to be very close to one end of the winding; the resistance to ground as measured by the 427 is way under 1Ω in that end, while a few percent, at most, off of the whole-winding resistance in the other.

(Attachment Link)

Very close to the end indeed.  Our friends at -- hp -- apparently had some assembly quality issues with late-model 428b's. First we read about the non-finished solder job here and then this. This, of course, is much harder to locate; if the insulation is still soft and not squeezed out it will of course not arc over until the unit is shipped to the customer. I suspect it was the TE equivalent of a hangar queen where it was used. Just stood on a shelf and noone wanted to use it. 

Looking at the otherwise spotless chassis there is evidence of arcing where this was pressed onto chassis:

(Attachment Link)

Some thin shrink tubing and careful wire dressing should fix this, I hope. But first, I got so excited that I had to tell you.

Another assembly problem with the 428B then, that's three different problems now. 
Hopefully you can fix it, that transformer looks different to the one in my good 1960's 428B and what may have been in the spares one (which has two small round holes in the chassis for the wiring), how late is yours?

Here is my 428B (the meter glass didn't survive shipping) which came with no probe (nor did the spares one), it was sold together with the 411A in the first picture.





David