Test Equipment Anonymous (TEA) group therapy thread

[Specmaster]

You did indeed and deserve the credit

I got it dismantled this far down last time so it would be a crime to give up too!

You got the damn thing all broken down. It should be an easy fix. I'll bet you have some contacts that have lost some of their tension too.

I agree on both counts, it would be a shame to give up having got this far and also about the contact tension. Isn't that something that they could have used a normal wafer switch for?

[bd139]

The wafer switches have too much distributed inductance and capacitance. The Tek attenuators are basically very carefully designed transmission lines to maintain signal integrity. They were pretty much state of the art at the time. If you look at the 475A for example it has 250MHz bandwidth (one metric shit ton). You're going to get some weirdness there with any other type of switch. In fact I think they are good to quite a bit more than that.

I hope it is just some poking required. That'll be a fixed scope then. Then I can proceed to fix the 475A AGAIN which is now having problems with one sweep setting. Probably same issue. Although I don't fancy getting the timebase assembly out as is much harder to get out and it's got a DM44 wart on it too. Might leave that one for the Christmas period when everyone is at my sister-in-law's but I am not.

[xrunner]

Got this box-o-junque given to me today by the guy who operates our repeater. It was given to him by the previous repeater owner. Some interesting things such as attenuators, meters, preamps, box-o-ICs and a 10 MHz oscillator. The Simpson meters have no part numbers on the outside so have to open them up.

I thought the 10 MHz osc. might be interesting so I opened it up. Is it home-made? Someone carefully packed it with slices of foam. No schematic but upon a quick glance it has an LM78L09 regulator, and a 2N2222 for the output, among the crystal and other parts. It works and gets hot, but the output waveform is not that pretty. Might check the output transistor, but we'll put it aside for now.

Some broadband 10 dB amplifiers - checked one and it seemed to be working well.

Also an Advanced Receiver Research 144 - 148 MHz preamp. Be nice if it worked ... TBD.

 

[bd139]

Looks like an attempt at an OCXO if it's getting hot. Interesting box of stuff. Those Pomona boxes are worth their weight in gold even if the stuff in them is knackered 

Our local club only gives out biscuits, tea that gives you diabetes and lots of bad semi-religious advice.

[Specmaster]

The wafer switches have too much distributed inductance and capacitance. The Tek attenuators are basically very carefully designed transmission lines to maintain signal integrity. They were pretty much state of the art at the time. If you look at the 475A for example it has 250MHz bandwidth (one metric shit ton). You're going to get some weirdness there with any other type of switch. In fact I think they are good to quite a bit more than that.

I hope it is just some poking required. That'll be a fixed scope then. Then I can proceed to fix the 475A AGAIN which is now having problems with one sweep setting. Probably same issue. Although I don't fancy getting the timebase assembly out as is much harder to get out and it's got a DM44 wart on it too. Might leave that one for the Christmas period when everyone is at my sister-in-law's but I am not.

Dare we ask why it is that you'll be home alone this Christmas https://www.youtube.com/watch?v=ddXUQu9RC4U, practising your pranks? 

[0culus]

SWMBO

Na, not Ridder Haggard's 'She', Rumpole's 'She', Hilda Rumpole:



Ob Fact: Leo McKern (who played the very English Rumpole in the long running UK TV series) was actually an Aussie. Educated at Sydney Technical High School in Bexley, the other side of Sydney from where Dave is in Baulkham Hills.

Oh my, I remember this show.  Been ages....I should see if I can't get it somehow. I love old British comedy. Been laughing my arse off lately because Netflix has Monty Python's Flying Circus now. 

[xrunner]

Looks like an attempt at an OCXO if it's getting hot. Interesting box of stuff. Those Pomona boxes are worth their weight in gold even if the stuff in them is knackered 

Yep the boxes are very good to keep. I did find the documentation for the Simpson panel meters - they require a 120 VAC line for power and read out your selected DC voltage from inside your rack or wherever you want. I'll wire one up shortly.

Our local club only gives out biscuits, tea that gives you diabetes and lots of bad semi-religious advice.

"I don’t care to belong to any club that will have me as a member"

Groucho Marx

[mnementh]

mnem
That is all.

[nixiefreqq]

I thought the 10 MHz osc. might be interesting so I opened it up. Is it home-made? Someone carefully packed it with slices of foam. No schematic but upon a quick glance it has an LM78L09 regulator, and a 2N2222 for the output, among the crystal and other parts. It works and gets hot, but the output waveform is not that pretty. Might check the output transistor, but we'll put it aside for now.

can't really tell from the picture,   but it looks like your coax needs to be terminated in 50 ohms at the scope end.  bet that would clean up the trace.  then again.....i could be wrong.

[Berni]

I thought the 10 MHz osc. might be interesting so I opened it up. Is it home-made? Someone carefully packed it with slices of foam. No schematic but upon a quick glance it has an LM78L09 regulator, and a 2N2222 for the output, among the crystal and other parts. It works and gets hot, but the output waveform is not that pretty. Might check the output transistor, but we'll put it aside for now.

can't really tell from the picture,   but it looks like your coax needs to be terminated in 50 ohms at the scope end.  bet that would clean up the trace.  then again.....i could be wrong.

Yes does look a lot like a TDR waveform of a coax to me.

[bd139]

I just reverse engineered it from the picture and into DaveCAD and it's a textbook Colpitts VXO arrangement:



Doubt that has any match on the output near 50 ohms!

There may be an inductor in series as well with the capacitor. I couldn't quite make that out though. Pull it one way with L, then the other with C would make sense to get it in the right place if warmed up.

I'm not sure if it is effective though. Most crystals need to be cut for the temperature rather than heated then pulled. Would lead to frequency instability problems and drift.

Edit: that was the most fun I've had today 

Edit 2: This was probably designed to work with a step attenuator or something which would eventually give a better match to 50 ohms after enough attenuation.

[xrunner]

can't really tell from the picture,   but it looks like your coax needs to be terminated in 50 ohms at the scope end.  bet that would clean up the trace.  then again.....i could be wrong.

Perhaps I'll check it again today.

I just reverse engineered it from the picture and into DaveCAD and it's a textbook Colpitts VXO arrangement:



Doubt that has any match on the output near 50 ohms!

There may be an inductor in series as well with the capacitor. I couldn't quite make that out though. Pull it one way with L, then the other with C would make sense to get it in the right place if warmed up.

I'm not sure if it is effective though. Most crystals need to be cut for the temperature rather than heated then pulled. Would lead to frequency instability problems and drift.

Edit 2: This was probably designed to work with a step attenuator or something which would eventually give a better match to 50 ohms after enough attenuation.

Thanks - I'll see if I can get it to work a little better in the next few days. I'm sure the original man that this came from made it himself, there's a few other examples of home-made construction in the lot.

Edit: that was the most fun I've had today 

Let me see what else I can find for you in the junque then. 

By the way - anyone need a complete set of crystals for a PRC-6 radio (50 MHz)? 

[bd139]

That's just impressive to look at

Much nicer than a box of cigars! 

I nearly bought a whole crate of homebrew gear someone was selling a couple of years ago on ebay just to play with it and reverse engineer it. None of it was labelled at all. There were literally about 20 bits of test gear. I expected it to go for more than it did so I didn't bid. Went for £22 in the end. Idiot.

Random bit of homebrew stuff I saw a few years ago at at hamfest as well. A whole homebrew scope. Was £5. Wish I'd bought it to play with:



Edit: talking of amateur radio tat, I managed to snag this pile. As you know I'm not a particular fan of ARRL publications but there are some good earners here 



Also 33120A knobs finally arrived. Fresh knob installed!

[CJay]

Edit: talking of amateur radio tat, I managed to snag this pile. As you know I'm not a particular fan of ARRL publications but there are some good earners here 

oof, that bottom one, Solid State design, it's worth more than the rest put together.

[bd139]

Yes indeed. Solid state basics is an interesting one as well. That's sort of the precursor to it. I have two copies of that at the moment.

[mnementh]

https://archive.org/details/SolidStateDesignForTheRadioAmateur1986

Part of the Folksanomy HAM Radio Collection:

https://archive.org/details/folkscanomy_hamradio

Just on GP:

https://archive.org/details/folkscanomy_electronics

mnem
I need to go make myself a cup of coffee so I can see clearly enough to brew a pot.

[bd139]

Yep aware of those.  Have an archive of that ... Tangible copies are worth more to people. The books are technical classics really so seem to fetch a high amount of money.

You do get some proper loons though. One guy I sold a book to told me about the amateur radio doomsday prepper library he was building. I couldn't help picture him sitting there as a skeleton draped over his key because he forgot to pack food...

I'm just an honest businessman who knows how to put supply in touch with demand for a markup to pay for my TEA habit 



Talking of TEA habit, 5pm and no Tek scope bits have arrived yet. Was hoping to blow myself up on that 2235 power supply again this evening!

[mnementh]

Yeah, I figured that when you said they were "Good earners"... They're for the "collection". Just like anybody in the writing arts has a copy of the Scott-Foresman and a couple prehistoric dictionarii/thesaurii on the bookshelf.   

I just figured that for the casual reader here long after your copies have been sold, a pointer to those online resources would be a good idea.

mnem
A thesaurus is not an extinct reptile. Neither am I.

[Housedad]

Sheesh.  You guys with all the TEA.  You have more money than I to burn on this, and here TEE is my middle name!  (Really, seriously, Tee is my middle name.)  I am a Test Equipment Enthusiast by name.

So sad.  I'm way behind. 

[nixiefreqq]

I just reverse engineered it from the picture and into DaveCAD and it's a textbook Colpitts VXO arrangement:

Doubt that has any match on the output near 50 ohms!

There may be an inductor in series as well with the capacitor. I couldn't quite make that out though. Pull it one way with L, then the other with C would make sense to get it in the right place if warmed up.

I'm not sure if it is effective though. Most crystals need to be cut for the temperature rather than heated then pulled. Would lead to frequency instability problems and drift.

Edit: that was the most fun I've had today 

Edit 2: This was probably designed to work with a step attenuator or something which would eventually give a better match to 50 ohms after enough attenuation.

your point is well taken.   it is probably a waste of time to dork with the load match until you can characterize the source impedance.

BUT lots of things are a waste of time.  so here goes.

the first pic is the raw ttl driven time base output of the hp 5327 counter with only the 1 Meg input impedance of the scope as a load (on the end of a 3ft rg58 jumper).  looks pretty bad. 

the second pic is the coax terminated in 100 ohms.  looks a lot better. 

might be able to make it look even better with 100 ohm coax.  (but there is none in my junk box). 

what is the the source impedance of a ttl hex inverter output?  who the hell knows. (but 100 ohms seemed like a good guess)



ps  everything i know about "the law" i learned from rumpole.

pps  most of what i know about swmbo came from hilda.

[bd139]

True. Let's approach it analytically.

You can calculate the output impedance of that easy enough if you have both divider resistors. Could only see one here so we're sort of pissing in the dark a bit. From that divider and supply you can derive the quiescent current ((Vb-0.6)/150) and therefore small re from Ebers-Moll (26/Ie(mA)). From that you'll find that the emitter can quite happily source a fair bit of current there through small re. But it can't sink it easily as the entire sinking ability is defined by large Re which is 150 ohms here. It's not a push-pull stage. So the amplifier's ability is asymmetrical for low impedances. Really around 400-500 ohms or so is probably the lowest impedance you can chuck on that and get something reasonable looking out.

For me, if it had a common emitter amp on the output with Zin of 1K and out impedance of 1K into a 4:1 matching transformer you'd get a good 50 ohm match cheaply. In true coincidental TV style, here's one I made earlier. Well sort of. A few days back. It's a 7MHz double buffered FET then matching amp. This is a spot on match into 50 ohms confirmed with a return loss bridge (assuming that isn't total poo which it might be  )



Bob Pease style! (this is just a test rig, not the final thing!)

With an inverter you can usually stack them with a suitable series resistance. Say 200 ohms and 4 gates out and that will give you a pretty good 50 ohm match.

Edit: you can sort any matching problem out easily with attenuators though at the cost of amplitude. Stuff a 20dB in line! Problem solved. But that's lazy.

Edit 2: also EMRFD chapter 1. Excellent example of how to build matching networks in here: http://www.arrl.org/files/file/Product%20Notes/chapter_1.pdf ... see 1.21 (really damn good book!)

[nixiefreqq]

cool beans! 

would run to the bench.

but the mailman just went down the street and the replacement for my lost pocket knife is supposed to be delivered today.

(a buck 505)

i hope its here.......i hope its here........i hope its here.

[nixiefreqq]

now where did I put the sharpening kit?

[Wolfgang]

Bob Pease : Jim Kortge = Bronx : Manhattan.

[0culus]

Got my Hakko 301 today from Tequipment. Got the package deal with the iron stand and tips. It's going to be hard going back to manual pumps and braid now.