Test Equipment Anonymous (TEA) group therapy thread

[mnementh]

If you did that, a plagiarized version would find its way onto github in a week. Then every yo-yo with a smartphone would be buying your parts before you.

If you're talking about writing it just for yourself and not sharing... then *Shhhhhh...* the walls have ears. 


mnem
All hail the Hypno-Toad!!!

[med6753]

Well done, old Bean. 

My favorite kind of successful diag; where you do most of the work on the schematics and in your brain. So very satisfying. Although, I figured if anyone had devised a means to measure the crystal oscillation without disturbing the circuit, it would probably be you.

mnem
*Currently enjoying a pants-free day*

I didn't even try measuring the crystal because I knew as soon as I hung any sort of probe on it the damn thing would shift frequency instantly. So I can't take credit for devising a way to measure it without making it go ape shit.     

[med6753]

Old crystals are bastards, all of them. I was repairing an old counter for someone a few years ago, one of the very early LED heathkit ones and the damn thing just shot off all over the place occasionally. It took me three hours to identify the crystal. I was watching the supply lines and they'd spike heavily when the counter stalled so assumed it was power supply related (turned out to be Heathkit "economy decoupling"). Eventually I pulled every damn IC on the board out and added them back from the crystal oscillator forwards. Literally the moment I put poor little 7404 back in I could see it going mental. Looked at the crystal and it was a POS from 1958 that Heathkit had clearly bought in cheap. It was probably 15-18 years old when they shipped it in the kit. Replaced it with a modern one and changed the load caps so it was about right and it was fine after that.

I'll bet it was a 3.58Mhz color burst crystal, right? They were real cheap and a dime a dozen. I had a Heathkit frequency counter that used one. It was surprisingly stable and accurate right up to the day I sold it.

[mnementh]

Well done, old Bean. 

My favorite kind of successful diag; where you do most of the work on the schematics and in your brain. So very satisfying. Although, I figured if anyone had devised a means to measure the crystal oscillation without disturbing the circuit, it would probably be you.

mnem
*Currently enjoying a pants-free day*

I didn't even try measuring the crystal because I knew as soon as I hung any sort of probe on it the damn thing would shift frequency instantly. So I can't take credit for devising a way to measure it without making it go ape shit. 

That's why I said "If anyone"...

I have to admit... now that you've got me thinking about it, there HAS to be some solution to this conundrum that has plagued tinkerers and repair techs alike for like... EVER.    Maybe some misuse of one of those new micro inductive current probes? 

mnem
Fuck.    I just realized that I HAVE TO do something this morning that requires pants... *Sigh*

[bd139]

I'll bet it was a 3.58Mhz color burst crystal, right? They were real cheap and a dime a dozen. I had a Heathkit frequency counter that used one. It was surprisingly stable and accurate right up to the day I sold it.

Yep that was the one! They probably already had it in their inventory from their TV kits.

[mnementh]

https://youtu.be/Q7LKJXvf_do

Why am I imagining this as their warehouse, and this guy writing up the BOM?   

mnem
Yup. I went there. Can I please come back?

[Specmaster]

@mnementh Yes it is a bloody power amplifier in a IC but only a 2 channel one that is as you correct assumed, BTL and it is the only item that takes the full supply rail voltage of 37V +ve and -ve. I don't believe that the IC is shorting out internally as the outputs are taken through speaker selector switches which are mounted on the front panel and are connected via that flat 7 core cable. It also connects the headphone socket, nothing else is carried via that 7 core cable, it's just output from the amp IC 2 channels in and then 2 x 2 channels coming out again to speakers A and B. No permanent voltage should be but I was reading 36V on right channel, on the A setting, B setting appears to be fine.

If the IC was internally shorted then operating the A B speaker selector would move this 36V from Ch1 R to Ch2 R but it isn't. Anyway, its late now and I'm toddling off to bed for some well earned sleep, got to be up early tomorrow to collect Mother in Laws flower plants for her, and then shoot off doing a 40 mile round trip collecting more audio gear and a set of speakers to replace my smoked ones. I'll look into this more tomorrow.

Does the 36VDC on the faulty A speaker output go away when switched to B Mode? A+B mode?

If not, I still suspect the IC.  You can confirm easily enough by tracing that hot channel back to the IC and desolder the leg to see if there's still 36VDC at the leg. If so, little doubt as to the culprit.

Cheers,

mnem
ZZZzzzZZZzzzZZZzzz...

I'm out shopping at the minute, I'll try that when I get back later. I expect personally it is going to be leaking onto the track from AB switches to speaker terminals. If it was on the input to the AB switches then selecting B speakers would switch the 37V connection from A to B but it isnt

[med6753]

https://youtu.be/Q7LKJXvf_do

Why am I imagining this as their warehouse, and this guy writing up the BOM?   

mnem
Yup. I went there. Can I please come back?

I like British humor (humour) but in this case the accents are too strong for me to understand it and fully appreciate it.    To my Yankee ears it's almost as if he isn't speaking English. 

[Specmaster]

https://youtu.be/Q7LKJXvf_do

Why am I imagining this as their warehouse, and this guy writing up the BOM?   

mnem
Yup. I went there. Can I please come back?

I like British humor (humour) but in this case the accents are too strong for me to understand it and fully appreciate it.    To my Yankee ears it's almost as if he isn't speaking English. 

Funny that we gave have much the same with some Americans as well [emoji23]

[bd139]

https://youtu.be/bIaUfBjHjpI

[med6753]

Check this out. Genuine boatanchor. Very sensitive differential inputs but only about 1Mhz bandwidth. Appears to be in good shape but I see the upper right cover fubar.

$100? Maybe. $200? Are you nucking futs?    I wish him good luck. I'll be watching to see if the price drops.

[med6753]

https://youtu.be/bIaUfBjHjpI

I don't understand that red neck either. After saying "911" it was pure gibberish. 

[mnementh]

Having spent some time living barely surviving in a little shithole town on the Georgia/Alabama border, I can tell you firsthand that not one of the characters in that show is in any way an exaggeration.    The only thing wrong with the show is the Hill family themselves; they're entirely too uptown. 

Okay... anybody on the other side of the pond gonna fess up and admit the same of their barely intelligible rednecks?

mnem
I barely escaped. That place, that life... it sucks you in like a black hole of despair. *shuddder*

[bd139]

Yeah we’ve got ours. Wanna buy a dag?


About to commence 8012A mods. Starting with power supply measurements. Fingers crossed

[tautech]

About to commence 8012A mods. Starting with power supply measurements. Fingers crossed

Don't just cross it, tape that penis finger away so it can't be blamed. 

[bd139]

I’ve got ten of those fingers 

[Specmaster]

I’ve got ten of those fingers 

[mnementh]

Yeah we’ve got ours. Wanna buy a dag?

Umm... no thank you. All my sheep are vinyl, so highly dag-resistant. 

mnem
Yes, you may touch it.

[tautech]

I’ve got ten of those fingers 

Some days.........me too ! 

My deepest sympathies.

[mnementh]

I’ve got ten of those fingers 

They make a special prophylactic for guys like you...















...but in your case, these might be safer. 

mnem
You read my PS. That's enough interaction for one day.

[factory]

Check this out. Genuine boatanchor. Very sensitive differential inputs but only about 1Mhz bandwidth. Appears to be in good shape but I see the upper right cover fubar.

$100? Maybe. $200? Are you nucking futs?    I wish him good luck. I'll be watching to see if the price drops.

Also the CRT is a proper dual beam one too & mostly vacuum-bulb based apart from a couple of transistors in the low voltage supply & another in the sweep circuit.

Mine followed me home from a local yesteryear steam rally last year, rescued from the local house clearer's stall, sadly it had been left out in the rain for the previous day and had clearly spent a long time in a damp shed too.
It dried out OK and after several cracked ex-vacuum bulbs were replaced I got a trace, unfortunately this was quickly lost after fixing the power supply & replacing the snapped intensity control. 
That transistor in the sweep lockout circuit was made the gold plated rust too.  Still haven't got round to replacing it yet.

David

PS. This was my first Tek purchase too, the price was a more sensible £20..

[bd139]

Fluke 8012A update. ALL of the damn electrolytics have spewed their guts on the board. Major cleanup. Surprised it even came up.

[mnementh]

Got you covered, bd. 


@factory:  Damn you... more hand-drafted schematics. And these have character out the yodel-ay-hee-hooo...  *wibble*

mnem
Remember when you could be in business and still have a soul?

[Specmaster]

Been working on the Technics amp tonight and I have no idea what I was doing / thinking last night, must have been distracted or someit  , pulled the main board out and checked all tracks for bridges, could I see any, nah.

Refitted it and powered up without speakers this , connected 4 meters up (internally this time) and checked to see if the 36V was being switched by the AB speaker switch and darn it, it was so WTF did I not spot that last night???

Got a 5th meter and probed the main amp IC pins to see if there was an internal short when powered, and bugger it, there was (you were 100% bang on menmenth)  the right hand channel was instantly raised to 36V, left channel, not a problem.

New power amp IC on order, just got to wait for that slow boat to come now. Arh well onto the next item in line for the bench, an Aiwa XR-M500 whose CD player skips on tracks 1 and 2 and the tape drive doesn't work, otherwise performs really well. 

[bd139]

Nasty. No stonking great big DC blocking cap I bet either!


8012A power supply and caps done. Didn't notice it was a bit slow to start up initially. Turns out bad caps!

The regulator is a simple LM317 but in constant current mode. I converted it to constant voltage mode. I cut the trace to the adjust line to the regulator and bridged the old current limiting resistor:



Then on the bottom of the board I used a 240 and 330 ohm resistor to provide the reference voltage divider for the regulator. This gives 3.0V out which is close enough to the 2.8 of full NiCads. I didn't have a 120 ohm resistor or I would have used 120+150. Since taking this photo I've added a small run of epoxy on the bottom of the board to support the floating resistors better.



And then all the caps were swapped out. Fortunately I had all of them in stock already. Joy to buying way too much of everything. Check that big one's bum. Nasty stuff!



And I'm off to bed now. Display replacement tomorrow!