Test Equipment Anonymous (TEA) group therapy thread

[DC1MC]

Accidently listed a TDS 3064B for 12€. 

Did you sold it ?

[Ice-Tea]

Within a minute or two. I cancelled the sale, which is a dick thing to do but I guess a normal person will understand it's no fair to expect a 99% discount.

[TERRA Operative]

I'll give you 13€

[Ice-Tea]

Anyone know if there's a way to "force" SPC on a TDS 3014B if there's an error?

[DC1MC]

Anyone know if there's a way to "force" SPC on a TDS 3014B if there's an error?

No, you have to cancel the sale and relist it with the right price, why would you want to recalibrate it ? 

[Ice-Tea]

Anyone know if there's a way to "force" SPC on a TDS 3014B if there's an error?

No, you have to cancel the sale and relist it with the right price, why would you want to recalibrate it ? 

Heheh... different machine 

[Vince]

QUAD 303 AMP REPAIR

I just replaced the pass transistor, with a 2N4347.
The heat sink assembly actually comes off extremely easily, it's well designed. Just a couple screws on each side of the chassis and that's all.

So I soldered my 4 big snap-in caps... again, which is a huge pain, their terminals just are not made for that kind of wiring. So it's butchery for the most part... but the electrons can flow none the less so I don't think they care too much.

So I fired the amp up, again through the dim bulb tester of course and..... the light bulb lights up very bright and solid. This amp still has a massive short.

Maybe the push-pull trannies ?!

I know squat about amps, but I have the schematic so hopefully I should be able to figure it out....stay tuned.

But that will be tomorrow's next installment.

 

Have you measured the resitance across the power rails with a DMM? Power off of course and using both normal resistance and diode test mode?
If a short is shown tracking it down power off is easier and safer.

OK so  for this evening's installment :

- Found that on one channel, the two push-pull power transistors were shorted. I replaced them with 2N3055... this time I am allowed to use them because... schematic says that's what QUAD used ! 

- Measured supply rail resistance.... not shorted, a few kilo-ohms.

- Applied power : DBT stil lit solid 

- Checked again all 5 TO3 transistors on the heat sink.... still all good ! Good... problem is : looking at the schematic, shorted output transistors is the only I see for this amp to produce a massive short !

- So I did what one of you suggested I think : pulled the plug, and instead powered the amp from my lab power supply. This way I can control things precisely, ramp up the voltage very slowly and monitor the current at the same time. So I did that. Put the two outputs of the PSU in series to get 2x30V. 60V is not quite the 85V the amp produces, but if the voltage regulation board does its job and saturates the pass transistor, then I should about 60V also to feed the power boards... close enough to the nominal 67V. At least good enough I thought, to get shorts to manifest themselves.
So I slowly ramped up the voltage... at first I got almost one amp,  but then as I kept increasing the voltage, current quickly dropped and settled at 80mA or so.

So 60V in the amp, and only 80mA being drawn.... this amp is not shorted at all !!! 

SO I am thinking that, for the second time (happened when I was fixing my Tek 2232 scope years ago), the DBT is CREATING the problem when there is none !
Maybe because the light bulb I installed is not powerful enough, I don't know.

So I am now thinking that maybe I could take the risk of powering the amp from the mains, withOUT the DBT... and that maybe it wil work just fine.... but I am scared none the less, so I wanted to post this before attempting it ! Seeking approval I guess....

This further motivates me to get a bloody VARIAC ! It would solve that kind of problem quickly.... DBT is all of nothing and can fool you into thinking there is a problem when there is none. Whereas with a Variac you just increase the voltage slowly and watch the current in real time and see for yourself how the DUT is behaving !

Variac now high on the shopping list...


23H18 here..... might try powering without the DBT just before going to bed, just so I can see that yes, it does not smoke, and this way I can go to bed very content and sleep on both ears 

[tautech]

DBT usage requires a little thought Vince and a few different wattage bulbs too. 
When I first built mine I checked with many different appliances so to know what to expect with correctly working devices. < That I recommend you do.

[Vince]

OK I think I found something !!

I wanted to do better than 60V, so I added my big ass Adtech PSU on the bench. Wired its 12V output in series with my lab PSU for a total of 72V then.
Still not quite 85V, but getting closer. Idle current increased a bit, to 100mA or so.

After a minute, I noticed a puff of smoke coming from one of the two power boards and current skyrocketed... it all lasted for a split second only then puff disappeared and current moved back to its normal 100mA idle.

However if looking closer, and paying really extreme attention I could still see some veeeeeery faint smoke coming out of  an area on that board.  Tried to pin point it, not easy as the smoke is so thin and hard to spot, it seems almost like it's coming from nowhere, weird.

I "looks" like liek it's emanating from the surface of one of the two small heat sinks mounted one that board, strange.
That H/S is super hot to the touch, when its counter part on the same board, is not. Also, under that H/S is hiding a resistor that's clearly cooked !

Also I measured voltage on the body/collector of these two heat sinks. I get 5+ Volts, when on the good /other board, I get Vcc / 2 , which makes much more sense to me (it's an asymetrical supply, so the speaker needs to be biased at half Vcc I guess...).

sorry it's too late and I am too tired to post pictures and annotated schematics, it's midnight here.
I will do tomorrow.

Basically, these two transistors on the power board, are the ones that drive the base of the big TO3 trannies, the ones that drive the speakers.
Also, the two TO3 that I replaced earlier today... do belong to that very faulty board ! It's starting to add up, I like that...

Anyway, clearly something is wrong with that area of that particular board/channel, so I will look at that tomorrow, and post the schematic and pics to show you exactly what I am talking about.

But I can finally sense the smell of victory, I would say !!! 

[Specmaster]

@Vince, just as a point of interest, what size of bulb did you fit in the DBT then? Clearly if you go to a low wattage, then that filament is going to restrict the available power to the DUT.

Sort out the charred resistor tomorrow and test again, and as for the variac, I agree, get one as soon as you can, then you have the ability to control the voltage in small steps and observe the results until you are able to give it the full beans. 

Edit:
It sounds very much as if you are needing to get yourself a few decent power supply sources so that you can put a few in series to achieve the 67v you need for those power rails, and then you will also have of course some adjustable current protection build in. In this way, you can isolate the built-in supply voltage regulation and feed the power amp directly and slowly raise the current via the power supply unit's controls. This would then prove once for all if you have cleared the issue with the amps. Then with a variac and the internal supply connected, but the voltage regulation section isolated, you can see if the main unregulated supply is capable of being brought up to full input voltage without any issues. Then reconnect the regulation section to the power and see if that passes the test. Once you have proved that they are OK, then reconnect the main amps, and it's all done, and you have a saleable item on your hands

It's always a good thing to have many power supplies to hand to be able to provide clean current limited supplies to DUTs in order to assist fault-finding, even short circuits can be found in this fashion by limiting the current to a very low level and observing which components show signs of heating up, infra thermometers or thermal imaging cameras are very good for detecting areas of concern.

[TERRA Operative]

I don't have one of those testers (most of my tinkering is on gear with SMPS) but I heard you should use a light bulb with a similar wattage to the DUT.

[McBryce]

Looking for stocking filler present for someone, but they already own an AVO converted to a lamp? No worries, get them one of these: https://www.amazon.de/Handwerk-Porthole-Radior%C3%B6hre-Steampunk-Industrial/dp/B01FXGOD76/ref=sr_1_4?geniuslink=true&keywords=SlavaTech+Pentode+USB+flash+drive&qid=1669201738&sr=8-4&language=en_GB 

McBryce.

[m k]

Variac now high on the shopping list...

Clearly you should use one more of those TO-3 stones and make adjustable pre-regulator,
with extra power switch and adjustable overcurrent protection.

[TERRA Operative]

Anyone got setup disks for a Dell Precision 410MT workstation by chance?

I need a working ISA Config Utility (ICU) disk....

[m k]

Wow, quite a motherboard.

E,
torrent
https://archive.org/details/dell410mtdiagdisks

[TERRA Operative]

Yeah, I found that link, but it's just some system diagnostics that doesn't really work to well.

I'm running 1Gb ECC RAM with dual 600MHz PIII slot-1 CPU's and a Quadro FX3000 AGP GPU, so I'd love to get one of my ISA National Instruments GPIB card working to calibrate my Tek TDS600 and TDS700 series scopes....


Too bad the stupid motherboard needs the dumb ISA Config Utility boot disk to alter settings, rather than just doing it in the BIOS like normal sane motherboards do...

[PA0PBZ]

Anyone got setup disks for a Dell Precision 410MT workstation by chance?

Did you have a look here? https://www.dell.com/support/home/en-us/product-support/product/precision-410/drivers

There is something called "Dell System Utilities, v.1.0, A01", not sure if that is what you are looking for.

[TERRA Operative]

Yep, it's a diagnostic utility, not ISA config.

I have found one ISA config utility disk image, but it don't work...

[m k]

Link or upload?

[TERRA Operative]

This is the closest I've gotten.

https://www.dell.com/support/home/en-us/drivers/driversdetails?driverid=r19145

[m k]

Not working how?

It's a Disk eXPress V2.34 image and needs to be written back to static locations.
Also possibly including several CRC-32 checked parts so extracting manually can be time consuming.
The creating DXP software can also extract images.

The file DE000001.EXE should have at least /d and /t options.
(description and test)

Since static locations are needed maybe you should create a virtual disk as a destination.
DXP has also a disk interleave option, that's not optimal since it can be set to what ever.

It's a bit old stuff but some old Windows DOS box support may be available.

[Vince]

QUAD 303 AMP REPAIR

OK worked a bit on the thing.

I meant to resume work on that defective power board, but before doing that I wanted to do a quick little test.

Since I have 3 PSU in series, I can now feed up to 72V to the amp, which is 5V more than the 67V the regulator board is supposed to regulate at.

72V is not quite the nominal 85V of course, but I thought it should still be enough for the board to operate and regulate the voltage at 67.

So I wanted to check for that.

Didn't work : the pass transistor is always turned on hard, Vce = 0.6V or so, no matter the input voltage, it passes it straight through !

So I decided that I would try to fix that before returning to the power board.

Remember on that regulator board, a resistor smoked and charred. It was the dropper resistor for the Zener diode.
Schematic says it should be 10K but from what's left of the body of this resistor, can only see the first colour band now, I would say it was either 6.8K or 8.2K, so a bit less than 10K.
Also, despite being charred, it still measured IIRC between 5 to 10K, so good enough.... it should not cause the board to fail massively like it does.

Still, in doubt, I replaced it anyway. Meant to go back to the 10K value showed in the schematic, but would you believe it, it looks like I have ZERO 10K left ?! 
How can this be ! Need to buy some !
So instead I put two 18K in //, 1Watt jobbies.... just in case the old one smoked because its power rating was a bit miscalculated...
But still no joy.

So I measured the reverse voltage across the Zener diode. I had already checked it much earlier, power off, in the forward direction, and it read OK.
But powered up, it drops only 0.9V or something in reverse !   
That can't be right.... way too low.
Schematic does not say what value it is, and the parts list shows two numbers but none of them return anything on Google.
However they looked like it might be a 12V Zener. On the diode itself the markings are a bit faded but it seems about consistent / plausible...
So I put a new 12V Zener in there.... no joy, still drops 0.9V ! 

Oh dear.... what the hell is wrong with this board !!!
I had already buzzed every component on that board, they were OK !

Anyway, turned out I must not have checked properly ONE component... the trim pot that adjusts the output voltage !
Supposed to 4.7K, but read 2 or 3 times that in circuit ! So I pulled it off the board to measure it properly... read open circuit whichever way you test it !
Not just the wiper connection, but even the track itself was open circuit ! 

So I went to replace that trimmer... looks like your typical cheap plastic black/white horizontally mounted trimmer, have plenty of those ! 
... but of course Murphy made it that I had plenty of 470 and 47K but not one single 4.7K.
After checking all my stock one by one... the best I could manage was a 10K one, but VERTICAL not horizontal !
Managed to butcher it on the board anyway, helped by the fact that the trimmer is used as a variable resistor not a potentiometer, so I only needed to shove 2 of its 3 pins into the board.

I am not gonna lie, it's butchery, but I do what I can with what I have, and this is for troubleshooting... I can always put a better replacement later.
So please be forgiving.

Anyway, now the board WORKS !!! 
It does regulate at long last. I can adjust the voltage as I wish. However with a 10K trimmer instead of 4.7K, it's very touchy of course ! I managed to set it to 67.1V and it's gonna stay like that ! If I try to make it spot on 67.0V it could take me hours of back and forth, no thank you !

So that's cool, one of the two big problems in this amp is now solved !

I can  trust the PSU section now...

OK it's 20H00, I still have some time left to resume work on that faulty power board now....

[tautech]

 
Good job Vince !
10k and 4.7k trimmers here. 

[Vince]

Yes, so many trimmers and the one time I really need one... I can't find the correct one ! 

I think I need to do some more component sorting and inventorying... need to inventory all these trimmers so I know what I need to buy to fill the gaps.

And some 10K 1/4W resistors as well...

[PA0PBZ]

I have found one ISA config utility disk image, but it don't work...

Try this, it did put up a fight and I switched virtual machines a lot but it looks promising.