Test Equipment Anonymous (TEA) group therapy thread

[bd139]

The USB power will not be wired to the rail directly. There will be a host controller in the IC that handles all that faff including power negotiation.

3.3v is interesting through. It should be 5V. Check it with a scope to make sure it is actually that and not the average voltage you are reading.

Thanks. OK that means we back again to incriminating that freaking CPU ! It's not looking good then 

OK so I really need to pull the datasheet for that CPU, see how it handles USB power. If it requires with some luck the help of external discrete components that might have failed... if not and it's the CPU direct, then it's toast 

23H56 here... will work a short while on it then 

I hate debugging that sort of stuff. I just throw it in the trash. Don’t even strip it for parts now.

It’s always either a cost cutting measure that blew up or a lifespan limiting design feature. And I can’t be arsed to reengineer Chinese shit these days. I’ve done it enough to know that it’s not worth the tears.

Edit: aka life is too short.

[Vince]

Yeah I would not bother either ! Well I would not own such a piece of junk to begin with.... but as I explained I am under a moral obligation to go the extra mile to rescue this thing.


However after what you and Dwagon said, it's realllly looks like it's that cheap disposable CPU that's toast USB wise, so the "extra mile" is 99% covered already....

All I can do now to be 100.0000% sure it's the CPU, so I can look my friend in the eyes... is to make 100% sure there is zero discrete component between the CPU and the USB connector power pin.

So I need to find the datasheet for the CPU... and sadly I just spent 10 minutes on Google and NO LUCK ! This CPU is so crap apparently that you can't even find a datasheet for it....

So looks like I have reached the end of the road.... pfff....

People keep thinking/assuming somehow that electronics and simple and can be fixed easily in 2 minutes for free. I keep telling them that no, it's the other way around : there is a millions things that can prevent you from repairing a device, and that a quick cheap repair is not the rule it's the exception ! But when I tell them that they look at me suspiciously, like they are thinking I am either incompetent or trying to find excuses because I am lazy and don't work to spend  any time on their stuff...

[mnementh]

JUNK STEREO REPAIR UPDATE

Anyway. Regardless, it looks like USB/SD are having power supply issues, be them with 3.3V or 5V doesn't matter. They don't get proper power and I need to investigate that don't I...  The CPU works fine and the output of the 3.3V regulator is solid, no issues there. So even if USB was indeed using 3.3V rather 5V, then the problem is not with the 3.3V regulator. There must be some discrete components in between the regulator and USB/SD connectors. That's plausible because when I do a continuity test, I don't get continuity between, the power pins of Sd and USB, nor do I get continuity between USB and 5V rail or SD and 5V rail.

tried looking at traces around the USB power pin. did some buzzing. I see an MLCC cap between that power pin and ground. But it's not shorted nor low impedance (could try removing it anyway just to see...). then there is an inductor in series with the pin, then it buggers off to I don't know where because the other side of the inductor goes to a buried via... does not come out the other side of the board  .....

OK back to the bench, oh dear....

SD card is 3.3V, USB is 5V. They can be and often are powered from the same 5V source; I'd look for a 3.3V LDO regulator in SOT-23 or SOT23-5 footprint. Yes, just like a transistor.

mnem
virtual

[mnementh]

The USB power will not be wired to the rail directly. There will be a host controller in the IC that handles all that faff including power negotiation.

3.3v is interesting through. It should be 5V. Check it with a scope to make sure it is actually that and not the average voltage you are reading.

Naaahhh... these POSes don't do power negotiation... they're just gonna have 5V@300-500ma to power a USB memory stick and nothing else. And often likely a 3.3V LDO on that rail to power the SD card slot, if it doesn't draw off the 3.3V to the CPU.

Think Android 4.0 grade hardware.

mnem
 

[bd139]

Yeah I would not bother either ! Well I would not own such a piece of junk to begin with.... but as I explained I am under a moral obligation to go the extra mile to rescue this thing.


However after what you and Dwagon said, it's realllly looks like it's that cheap disposable CPU that's toast USB wise, so the "extra mile" is 99% covered already....

All I can do now to be 100.0000% sure it's the CPU, so I can look my friend in the eyes... is to make 100% sure there is zero discrete component between the CPU and the USB connector power pin.

So I need to find the datasheet for the CPU... and sadly I just spent 10 minutes on Google and NO LUCK ! This CPU is so crap apparently that you can't even find a datasheet for it....

So looks like I have reached the end of the road.... pfff....

People keep thinking/assuming somehow that electronics and simple and can be fixed easily in 2 minutes for free. I keep telling them that no, it's the other way around : there is a millions things that can prevent you from repairing a device, and that a quick cheap repair is not the rule it's the exception ! But when I tell them that they look at me suspiciously, like they are thinking I am either incompetent or trying to find excuses because I am lazy and don't work to spend  any time on their stuff...

Had that for years. I impolitely explain my time is worth more than the device is worth. That usually gets the point across. Same with computer problems.

Had a recent one with my uncle who wanted his laptop repaired. It’s about 8 years old and quite frankly disgusting. I don’t even want to touch it. I impolitely explained that it doesn’t matter what you want me to do you will have to pay me the hourly rate for what I could have been doing instead because my time isn’t worth it. He bought a new one.

And that’s the reality of a lot of repairs these days. I only play inside old test gear for fun.

[bd139]

The USB power will not be wired to the rail directly. There will be a host controller in the IC that handles all that faff including power negotiation.

3.3v is interesting through. It should be 5V. Check it with a scope to make sure it is actually that and not the average voltage you are reading.

Naaahhh... these POSes don't do power negotiation... they're just gonna have 5V@300-500ma to power a USB memory stick and nothing else. And often likely a 3.3V LDO on that rail to power the SD card slot, if it doesn't draw off the 3.3V to the CPU.

Think Android 4.0 grade hardware.

mnem
 

Straight in the WEEE bin…

[Vince]

The USB power will not be wired to the rail directly. There will be a host controller in the IC that handles all that faff including power negotiation.

3.3v is interesting through. It should be 5V. Check it with a scope to make sure it is actually that and not the average voltage you are reading.

Naaahhh... these POSes don't do power negotiation... they're just gonna have 5V@300-500ma to power a USB memory stick and nothing else. And often likely a 3.3V LDO on that rail to power the SD card slot, if it doesn't draw off the 3.3V to the CPU.

Think Android 4.0 grade hardware.

mnem
 

No datasheet for this CPU anywhere so... been buzzing the CPU instead. I was able to find the two CPU pins that are connected to the two data lines of the USB connector. So it's direct connection, no circuitry in between. Then I buzzed the USB power pin.... didn't appear to be connected to any of the CPU pins !  That's good hey ? Means there is some hope ? Either there are discrete components between the CPU and power pin, that prevent a continuity tyest from buzzing, or it's not powered by the CPU as you said, so the offending components are instead between the 5V rail and the USB connector.

So a tiny ray of hope... let's do some more buzzing...

[mnementh]

Yeah I would not bother either ! Well I would not own such a piece of junk to begin with.... but as I explained I am under a moral obligation to go the extra mile to rescue this thing.


However after what you and Dwagon said, it's realllly looks like it's that cheap disposable CPU that's toast USB wise, so the "extra mile" is 99% covered already....

All I can do now to be 100.0000% sure it's the CPU, so I can look my friend in the eyes... is to make 100% sure there is zero discrete component between the CPU and the USB connector power pin.

So I need to find the datasheet for the CPU... and sadly I just spent 10 minutes on Google and NO LUCK ! This CPU is so crap apparently that you can't even find a datasheet for it....

So looks like I have reached the end of the road.... pfff....

People keep thinking/assuming somehow that electronics and simple and can be fixed easily in 2 minutes for free. I keep telling them that no, it's the other way around : there is a millions things that can prevent you from repairing a device, and that a quick cheap repair is not the rule it's the exception ! But when I tell them that they look at me suspiciously, like they are thinking I am either incompetent or trying to find excuses because I am lazy and don't work to spend  any time on their stuff...

Yeeaahhh...

When I usedta still do this on the side I charged $70/billable hour, and minimum diag fee if they brought it to me was 1/2 hour. If I was able to fix it in that time, then Yay! They got out of it for that plus parts and shop supplies fee (if I used appreciably more than a few cm of solder and a squirt of IPA). If not, then we'd revisit for further diag time and or repair.

When people started hemming and hawing over that pittance, I quit taking on outside work and started selling repaired gear as a side gig. Lots of people would beg me to fix stuff for them instead of buying it, and I politely explained that I couldn't afford to do that anymore. I'd been stiffed on too many expensive parts orders; nobody wanted to pay enough for repairs to equal a living wage. That was what I was doing for a business when I met my wife in a online chat room a couple decades ago...

mnem
"The long and winding road... that leads to your door..."

[mnementh]

The USB power will not be wired to the rail directly. There will be a host controller in the IC that handles all that faff including power negotiation.

3.3v is interesting through. It should be 5V. Check it with a scope to make sure it is actually that and not the average voltage you are reading.

Naaahhh... these POSes don't do power negotiation... they're just gonna have 5V@300-500ma to power a USB memory stick and nothing else. And often likely a 3.3V LDO on that rail to power the SD card slot, if it doesn't draw off the 3.3V to the CPU.

Think Android 4.0 grade hardware.

mnem
 

No datasheet for this CPU anywhere so... been buzzing the CPU instead. I was able to find the two CPU pins that are connected to the two data lines of the USB connector. So it's direct connection, no circuitry in between. Then I buzzed the USB power pin.... didn't appear to be connected to any of the CPU pins !  That's good hey ? Means there is some hope ? Either there are discrete components between the CPU and power pin, that prevent a continuity tyest from buzzing, or it's not powered by the CPU as you said, so the offending components are instead between the 5V rail and the USB connector.

So a tiny ray of hope... let's do some more buzzing...

Empirical Engineering: one of the most useful skills any fixer can develop. 

mnem
 

[mnementh]

Yeah I would not bother either ! Well I would not own such a piece of junk to begin with.... but as I explained I am under a moral obligation to go the extra mile to rescue this thing.


However after what you and Dwagon said, it's realllly looks like it's that cheap disposable CPU that's toast USB wise, so the "extra mile" is 99% covered already....

All I can do now to be 100.0000% sure it's the CPU, so I can look my friend in the eyes... is to make 100% sure there is zero discrete component between the CPU and the USB connector power pin.

So I need to find the datasheet for the CPU... and sadly I just spent 10 minutes on Google and NO LUCK ! This CPU is so crap apparently that you can't even find a datasheet for it....

So looks like I have reached the end of the road.... pfff....

People keep thinking/assuming somehow that electronics and simple and can be fixed easily in 2 minutes for free. I keep telling them that no, it's the other way around : there is a millions things that can prevent you from repairing a device, and that a quick cheap repair is not the rule it's the exception ! But when I tell them that they look at me suspiciously, like they are thinking I am either incompetent or trying to find excuses because I am lazy and don't work to spend  any time on their stuff...

Had that for years. I impolitely explain my time is worth more than the device is worth. That usually gets the point across. Same with computer problems.

Had a recent one with my uncle who wanted his laptop repaired. It’s about 8 years old and quite frankly disgusting. I don’t even want to touch it. I impolitely explained that it doesn’t matter what you want me to do you will have to pay me the hourly rate for what I could have been doing instead because my time isn’t worth it. He bought a new one.

And that’s the reality of a lot of repairs these days. I only play inside old test gear for fun.

Yup. Same for me and old computers. A while back I bought a "WalMart Special" Acer laptop with  N2840 CPU/4GB RAM/500GB HDD. I took on the usual warnings in here that it wasn't worth the money to fix it up, but I wanted to see what was inside of it and fix it up to keep my diag skills sharp.

Right now I'm in it for $67 and it runs half-decent but still spinning rust slow; I'm toying with the idea of popping in a old 120GB SSD I unearthed the other day to make it more attractive before I flip it.

But this was all for the fun of fixing and keeping my skills sharp; I know when I do flip it I'll get $100-125 for it, so not even breaking even on my time.    But the difference is it's my toy, my time and my choice whether to spend it that way or not.

mnem
 

[Specmaster]

The USB power will not be wired to the rail directly. There will be a host controller in the IC that handles all that faff including power negotiation.

3.3v is interesting through. It should be 5V. Check it with a scope to make sure it is actually that and not the average voltage you are reading.

Naaahhh... these POSes don't do power negotiation... they're just gonna have 5V@300-500ma to power a USB memory stick and nothing else. And often likely a 3.3V LDO on that rail to power the SD card slot, if it doesn't draw off the 3.3V to the CPU.

Think Android 4.0 grade hardware.

mnem
 

No datasheet for this CPU anywhere so... been buzzing the CPU instead. I was able to find the two CPU pins that are connected to the two data lines of the USB connector. So it's direct connection, no circuitry in between. Then I buzzed the USB power pin.... didn't appear to be connected to any of the CPU pins !  That's good hey ? Means there is some hope ? Either there are discrete components between the CPU and power pin, that prevent a continuity tyest from buzzing, or it's not powered by the CPU as you said, so the offending components are instead between the 5V rail and the USB connector.

So a tiny ray of hope... let's do some more buzzing...

Don't lose sight of the fact that some meters' continuity buzzer / beeper can still work through resistances etc of around 200 to 300 ohms and also often you can get an initial buzz / beep on circuits that have caps in them as well, so make sure that you can get the signal for a few seconds rather than a quick sounding.

[mnementh]

To risk or not to risk:   
https://www.ebay-kleinanzeigen.de/s-anzeige/tektronix-577-d2-defekt-/2123531071-168-3793

DOA, Local pickup only, and some gorilla has already tried to fix the PSU... plus it's 4 1/2 hours away in Gera...? 

Even Neo would think twice on that one, even if it were free...

mnem
OTOH... OktoberFest ROAD TRIP!!!   

[BU508A]

That digimess stuff is Chinese rebrand garbage. Not even decent Chinese garbage but rock bottom stuff 

Some of it might be, idk I've not had any in my hands. This however, does not look like Chinese made, it looks low volume, low R&D budget, low design budget, mid-quality euro-tryhard. In fact, it reminds me of early TTi stuff.  https://www.ebay.co.uk/itm/234578724312



It's from rchaudio so it's overpriced. Just to prove the Grundig lineage: https://www.farnell.com/datasheets/104629.pdf

I'd pay £50 shipped.

Hmm, I had a Grundig satellite box a few year back and that was utter crap, so I'd expect this to be no better, it has a certain rarity value and that's it.

Now I know, about what that design of the front is remininding me: The Grundig PN 300 powersupply.


The Radiomuseum has some more details about it: https://www.radiomuseum.org/r/grundig_programmable_power_supply_pn300.html

Looks like, RS had the Digimess rebranded in their sortiment: https://de.rs-online.com/web/p/labornetzgerate/1590504

Rosenkranz electronic (surplus dealer) has it and is asking 199 Euro for it: https://www.rosenkranz-elektronik.com/de/messgeraete/261928/grundig-pn300


As always: nawts

[Specmaster]

If you all are wondering where have I been over the last few days, I have been to the local steam train gala on the Epping Ongar railway, something that I have had on my bucket list for sometime to do, and is now ticked off. For those that don't know, this is a 6.5 mile long track that was once part of the London Underground network. It was shut down and acquired by enthusiasts who started their own little service but also ran into trouble and the track lay idle for a while and 10 years ago a new group was formed and converted it over to diesel and steam powered locos, and this weekend saw its 10th anniversary with a couple of visiting locomotives.

I took lots of video, and now I'm grappling with trying to learn how to effectively edit the footage to remove all of my shaky camera work etc for my YT channel, using Da Vinci Resolve, which is a steep learning curve, but so much better than the basic tool that YT give you.

[vk6zgo]

Today, I decided to do some "ham Technicianing"!

I scratched my bum, donned my bib & braces overalls, picked a few fleas out of my beard, & headed off to the "shack".
Once there, I let off a stentorian fart, (my dog sniffed the air, & quickly vacated the scene), settled down on the long suffering "operator's chair", moaned loudly, & set to work.

The"patient" is an elderly Yaesu FT707 transceiver.

This rather nice looking device has a strange fault, with both the "carrier" control which adjusts output power in the CW (morse) mode, & a similar one in the SSB mode associated with the "mic gain" control.
In both cases, the control seems to go from "zero" to "flat out", if the control is moved, which makes the controls, & hence, the radio useless.

My main fear was that both pots were faulty, & initial tests, looking back at them, seemed to bear this out, with the resistance between the moving contact & the chassis, measured with my Fluke 77, jumping around as the pots were turned.
This was worrying, because they are pretty much "unobtainium".

After digging deeper into the manual I downloaded, I found several pages with better representation of the circuitry involved, so I resolved to try again, today.
This more thorough check still showed the resistance reading "varying all over the place", but, knowing that the "77" was a bit ancient (& digital) , I decided to call upon an even older device-------my HP 410C!

On Ohms, the 410C showed the resistance varying smoothly on both pots, ----What the hell?
Trying the Fluke again, showed the previous situation.

Slowly, my ancient solder smoke addled brain found the answer ---"Dawk!" the Fluke was auto ranging, hence the weird readings!!
Pushing the range hold button changes the readings whilst turning the pots  to something similar to that of the 410C!

Definitely, a "Trap for Old Players"!

If the pots are ok, what's your next suspect?

All the above stuff is situated on, or connected to, the "AF board", then the selected mode of signal heads off to the "IF board".
I have unplugged the connector involved to stop the Tx trying to produce excessive power & shutting down, so will be able to "key" the thing in each mode & see what the output of the AF board looks like.
The two sections on that board which are common to all modes are the 8V supply & the discrete diode balanced modulator.
(They unbalance it for CW & AM).

I was pretty "chuffed" to find out the pots were OK (I say "pots" but they are really two sections of a concentric dual pot.)

[BU508A]

Here's the 8000A +5V PSU mod installed. Somewhat bodged into place but there isn't a lot of room to work with, especially when it came to installing a 2nd transformer. It's been in there for almost 5 years but if I had to do it over again I'd try to come up with a neater arrangement. The 7815/7915 regulators are just to the left of the heat sinked 7805 and didn't require heat sinks.

I'm a bit confused about that transformer of your bodge:
is it really a 115V to 60V transformer for your +5V power rail?


If it is the other transformer, what are the specs of this one?
Just curious ..

[Cerebus]

That digimess stuff is Chinese rebrand garbage. Not even decent Chinese garbage but rock bottom stuff 

Some of it might be, idk I've not had any in my hands. This however, does not look like Chinese made, it looks low volume, low R&D budget, low design budget, mid-quality euro-tryhard. In fact, it reminds me of early TTi stuff.  https://www.ebay.co.uk/itm/234578724312



It's from rchaudio so it's overpriced. Just to prove the Grundig lineage: https://www.farnell.com/datasheets/104629.pdf
I'd pay £50 shipped.

Hmm, I had a Grundig satellite box a few year back and that was utter crap, so I'd expect this to be no better, it has a certain rarity value and that's it.

Alba plc and Beko jointly took over Grundig's consumer electronics business in 2004. Since then the Grundig name has devolved to being a badge on Alba or Beko products when they want a name with a bit more class to it. At the end of 2007 Turkey's Koç Holding (the parent of Beko) took over the whole shebang, but Alba still have licence to use the name. I can't trace what happened to the bit of Grundig that made professional equipment.

People outside the UK have probably never come across Alba, but if you wanted the lowest quality consumer electronics equipment then Alba is your go-to brand.

Picture of the inside of a random bit of Alba consumer electronics:

[Cerebus]

Perhaps we should all offer him £1 for it 

50p would be my limit.

[med6753]

Here's the 8000A +5V PSU mod installed. Somewhat bodged into place but there isn't a lot of room to work with, especially when it came to installing a 2nd transformer. It's been in there for almost 5 years but if I had to do it over again I'd try to come up with a neater arrangement. The 7815/7915 regulators are just to the left of the heat sinked 7805 and didn't require heat sinks.

I'm a bit confused about that transformer of your bodge:
is it really a 115V to 60V transformer for your +5V power rail?


If it is the other transformer, what are the specs of this one?
Just curious ..

Yep, I have no clue as to why it's labeled as "60V" when it's nothing more than a standard 12.6VAC secondary C/T transformer.

[Vince]

JUNK STEREO REPAIR UPDATE

Don't lose sight of the fact that some meters' continuity buzzer / beeper can still work through resistances etc of around 200 to 300 ohms and also often you can get an initial buzz / beep on circuits that have caps in them as well, so make sure that you can get the signal for a few seconds rather than a quick sounding.

Yep ! Think my Fluke 11 sets the bar at 150 ohms IIRC, but whatever... yes it's taken into account. That meter has an excellent continuity tester. Super fast. You just can't mistake a quick blip with a positive/true continuity. I just love my Fluke 11, still my go to meter 25 years after I bought it !


Anyway, I have spent lots of time buzzing again and I got somewhere....  finally found where that USB power pin was going to. It goes to the drain of a tiny P-channel MOSFET that's miles away, near the edge of the board right by the audio aux connector. Yes, I spent ages checking every single component on the board before eventually checking that "unlikely" area of the board...
MOSFET has 5V on the source and gate, hence not conducting, so not being asked to. Took it off the board anyway, with the hot air station, then put it on the chinese tester, it has SMD pads. still was a challenge trying to hold it steady with one hand with tweezers, while trying to press the push button with the other hand. Took a few attempts... but the FET tested good.
However before soldering back to the board, first I did a quick sanity check : I shorted drain and source with a piece of component leg that I soldered. Wanted to see I could force 5V to the USB connector. Yes I can, I now have a rock solid 5V at the USB connector !

Overjoyed, I rushed and plugged my USB stick.... the 5V supply still holds fine, yes !  ..... but still doesn't work, no activity going on on that stick ! 

tried the SD card, still no go even though the power is good.

I give up. The fact that both USB and SD fail is suspicious to say the least, especially given everything Dragon said about this very problem on these very ALI CPU's....

I think I have run out of options and hope now... time to move on. It's 2H30 AM, I going to bed. Will phone my friend tomorrow to tell him all about it....

No regrets, tried my best.... plus it was a learning experience, I know more than when I started on it 12+ hours ago.

Thanks every one for your help, much appreciated...

[TERRA Operative]

DOPAMIIIINNNNNEEEEEE!

I now have a fully working TDS794D!
I tracked down two bad acquisition RAM chips, U405 and U413 on channel 2. Replaced them with parts pulled from a scrap board (a smashed unit with a physically damaged acquisition board, well and truly for pulling parts..)

Now it passes all self-tests and no spurious spikes on the display. Yaaaaaaaaayyyyyy! 

Wow that was quick !!  Your intuition was spot on then, you are getting good at this !

But the report lacks detail : how did you find the bad chips ? Brute force, replacing them one by one till the fault cleared  ? Or you did some more troubleshooting to try to identify the bad chips, if so how did you proceed ? Maybe just look at the data bus of each RAM chip searching for the ones that are stuck on FF, simple as that maybe ?

Congratulations in any case !

Alrighty, so short story short.... I posted on the tekscopes groups.io and was given some memory address-to-RAM chip Reference Designator info, so I found the location on the PCB of the RAM chip that matched my error and swapped it out with a hot-air pencil and soldering iron as usual.

Ran the full self-check again and got another error on another RAM chip, so using the same info as previously, I located that bad RAM chip and replaced it too.

Ran the full self-check yet again fully expecting more issues, but nope, it passed everything and all the traces look nice and clean!

So, it seems the two RAM chips on Channel 2 were the total problem, and it also seems that as the scope interleaves the memory with the other channels when only 1 or 2 channels are in use for greater memory depth, it was causing the same error to appear on other channels in certain horizontal settings, depending on what RAM chips were being used at a particular time.


So all good now!


Next thing to do is finish getting the Compact Flash card conversion working to replace the HDD, then poke at the RAM banks a bit to map out the complete memory addresses to the RAM chip Reference Designators on the silkscreen for future reference and repairs.

Actives or passives ?

I'll probably keep an eye out for something like a P6339A or two.

[tautech]

Actives or passives ?

I'll probably keep an eye out for something like a P6339A or two.

One of these ?
https://www.ebay.com/p/1100242214

Seems expensive but buffered and passive, how does that work ? 

[TERRA Operative]

Actives or passives ?

I'll probably keep an eye out for something like a P6339A or two.

One of these ?
https://www.ebay.com/p/1100242214

Seems expensive but buffered and passive, how does that work ? 

 

[tautech]

Actives or passives ?

I'll probably keep an eye out for something like a P6339A or two.

One of these ?
https://www.ebay.com/p/1100242214

Seems expensive but buffered and passive, how does that work ? 

 

LOL, ah sooo, it's really an active probe ! 

[vk6zgo]

The USB power will not be wired to the rail directly. There will be a host controller in the IC that handles all that faff including power negotiation.

3.3v is interesting through. It should be 5V. Check it with a scope to make sure it is actually that and not the average voltage you are reading.

Naaahhh... these POSes don't do power negotiation... they're just gonna have 5V@300-500ma to power a USB memory stick and nothing else. And often likely a 3.3V LDO on that rail to power the SD card slot, if it doesn't draw off the 3.3V to the CPU.

Think Android 4.0 grade hardware.

mnem
 

No datasheet for this CPU anywhere so... been buzzing the CPU instead. I was able to find the two CPU pins that are connected to the two data lines of the USB connector. So it's direct connection, no circuitry in between. Then I buzzed the USB power pin.... didn't appear to be connected to any of the CPU pins !  That's good hey ? Means there is some hope ? Either there are discrete components between the CPU and power pin, that prevent a continuity tyest from buzzing, or it's not powered by the CPU as you said, so the offending components are instead between the 5V rail and the USB connector.

So a tiny ray of hope... let's do some more buzzing...

Don't lose sight of the fact that some meters' continuity buzzer / beeper can still work through resistances etc of around 200 to 300 ohms and also often you can get an initial buzz / beep on circuits that have caps in them as well, so make sure that you can get the signal for a few seconds rather than a quick sounding.

That's why I always do my continuity testing the "old school" way---On the "Ohms" range!

I was brought up on AVO 8s, which "didn't have no steenkin' continuity buzzer", although their "Ohms" range will turn on semiconductor junctions, just to confuse things.
The fluke 77 Ohms range won't turn them on, but "continuity" will, as it is the same function as "diode test".