UNI-T UT61E Diode mode repair?

[Per Hansson]

Hi, I once measured a 325VDC bus with my UT61E set in resistance mode by mistake.
There was a small spark at the tip and the instrument shutdown however it worked fine afterwards it seemed.
Only a few weeks later did I notice the diode test function is broken:
If I switch the meter to diode test it shows 1.4600v even with nothing connected which slowly goes down in value to ca 1.4500v
If I connect probes and take them together the buzzer beeps and the instrument shows 0.0004v (seems correct)
I assume this might be some input protection that is gone.
Mine is bought from Elfa in Sweden and does have the German TUV and GS safety logos on the front, it is rated to 300v CAT-III
It also has proper ceramic fuses.
I attached a photo of it which I took when I bought it some years ago.
I tried to follow the schematic that exists online of it but it makes me crazy with the range switch logic to make heads or tails from it.
Does anyone have some idea, is it an external component or the MCU itself?
The instrument works fine except for the diode test function...

[Per Hansson]

I found a way to better comprehend the schematic: I drew the path with a brush for the diode test mode as I could understand it.
This led me to suspect Q5 and Q6 but they seemed ok, next I checked D1 and D3 but they also seemed ok.
Finally Q1 and Q8, the failure was in Q1
I have attached two pictures of Q1 and Q8 on my transistor tester:
Q1 has a little extra diode between emitter and collector, really fachinating
I desoldered them and the DMM can now read a diode properly!
As for replacements: I can't find a SS8050 transistor on Elfa Sweden.
Could I substitute it with something else or is the exact type really important?
The marking is "Y1" and according to my SMD databook this is Weitron SS8050LT1-H or SS8050LT1-L
In the schematic the designation given is: 8050(BKD)
Can someone please suggest a good replacement?

[retiredcaps]

Hi Per,

I see ebay has them for 99 cents (qty = 30) + free shipping (at least to Canada, but I suspect worldwide).

https://www.ebay.ca/itm/171734273697

Alternatively, I'm sure there is an equivalent part with similar specs.

[fzabkar]

"In the schematic the designation given is: 8050(BKD)"

KTC8050S, KEC, NPN transistor, 30V, 800mA, 350mW, hfe = 150 - 300, marking BKD, SOT-23:
pdf.datasheetcatalog.com/datasheet/kec/KTC8050S.pdf

Pinout is Emitter-Base-Collector. Most transistor pinouts are BCE, so take care with substitutes.

[magic]

Pinout is normal, it seems they swapped pin 1 and pin 2 numbers.

[Per Hansson]

Hi Per,

I see ebay has them for 99 cents (qty = 30) + free shipping (at least to Canada, but I suspect worldwide).

Thanks but Swedish post has added a "fine" for all outside EU packages since about 3 years now.
It means I have to pay tax (only 25c in this case) but also a set import fee of 75SEK which is around $8
This makes allot of repairs uneconomical, but also because of the slowass postal service it takes weeks extra to get your package
Also some less honest Chinese sellers list addresses in the EU now, however still ship from China!
I've been bit by that enough times now that I am very careful...
I'm also leary of fake components from China, especially from second-hand sources.

"In the schematic the designation given is: 8050(BKD)"

KTC8050S, KEC, NPN transistor, 30V, 800mA, 350mW, hfe = 150 - 300, marking BKD, SOT-23:
pdf.datasheetcatalog.com/datasheet/kec/KTC8050S.pdf

Thanks but this is wrong, the marking on the transistor itself is Y1 as I wrote in the second post and can be seen in the picture of my unit.
I'm thinking maybe the schematic is the one with less input protection vs the TUV / GS rated meter that I have?

I remembered I had seen a video of this from Dave, seems the important specification is VEBO which is listed as 6v.
That is available as a parametric search on Elfa however all matches are only rated for Ic of 100mA vs 1500mA of the original part, I guess that is a no-go.

[Per Hansson]

To provide a conclusion to this I bought the following from within the EU:
https://www.ebay.com/itm/SMD-transistor-SS8050-Y1-SS8050LT1G-S8050-SOT23-NPN-SMD-Y1-25V-1-5A-B71-1/182845725607

The device now works fine and the transistor tester reported these with similar values to the original Q8 transistor that had not failed.
Thanks @retiredcaps for the idea about eBay

[retiredcaps]

Thanks @retiredcaps for the idea about eBay

Glad to help out my fellow badcaps member.

[Per Hansson]

Almost two years later and I killed the input protection again.
This time I was connecting up a variac and wanted to make sure I get the neutral hooked up to where I wanted it.
(We have the Schuko plug in Sweden that can be put in any way...)
And by mistake I left the meter in ohms mode after I had checked where the neutral was in the IEC cable when I measured the 230VAC outlet.
This time the fault was different: at first it showed ca 60Ω continuously in ohms mode, and a 0.2v diode drop in diode mode.
So I replaced Q1 and that got rid of this issue, but the meter was reading resistance wrong, with a offset, for example 10Ω read around 3Ω.
So I changed Q8 too and then it is mostly back to normal, with one strange caveat: it can't autorange when measuring the variac.
That is from Line to Neutral connectors in the variac I have 51Ω and the display just flashes.
But if I measure on the variacs output and bring it up slowly it has no problem to autorange from 0 to 51Ω.
However the issue starts at around 34Ω if I remove and put back the probes there it is unable to autorange.
But I also realized an even stranger artifact: if I leave the probes hooked up and turn off and on the meter then it is able to autorange just fine!
It is also able to measure some common E12 range resistors around that range with no autorange issues.
I'm just curious if anyone else has the same shitty experience with the input protection?
This is afterall the model with the German TÜV Rheinland GS logo on the front!