TT/Mega STE power supply headaches

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czietz
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Re: TT/Mega STE power supply headaches

Post by czietz » Sun Apr 22, 2018 3:20 pm

Exxos, the changes you propose (e.g. changing the resistor divider) are on the input side of of the TLP431, which in this circuit is only operating as a comparator with integrated reference to perform "power fail" detection. Do you think the input side is at fault? In my opinion, damanloox's measurements on a good vs. bad PSU show that the TLP431 is working as intended, its cathode voltage being even slightly smaller in the bad PSU.

damanloox: If you unsolder one pin of ZD3, thereby cutting any input to Q2's base, is there still ca. 0.7 V at the base? Then it could only come from Q2 itself.

damanloox
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Re: TT/Mega STE power supply headaches

Post by damanloox » Sun Apr 22, 2018 3:28 pm

with Z3 desoldered there's still 0.6v on Q2 base (and there's no short or anything - Q2 base is definitely disconnected from anything)...
I think we have the offender... I guess I'll have to wait for the delivery and see what happens...
edit: is there a way of reliably testing a transistor (with basic multimeter only)? I'm pretty sure I tested it (as two separate diodes) and it seemed to be OK...

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Re: TT/Mega STE power supply headaches

Post by damanloox » Tue Apr 24, 2018 2:48 pm

The supplies arrived, Q2 replaced and... no luck. Still the same thing.. And (!) still 0.6v on Q2 base with ZD3 removed...
I checked again (and probably 50 times by now) for any signs of a short anywhere near SHR2, R28, ZD3, Q2.
How is that possible (ie. o.6v on the base that's not connected to anything..?
I think I'll give up on this one...

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exxos
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Re: TT/Mega STE power supply headaches

Post by exxos » Tue Apr 24, 2018 2:56 pm

Send me the PSU.. I will soon get to the bottom of it.
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

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damanloox
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Re: TT/Mega STE power supply headaches

Post by damanloox » Tue Apr 24, 2018 5:54 pm

ok... here's the surprise. The base of Q2 is _definitely_ disconnected from the rest of the circuit (after multiple desoldering/soldering operations the track has lifted - that's not a big deal as it's only connection is ZD3 so can be fixed easily). Anyway - base disconnected and 0.6v present..
In an act of desperation I cut the base from the PCB and on the part of the leg that attaches to the transistor voltage is now 0 (and PG is obviously OK).
But - on the leg that's still in the board there's 0.6v...! I checked 10 times to make sure it's actually correct leg and it is..! Just to be clear - a bit of metal that's not connected to anything on either side and there's 0.6v there... All the time..
One more thing (that may be relevant which I don't think I mentioned in this forum - that PSU had 5v shorted when I got it (ie. 5v wire was touching the metal enclosure). That has been fixed right after I got it. I'm not sure if it did any damage but again - all voltages seem to be OK now (and by al I mean +/- 5V and +/- 12V)
@exxos - thanks for the offer but shipping the PSU back and forth (and postal services in Ireland are very expensive) plus your time is not worth it... I'll probably invest in flex atx if anything ;)

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Re: TT/Mega STE power supply headaches

Post by exxos » Tue Apr 24, 2018 6:47 pm

So the base doesn't have 0.6V but the PCB does ? I would try soldering 100K pull down on the base (reconnect the transistor base to the PCB).
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

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damanloox
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Re: TT/Mega STE power supply headaches

Post by damanloox » Tue Apr 24, 2018 6:55 pm

I'll try that (I don't understand the point though.;))
What worries me is that there's a voltage (0.6v precisely) at some random point of the PCB that's not connected to anything... And apparently that voltage is turning the transistor on... If base is connected to that point of the PCB the transistor turns on. If I cut base leg - transistor turns off and all works OK (but that's not the real solution as I could as well remove the transistor...).
Again (just to make sure we're all clear on current situation): ZD3 is desoldered, Q2 base leg is cut in half and moved away from the board.
0V on base (which I'd expect as it's not connected to anything) and 0.6v on the remaining part of the leg that's still in the PCB (but not connected to anything) - that is something I wouldn't expect...

BTW - I updated my previous post (re: 5v shorted).
And one more thing I noticed (though it might be my imagination) - I think these dummy load resistors are heating up much faster on the non-working PSU. On the good PSU it takes about 30s for them to get hot, on the bad one it seems to be 15-20 seconds...

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Re: TT/Mega STE power supply headaches

Post by exxos » Tue Apr 24, 2018 7:02 pm

damanloox wrote:
Tue Apr 24, 2018 6:55 pm
BTW - I updated my previous post (re: 5v shorted).
And one more thing I noticed (though it might be my imagination) - I think these dummy load resistors are heating up much faster on the non-working PSU. On the good PSU it takes about 30s for them to get hot, on the bad one it seems to be 15-20 seconds...
Well im confused..

Is the 5V actually 5 volts ? not like 5.5v or something ? Similar with the 12V is actually 12V ? not 13V or something ?

I'm not following what your doing with the base of the transistor... when its not in the PCB its 0v and it works ?

When in the pcb without the zener connected the pad measures 0.6v ?!
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxoshost.co.uk/atari/last/storenew/ - All my hardware mods for sale - Please help support by making a purchase.

czietz
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Re: TT/Mega STE power supply headaches

Post by czietz » Tue Apr 24, 2018 7:05 pm

damanloox wrote:
Tue Apr 24, 2018 6:55 pm
Again (just to make sure we're all clear on current situation): ZD3 is desoldered, Q2 base leg is cut in half and moved away from the board.
0V on base (which I'd expect as it's not connected to anything) and 0.6v on the remaining part of the leg that's still in the PCB (but not connected to anything) - that is something I wouldn't expect...
Check for dirt, flux or glue residue or (since you mentioned that the PSU had been overloaded/shorted) any hints of a burnt PCB. All these things can get conductive and be the cause for voltages on seemingly totally disconnected pins.

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Re: TT/Mega STE power supply headaches

Post by damanloox » Tue Apr 24, 2018 7:16 pm

when I say 5v I mean 5.11 exactly and 12v is about 11.8v. -5v line is -5.2v and -12 is -12.1V
when the base is not connected to the pcb it reads 0v (around 100mV, hard to say exactly) and there/s 0.6v on the pad (Zener diode is desoldered so the pad just a piece of copper on the pcb disconnected from anything).
@czietz: I did that - I don't see anything (and I checked a hundred times by now...). But i'll do it all over again... I'm just really (and I mean really) surprised to see that 0.6v. That pad is definitely not touching anything...

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