TT/Mega STE power supply headaches

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exxos
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Re: TT/Mega STE power supply headaches

Post by exxos » Tue Apr 24, 2018 7:19 pm

damanloox wrote:
Tue Apr 24, 2018 7:16 pm
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).
Forgive me if this sounds dumb.. but have you tried moving your scope probes and leads and even meter away from the psu ? probes can pickup RF from the circuit causing voltages which are not really there.. one angle on the probes may pickup RF, another angle may not.. also some RF can even cause the meter display itself to register voltages which are not really there.. also make sure you are not touching any part of the circuit / probs (can touch gnd but not the probes themselves) Just a thought...

EDIT:

Its also possible the small track length on the pcb could be picking up RF which may explain why you see voltage there.. it may not even be 0.6V, its just RF noise which your meter is seeing and translating to 0.6V on the display.. I don't know how long the track is.. but nothing surprises me much anymore..

EDIT2:

Which side of the zener did you disconnect, the transistor side or other side ? The zener could act as a antenna and float the Base voltage up, may be then internally clamped to 0.6V in that case... again just a thought..
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 7:44 pm

The meter seems to be ok - it doesn't show any voltage (maybe a few mV) a few mm from that spot...). I thought it might be some sort of transparent residue that is conducting but I cleaned the pad area and checked multiple times - nothing.
The track is maybe 1cm at most and again - nothing that could conduct around it...
Zener is removed completely - the pad is not connected to anything (apart from that 1cm of the track).

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Re: TT/Mega STE power supply headaches

Post by exxos » Tue Apr 24, 2018 7:49 pm

damanloox wrote:
Tue Apr 24, 2018 7:44 pm
The meter seems to be ok - it doesn't show any voltage (maybe a few mV) a few mm from that spot...). I thought it might be some sort of transparent residue that is conducting but I cleaned the pad area and checked multiple times - nothing.
The track is maybe 1cm at most and again - nothing that could conduct around it...
Zener is removed completely - the pad is not connected to anything (apart from that 1cm of the track).
What about lift the transistor base and solder on 2cm of wire and lay it as close to the pcb track as possible ?

I think you should really send me this mystifying psu to look at.. This issue really needs to be looked at as if someone else has it in the future, then we (* I *) will be going through all this again.. i would pay return shipping... guess about £12...
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 » Sat Apr 28, 2018 3:06 pm

damanloox wrote:
Tue Apr 24, 2018 7:44 pm
The meter seems to be ok - it doesn't show any voltage (maybe a few mV) a few mm from that spot...). I thought it might be some sort of transparent residue that is conducting but I cleaned the pad area and checked multiple times - nothing.
The track is maybe 1cm at most and again - nothing that could conduct around it...
Zener is removed completely - the pad is not connected to anything (apart from that 1cm of the track).
You could (with the PSU switched off, of course) use the resistance mode of your multimeter so see if there are any connections (maybe in the hundreds of kOhms range) between the disconnected track and surrounding tracks or components.

damanloox
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Re: TT/Mega STE power supply headaches

Post by damanloox » Wed May 02, 2018 8:43 pm

It took a while but here's a little report...
First of all - it's fixed!
There was definitely something wrong with the pcb around Q2 - no matter what I did there was 0.6v at the base of Q2. Even when I desoldered the transistor - there was still 0.6v at the pad (which was not connected to anything!). Whatever it was - some sort of residue, moisture - I don't know. What I did I cleaned it the best I could (although it was cleaned several times before because I suspected it might be the problem..), I then used putty and filled it all up. After 48hrs I drilled the holes back where they should be, installed rivets, soldered Q2, crossed my fingers and powered the thing up. After about 0.5s to 1s - PG was at 5v and all voltages exactly as they should be. No more 0.6v at random points around Q2... Checked multiple times - it all works exactly as it should. The PSU is now sitting in my TT and all works perfectly fine.

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