TT/Mega STE power supply headaches

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damanloox
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Re: TT/Mega STE power supply headaches

Post by damanloox » Fri Apr 20, 2018 7:46 am

voltage on collector of Q2 remains pretty much unchanged. I'll check voltages on all 3 transistors later this evening.
Voltages on R29, R31 and R33 is correct

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exxos
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Re: TT/Mega STE power supply headaches

Post by exxos » Fri Apr 20, 2018 8:39 am

So capacitor is not charging which is the problem... you 100% sure you have put in pcb correct way around ?

You could try removing Q2 and see if PG works then...
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damanloox
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Re: TT/Mega STE power supply headaches

Post by damanloox » Fri Apr 20, 2018 8:47 am

exxos wrote:
Fri Apr 20, 2018 8:39 am
So capacitor is not charging which is the problem... you 100% sure you have put in pcb correct way around ?
yes - I checked that a hundred times by now ;)
You could try removing Q2 and see if PG works then...
I'll do that later this evening and report... I'll also replace the cap just to be sure. I also ordered a set of replacement transistors for a pound so will replace all three when they arrive...

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Re: TT/Mega STE power supply headaches

Post by exxos » Fri Apr 20, 2018 9:02 am

damanloox wrote:
Fri Apr 20, 2018 8:47 am
I'll do that later this evening and report... I'll also replace the cap just to be sure. I also ordered a set of replacement transistors for a pound so will replace all three when they arrive...
I don't really get whats going on with Q2 anyway.. linked to the transformer winding directly, not even checking -12V really..

Does the Fan run after turn on ?
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damanloox
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Re: TT/Mega STE power supply headaches

Post by damanloox » Fri Apr 20, 2018 9:45 am

yes - the fan starts about a second after power on (as does power led). But the TT is not starting until PG goes to 5V (and that happens about 30 seconds after power on)

I'm no expert in analog circuits but my (infantile) understanding (and correct me if I'm wrong) is that Q2's role is to make sure 5v isn't too high? Ie. If there was 10v on 5v line current would flow via ZD3 and Q2 would be open? But there's also the whole SHR2 and resistors contraption which I don't understand... but anyway - according to my measurements - Q2 seems to be closed (as I'm reading 1.1V on Vce) so it all should work (but it doesn't..)
Again - all the above is probably completely wrong ;))

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Re: TT/Mega STE power supply headaches

Post by exxos » Fri Apr 20, 2018 9:51 am

Could be yes.. I guess if voltage on the core was to high, Q2 would turn on and prevent C8 from charging... (which seems like the problem you are having).

Have you check -12 voltage ?
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damanloox
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Re: TT/Mega STE power supply headaches

Post by damanloox » Fri Apr 20, 2018 5:33 pm

OK, I removed Q2 and PG comes up within 1 sec (exactly as it should) but (big but...) - without the whole Q2 circuit I can see 12V on 5V line for a split second... And - as I guessed - it seems that the whole Q2/SHR2/ZD3 circuit is to turn PG off when 5V line is above 5V... So I won't even try to connect that PS to my TT without Q2... And Q2 seems to be OK (ie. when measured as two diodes)...
re -12v - yep - measured it and it's perfectly OK... I'm stuck again. I need to find equivalents of SHR2 and ZD3 and try replacing them... My love to the TT starts to fade ;))

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Re: TT/Mega STE power supply headaches

Post by exxos » Fri Apr 20, 2018 6:00 pm

PG only seems to be checking overvoltage on a transformer tap which drives the -12V rail... Think over volting on the other rails are not going to be detected..

So I think it is clear the problem is circulating around Q2.. You could try another diode in series with D10.. And see if that works..
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damanloox
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Re: TT/Mega STE power supply headaches

Post by damanloox » Fri Apr 20, 2018 6:06 pm

that spike comes from 5v (as I can see it on 5v rail and PG). What I'm saying is that Q2 will probably be open when anything over 5.2 (or whatever zener voltage is for ZD3) - and that will close Q3 which in turn will open Q4 (which in turn will cause PG voltage to go to 0). Without good PG TT won't turn on and that (I think) is the mechanism protecting the TT from big 5v spikes...
without Q2 PG goes to whatever 5V rail is... At least that's what I'm seeing...
all the above must be taken with a grain of salt - as I said I know nothing about circuits...

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Re: TT/Mega STE power supply headaches

Post by exxos » Fri Apr 20, 2018 6:16 pm

I think the circuit around Q2 is shorting out C23 when it sees overvoltage on the -12V tab on the transformer..

So for some reason the voltage there I assume is slightly too high pushing the Q2 base voltage high enough stop C32 from charging...

Is why I suggested adding in another diode series with D10 as it will drop the voltage there by about half a volt, which is basically nothing, but it could be enough to solve the problem..

Also you could try something like a extra 1,000uF across C17 to see if that will clamp the voltage spikes more..

Course it could be possible something may be faulty, but my first guess is is just something which has gone out of tolerance enough to stop the power supply starting up correctly..
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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