Issue 4a docs needed

eccles
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Re: Issue 4a docs needed

Post by eccles » Sun Feb 09, 2020 9:11 pm

I'll hoike out the coil next, then

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1024MAK
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Re: Issue 4a docs needed

Post by 1024MAK » Sun Feb 09, 2020 10:03 pm

A coil with shorted turns tends to kill TR4 (as in cooks it until it goes short circuit)...

What does TR4 show when tested? Or have you binned it?

One other test before removing the coil. Is your current limited supply (is it a bench adjustable supply? If you have already said, sorry, please remind me...) capable of 12V with a current limit of between 100mA and 166mA? If yes, what happens if you use it to power up the +12V rail ONLY. That is, don’t power up the +9V or the +5V. Obviously the 4116 DRAM chips should not be fitted.
What current is drawn from your bench PSU when connected as described?

Also test the diodes in the +12V rail and the -5V.

Mark

eccles
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Re: Issue 4a docs needed

Post by eccles » Mon Feb 10, 2020 11:26 pm

Hi Mark,

Haven't had a chance to look into this further, but TR4 shows fine when tested.

The bench power supply allows me to set voltage and current limits - is there anywhere you suggest I inject 12v, and should I remove anything first from DC-DC converter stage to avoid issues?

The only diode that I tested in circuit that looks funky is D15, so before i go injecting anything, i'll unsolder one end and retest that.

What is the logic of the 12v/100mA test? Presumably if it trips the current limiter, then it's something on the 12V line shorting to ground?

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1024MAK
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Re: Issue 4a docs needed

Post by 1024MAK » Mon Feb 10, 2020 11:46 pm

This:
eccles wrote:
Mon Feb 10, 2020 11:26 pm
What is the logic of the 12v/100mA test? Presumably if it trips the current limiter, then it's something on the 12V line shorting to ground?
Yes, to see if there is something else shorting out the +12V supply.

Mark

eccles
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Re: Issue 4a docs needed

Post by eccles » Wed Feb 12, 2020 9:47 pm

I've checked all my diodes, and they're good.

Where would you reccommend injecting 12V? Positive side of C44 goes to the +12V DRAM supply, Positive side of C45 goes to +12VA(Analogue supply) (or either side of R62)

Now to try and find some spare banana plugs to make up a jumper cable...

eccles
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Re: Issue 4a docs needed

Post by eccles » Wed Feb 12, 2020 10:18 pm

OK, went +ve to positive of C44, and ground to negative of C45

Both at 100mA and 130mA, the current limiter of the bench supply is tripped.
So it would appear then I have a short on the 12V line

Looking at the schematics, the stuff I ought to be looking at are:
IC6-13 (already removed, currently have sockets)
C1-C8
Then starting with IC14 (LM1889), C48, R42... dunno how much further past there i need to go

I seem to recall someone on Youtube suggesting if all else fails, removing components in such a way to split the remaining circuit in half to see if the short remains

Thoughts?

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Re: Issue 4a docs needed

Post by 1024MAK » Thu Feb 13, 2020 5:16 pm

Lifting one leg of R62 will split the +12VA (analogue) supply from the main +12V supply.

C1 to C4 are for the -5V supply, C5 to C8 are on the +12V supply.

Mark

eccles
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Re: Issue 4a docs needed

Post by eccles » Fri Feb 14, 2020 9:21 pm

Having cocking the leg of R62 (and confirming +12VA was now seperated), I tried injecting as before +ve to +ve of C44 and gnd to -ve of C45

12v at 130mA still tripped the supply

Would I be right in thinking that if I were to lift R59 and the -ve lead of C44, this will completely isolate the DC-DC converter from the rest of the board supplied by 12V?

Would this be a sensible next step to seeing where the short is?

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1024MAK
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Re: Issue 4a docs needed

Post by 1024MAK » Sat Feb 15, 2020 3:10 pm

eccles wrote:
Fri Feb 14, 2020 9:21 pm
Would I be right in thinking that if I were to lift R59 and the -ve lead of C44, this will completely isolate the DC-DC converter from the rest of the board supplied by 12V?

Would this be a sensible next step to seeing where the short is?
No.

If R59 is okay (1.8kΩ, although an in circuit test will produce a lower result) then that will only let through less than 7mA at most.

C44 negative (-) lead being connected or disconnected (unless C44 is short circuit itself) will make no difference to a DC current flow.

It now looks like either one of C5 to C8 is faulty, or there is a short circuit between one of the +12V tracks and a 0V/GND track. So I suggest a good visual examination of both top and bottom of the board using a good light and a magnifying glass. Also have a good look at C5, C6, C7 and C8. If any of them look to have any visible damage, remove the damaged one(s).

If you still have the fault, then remove C5, C6, C7 and C8 one at a time.

Note: all of the above assumes that diode D15 is good and not short circuit, that none of the 4116 chips are fitted, the modification that is shown here has been carried out correctly and that resistor R62 still has one leg lifted.

Mark

eccles
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Re: Issue 4a docs needed

Post by eccles » Sun Feb 16, 2020 7:18 pm

(Reminder TR4 is still out of circuit)

Removed C5-C8 - short remained
Disconnected R59 and C44 - current draw went to zero
Replaced C5-C8, current draw still zero

Suggests to me the issue still lay in the DC-DC converted board somewhere....

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