Sinclair ZX80 / ZX81 / Z88 Forums

eccles wrote: ↑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....

Where are you applying the +12V and 0V/GND leads to now ?

So have you tested the resistance of R59 ?

And have you tested C44 ?

Mark

R59 is almost smack on 1.8K
C44 checks out at 100uF 0.9ohm ESR

+12V lead to where the +ve of C44 was
GND lead to -ve of C45

EDIT:
I got confident (not earned, as i'm a novice)

I reconnected R59, and removed R58 and TR5 - short gone
Reconnected R58, short returns (R58 checks out at 1K)

I’m confused. There should be no connection from the +12V rail to R58. And even if there was, you’ve just said that it tests okay at 1k. 12V applied to a 1k resistor will only result in 12mA flowing through it.

Hence why I’m confused....

R59 and R58 are supposed to form a potential (voltage) divider to to feed the base of TR5 with a proportion of the voltage on the +12V rail. TR5 then compares this to the voltage on the +5V rail. If the voltage at the junction of R59, R58 and the base of TR5 is low, TR5 supplies more current to TR4 base. TR4 will then work harder to push more power through the coil. More power through the coil should result in the voltage on the +12V rail going up.

Note that the above explanation is rather simplified.

Mark

I may just remove the coil temporarily to see what happens - it's the last major component left that could be shorting to ground before I start pulling the remainder one thing at a time.

Edit:
Having removed the coil, the short went away. Inspection of the underside now the coil is gone might lend a clue to the possible cause. There is a trace that runs right past the coil solder point where the mask has been lost (the trace itself is fine) - i'm wondering if this was shorting across to the solder pad next to it.

I suppose i ought to get some solder mask to clean it up.

I carefully nailpolished the bare bit of trace and then resoldered the coil.

I noticed the construction of the coil is such that part of the winding wire is present on the underside of the coil, such that if pushed down on the board, this part of the winding wire would touch any bare metal on the top side of the board. Given there exists such bare traces on the top of the board under the coil, I thought it possible that could a source of a short if coating on the winding wire was poor.

So I made sure there was a modest amount of clearance between the coil and the top of the board when I resoldered it. Tested - no short

So this is a definite improvement - i'm tempted to refit the lifted resistor and removed capacitor, plus TR4 and TR5 and see whether it still freaks the 9V supply....

So I refitted all lifted/removed components, including a ZTX213 for TR5 and a ZTX650 for TR4 (both brand new and tested before fitting)

9v with 700mA limiter. Triggers overcurrent protection every time

Sad face

I'm entertaining suggestions once more.....