48K Issue 2 ZTX650 running hot/overcurrent

[linker3000]

Hi folks,

Here's where I'm at with an issue 2 48K board:

Recapped + Switching 5v Reg fitted = working but screen glitches/colored blocks which got worse and then the board died.

ULA/CPU tested OK in a working issue 3.

All board/RAM voltages checked out OK.

Removed upper RAM = booted to a good screen but no copyright message.

Removed lower RAM = Got the expected 'all RAM removed' screen.

Replaced all 4116s with NOS from UTSource (no upper RAM fitted) = glitchy 'RAM fault' type screen - lots of vertical stripes.

Noticed coil looked very toasty (but did not see or smell anything when I was working on the board - huh?).

No -5V. Suspect lower RAM now very unwell so I removed it again (from the sockets I fitted). Ordered replacement lower RAM module - but not fitted yet because....

Here's where I am now:

All RAM sockets show no shorts between power rails or between data/address bus.

Coil carefully rewound. ZTX650 (blown) replaced with ZTX651.

ZTX213 removed and tested OK. Tried replacing it with the suggested BC557 but no oscillation. Re-fitted the ZTX213.

Issue 2 oscillator circuit resistor/capacitor mod fitted.

Using a current-limited PSU set to 9V and ALL chips removed (except the LM1889 still in situ), the board hits the current limiter (set to 150mA) and the supply voltage drops accordingly, implying an overcurrent situation. ZTX651 is hot but the current limiting is protecting it. Current limiter cranked to 250mA before I decided to have pity on the ZTX651 and turned it down again.

-5V rail IS present and the 12V rails have made it to 7V (PSU current limiting has dropped the input voltage to about 4V) - so things are kinda working

Coil is whining so I have oscillation (but not scoped it yet).

NO shorts on power rails.

I have removed and tested all diodes/resistors in the power circuit and replaced C49 (axial 100nF) even though it tests OK. I have also run the board with various diodes/resistors removed to isolate the coil circuit (12V rails and the -5V rail) from the rest of the board and this makes no difference - current limiter kicks in.

Multiple visual inspections and multimeter probing doesn't show up any shorts.

Here's where my speccy-specific electronics knowledge runs out.

I'll keep testing and I suspect either:

Problems with the rewound coil

ZTX213 gone funny?

ZTX651 will get hot when there's not much load on the coil circuit and so I should just plug everything back in and see what happens!?

Something obvious I have not spotted yet

So - any advice?!

I'd love to get this board running. My plan B is to strip off the coil/inverter circuitry and use a small boost converter for the video 12V - or considering that the original PSU (which tests and works fine with another Spectrum) puts out >14V no-load, I was wondering whether I'd get away with a low drop-out 12V regulator for the video chip supply?

Words of wisdom gratefully accepted! Thanks.

[RMartins]

Don't take this the wrong way, because you seem knowledgeable enough to avoid this, but I have seen it happen before.
Have you confirmed that the coil copper wire you used, is properly insulated with some kind of varnish, i.e. are you sure it is enameled wire ?

Also, have you checked you put the transistors back in place in the correct orientation ?

Some boards have the silk screen wrong.
Some equivalent transistors have different pin-outs.

I hope you took a picture before disassembly.

[linker3000]

Hi,

Thanks for the reply. Yes:

Enamelled copper wire for the coil; rewound carefully in the same direction.

Photos taken and checked.

...to be continued!

[1024MAK]

There must be at least some load for the coil to unload it’s magnetically stored energy into. But it does not have to be a particularly big load.

TR5 acts as a comparator comparing the actual +12V rail voltage (via R59 and R58 which act as a voltage divider) with the +5V supply (which is used as a reference). If the +12V is low, TR5 will drive TR4. If/when the +12V rail gets to +12V, TR5 will start turning off, which reduces the amount of power that TR4 can put into the coil.

You should also have a look at my blog page where I detail the recommended Sinclair DC/DC converter modification.

The BC557 does have a different pin-out to the ZTX213.

Did you test the ZTX213 out of circuit?

A ZTX650 / ZTX651 should not get hot in normal operation (slightly warm yes, but not hot). A ZTX650 / ZTX651 is rated at a maximum of 1W.

Also visually check for any shorts on/near the coil on both sides of the PCB.

Mark

[payty]

Hi. Just an idea...

Since the 12v drops when connected to your adjusted curent psu, could the lm1889 be the problem? If the lower RAM has been removed, this is the only one powered by the 12v line.

[1024MAK]

LM1889 chips rarely fail. But if course anything is possible.

If the power is being injected on the nominal +9V input, you can measure the voltage across the two legs of R62 (15 ohms).

Then the current flowing to the video / analogue section (the LM1889) will be:

Voltage divided by the resistance (15 ohms) = current

Mark

[payty]

I think it's an easy task to remove power from this chip and check the 12v rail again.

[1024MAK]

LM1889 chips rarely fail. But if course anything is possible.

Should have said: LM1889 chips rarely fail. But of course anything is possible.

Mark