Resurrecting HB75, black screen, fuse? | MSX Resource Center (Page 2/2)

Supporter (10)

08-11-2022, 03:08

I bought all items in circuit. I thought I might be methodical and replace the diode and R11 which was only showing 0.3 on one side. No -5V. Moved on and replaced C6 and R12. At this point I thought it best to replace c2, but silly me purchased a 4.7 not 0.47. Anyway, it still doesn’t work, so SURELY must be C2. If not, I’m stumped. Unless a shorted VRAM can “halt” the voltage... or something?

By Wierzbowsky

Guardian (3603)

08-11-2022, 21:46

Indeed, any capacitor can cause a short circuit. Those resistor-shaped ceramic capacitors near the RAM should be checked. If your multimeter working in continuity mode beeps on measuring a capacitor, it's a good idea to replace it.

By gdx

Enlighted (6215)

09-11-2022, 01:36

Wierzbowsky wrote:

Indeed, any capacitor can cause a short circuit.

Capacitor can cause a short circuit when it is defective. That's what sometimes causes it to inflate.

By Wierzbowsky

Guardian (3603)

09-11-2022, 14:18

Even ceramic caps can cause short circuit. And it's hard to notice. It could have microcracks as a result, but this is visible only with high magnification. Tantalum caps usually loose their color, sometimes they burst emitting smoke and soot. Electrolytes inflate and leak, sometimes even blow up.

My experience tells me that every old tantalum cap on a 12v rail must be replaced ASAP. Seen too may of them causing various problems in vintage machines, starting from CBM and ending with MSX. And you are lucky if you notice the problem before something else blows up.

By Dd84

Supporter (10)

10-11-2022, 08:40

Hi again. I replaced C2, the final component on the -5 supply side. No change.

Are the tantalum caps the green disc ones? And the ceramic ones are the ones which look like resistors?

Perhaps I should upgrade multimeter for further testing. No sound effects on mine.

I did notice when I switch the unit off, voltage spikes up near to where it needs to be

By Dd84

Supporter (10)

23-03-2023, 09:45

Hi again, are the tantalums the green discs beside the chips?