Resurrecting HB75, black screen, fuse?

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بواسطة Dd84

Supporter (9)

صورة Dd84

01-11-2022, 12:24

Hi, new member. I’ve pulled my old Sony msx out of storage. It worked well when boxed up, but now just gives a black screen. I’ve done some googling and have seen reference to a 12V fuse which isn’t in the service manual. I see the 240V fuse and that is fine.

Thank you

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بواسطة Wierzbowsky

Guardian (3541)

صورة Wierzbowsky

01-11-2022, 14:17

Could be a voltage regulator. Check all voltages on the board: -5,+5, -12 and +12. If all is fine, RAM will be the next suspect. Does the machine make any sound (click) when you turn it on?

بواسطة Dd84

Supporter (9)

صورة Dd84

01-11-2022, 23:51

Thank you. I checked at the top board with the RF socket and voltages seem ok. Where is this 12V fuse people speak of, that isn’t in the workshop manual ?

No click but do get the red power light

بواسطة Dd84

Supporter (9)

صورة Dd84

02-11-2022, 03:34

Actually... I’ve just checked voltages again, at the top board, direct from the power supply, and get about 3.5V at the 5V and 1V at the 12V... I could swear last time I checked, the voltages were ok...

بواسطة Wierzbowsky

Guardian (3541)

صورة Wierzbowsky

02-11-2022, 18:23

Those big voltage regulators are no longer reliable. I've changed a few of them during the last year. Luckily, they are still available for purchasing on AliExplress.

They tend to break and either start providing too low voltage (if you are lucky) or too high voltage. In one case I had almost 17v on a 12v bus. Luckily nothing burned down, but that scared me pretty well because this also affects the 12v on the slots.

بواسطة Dd84

Supporter (9)

صورة Dd84

02-11-2022, 23:29

Do you mean the power transformer? If so, I’ve had a bit of a look on aliexpress but the closest dual voltage ac to ac is 6 /12

Where is this hidden 12v fuse?

بواسطة Dd84

Supporter (9)

صورة Dd84

02-11-2022, 23:32

I noticed there’s no continuity between 5V and G but there is between 12V and G. I haven’t isolated the transformer to check that separately

بواسطة Dd84

Supporter (9)

صورة Dd84

03-11-2022, 08:19

Apologies for my barrage of contradictory posts. Electronics aren't my forte.

I was testing the voltages at the power/RF board on the input side. I looked at the service manual and measured at the output of the RF/power board. The voltages are correct.

Next I measured voltage at the 4116 chips. No -5V on pin 1. Based on a previous thread, does this bring it back to the mystery -12V fuse?

بواسطة Wierzbowsky

Guardian (3541)

صورة Wierzbowsky

03-11-2022, 17:19

Check the 79L12 voltage regulator's input and output. If it's broken, replace it. If -12v appears, then -5v should also appear. If not, then you need to check the zener diode.

بواسطة Dd84

Supporter (9)

صورة Dd84

04-11-2022, 00:18

Thank you. We do have -12V

I found the reference in the book which shows how -5V is made. Is that the diode I should replace, D1 5.1V (near the plug to the power/rf board)?

I checked the other components mentioned and all seem to have continuity, though I realise this isn’t the best test for capacitors (and it’s a cheap multimeter which I don’t think tests diodes with any accuracy). What was interesting is that one side of R11, which I believe comes directly from the -12V plug/wire, is -12V on one side and 0.3 on the other. I had presumed the voltage here would be much higher than that. Resistance across measures at 890, spec is 1000, doesn’t look burnt etc.

بواسطة Wierzbowsky

Guardian (3541)

صورة Wierzbowsky

05-11-2022, 10:03

Zener diode works together with the resistor, so check both and replace if necessary. Also, check the caps in the same circuit, they are unlikely to be bad, but still.

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