Power Supply Replace

Supporter (12)

06-09-2016, 08:04

Hi to all, i have one Fenner SPC-800 with damaged power supply.
Is possible to replace a original power supply with this model or similar?
http://www.ebay.com/itm/30W-5V-12V-12V-Triple-output-Regulat...
The total power consumption of Fenner SPC-800 is 21W the power supply have 30W output max.

Thanks in advance

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By Wierzbowsky

Guardian (3603)

06-09-2016, 09:24

It's easier to fix the original power supply IMHO. Unless it's physically broken (charred or broken board or transformer. Part-wise the repairs may cost just a few bucks.

Please show the photo of the damages if they are visible.

By agnul75

Supporter (12)

06-09-2016, 11:03

No parts damages only missing +5v, 12v + & - is OK
The computer was in poor condition, i have open it and remove keyboard, mainboard and tv tuner and clean all parts.
After i have tested the power supply without connect the mainboard and 5v+ is missing.
power supply 1
power supply 2

By Wierzbowsky

Guardian (3603)

06-09-2016, 15:07

I don't have the schematic at hand, but those D3 and D4 must not be like that. They look crossed and they should stand away from each other. Please fix this by carefully returning them to their original position. Then check 5v again. If it's still not there, then the 7805 voltage regulator (that is one of those 3 big regulators connected to the heatsink) is most likely faulty. Replacing it will solve the problem with a large probability.

And just in case please check the continuity between 5v and ground. If the 5v line is shorted to the ground, check all the parts in the 5v circuit - one of them could cause the problem.

By Lord_Zett

Paladin (807)

06-09-2016, 17:05

https://www.conrad.nl/nl/acdc-inbouwnetvoeding-open-mean-wel...

i used this one for projects

By agnul75

Supporter (12)

06-09-2016, 17:59

OK how to test the 7805 voltage regulator? i have only classic tester.
the first green wire called R.C. what is needed? in anothers MSX i dont have this in the power supply.

By Wierzbowsky

Guardian (3603)

06-09-2016, 18:40

Ignore the green wire, it's probably the inhibitor.

Did you fix the diodes? Still no 5v? Did you test continuity between 5v and ground? No shorts?

On 7805 check the input voltage first (measure between middle and left pins). It should be above 8v. Then check the outgoing voltage (measure between middle and right pins). There should be around 5v or a bit more. If there's no input voltage then the bridge diodes or the transformer may be broken. If there's no output voltage, then the 7805 is fried or something after it is shorted. See the pinout:

http://www.mpja.com/images/0359-1.jpg

By agnul75

Supporter (12)

06-09-2016, 19:06

Main transformer have 17 volt AC output in first and second line
imput voltage for all 7805 is 23 volt
output voltage for the first 7805 to the left is 23 volt
output voltage for the central 7805 is 12 volt
output voltage for the first to the right 7805 is 10.8 volt

By tvalenca

Paladin (747)

06-09-2016, 19:21

Alexey wrote:

If it's still not there, then the 7805 voltage regulator (that is one of those 3 big regulators connected to the heatsink) is most likely faulty. Replacing it will solve the problem with a large probability.

I didn't looked at the schematics, but this seems strange. a 7805 regulator only provides about 1A, which is way shorter than a MSX need. Isn't that a 7905 (-5V regulator)? If this MSX uses 4116 memories for VRAM, its PSU should provide -5V, and one way to get this is a 7905 regulator on the -12V line. It may not be enough to prevent the PSU from working thou.

Some MSX PSU have a inhibit feature: if you don't ground a specific wire (which is wired through the slot protection mechanism) the PSU won't power up the MSX. check this wire, if it's there it will be connected to a switch behind the cartridge slot cover and slot pin 44. Some MSX won't power up if this mechanism isn't working.

If there's a short on +5V line as Alexey suggested, PSU self-protection circuit will prevent it from working, so use the continuity test or the ohmmeter to check for continuity.

By RetroTechie

Paragon (1563)

06-09-2016, 19:49

Welcome to this forum, agnul75!

Alexey wrote:

I don't have the schematic at hand, but those D3 and D4 must not be like that. They look crossed and they should stand away from each other.

Silkscreen suggests they're positioned correctly... But better check that leads are not touching.

Either of those linked psu's should do fine. But like Alexey said, fixing the original supply is probably easier.

I'd also suspect the +5V regulator IC. Is it possible to post a photo of the bottom of that pcb? Would make it easier to check how the psu's components are connected. And type numbers of those 3 IC's stuck to the cooling block (probably 7805 / 7812 / 7912, but confirmation would be nice).

By maxis

Champion (512)

06-09-2016, 21:00

Sir,

Are you sure, that actually you have a 7805 regulator?

IMHO, +5V regulator on this board is not linear, but rather switched-mode. Please, read carefully the markings on the transistors/ICs too.
To me it seems, like the very first thing to the left on the heat sink is not 7805. It is a transistor (must be marked Qxx, B C E)
So, the other 4 small transistors will create a voltage reference and a PWM oscillator/modulator.

Very important: disconnect the computer motherboard from the power regulator before continuing. On the power supply board you have a VR8 resistor (to set the output voltage). Turn it several times left-right-left-right... it might have corroded. Observe the +5 output. Does it change?