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britlord
14th March 2015, 13:45
This is a question for the electronic techs here. I have a AT PSU thats not working correctly (Low voltage, fan spins slow too) You see I did a recap and notice that a diode is blown on the controller PCB (Gone black). But I don't have a clue what type it is (I have removed it). All I know is that it some sort of zenner diode. The board has 5 other Zenner diodes, and I bet they are all the same type. Anyone know what the type of Zenner would used on this board.. I do have another AT PSU for my 1200, and that works fine (The same type). But I want to use this one for my 500+....

roy_bates
14th March 2015, 13:54
dont assume the diodes are all the same.

also what does the diode do in the circuit and where does it lead?

then we might be able to work out what replacement you need.


also,cant see any zeners on the board you posted.its not close enough to see much...


you also have to realize when diodes go bad they sometimes take out other parts in circuit before and after it.

britlord
14th March 2015, 16:16
I get what your saying. All I can tell it that is that diodes are of the glass type, and the markings in all the diodes start with 2N (Can't read the rest).. Oh diode that I removed was on the top left side (DZ)

roy_bates
14th March 2015, 16:43
hang on a sec...i think that ic is part of the power good circuit?

EDIT:yeah it is i think,it wont help you much unless you understand what im looking at.

britlord
14th March 2015, 16:49
hang on a sec...i think that ic is part of the power good circuit?

EDIT:yeah it is i think,it wont help you much unless you understand what im looking at.

Well it is a PWM Power Supply Controllers IC as I have some on order, so I can socket it...

Oh the other IC is marked M51957A

Oh again. LOL Roy. (Off topic).. What's the size of the square (Gold) in the middle of a 68030 processor as I need to get a heatsink..

roy_bates
14th March 2015, 16:59
have you got a multimeter?

test either side of the missing diode and see if either is a ground.
but do it while its off and connected to the rest on the supply.

ill have to assume you know where ground is on the supply,not earth

ill wait.


the other part your looking at is this:

http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCIQFjAA&url=http%3A%2F%2Fdocumentation.renesas.com%2Fdoc%2Fproducts%2Flinear%2Frej03d0778_m51957ads.pdf&ei=EGkEVa2LN6fQ7AboWg&usg=AFQjCNGTPKfKWdcNu7oy5Yahkpajr1Ip4A&sig2=tlWXi43pxDgV6AaaIg369A

britlord
14th March 2015, 17:03
Roy how can I tell, as I get a reading both ways..

roy_bates
14th March 2015, 17:14
never mind that right now...i can see where ground is its the middle pin on the m51957a pin one is the voltage to run it.it cant be more than 18 volts max.


you say you have one just like this one?...as in the same one exactly?

i think the diode is a limiter.it could well be a zener

britlord
14th March 2015, 17:32
If You mean the diode. Nope I don't have any yet, or that IC. If the IC then it is exactly the same one as in the link...

And here's pics of the AT PSU that I have converted for Amiga use. I have even painted the metal cases white too (Just have to fit rubber pads under the case)...

roy_bates
14th March 2015, 17:35
i meant the power supply

britlord
14th March 2015, 17:41
Well I have ordered another one (Thank god that I found another exact one), so I can use that PCB board. I'll just have to remove it, and solder a header to it.. You see these AT PSU are very easy to convert for Amiga use (I have one for my A1200), and they don't need any load to run (Power good wire soldered to +5v). But there's not many of this type around even on eBay (I only found one seller. Now he just has 3 left)

roy_bates
14th March 2015, 18:14
i think the top of the 030 total is about 35mm square,so the gold part is about 20mm square i think.

if you mean the pga version.

britlord
14th March 2015, 18:32
Yep The PGA, so I'll get a 35mm square one. You see I'm planing on over clocking a 50MHz one to 54MHz (I have the crystals). Will it it work with a passive heatsink fitted..

roy_bates
14th March 2015, 18:34
dunno mate,depends on alot of things...


let me get a calculator...

EDIT: probably will,but i dont think its worth it for 4 extra mhz on these cpus.
if you start getting instability problems and memory errors clock it back..dont say i didnt tell you,lol

britlord
14th March 2015, 18:46
Well it's just 4MHz over. Now back to the topic lol. Roy I've ordered these. But what voltage should I use..

http://www.ebay.com/itm/360784776930?_trksid=p2059210.m2749.l2648&ssPageName=STRK%3AMEBIDX%3AIT

roy_bates
14th March 2015, 19:40
the easiest way is to wait for the replacement to turn up and get your multimeter, set it to dc volts and put the com probe to ground and the other to one side of the diode to see what voltage it is on the working one.

britlord
14th March 2015, 20:19
Ok Will do... Thanks..

britlord
15th March 2015, 00:54
dunno mate,depends on alot of things...


let me get a calculator...

EDIT: probably will,but i dont think its worth it for 4 extra mhz on these cpus.
if you start getting instability problems and memory errors clock it back..dont say i didnt tell you,lol

Roy. It should work, because I have a Cobra board with SMD 33Mhz 68030 (Plastic version), and I have overclocked it to 40MHz and the Cobra runs without any problems (Heat sink on it though), so I'll used a 56MHz Oscillator (Not 54MHz) on my Midget racer board (That has a 50MHz fitted)...