Power supply with only negative voltages

[roadrash]

I am trying to get a old Osborne 1970's computer working. I turn it on but there is no life at all and no noises or smells. The PSU looks in great condition and all caps and filters look like new with no visible damage. I have aslo desoldered some of the caps and tested them on a ESR meter and they seem ok too. If I turn it on and the PSU slowly warms up but still no life from the computer or its built in display. I am struggling to find anything wrong with it but I have noticed it is only producing -12 & -5v at 2 of the 4 power output connectors to the logic board. I thought there should be both + & - 12 and 5v. I am assuming its a negative earth. Can someone help me with this? I have got a manual for it and a schematic if needed.

[stj]

Re: Power supply with only negative voltages

check the diodes and upload some foto's

[roadrash]

Re: Power supply with only negative voltages

Thanks STj, I Took out some more parts today to give them a test like the diodes you suggest. I have removed quite a few so far but all test as ok. I removed a transistor to test and dropped it on the floor and now cant find it anywhere. Aint that typical! Ive ordered another in case it dont turn up. I have also re-tested some of those caps again with a new tester I got recently and can I ask you what tolerance is acceptable on these electro caps as I have one 25v 1000uf that tests at 1092uf and another at 1038uf. Also what is a acceptable VLos percentage? Here is the link to the diagram (PSU is on page 5):

[stj]

Re: Power supply with only negative voltages

esr is more important than capacitance

[stj]

Re: Power supply with only negative voltages

upload the manual here - i'm not signing up to dropbox.

[roadrash]

Re: Power supply with only negative voltages

upload the manual here - i'm not signing up to dropbox.

Sorry didn't realise you had to sign up to dropbox to view a shared folder or document. manual is too big for this forum so took screenshot of main PSU part of Schematic and uploaded it.

[roadrash]

Re: Power supply with only negative voltages

esr is more important than capacitance

Ok just read up on it "equivalent” series resistance. But again no figure is mentioned. Like what is regarded as too high a resistance. There must be a cut off point when its regarded as out of acceptable levels. And also checked on the tolerance of caps and now understand that it is 10 - 20% depending on size and is usually written on the cap as + or -%. I didn't realise this although I knew of the tolerance of resistors. Apologies I am a dump Motor mechanic but love repairing things not throwing them away so I need to learn more.

[stj]

Re: Power supply with only negative voltages

esr is listed in capacitor datasheets.

[stj]

Re: Power supply with only negative voltages

ooo - bypassing those dropbox assholes is a bit easy!

[R_J]

Re: Power supply with only negative voltages

I think your probes may have been reversed, or you used the wrong ground, They are using the +5 volts and the + 12 volts but the -12 volts at test pint 46 is not used (nc) no connection.
I would believe the the power supply is ok and the problem is elsewhere

Check page 5 (part of main logic board) that is where it gets its negative voltage, see if there is -5.1 volts and -8.2 volts if they are missing I might suspect C72 15uf is bad
also check the + 4.94V and the +8.7v

[roadrash]

Re: Power supply with only negative voltages

I think your right too the problem is something else but my probes were not reversed but maybe was using wrong ground. PSU was weak and wasn't producing enough current to power the chips on the logic board and if I checked the voltages on the logic board with a multimeter the voltages disappeared and only a power off and restart restored them. So something else is wrong. I've had the transistor I lost turn up today and just waiting for all the new caps to come now and I'll reassemble it and test it again. I have also tested the bridge rectifier and it checks out ok. Also removed a Diode block that appears to contain 4 diodes that I will test to. Other than that there isn't a lot more I can test as all the other diodes tested ok.

[roadrash]

Re: Power supply with only negative voltages

Some of the Caps arrived today but I found it hared to get caps anywhere near the size of the original caps. The originals were 16x33mm and the biggest I could source from ebay was 10x20mm. Does this matter greatly or should I try and source some others somewhere else.

I also have 2 other caps that look perfect and my esr meter tests them as so:

25v 470uf- tests as: 467uf, vlos=1.4, esr=21ohm

16v 2200uf - tests as: 2268uf, vlos 1.8, esr=25ohm.

I'm sure from those figures that the 470uf cap is ok but not sure about the 2200uf one. opinion someone is it usable?

[ChaosLegionnaire]

Re: Power supply with only negative voltages

errr is the esr measurement really in ohms or milliohms? 21-25 ohms for a cap's esr is quite high! the cap may be dried out...

[roadrash]

Re: Power supply with only negative voltages

errr is the esr measurement really in ohms or milliohms? 21-25 ohms for a cap's esr is quite high! the cap may be dried out...

I should have said 0.21 & 0.25 ohm or that's what my meter shows.

Well I am really running out of things to check now. I have checked all the diodes and none were faulty. I have also checked quite a few resistors too and they check as ok. I also tested the transistors including the large power transistor (Q2) and they are ok. I removed the Bridge rectifier (KBP-10 8128) and it was testing as ok as well. I checked the windings in the transformers and they seem to all right although I don't have the resistance figures for them but they are not open circuit and have resistance. I did find out this was a PSU made by a company called "Astec" model: AC8151 and I found a company repair manual for it too and its a lot more helpful than the Osborne one. I have attached it for you to see. Hopefully now with some help from here I can get this fixed now. Does anyone know what the Power transistor is as I can see any component ID anywhere.

[PeteS in CA]

Re: Power supply with only negative voltages

Astec was a good company, making good stuff. They also made the Apple II+ power supply. That's a discontinuous flyback design, very stressful on the output caps. If you are testing it outside of the computer it does need to be loaded to perform well.

Astec had the unpleasant habit of wire-brushing off the markings of their switch transistors. On one P/S I saw they did a poor job with the wire-brushing, and I made out the part number. It was a horizontal flyback transistor used in TV sets, nothing exotic. For 38W, the output power rating of that power supply, an MJE13005 would probably suffice, if it's a TO-220 package device.

[roadrash]

Re: Power supply with only negative voltages

Astec was a good company, making good stuff. They also made the Apple II+ power supply. That's a discontinuous flyback design, very stressful on the output caps. If you are testing it outside of the computer it does need to be loaded to perform well.

Astec had the unpleasant habit of wire-brushing off the markings of their switch transistors. On one P/S I saw they did a poor job with the wire-brushing, and I made out the part number. It was a horizontal flyback transistor used in TV sets, nothing exotic. For 38W, the output power rating of that power supply, an MJE13005 would probably suffice, if it's a TO-220 package device.

Yes you are right. look at the attached pics and you will see the markings on the transistor are ground off. There is no reference to what this isn anywhere in any of the manuals either. Do you know what this is?

[stj]

Re: Power supply with only negative voltages

BU208a maybe.

[R_J]

Re: Power supply with only negative voltages

In your original post you said you were getting 15 and 5 volts, although you said they were negative, what happened? are you now getting no voltages at all?

[PeteS in CA]

Re: Power supply with only negative voltages

Yes you are right. look at the attached pics and you will see the markings on the transistor are ground off. There is no reference to what this isn anywhere in any of the manuals either. Do you know what this is?

Well, I didn't expect that package type. Commonly referred to as a TO-3 package, it's actually a TO-204 package. And the part I referred to earlier was in that package. So that is probably a TV horizontal flyback output transistor. The part I figured out was made by Fujitsu, I think, and had a 2SC#### type part number. If you had to replace it a 2N6545 would work. At the die level, the 2N654x and MJE1300x series were of the same family, just in different packages.

[roadrash]

Re: Power supply with only negative voltages

In your original post you said you were getting 15 and 5 volts, although you said they were negative, what happened? are you now getting no voltages at all?

Yes I lost the voltages i was getting. Dont know what happened because i when i first powered it on connected to the logic board & crt monitor I got those negative voltages but as soon as i tried to test for voltages on any of the chips on the logic board they would disappear until i turned it off for a few minutes then when powered up came back until the next time i tested for any voltages on the logic board again. eventually I lost all voltages. Whatever power was going to the logic board and crt monitor was very small and not enough for them to power upp..