Power supply with only negative voltages

[roadrash]

Re: Power supply with only negative voltages

I don't own a Osborne, I only have the power supply, these things were used everywhere
why don't you make sure the power supply is working properly before connecting it to the logic board, It should operate by itself and you can make sure the voltages are correct before connecting it. It's hard to say why the cap went, It could have been bad or it could have received a voltage higher than it could handle, I would replace it and then connect a meter to it so you can check the voltage thats going to it but I suspect it was just the one cap. or there would have been more damage
That whine noise was from the power supply because there was a short on one of its outputs

I did wonder that after the cap blew. I have had this happen before especially on the Original IBM XT & AT computers that have lots of these little tantallum caps and do every so often pop one. I guess its an age thing like you get with electro caps. What would happen if I replaced this with a electro cap, would it be better or maybe just to test it again while I wait for the replacements to come? I don't think there was over voltage though like you say because only this typical tantallum cap blew and nothing else.
I will get it hooked up to a voltmeter before I turn it on next time ,

[Agent24]

Re: Power supply with only negative voltages

stay away from NTE, they just buy surplus junk and re-print the numbers

If that were true I doubt they would still be in business.
I suspect they are no worse than any other generic replacement or second-source manufacturer, just more of a rip-off.

I did wonder that after the cap blew. I have had this happen before especially on the Original IBM XT & AT computers that have lots of these little tantallum caps and do every so often pop one. I guess its an age thing like you get with electro caps. What would happen if I replaced this with a electro cap, would it be better or maybe just to test it again while I wait for the replacements to come? I don't think there was over voltage though like you say because only this typical tantallum cap blew and nothing else.
I will get it hooked up to a voltmeter before I turn it on next time ,

Quite possible it had just internally shorted due to age.

Tantalum have lower leakage current so if they chose tantalum for that specific reason, then it would be wise to use another tantalum or a low-leakage aluminium electrolytic. But they probably didn't, I suspect they just figured tantalum would last longer, no liquid to dry out. Likely used for decoupling, I suspect it wouldn't matter which one you used.

[R_J]

Re: Power supply with only negative voltages

Not knowing where that cap is in the circuit I dont think its that critical, Just put a electrolytic in its place, you can always change it to a tan. later
As for the NTE brand, years ago we tried using them when we could not get the original or an ECG replacement, usually to replace a horzontal output, we found they might work but would almost always get very hot where the ECG would work better, In all cases the original would run very cool, I suspect the NTE were just not switching fast enough. I never use them anymore as you can get original parts so easy now.

[Agent24]

Re: Power supply with only negative voltages

I have read on RepairFAQ that in general, substitutions for a HOT are a bad idea because the circuits are often designed very specifically for a certain transistor. Sometimes they only work reliably with parts from the same manufacturer, too. Your experience seems to back that up.

That said I have no actual experience with substitutions for HOTs, so I can't say anything.

NTE parts cross-reference so many others there is no way they are going to be the same as the original, so for a HOT - if they are indeed so specific - it makes sense that NTE or any other substitute parts would be much less likely to work.

Of course any substitution is just that - and there's no guarantee that it will work like the original in a particular circuit unless you know exactly why the original part was chosen.

[roadrash]

Re: Power supply with only negative voltages

Interesting, in a new project I have just started, a old Apricot PC (on another thread) is also using one of these Astec psu's and on this one they haven't ground off the markings on the power transistor and its using a Nec C1325A. I wonder if this is the mystery power transistor used in this Osborne power supply.

[PeteS in CA]

Re: Power supply with only negative voltages

Interesting, in a new project I have just started, a old Apricot PC (on another thread) is also using one of these Astec psu's and on this one they haven't ground off the markings on the power transistor and its using a Nec C1325A. I wonder if this is the mystery power transistor used in this Osborne power supply.

That would be a 2SC1325A. With Asian semi companies it was/is common that a part number like "A####" or "C####" or "K####" is actually "2S_____".

It is very possible that that is the part used in the Osborn P/S. The 2SC1325A was designed for horizontal deflection use in large screen (probably 23" and 25", in that time frame) color televisions. I don't know about PAL, but the horizontal frequency for NTSC was ~15.75 KHz, not much lower than the 20-30 KHz switch frequencies common in that era. Astec used such parts because they were adequate, reliable, and inexpensive (due to high volume usage). Astec was all about good performance and quality at a very low price, and component choices like this were a key factor.

[roadrash]

Re: Power supply with only negative voltages

The new Tantallum caps arrived and i soldered one in and connected it uyp up again but the PSU is now doing the chirping noise whether connected to a load or not. What should I be looking at on this old PSU?

[R_J]

Re: Power supply with only negative voltages

Usually the chirping sound is the primary trying to start but seeing a short somewhere in the secondary, I would start by checking the secodary diodes,
was the shorted cap on the 12 or 5 volt line? check the components that supplied that voltage

[roadrash]

Re: Power supply with only negative voltages

Usually the chirping sound is the primary trying to start but seeing a short somewhere in the secondary, I would start by checking the secondary diodes,
was the shorted cap on the 12 or 5 volt line? check the components that supplied that voltage

the cap (C71) is connected to a CA555CE timer which according to schematic has three voltage inputs of 6.14, 7.45 and 8.09 and the cap was rated 16v. I did before this cap blew check all the diodes on the entire pcb so I guess I will have to recheck all the diodes again in the primary section is that right? Does this include the three big diodes on a heatsink?

[R_J]

Re: Power supply with only negative voltages

I thought the cap that blew was on either the +12 or +5 coming from the power supply, Can you disconect the power supply from the main/logic board and see if the voltages come up? if they don't then I would check for something shorted on the secondary of the power supply, ie diodes

[roadrash]

Re: Power supply with only negative voltages

I thought the cap that blew was on either the +12 or +5 coming from the power supply, Can you disconect the power supply from the main/logic board and see if the voltages come up? if they don't then I would check for something shorted on the secondary of the power supply, ie diodes

I just checked every single diode and something called a diode block (3xdiodes) and all check out as ok. so more investigation needed i guess. This is very annoying as I already checked every single component at the start of getting this supply working.

[roadrash]

Re: Power supply with only negative voltages

Ok brilliant ive got it all working and have even had the whole computer up and running now. Ironically I found a transistor that had a bad connection on the emitter. I found it when i was removing it to re-test it after all the diodes checked as ok. I dont know how I did that. Anyway all this has taught me lots and I am very grateful to everyone on here who has helped me.