BTW, anyone looking for a good sunday talk? Incredible I have no one from this list on MSN. If you're looking for some chatting, add webmaster at pinball-taito.com.br to your MSN and lets have a bit of talking :D Boring day... Greetings from Brazil Alexandre Souza, PU1BZZ --- Enviado do meu Motorola PT550 Meu site: http://www.tabalabs.com.br

On Sun, May 1, 2011 at 5:53 PM, Michael B. Brutman <mbbrutman-cctalk at brutman.com> wrote: I recommend checking out both channels. If #vc is the computer club meeting, #classiccmp is probably the meet-up in the bar [1] afterward ;) [1] Or pub, since we have a few Euro types, too. -- S.7k

So the 555 timer is getting power, and 8.6V is enough to run a 555. So why isn't the PSU working? OK, I know what I would do next : 1) Remvoe and test (properly) the chopper transistor. It's the most likely part to fial after all. 2) Remove the Bias and Interface Board and find some way of powering that control circuitry from a bench suppy (or even a 9V battery). Of course you can't then link it to the mains parts of the H7140 (like the chopper transfromer), but now you can connect a 'scope to it and see if that 555 is oscillating, if the chopper transistor is being driven, if the regulation feedback circuit works, etc. -tony

Please don't take this as an insult, but I am going to make the same recoemndation that I made to another list member a few days ago. Read 'The Art of Electronics' by Horrowitz and Hill. I think it will help you to understnd some of these circuits. The diode test range on most digital multimeters is quite simple. It supplies a contanst test current (a few mA) to the probes. The display shows the voltage between the probe. When connected to a working diode junction, connected the right way round (normalyl with the red probe to the anode), that voltage will be about 0.7V. The other way round you'll get 'OL' or however your meter shows an over-range conditon An open junction (or nothing at all) will show 'OL' bot ways round. A shorted jucntion will show 0.00 both ways round. Now, a transistor can be consideded for the purpose of this test to be 2 diode junctions -- one from collector to base and one from emitter to base. Bo, you can't make a transistor by connecting up 2 diodes, (there's some magic that goes on in the thin base layer), but you can test the 2 junctions seprately. Having 2 good juntions don't mean the trransistor is necessarily good, but if you don't have 2 good diode juntions, it can't be a working (bipolar) transistor. The first thing to do is to fidn out the transistor pinouts. Often sites like http://www.datasheetarchive.com/ and http://www.digchip.com/ are useufl. Type i nthe part number and see what turns up. I often use a book called 'Towers International Transistor Selector', but it's quite expensive and probably not worth buying unless you are as insane as I am. Actually, for the sort of small power trnasistor you're looking at, in what's called a 'TO220' pachaga, the pinout is ----------------- | | |------- emitter | O | |------- collector (also connected to the metal tab) | | |------- base ----------------- That shows the transsitor with the metal face downwards (Plastic 'bump' uppermost). The tab is on the left with the mounting hole. Now, this is an NPN transistor which will test as a pair of diodes with a common anode connection. Assuming your meter is conventional : Red Black Probe Probe Reading ---------------------------- b e 0.7V e b OL b c 0.7V (Often this is a little higher than the b-e reading) c b OL c e OL e c OL As I said, that's doesn;'t prove it's a working transisotr, but it's a start, There are much more elaborate specialist transisotr testers out there, going uop to things like 'curve tracers' which will display a graph of say collector current against collector voltage for various vlaues of base current. Tektronix 575 or 577 for example. I asusme yuo don't have an adjustable bench power supply either. Let me first explain what I would do. If you look at the Bias and Interface Board scheiamtic, you will see thatwhen the supply is running, this circuitry is powered from a feedback widnign on the transformer (T1 on the PSU motherboard). The output of this winding firstly powers the osiclalator cuircuit, secondly it's connected to a zener diode to the base of a transistor (IIRC bottom left corner of the diagram, I don't have it in front of me). The idea is that if the output votlage rises too high, this transitor affects the opeation of the 555 oscillator, reducing the output votlage. In other words a votlage regualtor. I have looked up the characteristic of that zener diode, it's rated at 12V. Together with the 0.7V b-e drop in the assoctiate transistor, thsi thing is going to regualte at about 12/7V (DEC call it the 13V line on the diagram, which is reasonable). I owuld therefore connect an adjustable PSU to the pins on the connactor on the Bias and Interface Board (removed from teh PSU) that normally to this transformer widning. With the supply set to 10V or so, the oscilator should run, I would check this at the output (pin 3) of the 555. Then, carefully increase the output of the supply and see if the regualtion circuit operates as it should do. If the oscillator is running, then I'd check the circuitry coupling it to the base of the copper trnasistor. Incidentally, it would be worth chercking that the widings of the transfortmer T1 are continuous. An ohmmeter chceck between the appropraite conenctions will do this. In a word 'Tektronix' :-). More serieously, there are 2 schools of thought when it comes to cheap scopes. Some people prefer a cheap-nasty fairly modern thing which will progagbly work as well as it ever did and will keep going with no problems for a few years. Others, including myself. prefer an older, top-quality instrument. It'll be large, it'll probably use valves, and it ill need a bit of tweaking to keep it running. This can be fun (and to be fair, a good older 'scope will be relaible if you look after it), but equally, if you're just starting out you may not want to laearn to fix a 'scope just so you can fix your PDP11. But it's something you may well want to do later. As regards bandwidth, for digital work you want a bandwidth of at leaast 3 times the highest frequency you want to look at. But acutally, a 'scope is not that useufl an instruemtn when dealing with purely digital problems (like debuggign the CPU itelsf), a logic analyser is more useful there. I find I use the 'scope more for looking at SMPSUs, motor drivers, that sort of thing. For qhich a 20MHz bandwidth is adequate. Dual trace is almsot essential, dual timebase (only found on a few models, and no modern cheap ones!) is very useful. A good trigger system is probably the msot important thing of all, if you can't keep the trace steady, you can't measure from it. Alas this is one thing that is rarely mentioned. -tony

so For "our" purposes, 1st or 2nd edition? 2nd edition is a lot more expensive. Regards Rob