related parts list, Selector, The along suspect a After my stupidity it started to run open (chatter) in local mode as well as line mode. But this could simply be because two of the fuses blew. I have new fuses on order and I hope they will be here soon. It was suggested to me (and you said it too) that transistors fail faster than fuses and that this would be an obvious one to test. I thought the voltages I reported to you were suspicious, didn't you say they indicated it is leaky? I am wondering what my first step should be once I get the fuses. Just turning it on would seem a bit risky. I could do the light bulb trick to limit current perhaps, someone suggested I do this, but disconnect the motor as it will try to draw too much current. Regards Rob

Yes, it might well be. One of the big differences between most UK(Creed) and American (Teletype) amchiens is that the fobmer use double-current signally (a current one way for a mark, the other way for a space), the latter use single-current signally (curent = mark, no current = space). In the case of the Model 33, if the receiving magnet driver loses powre (maybe because the fuse has blown...), there will eb no current through the receiving magnet and the machine will 'run open' (or 'space out' or whatever you want to call it). It looks leaky, but it might well be germanium and they tend to leak anyway. I don't think you wil ldo any real harm by fitting the correct fuses and powering up. There are no custom, delciite semiconductors in a Model 33, You are not going to burn out the receiving magnet either. The worst that will happen is the new fuses will blow and maybe some small components in the manget driver. Amazingly (given my normal caution) I would fit the fuses, power up and see what happens. -tony

There are several reasons why I don;t think you need to be over-cautious this time. The first is that although fuses blew, there was a definite reason for them to blow, you applied too high an input voltage, so probably the mains transformer core would ahve saturated, the current would have gone sky-high. The socnnd, and more importent, reason is to consider what could be damaged if things went wrong. In the case of an IC-based computer, a failure int he PSU can wipe out all the ICs -- expensive and torublesoem to fix (fi there are rare/custom ones invovled). So I would always check the PS on a dummy load first. But in the case of the Mdoel 33 Teletype, what can happen if that transistor has failed? If it's open, there's no current in the receiver magnet, the machine 'runs open'. OK, it's probsbly not good to run it continuously like that, but it wil ldo no harm for the few minutes it takes for you to nice that it's running like that. If the transistor is shorted, well, since the normal state of that transistor is saturated, there will just be the normal current through the receiver magnet, the machine will never do anything. Bu no damage. I suppose it's posisble for a failure here to damage otehr parts in the selector magnet driver, but those parts are cheap and esasy to get -- R's C's and a transistor. And I think such damage is unlilely anyway. So yes, try it and see what happens. -tony

I would never think of looking on Ebay for fuses... If in a hurry, Maplin do a small range, Cricklewood Electornics have rather more types in stock. If not in a hurry, there's RS and Farnell... But don't be tempted to short out the fuseholders to try it now. If there is a serious problem you could end up buring out the transformers, the drive motor, etc. Not pleasant. -tony

I am suprised about that. I thought the fuses in a Model 33 were all common types that you coudl get almost anywhere. What is this difficult-to-get one? ricklewood are useful to me becuase they're near enough for me to go there and pick u pthe bits I need. Very useful when I've run out of 74LS00s or something. I am not sure I;d use them for mail orde -- not that I don't trunt them, I certainly do, but if I am going to go to the toruble of making and order nad waiting for it to come I might as well go to Farnell. -tony

I always remember them as 1+1/4"... To me the standard fuse lengths are 5/8", 20mm, 1", 1.25" I suspect a 250mA fuse wouldn't last. If necessary I would sensibly over-rate the fuse, up to say 500mA, at least for somenthing I was goign to be keeping an eye on (like a model 33 I was testing/repairing. But fittign the right one is always a good idea if you can get it. -tony