On Sat, Oct 10, 2015 at 8:44 PM, Josh Dersch <dersc...@gmail.com> wrote:
> On Sat, Oct 10, 2015 at 2:38 PM, tony duell <a...@p850ug1.demon.co.uk> > wrote: > >> > >> > From a quick glance at the schematic I think that this part is related >> to >> > the fan driving circuitry. The diodes look like some kind of protection >> >> Yes. >> >> > diodes which is good to have when switching inductive loads. I have >> really >> > no idea why they have been removed. But I don't think it is related to >> the >> >> Remember that said diodes do slow down the decay of the current (and thus >> the magnetic field) when the transistor turns off (this can be a problem >> with >> solenoid drivers for paper tape punches, etc). It is possible that with >> the diodes >> in-circuit the fan drive waveforms were not right. >> >> > PSU not working at all. >> >> Correct. The fan driver is powered by the 36V output of the 'memory' >> SMPSU, >> Even if it is defective, the bias supply should run, the logic supplies >> should come >> up in the ON position of the key, and so on. >> >> That 'bias/interface' PCB contains (at least) the bias supply chopper and >> control >> circuit; the fan driver; and the control logic to start the 2 main PSUs >> in the correct >> posiitions of the keyswitch, >> >> -tony >> >> >> >> /Mattis >> >> >> >> > - Josh >> > >> > >> > > -tony >> > >> > >> > > Thanks for the explanation. I tacked on some wires to the bias board (to > bring them out for safe probing); the wires are connected to the 5V logic > supply and the +12V bias supply and I get nothing at all out of them when > the H7140 is turned on. > > I'm slowly coming to terms with the schematic but I'm not quite clear > where these two voltages are being generated. I note from the block > diagram that the +15 and +12 are rectified on the motherboard (backplane) > -- which should make it GREAT fun to probe. So it looks like some careful > disassembly is now required (unless there's a trick to probing things on > the backplane...) > > Thanks, > Josh > > > And a quick correction, it's -15V, not +15; I need to proofread more :) - Josh
