F100 issues

Just got an F100 in what appears to be fairly good, but obviously old condition. When it first arrived, I tested it out, and it seemed to work beautifully. Took it to it's first gig (mostly just to try it out in practice, I didn't really need it), and hooked it up. It heated up fine, but the pump wouldn't run.
Now, back in the shop, I measured the voltage on the pump leads, and it seems to be 110VAC, whether or not the run button is pressed (timer is turned off on the timer remote). I also measured with a scope, the input voltage to the pump optocoupler. You can see a train of pulses when you press the Run button.

So, to me this says that the problem is either the SCR, the pump, or the optocoupler.

With the power off, the resistance across the pump windings is high, but not open (about 1 Meg).

What should the resistance of the pump windings be? Is this indicative of a bad pump?

If this is the case, does anybody here have a surplus pump they wouldn't mind selling?

I haven't really worked much with SCRs but I seem to remember they require a load to work properly, so is this why the voltage is 110VAC? Is the pump not giving enough load to the SCR for it to work? Or is this a bad SCR?


Thanks,

Jeff
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    If the SCR were shorted, the pump would likely run all the time, or overheat if it's a pair and only one side failed.

    I suspect the pump myself - 1 meg is very high for any device that will run on 110V. Since these are magnetic, the coil should read closer to a short circuit on a typical meter that tests with DC. I suspect the 120V you are seeing is a combination of the SCR being fired, and no load - they do have a bit of bleed.

    - Tim
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    Turns out it's not the pump. Supplying power temporarily directly to the pump causes it to work (found this out accidentally by shorting the SCR with a meter probe while trying to measure the output of the SCR).

    I actually think the problem might be the optocoupler at this point, but I think I'm going to just replace that entire portion of the circuit (because I'm pretty sure I burned up the SCR with my shaky hands) from the opto to the scr. I see what appears to be good signal going into the opto, but nothing on the output. won't cost much, and that will catch any other wierd things like partially burned resistors or bad diodes that may have either been originally contributing to the problem, or may have gotten burned when I shorted the SCR. Luckily, the traces all seem ok.
  • 0
    ok, so now i'm even more baffled. I have 2 mainboards.. one from my unit, and one from a freind's (muvement on this board).

    On my board:

    I replaced the opto, the SCR, the diode (D4). The pump works if connected directly to 110V. When the fog button is not pressed, there is no signal on the input to the opto. When it is pressed, there is signal (120Hz with pulse width variable by the output pot), but also, when it is pressed, R17 burns up (pretty spectacuarly, with lots of smoke and a puff of flame). I can't seem to find any shorts or bad parts in that section of the circuit.
    Oh, and in steady state (when the button is not pressed, before R100 burns up), I measure ~120VAC across the pump leads (but the pump isn't running).

    On Neil's board:
    Replaced the opto, and the SCR. D4 measures fine. R16, R17, and R18 all seem fine. Using it in the same chassis as above, so the pump is the same. Have signal on the input to the opto when the button is pressed, just like on my board. Can't detect any changes on the output. Steady state voltage across the pump leads is a low AC voltage (8vac or so, whether the button is pressed or not). Haven't been able to find any open circuits or bad components. This seems more like an open than a short to me, where the other board feels like a short, but i can't find either of them...

    ideas?? things to check??
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    Barring a gate to main lead short in the SCR, on the first unit, I would suspect cap C5. If it should fail shorted, it could allow a lot more than normal gate current to flow in the SCR, and perhaps take out R17. I would think it would have effect on R16 as well, but if the SCR never conducted, perhaps not. Myself, that part of the circuit is so simple, and since you have already changed the parts with any $$$ involved, I would be inclined to just change the works - at about $.02 per resistor, and probably not more than $.10 for the caps, it's cheaper to "shoot the works" than to spend the time.

    Also, if you jump either across the opto, it looks like pins 4 and 6, on the second, any life? If not, then how about if you jump across the SCR, does the pump run? If the latter, then the SCR out is good, and you need to look at the SCR towards the opto. If jumping the opto works, then obviously it is either the opto, or the drive going into it.

    On the second unit, what do you see on the SCR gate when the run button is pressed? And by "R16, R17, and R18 *seem* fine", please define "seem" in this context - did you meter them, or are you just noting that they are unburned?

    - Tim
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    figured it out... bad connection to the pump... I must've reconnected the pump badly multiple times, because I disconnected the leads from the phoenix connector multiple times to test the pump, but this last time, when I reconnected it, it started working.

    Haven't tested the first board in the unit yet, but I believe it was a short that resolved when the trace burned (the burned trace has been cut out and bypassed by a jumper now, so no short exists anymore), in addition to the bad connection to the phoenix connectors.

    the strange thing is that I tested the resistance between the 2 traces that shorted, and they didn't seem to be shorted. Yet when I replaced the problem trace with a jumper, the problems went away... (no more burning parts or traces, but this still had the problem caused by the phoenix connector/pump connection).

    In any case, I have a working f100 now... and likely will have 2 soon...
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