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  1. Replies
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    This looks to be exactly what I am dealing with,...

    This looks to be exactly what I am dealing with, even the layout is pretty close. The mosfet isn't named, but it is a 12v 1 watt zener, so steady as she goes. I will install the 12v when I get it,...
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    Juan, you know more about this than I ever will,...

    Juan, you know more about this than I ever will, so I submit the following evidence.

    54829
    in diode mode in one direction

    54830
    in diode mode in the other direction

    54831
    voltage measured...
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    I scored a good used charger for $15 shipped. It...

    I scored a good used charger for $15 shipped. It doesn't get nearly as warm, even while charging. Buh-Bye potential fire hazard!
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    Both. 0.4v in one direction, 0.5v in the other...

    Both. 0.4v in one direction, 0.5v in the other out of circuit on diode mode. In series with a 1K resistor on a power supply, it measures 6.1v across itself.

    I am now confident saying the zener is...
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    I am kind of concerned that both those drawings...

    I am kind of concerned that both those drawings show 6v 1 watt zeners, and the labeling on the one I have I THINK makes it a 12v 1 watt. And the one I have drops ~6v. Should I be worried?
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    It sounds like this zener is the problem, so I...

    It sounds like this zener is the problem, so I will get it ordered. Thanks guys! I will report back when it arrives. I had not heard of the Hall VVR. Pretty disappointed in Morgan for dropping the...
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    OK, I can accept that. Can someone explain why a...

    OK, I can accept that. Can someone explain why a defective zener would disable this circuit? I realize not having a schematic is problematic, but I am not adept at drawing one.
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    Why won't this power scaler circuit work?

    I have a Morgan AC40 in, stuck in 1/2 watt mode. Morgan is no help, after a couple of emails promising to send me parts, he has stopped responding. There is a little, nearly impossible to get at...
  9. If you don't use the stereo option, you could...

    If you don't use the stereo option, you could ditch that board and switch and just wire two standard Switchcraft #11 mono 1/4" jacks in parallel to each other and the speakers. Done. If you must have...
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    I have a pair of these. This has been on ongoing...

    I have a pair of these. This has been on ongoing fear.
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    It is always good to hear from you kiwi.

    It is always good to hear from you kiwi.
  12. Thanks fellas!

    Thanks fellas!
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    I had a JTM30 in last year with the same...

    I had a JTM30 in last year with the same complaint, low output. It was the OT.
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    "Is the standby hot switching the reservoir cap...

    "Is the standby hot switching the reservoir cap on to the rectifier?"

    I didn't draw anything, and the amp is being picked up soon, but I can tell you the B+ goes from rectifier tube to standby...
  15. This is almost exactly my experience with a bad...

    This is almost exactly my experience with a bad HT fuse holder that followed a standby switch on a Dr Z just today. All kinds of chaos remedied by a new and better fuse holder on the B+ feed.
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    "I rather assume that for some reason...

    "I rather assume that for some reason (temperature, vibration, mechanical shock, corrosion) the effective airgap of the core has increased. This would explain increased input current and transformer...
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    Interesting problem and solution

    A Dr Z MAZ Jr came in with no output, not a peep. One EL34 wasn't lighting up, and the HT fuse was blown. I replaced the dead tube with a good used, and put in a new 1/2A fuse. Now it was making...
  18. OK, so shorting past the power scaler board I...

    OK, so shorting past the power scaler board I have a functioning amp, and 9.5 volts across the 62 ohm cathode resistor. This is the math I am seeing:

    9.5v = 0.153A * 62 ohms

    1.45W = 9.5v *...
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    "Is the pot either VR1 or VR2 in the schematic...

    "Is the pot either VR1 or VR2 in the schematic below? (Both in the feedback loop of an opamp.)"

    Yes, it is VR1.

    "Not quite. A linear 50k pot with the wiper tied to the end gives 25k in middle...
  20. I think that is how it works, yes, by lowering...

    I think that is how it works, yes, by lowering B+. No schematic of course, but it does appear that it adjusts the plate voltages. I'm not solid on it, but with the mosfet failed it is sitting at the...
  21. Enzo, that makes sense, but I wonder if the fact...

    Enzo, that makes sense, but I wonder if the fact that the MOSFET driven power scale circuit isn't working at the moment will give me accurate measurements. It is stuck to the lowest setting, approx...
  22. How do I figure this cathode resistor wattage?

    I have a Morgan AC40 Deluxe in that blew up. I think a tube shorted and cooked the cathode resistor, which melted the wrap off the F&T 250uF bypass cap. The resistor measures 62 ohms, which believe...
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    volume pot subbing question

    I have a Hartke HA3500 head in with a broken volume pot VR1. It is a long shaft 50K linear pot with the wiper tied to one leg. Full Compass has this pot in a 100K linear, can I get away with...
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    So what is the bottom line here? I don't remember...

    So what is the bottom line here? I don't remember it always getting hot like this. Is the transformer malfunctioning? I have included a couple of shots of it, but it doesn't look to me like there is...
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    Wait, what? LOUD is in Europe, when did that...

    Wait, what? LOUD is in Europe, when did that happen? And I thought Ampeg was sold to Yamaha? I'm always the last to know.
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    "I think we may be seeing a limitation of the...

    "I think we may be seeing a limitation of the neon bulb test for transformers, as you said it passed. "

    I was thinking the same thing.
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    I have 125v here, puts it at 11.6W. Laminate...

    I have 125v here, puts it at 11.6W. Laminate dims are 2.5" x 2.25" x 1.15".

    I just ordered a Kill A Watt meter.
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    Current measured by placing ammeter in AC line is...

    Current measured by placing ammeter in AC line is 0.093A loaded and unloaded, that is with secondary fuse in/out, but not charging a battery. 11.6 watts.
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