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Thread: SVT classic transformer color code

  1. #36
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    Those screws are definitely Allen head screws. Or as the Air Force trained us,internal hex head screws. My brother had a 1946 Chevy pickup that used clutch head screws for the interior. The heads looked more like an hourglass. Now some of the older Ampegs like the VT 40s,VT 22' V2s and V4s used the clutcheads.Every SVT classic I've worked on has had the Allen screws for the o/p tube res trainers. Maybe the factory uses air tools. They probably do to speed things along.
    Now,back to the problem. J36 has no voltage. J 35 has neg 1.03 volts. IC 2 has little or no voltage. Pins 1/2/5 have no voltage. Pins 4 has neg. .003 volts. Pin 6 has pos .8 volts. Pin 7 pos .02 volts. Pin 8 is pos .003 volts. Connector J 19 has 15 volts AC at driver board.
    What does IC 1 chip do? Does it just switch the led on the front panel? All the tubes have heater voltage. Should I be looking at the pre amp circuit closer? The power and standby switches are working. If the customer did plug this into a 240 volt outlet,like he said, shouldn't there be obvious damage confined to 1 or 2 parts of the circuit or something is fried.
    I'll still be throwing more info at you as I continue to follow the schematic and check things. The opamps should have operating voltage shouldn't they? Both negative and positive? I've seen loss of voltage at opamps before because of a shorted zener. Thanks.

  2. #37
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    It seems I've lost the low voltage in the circuit. The 15 volts. I'm pretty sure of that. I can't find positive or negative 15 volts anywhere in the circuit. Now, I think the low voltage enters the circuit at connector J 19 , without looking at the schematic. I have 15 volts AC at the connector. So i assume the filament transformer is ok. After J 19 there is a series of 4 or maybe 8 rectifier diodes. Can't remember without looking,but I checked those,I believe,out of circuit and they all check good. I'm going to check those again. This is where it gets fuzzy for me. If those diodes are good,then it's a shorted opamp maybe. It looks like to me something is pulling down the 15 volt rail. Now I have to find it. I wound up with this amp because no one else around here would touch it with a ten foot pole. The kid is desperate. His band is going on tour next month. Like most bands starting out,he doesn't have much money. I'm not going to let him down,if I can help it. Anyway, any suggestions on anything that may be pulling down the 15 volt rail? Thanks.

  3. #38
    Senior Member Enzo's Avatar
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    Well don't guess, look at the schematic. There are four diodes in a simple bridge, at the DC corners you should find around 20v of each polarity. Do you? R60,61 are 30 ohm 5 watt resistors. One end of each should see the 20v supplies, the opposite ends face 15v zeners, exactly where is the voltage disappearing?

    You are saying something is pulling down the 15v rails, but that is very different from missing 15v rails. J20 is right near those resistors, if you disconnect the cable there, it takes most if not all the load off the 15v rails anyway. So that is one way to decide which type problem we have.
    Education is what you're left with after you have forgotten what you have learned.

  4. #39
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    I can tell you Enzo, the 20 volts is missing as I remember. Let me verify that though. I'll unplug J 20 and see what that does. I had to start writing down what I'm doing. As far as testing components with an ohmmeter,I was told to remove them from circuit. Is this true with rectifier diodes?

  5. #40
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    Well Enzo, looks like we got it. I checked the two zeners out of circuit and sure enough,they test shorted. The resistors are OK. This amp has 47 ohm 5 watts but I don't think that's going to make a difference. I have the zeners in stock so I'll pop those in the circuit. I'll get back to you and let you know if that fixes it. Thanks everyone for the input.

  6. #41
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    So far it works like it's supposed to. Passes signal and no nasty surprises.Thanks everyone for the help.
    g1 and The Dude like this.

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