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2213 PS Voltages All Too High
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In getting started troubleshooting a 2213 with low gain on one channel, I figured I should probably check the supplies just to make sure they were okay -- they aren't.
They are all significantly high and won't adjust (-8.6 is -9.5, 100 is about 110, etc.) The preregulator is good at about 42.7. I checked the primary on the transformer, figuring there is nothing on the secondary side that could cause such a problem, and it is being driven at about 175VAC whereas in comparison to one that is not running at overvoltage I got around 154VAC. This makes sense -- if the primary voltage is too high, the secondary voltages will all be too high too and that is exactly the situation. The transformer windings ohm out okay (low resistance, no shorts and exactly like one that I compared it too) So, in troubleshooting the inverter with power off I can find nothing wrong with it either. With power on, measuring the voltages on the smaller transformer connected to the switching transistors Q40 and Q42 I see no difference compared to one that is working normally and I see little difference in comparatively measuring the voltages on the collector, emitter or base of those transistors either. I even followed the troubleshooting chart in the manual as well and it says to check the error circuit in the inverter (which I already did because I was sure I would find an obvious problem there) but no dice. But, I don't have an isolation transformer and Variac so I can't do it quite the way it recommends. I'm stumped (for now). Phil... P.S. I could still use some advice on whether to build an HV multiplier or I should just bite the bullet and scrap one defective scope (maybe this one!) and cannibalize it for that and other parts. |
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This is opposite the usual problem of having something overloaded by a bad cap or rectifier, for example. If all of the secondaries are in the correct proportions, then look to the chopper stage. As I recall, there is a regulator loop that measures the collector voltages via diodes, feeding back to an analog loop that sets the on-ness of the chopper transistors, which in turn sets the transformer primary voltage - the secondaries are indeed regulated by virtue of the primary voltage.
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On some 2200 models there is instead a series pass MOSFET that provides the equivalent function. Either way, the primary voltage is what's regulated by the control loop. A bad (open) collector voltage sampling diode, for example, could make everything too high by not contributing its share of feedback signal, while the working one still allows it to get close. Or, there could be a control loop failure that forces the output to the maximum value. Study the circuit to see what it should do - some simple DVM measurements should point the way without needing to scope it or worry about isolation. As long as it runs - sort of - DC measurements should be enough to diagnose the control loop function. If this fails, and it is necessary to isolate the stage, there is a common trick that many have used, powering the chopper stage directly from a DC supply, without using the line-powered preregulator of the scope. There are lots of references to this in the archives. Ed --- In TekScopes@..., "Philip" <ndpmcintosh@...> wrote:
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Did you checked the supplys for ripple voltge with a scope ? If ripple if excessive the DC can look higher for your multi meter. Is adds the average AC from the ripple to the DC.? Fred PA4TIM Op 19 mrt. 2013 om 03:46 heeft "Philip" <ndpmcintosh@...> het volgende geschreven:
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