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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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CTC211
No print.
It only has about a volt drive to the HVOTs. These are FETs so that is obviously not enough. The drive is supplied by a totem pole arraingement like an audio output stage. Actually I wonder why they bothered to bias it, but the did and there are emitter resistors. This is where it gets wierd. On the diode scale at the gates of the FETs I read about 0.6 to ground. On resistance it reads like 47K ohms. when I lift the base of the bottom (PNP) transistor it is open circuit, however what perplexes me is that the base of the transistor reads about 1.4 to ground in circuit. How can it cause more of a short than it has. The set is off and powered down. I alsmost thought the transistor might be amplifying the current from my DVM, but that doesn't fly. The IC that drives this mess is an MC 1391, and has an open collector output. The set is off, there is no current to turn on it's interal output transistor. I changed it anyway with no change in readings or symptoms. I can understsand that the reading at the base is caused by the 220 ohm pullup resistor loading it to a supply that is off. But how does that cause a 0.6 reading at the emitter ? I have now changed both transistors and the IC, and there doesn't appear to be anything else in the circuit. What am I missing here ? I see no jumper wires, and it is not a two sided board, the circuit is simple, is this a brainfart or something ? The circuit is simply not that hard to figure out without a print, but who knows. I see there is a foil going off to who knows where, but none of this has any continuity to it and it seems to not be used at all. Perhaps I should start getting up in the wee hours and make donuts. Dammit, even if that PNP transistor was conducting the emitter reading would be higher than the base reading. Doesn't that make sense ? But when you disconnect the base, the emitter then reads open. How the hell can something do this ? The collector of the transistor is grounded, but even if it did some amplification,,,,,,,,,,, this kinda had my head spinning. First time I got screwed up readings that made no sense was in those old NECs with a coolant leak, and I did indeed check for coolant on the board, it is dry. The diode and resistor that provide bias for the stage might be over a ground trace, so I thought of the glue, but that still doesn't explain the lower reading at the emitter. If I don't get a lightbulb over my head I guess Monday I'll call my old shop and see if they have the print, if so maybe they'll email me the DWG files. I am fairly sure they have it for the DTC307 which I am pretty sure uses the same configuration. Any ideas ? Thanks in advance. JURB |
#2
Posted to sci.electronics.repair
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CTC211
On Jun 8, 7:11 pm, wrote:
No print. It only has about a volt drive to the HVOTs. These are FETs so that is obviously not enough. The drive is supplied by a totem pole arraingement like an audio output stage. Actually I wonder why they bothered to bias it, but the did and there are emitter resistors. This is where it gets wierd. On the diode scale at the gates of the FETs I read about 0.6 to ground. On resistance it reads like 47K ohms. when I lift the base of the bottom (PNP) transistor it is open circuit, however what perplexes me is that the base of the transistor reads about 1.4 to ground in circuit. How can it cause more of a short than it has. The set is off and powered down. I alsmost thought the transistor might be amplifying the current from my DVM, but that doesn't fly. The IC that drives this mess is an MC 1391, and has an open collector output. The set is off, there is no current to turn on it's interal output transistor. I changed it anyway with no change in readings or symptoms. I can understsand that the reading at the base is caused by the 220 ohm pullup resistor loading it to a supply that is off. But how does that cause a 0.6 reading at the emitter ? I have now changed both transistors and the IC, and there doesn't appear to be anything else in the circuit. What am I missing here ? I see no jumper wires, and it is not a two sided board, the circuit is simple, is this a brainfart or something ? The circuit is simply not that hard to figure out without a print, but who knows. I see there is a foil going off to who knows where, but none of this has any continuity to it and it seems to not be used at all. Perhaps I should start getting up in the wee hours and make donuts. Dammit, even if that PNP transistor was conducting the emitter reading would be higher than the base reading. Doesn't that make sense ? But when you disconnect the base, the emitter then reads open. How the hell can something do this ? The collector of the transistor is grounded, but even if it did some amplification,,,,,,,,,,, this kinda had my head spinning. First time I got screwed up readings that made no sense was in those old NECs with a coolant leak, and I did indeed check for coolant on the board, it is dry. The diode and resistor that provide bias for the stage might be over a ground trace, so I thought of the glue, but that still doesn't explain the lower reading at the emitter. If I don't get a lightbulb over my head I guess Monday I'll call my old shop and see if they have the print, if so maybe they'll email me the DWG files. I am fairly sure they have it for the DTC307 which I am pretty sure uses the same configuration. Any ideas ? Thanks in advance. JURB The parts are cheap enough, so buy some generic ones (not NEC) and replace them all at the same time. Do a power on test (forget your meter for now). I don't know what the symptoms are, but sometimes we can think this stuff to death when the fix is really the most obvious. Then, look for something odd at the imput or output (load?). Bob |