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Sets, Parts and Service Information Wanted (private buyers only, no swaps) If you need help obtaining components, sets or equipment, post a message here. Private buyers only - no traders. No swaps.

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Old 13th Jun 2019, 11:01 am   #21
'LIVEWIRE?'
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Default Re: Information about a plastic encapsulated (2S)c107

I don't, with all due respect, think so, Peter. The symbol, as drawn, is a bipolar NPN Transistor with built in resistors, one in series with the base connection, the second shunted across the B-E- junction. Since the emitter is earthed, the two resistors form a conventional base-biassing circuit, which turns on the transistor when +5v is present, thus operating the relay. There are many transistors of that general type, as you no doubt know.
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Old 21st Jun 2019, 11:28 am   #22
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Default Re: Information about a plastic encapsulated (2S)c107

According to the Data Sheet, the KRC107M to 109M series are 'Epitaxial Planar NPN Transistors', which is how the device is shown in the circuit diagram of the TEAC CR-H257.
Since my last post I have checked further and found that the base of the transistor is connected to pin 61 of the uP IC (IC41-ST92F124) and is marked 'Power-Hi', but there is zero volts anywhere along this connection. Bridging the relay contacts allows the main PSU to function and all DC voltages are present. I replaced the smoothing capacitors C223 & C224 on the 9v. lines as C223 was bulging, and when checked on a Chinese tester it read as if it were a diode!, so in went 2 x 1200uf, 16v caps, but There is still no display and no functions work, so I'm beginning to suspect that the processor (IC41) has failed, in which case, the unit is probably scrap as it's over 10 years old.
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Old 21st Jun 2019, 2:34 pm   #23
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Default Re: Information about a plastic encapsulated (2S)c107

I'm always reluctant to condemn uP's. The 'big chip' is usually bottom of the list of suspects. It would pay to go around all pins of the uP with a scope and verify you see what would be expected. Supplies, oscillator, reset etc.
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Old 21st Jun 2019, 6:43 pm   #24
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Default Re: Information about a plastic encapsulated (2S)c107

I've been around the pins with my DMM, and have found various voltages more or less ass and where I'd expect them, except that there is no sign of oscillation at the 4Mhz Crystal, and the DC voltages there are lower than I'd expect, being less than 1 volt on both pins. Now the Xtal could I suppose be faulty, as indeed could either or both of the SMD capacitors associated with it. I guess the next thing to do will be to remove the crystal.
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Old 21st Jun 2019, 7:27 pm   #25
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Default Re: Information about a plastic encapsulated (2S)c107

Are you using a divider probe? If not then I suppose its just possible the capacitance of a normal 1:1 probe could stop the osc.
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Old 21st Jun 2019, 9:45 pm   #26
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Default Re: Information about a plastic encapsulated (2S)c107

I was using my Frequency counter with a normal Coaxial Cable terminated in crocodile clips. IIRC The factory made cable, which is fitted with a BNC plug is 150 ohm impedance it came with my AM/FM signal generator, but I've often used it on the Freq. Counter. The workshop 'scope is out of order, so I'll have to take my large Philips one from home.
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