Question about Tape Decks
My tape deck developed intermittent speed control problems and ran significantly slower. The schematics I have from Pioneer on the deck have some errors in the capstan motor power supply areas.
It looks like the capstan motor runs from a pulse generated AC with a variable frequewncy for speed control. While checking this out I accidentally shorted the emitter of Q245 and lost all power to the capstan motor. Q245 pulse driver? shows zero vdc at base and emitter. The Darlington driver Q214 has 0 vdc as well since it's connected to Q245 base. I pulled both transistors. They seem to show normal junction behavior. The voltages at the open circuit points seem unaffected. My scope shows no input pulses going to Q245 of the capstan motor assembly board. I don't have exact matches for the transistors so would probably substitute others for them.
Meanwhile, I reinstalled the original transistors and got no change in results. All the electrolytic caps and current limiting resistors in that portion of the circuit seem OK (no shorts or opens).
Do you think the DC bias to Q245 base and emitter come from Q214 or somewhere else?
Could be a nasty fix this one. you will have 2 faults. Firstly the original fault, and secondly the man made one. I hate it when that happens. You dont mention what collector voltages you have. Check DC first is the oldest addage for electronics repairs. It is possible that you may have lost power supply to this stage when the short occurred by failure of a resistor somewhere in the supply line. It can be hard to check the junctions of a darlington transistor without replacing it. The transistor you mention is a series regulator that wont be providing pulses, but regulating to 24 volts. Check the Zener diode in the base of it(should have 25 volts here) and 37 volts on the collector. You wont have voltage at the motor but rather a AC drive generated and controlled by the the IC and FG coil and the 4 transistors q210-213 . You may need to get back to me to help fault find the circuit. I got a schematic at http://www.eserviceinfo.com/download.php?fileid=6302 Get back to me here if you can/cant get the 24v reg back up and we'll attack the speed problem:)
Posted on Jan 26, 2008
Check for pico fuses on the 12v motor line or protection fuses that look like transistors but infact are fuses. they may have board location numbers like PSxxx, or PRxxx.
Posted on Jan 25, 2008
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