I recently purchased a DENON DP-30L at a hamfest for $20, and it actually plays. It seems like quite a nice turntable. The main problem with it is that when the tonearm is done playing the record and is at the center, the tonearm does not pick up automatically like it should and the turntable does not stop.
So my understanding is that there is an optical sensor, a cadmium sulfide photocell, which is illuminated by a red LED when the record is finished. The photocell is covered by a shutter that is rotated by the tonearm, so that when the tonearm reaches the center of the record, the photocell should be uncovered by the shutter and the resistance of the photocell reduced by the LED illumination. I can measure the voltage at the testpoints 15/16 and indeed the voltage rises from only about 0.4 V when the photocell is blocked to about 3.2 V when the photocell is unblocked. I haven't tried to fine adjust the position of the shutter yet.
There's a comparator #4 of IC5 which is part number HA17901P, but as far as I can tell it is basically the same as the LM339 quad comparator. When the motor is started, the output of the comparator (pin 2) is pulled high by a 560 ohm resistor (R26). When the stop button is pressed, this grounds pin 2 which is used to create a pulse on comparator #3 which lifts the arm. This works, as when I play a record, and press the stop button, the arm lifts and the record stops as it should. However, when I start the turntable motor, and hold the motor stopped (like it says in the service manual), and then observe pin 5 of comparator #4 IC5 which is the reference voltage, and pin 4 which is the voltage from the photocell, pin 2 is pulled high by R26, but is never pulled low by the open-collector output of comparator #2. The voltage on pin 4 is higher than pin 5 when the tonearm is at the center of the record, and so the output should be grounded. So while the voltage at pin 4 - is higher than pin 5 +, the output of the comparator does not get pulled low, and so the end of the record is never registered and the stylus continues to turn in the end groove. So it seems like the comparator is broken, or is at least unable to overcome the pullup of some other part of the circuit.
How likely is it for a comparator chip to fail like this? It seems to me that I'm more likely missing something, and I'd rather not desolder a chip on an old PCB without having a good reason, because I might damage it. It is 42 years old, but nevertheless I don't really expect such simple ICs such as comparators to fail. Perhaps someone has a similar Denon turntable with the optical end sensor and can advise. Thank you!
So my understanding is that there is an optical sensor, a cadmium sulfide photocell, which is illuminated by a red LED when the record is finished. The photocell is covered by a shutter that is rotated by the tonearm, so that when the tonearm reaches the center of the record, the photocell should be uncovered by the shutter and the resistance of the photocell reduced by the LED illumination. I can measure the voltage at the testpoints 15/16 and indeed the voltage rises from only about 0.4 V when the photocell is blocked to about 3.2 V when the photocell is unblocked. I haven't tried to fine adjust the position of the shutter yet.
There's a comparator #4 of IC5 which is part number HA17901P, but as far as I can tell it is basically the same as the LM339 quad comparator. When the motor is started, the output of the comparator (pin 2) is pulled high by a 560 ohm resistor (R26). When the stop button is pressed, this grounds pin 2 which is used to create a pulse on comparator #3 which lifts the arm. This works, as when I play a record, and press the stop button, the arm lifts and the record stops as it should. However, when I start the turntable motor, and hold the motor stopped (like it says in the service manual), and then observe pin 5 of comparator #4 IC5 which is the reference voltage, and pin 4 which is the voltage from the photocell, pin 2 is pulled high by R26, but is never pulled low by the open-collector output of comparator #2. The voltage on pin 4 is higher than pin 5 when the tonearm is at the center of the record, and so the output should be grounded. So while the voltage at pin 4 - is higher than pin 5 +, the output of the comparator does not get pulled low, and so the end of the record is never registered and the stylus continues to turn in the end groove. So it seems like the comparator is broken, or is at least unable to overcome the pullup of some other part of the circuit.
How likely is it for a comparator chip to fail like this? It seems to me that I'm more likely missing something, and I'd rather not desolder a chip on an old PCB without having a good reason, because I might damage it. It is 42 years old, but nevertheless I don't really expect such simple ICs such as comparators to fail. Perhaps someone has a similar Denon turntable with the optical end sensor and can advise. Thank you!
Just picked up the same turntable with the same issue. Did you manage to figure out how to fix it?
I'm going to start looking into mine shortly.
I'm going to start looking into mine shortly.
Well, I think I can now answer the OPs question. It is more than likely for a comparator chip to fail like that. My table had the exact same issue, measured voltage at photocell with arm at spindle and at the IC and all checked out that it should work.
I removed the old IC and soldered in a socket for a new chip. Chips just arrived today and in about 5 mins. I have a table with working auto lift. Pretty pleased that it was not an expensive fix for a great table. Hope this is helpful to anyone else with the same issue
I removed the old IC and soldered in a socket for a new chip. Chips just arrived today and in about 5 mins. I have a table with working auto lift. Pretty pleased that it was not an expensive fix for a great table. Hope this is helpful to anyone else with the same issue
I'm going to ask a dumb question. I have a DP-30L and works fine, but nothing lasts forever. I think I picked it up in 78 and have recently started using it again. If it fails, what process did you use to open it up. I'm thinking latch the tonearm, remove the platter and cover, rig up something that if I flip it over, the top of the base will be supported by this rig and keep the tonearm from being squished. I have the service manual, but it overlooks this initial step as I guess maybe so obvious, but wanted to check with someone who actually opened it up. Thanks.
Definitely lock the tonearm after removing the cover, matt and platter. I think there are three screws securing bottom cover as well as screws with the four feet. I flipped it upside down over an open box with the tonearm outside of the box. Sturdy enough to remove boards to unsolder parts for test/replace. The motor won't run well upside down or even great on it's side, so test quick (multimeter) in that position. I can add pics when I put mine back together tomorrow if you want. I had just started having a drink before the ICs showed up so I just plugged a new IC in and tested it and then very much enjoyed my drink.
If you get in there, you might as well clean and lubricate the motor too.
If you get in there, you might as well clean and lubricate the motor too.
