Old 96er
New Member
I just want to share some of my knowledge and experience with my Kenwood KD-750 turntable as a way of paying it forward to those on here who have helped me along the way. I’ve had this turntable for over 20 years and It just recently developed the dreaded speed control issue.
At the end of last summer (2019) my KD-750 started having intermittent speed control problems where the quartz lock lamp would blink off and on every so often as it tried to maintain speed. The problem began happening more frequently to the point where I could only listen to my records for about 10 minutes before it would reliably fail to maintain quartz lock. I tried cleaning the speed control trim potentiometers on the circuit board with DeOxit, which initially seemed to help but neither would hold an adjustment.
logically, I replaced all the 40-year-old electrolytic capacitors with Nichicon KL. The problem corrected and the unit maintained quartz lock reliably for several months until February, when it started acting up again. This time, the failure mode changed. The quartz lock indicator would flicker and the platter would coast to a complete stop for a period of time and then start back up and try to maintain speed again.
At this point I was perplexed and overwhelmed by how I would have to troubleshoot this thing. There’s virtually no information out there about this somewhat rare turntable, besides the service manual, to go on. No convenience of a YouTube video of someone else repairing one. Now, I’m no stranger to a soldering iron or a multimeter, but the complex circuitry of this speed control board is no joke. Logic gate ICs everywhere! 17 of ‘em. Some of them out of production for years now.
Feeling like troubleshooting this thing any further was beyond my experience level, I took it to a local stereo repair shop. After waiting for about a month, the technician called me and said he was unable to find the problem and to come pick it up.
Determined not to give up on this beautiful unit, I began ordering ICs from eBay and China that could potentially cause the problem. The service manual has a pretty good troubleshooting flowchart. IC3 and 5 seemed the most likely culprits. Nope. I ended up unnecessarily replacing many other ICs with no success. Then, some better judgment prevailed and I finally got underneath and started checking voltages and comparing them to the service manual. Bingo, I found the problem. IC13 (TA7179P) is basically a bipolar power supply / voltage regulator IC that supplies +15V and -15V to IC10, a quad op amp. I was seeing the voltage on both sides of IC13 drop out to ~1V, which coincided with the platter coming to a stop. I hit IC13 with some freeze spray and the problem corrected briefly. I put my finger on the chip and that puppy was HOT!
Of course, the TA7179P chip is obsolete with no cross reference, so I found a new-old-stock chip for $5 on eBay and ordered it. It arrived this past Saturday and I soldered that puppy in almost as soon as I closed the mailbox. I was shaking with anxiety as I put the unit back together for a trial run. I plugged it in, heart racing as I hit the power button, and I pressed the start button. I was never before so grateful to see one little red light glow continuously and reliably, never to flicker or blink again! That one chip, IC13 was the culprit. I listened to my records on this beautiful machine, with zero malfunctions, for the rest of Saturday afternoon and all day Sunday. Steady as a rock.
I have another “parts” KD-750 with a different symptom, and I found that it also has a bad IC13 (TA7179P), for which I’m awaiting a new chip from China. The IC13 of this unit failed in a different way than my other unit. Instead of both rails intermittently going to +1V, the -15V rail is reading +1.2V and the +15V rail is giving +15V like it should. The symptom that this “parts” turntable has is that the platter spins at very high speed. Everything else checks out except IC13. I’ll update my findings when I get that IC replaced.
At this turntable’s ripe old age of 40+ years, maybe It’s likely IC13 may be a common failure point in these models since I have two that happened to fail. I’m sorry for the length of this wall of text, but I hope my findings can help someone else out. I spent many, many hours poring over the lengthy service manual and theory to get to this point. Thank you for reading and I hope this helps someone out.
At the end of last summer (2019) my KD-750 started having intermittent speed control problems where the quartz lock lamp would blink off and on every so often as it tried to maintain speed. The problem began happening more frequently to the point where I could only listen to my records for about 10 minutes before it would reliably fail to maintain quartz lock. I tried cleaning the speed control trim potentiometers on the circuit board with DeOxit, which initially seemed to help but neither would hold an adjustment.
logically, I replaced all the 40-year-old electrolytic capacitors with Nichicon KL. The problem corrected and the unit maintained quartz lock reliably for several months until February, when it started acting up again. This time, the failure mode changed. The quartz lock indicator would flicker and the platter would coast to a complete stop for a period of time and then start back up and try to maintain speed again.
At this point I was perplexed and overwhelmed by how I would have to troubleshoot this thing. There’s virtually no information out there about this somewhat rare turntable, besides the service manual, to go on. No convenience of a YouTube video of someone else repairing one. Now, I’m no stranger to a soldering iron or a multimeter, but the complex circuitry of this speed control board is no joke. Logic gate ICs everywhere! 17 of ‘em. Some of them out of production for years now.
Feeling like troubleshooting this thing any further was beyond my experience level, I took it to a local stereo repair shop. After waiting for about a month, the technician called me and said he was unable to find the problem and to come pick it up.
Determined not to give up on this beautiful unit, I began ordering ICs from eBay and China that could potentially cause the problem. The service manual has a pretty good troubleshooting flowchart. IC3 and 5 seemed the most likely culprits. Nope. I ended up unnecessarily replacing many other ICs with no success. Then, some better judgment prevailed and I finally got underneath and started checking voltages and comparing them to the service manual. Bingo, I found the problem. IC13 (TA7179P) is basically a bipolar power supply / voltage regulator IC that supplies +15V and -15V to IC10, a quad op amp. I was seeing the voltage on both sides of IC13 drop out to ~1V, which coincided with the platter coming to a stop. I hit IC13 with some freeze spray and the problem corrected briefly. I put my finger on the chip and that puppy was HOT!
Of course, the TA7179P chip is obsolete with no cross reference, so I found a new-old-stock chip for $5 on eBay and ordered it. It arrived this past Saturday and I soldered that puppy in almost as soon as I closed the mailbox. I was shaking with anxiety as I put the unit back together for a trial run. I plugged it in, heart racing as I hit the power button, and I pressed the start button. I was never before so grateful to see one little red light glow continuously and reliably, never to flicker or blink again! That one chip, IC13 was the culprit. I listened to my records on this beautiful machine, with zero malfunctions, for the rest of Saturday afternoon and all day Sunday. Steady as a rock.
I have another “parts” KD-750 with a different symptom, and I found that it also has a bad IC13 (TA7179P), for which I’m awaiting a new chip from China. The IC13 of this unit failed in a different way than my other unit. Instead of both rails intermittently going to +1V, the -15V rail is reading +1.2V and the +15V rail is giving +15V like it should. The symptom that this “parts” turntable has is that the platter spins at very high speed. Everything else checks out except IC13. I’ll update my findings when I get that IC replaced.
At this turntable’s ripe old age of 40+ years, maybe It’s likely IC13 may be a common failure point in these models since I have two that happened to fail. I’m sorry for the length of this wall of text, but I hope my findings can help someone else out. I spent many, many hours poring over the lengthy service manual and theory to get to this point. Thank you for reading and I hope this helps someone out.
. Good job! Congratulations! I also use Kenwood ( KP-770D). And I like this company's devices .
