CD player pwr reg section with shorted 1000uF electro cap - Page 1
Forum section: electronics / repair

author: whatever
author_rank: Contributor
timestamp: January 04, 2024, 02:19:02 am
content: A recently acquired Kenwood DP-750 CD player (1986) has several weird issues. No laser power, one regulator (Q908) runs hot as does the DAC chip.It may be the -5V line, which I measure at about +1.4V.Going thru the orig caps, all were okay with no leaks and only mildly higher ESR than brand new. Those OEM Japanese caps!Except one:(C906) a 1000/16v electro, Almost a near short across it (about 1 ohm).NOTE: I tested and replaced caps BEFORE researching the troublesome issues.Anyway, after replacing that, I still have same issues -- even more as I have lost some button functionality (drawer open/close, etc).I am thinking that when that cap shorted, it took out nearby active components (Q909, Q910).Which I have not tested; the PCB is bit of a pain to take in and out!!Any guesses or speculation as to what that shorted cap may have done.Have a look at the schematic images below. Full schematic PDF also attached.

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 04, 2024, 04:34:46 am
content: C906 being shorted would have stressed D903, R907 (Also a fuse), and maybe the transformer T901.Check the voltage across the new C906 when the unit is powered up. What do you measure?

author: whatever
author_rank: Contributor
timestamp: January 04, 2024, 07:29:05 am
content: Quote from: Kim Christensen on January 04, 2024, 04:34:46 amR907 is blown -- nice catch!Yeah, I have checked the some of the other other components in that area, incl. D903, and they seem to be okay. So I guess R907 did what it was supposed to do. But still way to too early to tell what else may be wrong.Back to work ...

author: whatever
author_rank: Contributor
timestamp: January 04, 2024, 11:52:33 am
content: Something a bit odd.--despite the -5v line now repaired --is that while the DAC chip runs much cooler, and it and other chips that need -5V now have -5v... nevertheless, after about 5 mins of pwr up, all the voltage regs become quite hot running (even with heatsinks, can't keep finger on them). Even just idling with no CD playing. Also, the wattage/amp use also go up by about 2 watts, Not sure what could be soaking up so much currentReg. section with three 3-pin power trans, shown below. Note that I added a small heartsink to the one in back. It get very warm to hot. Power use about 7 watts -- idling. The back panel specs note 13watts at 117vAC, but I've never seen that much pwr uses .

author: Paceguy
author_rank: Regular Contributor
timestamp: January 04, 2024, 12:45:00 pm
content: When manufacturers locate electro caps right next and in such close proximity to heat sink(s), it's usually just a matter of time that the caps fail.

author: whatever
author_rank: Contributor
timestamp: January 04, 2024, 06:48:37 pm
content: Quote from: Paceguy on January 04, 2024, 12:45:00 pmActually, the OEM caps near those hot devices were in good ESR shape. Notice how bent they are away from the heatsink. I replaced a few in that area with higher-quality Panasonic FM, but and those silver solid Japanese electros. Way back, I'm going to assume the OEM designers knew about the heat issue and chose caps that would be robust. The single cap that failed was well away from any pwr hearsunk device.

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 04, 2024, 08:20:09 pm
content: Quote from: whatever on January 04, 2024, 11:52:33 amWhich transistor is that? Q904 or Q908?Does the CD player now appear to work even though the regulators are getting hot?

author: whatever
author_rank: Contributor
timestamp: January 04, 2024, 09:00:28 pm
content: Quote from: Kim Christensen on January 04, 2024, 08:20:09 pmThe device works fine up to a point; it seems to have a bad laser, too. That may have been damaged by whatever led to the shorted cap issue. Not sure.Q904 , Q908 and Q911 all get hot. All three same device: An NPN power trans, TO-220http://radiocom.dn.ua/image/data/pdf/2SD1413_SavantIC.pdfI think they are all plastic.Philips used to go full outboard some of their early CDP, with a large finny alumin. block that stuck outside the back of the unit. Mated to this were the 3-pin pwr regs. The Kenwood draws 80mA at idle, about 7W.Some of the old digital IC and microprocessors could draw as much as 200mAperIC! The Philips SAA7220 digital filter was one. 180mA. Usually not heatsinked as it was large DIP package. But quite finger during normal operation warm. I guess 4x oversampling math was quite a task those days.

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 04, 2024, 09:16:22 pm
content: Well, if we look at Q904 as an example:It powers the microprocessor (Q304) and also the LED display (D203) among other things. Lets just say, as a guess, that 200mA is the draw off that 5V rail. The test points around Q904 show 13V on the collector and 5V on the emitter which means it has 8V across it. 8V @ 200mA = 1.6Watts which would get pretty toasty with a small heatsink. So maybe "hot" to the touch is kind of normal.

author: whatever
author_rank: Contributor
timestamp: January 04, 2024, 09:54:00 pm
content: Quote from: Kim Christensen on January 04, 2024, 09:16:22 pmNot only that, but that -- "hot--is my subjective opinion. I did measure the ICs with a IR laser scanner. Not the most trustworthy of devices. I did get some reading as high as 120F. But mostly around 100 for the pwr regs. The DAC IC, before the cap issue was fixed, was 110F at highest (I did heat sink it then). After fix, it has cooled to 95F max.My main pwr and amps are measured using Killa-Watt device.

author: whatever
author_rank: Contributor
timestamp: January 05, 2024, 06:15:11 pm
content: This CD player is now fully working. Had some issues with flat ribbon cable which de-laminated, at the terminations; after about 38 years, maybe the glue or heat-tack gave up.Anyway, what I learned from making new connectors is that even if you double check your solder connection for continuity and shorts, things don't go as planned if you are at the mercy of 22 time consuming lines (and then mult 22 by 4 to give actual solder wires that have to constructed) . Not having continuity on motor and coil terminals can cause motor driver to run hot. Just like that DAC chip.All is working fine now, but and the unit can draw as much 10W (90mA). The three pwr regs with heat sinks continue to run hot (120F). But audio is clean and powerful.Another curiosity, which I may bring up in another thread, is the X_o osc. waveform, which looks fuzzy compared to a similar ( but not identical ) Kenwood CD player model dp-160, from same vintage, that uses a very, very similar circuit design, down to the ICs and regulators ... and a 67.7376 MHz xtal that is driven by the DAC internal Pierce (??) osc. (X_o, X_in). See schematic.

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 05, 2024, 09:16:34 pm
content: Quote from: whatever on January 05, 2024, 06:15:11 pmWell, you can't reliably measure the xtal oscillator with a scope probe on pins 10 or 12 of the DAC (Q501) because the scope probe will change the capacitive loading. Note that C503 & C507 are only 5pF, so therefore this part of the circuit is extremely sensitive to any additional loading.

author: whatever
author_rank: Contributor
timestamp: January 06, 2024, 03:35:00 am
content: Quote from: Kim Christensen on January 05, 2024, 09:16:34 pmI have very similar Kenwood dp-460 that uses almost the same circuit design--except that xtal (dp-460 seems to be a 16.9344 mhz) . The dp-460 look much cleaner and DIFFERENT than the dp-750;but both units sound and perform just fine. Strangely, I see a sawtooth waveform on both BCK and X_o pins on the dp-750. On the dp-460, X_o looks like a sharp sine wage and BCK looks like a square wave. I am used to seeing square waves in CD players.Unfortunately, I don't have a schematic for the dp-460.