Op-amp circuit troubleshooting help needed - Page 1
Forum section: electronics / repair

author: 4thDoctorWhoFan
author_rank: Regular Contributor
timestamp: March 17, 2017, 08:34:51 pm
content: The attached circuit is from an older Sony CD player and I am having some issues trying to figure out why I'm getting the wrong voltage on the input of a op-amp.I circled the amp in question which is IC203. The schematic shows what voltages I should be getting but I cannot figure out why I'm not getting the voltages listed on the schematic.Pin 6 should be 3.2V and I am getting the correct voltage.Pin 5 should be 2.6V but I am measuring 10.8V.The output pin 7 should be -10.5V but I am getting positive 11.1V.I have tested most of the components around IC203 and actually replaced the IC. I just don't understand what is going on here.Of course, if I lift resistor R218, the voltage drops to almost nothing.Any input would be appreciated.Thanks,Tony

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: March 17, 2017, 09:23:49 pm
content: that portion of IC203 is actually looking good and the voltages you're measuring are reasonable. check the circuitry that is upstream of IC203, i.e., the stuff that's generating CLVS.as soon as CLVS (= pin 5) goes below 3.2V (@ pin 6), pin 7 will flip to -10.5V, because IC203 is wired as comparator.

author: 4thDoctorWhoFan
author_rank: Regular Contributor
timestamp: March 17, 2017, 09:43:37 pm
content: Quote from: Le_Bassiste on March 17, 2017, 09:23:49 pmThanks for the quick reply. I looked further upstream and this is getting more complicated because the CLVS line goes into another op-amp. I attached another portion of the schematic. The CLVS line goes into IC412.I'm getting the correct voltages on the two outputs, pins 1 & 7. (11V & -10.5V)However, the voltages on the inputs are way off. Pins 2, 3 & 6 are in the 10V range.I am getting 1V on pin 5.Would you agree that the problem is probably even further upstream?

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: March 17, 2017, 10:34:10 pm
content: good. note that the voltage on IC412:2 actually should be the same as on IC203:5, because there is just a straight wire connection between those two.in case there is a cable connector between them:

author: Armadillo
author_rank: Super Contributor
timestamp: March 17, 2017, 10:36:33 pm
content: What is the Sony CD player "STATE" for the voltages shown in RED stated in the Sony CD manual. Because the voltage is dependent upon CLVS signal [servo phase synchronization signal] which suggested that the machine should be in a state of "playing". You will need to measure the voltages using the same state as in the manual.

author: 4thDoctorWhoFan
author_rank: Regular Contributor
timestamp: March 17, 2017, 11:01:08 pm
content: Quote from: Armadillo on March 17, 2017, 10:36:33 pmThe voltages shown in red corresponds to the unit being in STOP mode. If the voltages have parenthesis, then it's PLAY mode.I think I'm barking up the wrong tree. I'm fairly sure that I have to go much, much further upstream.

author: Armadillo
author_rank: Super Contributor
timestamp: March 17, 2017, 11:32:17 pm
content: Can you check Q402 base and collector voltage [sawtooth wave generator]. Are the voltage correct.? Looks like the collector is stuck at 12V?

author: 4thDoctorWhoFan
author_rank: Regular Contributor
timestamp: March 18, 2017, 02:42:14 am
content: Quote from: Armadillo on March 17, 2017, 11:32:17 pmThe base is .2V and the collector starts around 1V then varies between 1V & 3V.

author: Armadillo
author_rank: Super Contributor
timestamp: March 18, 2017, 03:57:43 am
content: Because after D401 cathode, the positive Peak should be around +ve 3.9v instead of +ve 10v, therefore the wave shape at Q402 becomes important to watch, since you are not getting the correct collector voltages. Can you take Q402 out of the circuit to test it if you cannot scope it.

author: 4thDoctorWhoFan
author_rank: Regular Contributor
timestamp: March 19, 2017, 12:48:26 pm
content: Quote from: Armadillo on March 18, 2017, 03:57:43 amAnother problem has popped up which is putting this issue on hold.I think the motor servo just went out. The STK6922 got firecracker hot and the laser sled stopped moving. I have to work on this first in order to get back to the original problem. Thanks for your help though.