Low voltage on startup resistor  - Page 1
Forum section: electronics / beginners

author: sarry86
author_rank: Contributor
timestamp: January 31, 2023, 07:17:56 am
content: Greetings, I have a low voltage on a startup resistor on my Sony DAV-DZ350, the resistor is 100k ohms and checking it on multimeter, it shows exactly 100k ohms, it generate only 2.44vdc which isn't enough to start up the power IC, any tip on how to fix this? I have replaced the control IC twice to no avail

author: inse
author_rank: Super Contributor
timestamp: January 31, 2023, 10:10:27 am
content: Disconnect R936 and see if it tries to generate output pulses.The voltage before the 100k is OK?

author: RoGeorge
author_rank: Super Contributor
timestamp: January 31, 2023, 10:40:01 am
content: Check the electrolytic capacitors, particularly in appliances with switched-mode power supply (SMPS). The electrolytic capacitors dry out with time and develop a high ESR (Equivalent Series Resistance). This leads to all kind of strange behavior (more often manifesting when when the appliance is cold).Replace any electrolytic with a bulged top. Sometimes the electrolytics might fail without any mechanical hints. Measure their ESR with a good ESR-meter that can measure in circuit, without needing to desolder the capacitor first.

author: sarry86
author_rank: Contributor
timestamp: January 31, 2023, 01:41:13 pm
content: I tried, there's 1.85v with or without R936 connected, if I desolder a startup resistor leg to startup pin, the resistor gives exactly 230~ volts! Does that mean it is open? 😳

author: nali
author_rank: Frequent Contributor
timestamp: January 31, 2023, 02:03:37 pm
content: Does the "~" refer to AC volts? That's wrong, you should be seeing a DC voltage, so it looks like C901 has probably failed (plus any other electrolytic which may be in parellel with it outside the cropped schematic)

author: inse
author_rank: Super Contributor
timestamp: January 31, 2023, 05:32:26 pm
content: I‘d expect that the startup resistor is connected to the positive leg of CE901.As the schematic says 330uF 200V, I assume the device is running at 120V mains.The rectified filtered mains voltage at that capacitor should be around 170V.Either the schematic does not match your device or your measurement is buggy.

author: wasedadoc
author_rank: Super Contributor
timestamp: January 31, 2023, 05:46:16 pm
content: Quote from: inse on January 31, 2023, 05:32:26 pmCE901 is only half of the capacitor on the bridge output. See attached. I would expect about 340 Volts DC.

author: sarry86
author_rank: Contributor
timestamp: January 31, 2023, 07:13:59 pm
content: No I meant to write "nearly" 230 vdc sorry

author: sarry86
author_rank: Contributor
timestamp: January 31, 2023, 07:15:40 pm
content: I am on 230 vac, I get around 230 - 270 vdc

author: inse
author_rank: Super Contributor
timestamp: January 31, 2023, 07:43:27 pm
content: Can you measure both AC and DC after the rectifier, please?

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 01, 2023, 04:37:50 am
content: The 100K (R935) only provides a bit of current to charge a capacitor and start IC903 operating. The power to keep things running comes from pins 3&1 of the transformer, is rectified by D904, and smoothed by C927. Check that C927, D904, C920, C926 & R937 are all OK. C927 is your prime suspect. The output capacitors (Not shown in your schematic) are also good suspects as they always tend to fail in SMPS.Note: Any major circuit fault can make the startup voltage appear low if you are measuring it with a multimeter. This is because IC903 will be continually resetting and the voltage at startup (pin 4) will be a sawtooth waveform. ie: The intermittent sawtooth wave could have a peak voltage of 10V but an average value of only 2V or so.

author: sarry86
author_rank: Contributor
timestamp: February 01, 2023, 06:50:59 pm
content: Sorry for the late reply, here are the photos you requested, the first ome is AC input, the second one is - ve and +ve of the bridge rectifier, the third one is between the filter caps in parallel

author: sarry86
author_rank: Contributor
timestamp: February 01, 2023, 07:03:49 pm
content: Quote from: Kim Christensen on February 01, 2023, 04:37:50 amI have already changed the C927 and the Q901 busted so i replaced it too, all R931 IC903, R936 and R930 all burned out so I replaced them too

author: inse
author_rank: Super Contributor
timestamp: February 01, 2023, 08:02:07 pm
content: Why do you only measure 205V on a 230V grid?Also the DC voltage makes no sense…Do you have confidence in your meter?

author: sarry86
author_rank: Contributor
timestamp: February 01, 2023, 08:09:25 pm
content: Quote from: inse on February 01, 2023, 08:02:07 pmIt's our power source's fault, it's not stable. Yes my meter has no problem, is it possible that these filter caps are fault? 😳

author: inse
author_rank: Super Contributor
timestamp: February 01, 2023, 09:48:39 pm
content: The filter cap should be charged to the peak value of the grid voltage, in your case 205V*1.414~289VSet the meter to AC and measure the ripple at the filter cap.Maybe it’s toast

author: wasedadoc
author_rank: Super Contributor
timestamp: February 01, 2023, 10:58:24 pm
content: I'm not sure that meter can measure the AC sitting on DC.

author: sarry86
author_rank: Contributor
timestamp: February 02, 2023, 06:54:28 am
content: Quote from: inse on February 01, 2023, 09:48:39 pmThis is what I get

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 02, 2023, 07:23:27 pm
content: Quote from: sarry86 on February 02, 2023, 06:54:28 amLooks like that meter can't read AC properly when DC is present... A double check would be to set it on the 200 VAC range and measure a good 9V battery. It should read zero after a second... I'll bet it reads something more than 9 which means that the 514 VAC reading across the capacitors is erroneous.An old school method on analog VOMs was to have a capacitor that went in series with the meter lead when you wanted to measure an AC voltage riding on top of the DC one. Not sure of your meter's input impedance (Most are 10M), but if you have a known good 0.1uF - 1uF capacitor with an appropriate voltage rating, you could insert it in series with the meter lead and try again. There will be a bit of an error caused by the capacitance reactance and leakage, but it should give you a ballpark reading. You'll have to wait for the reading to settle as the capacitor charges up. As always, be careful with loose bare wires and high voltages. Also, beware of the residual charge left on the capacitor when you are done.

author: sarry86
author_rank: Contributor
timestamp: February 03, 2023, 01:14:57 am
content: Quote from: Kim Christensen on February 02, 2023, 07:23:27 pmI didn't understand the part quoted in red, care to explain in the simplest way 🙆?

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 03, 2023, 03:42:06 am
content: To measure the ripple at the filter cap, you need to connect your meter like this:

author: sarry86
author_rank: Contributor
timestamp: February 03, 2023, 05:59:35 am
content: Quote from: Kim Christensen on February 03, 2023, 03:42:06 amThat should be a non polarized cap right?

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 03, 2023, 06:12:36 am
content: QuoteIt doesn't have to be, if you know which side of the circuit you are testing is positive.ie: connect the + of the capacitor to the positive side of the DC circuit that you want to check for ripple.

author: Nusa
author_rank: Super Contributor
timestamp: February 03, 2023, 06:40:54 am
content: 514 on the 500V scale of a meter marked max 500V could simply mean it's at the top of it's range.

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 03, 2023, 06:55:19 am
content: Quote from: Nusa on February 03, 2023, 06:40:54 amSorry, not in this case. The meter is set to the 500 VACrange. Line voltage where the OP is located is normally around 240VAC (He measured 205VAC earlier). The OP is measuring the rectified AC (DC with some ripple) so any AC reading should not be that high even if the filter capacitors were completely knackered.

author: inse
author_rank: Super Contributor
timestamp: February 03, 2023, 07:12:08 am
content: @Nusa:500V may only be the safety limit, the meter is principally able to measure above that.But nevertheless I don’t trust in this piece of equipment.