Power Supply Question - Page 1
Forum section: electronics / beginners

author: Crambone
author_rank: Frequent Contributor
timestamp: July 11, 2024, 10:42:00 pm
content: I know this is limited in information but manufacturer won’t release a schematic. Unit is an ACOM 1200S linear amplifier for 1.6-50MHZTo start the 1200S let out a pop and then dead. I opened it up and found C4 Safety Capacitor blew on power supply. Replaced with new and still nothing but RY1 clicking.I followed the 120VAc and it goes to 2 relays in Normally Opened state that are activated by 12VDC. The relays aren’t closing so I pulled them and tested and they close perfect with 12VDC .5A applied. Placed them back in and jumped the contacts from normally open to closed. Following the 120VAC I come to a Bridge Rectifier. 2 inner posts have 120VAC. The 2 outer posts are showing +130Vdc or -130Vdc depending on probe orientation. I’m also seeing 20Vac on the same outer posts. I don’t recall ever seeing both DC & AC out of a bridge rectifier.Questions1) if I’m seeing 12VDC on both relays why won’t they close? I didn’t measure the current.2) Should a bridge rectifier pass both DC & AC? Maybe just pulsating DC I may check the frequency.Andrew

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 12, 2024, 02:11:23 am
content: You test diodes and bridges with a diode test feature.A (cheap) multimeter may show everything at DC/AC modes.

author: Crambone
author_rank: Frequent Contributor
timestamp: July 12, 2024, 02:45:25 am
content: I have a partial schematic of the circuit in question now.RY1 (relay) (yellow) chatters when powered on and won’t stay closed.J200 & J201 (green) have 120vacK202 & 203 (blue) N/O relays won’t pull closed even though I show 12vdc across A1 & A2 coil. Pulled relays and tested good.If I jumper the relays so 120vac passes at the bridge rectifier (green) I have my 120Vac on legs 2 & 3 and I get 130Vdc & 20Vac on legs 1&4?what would cause relay K202 & 203 to not stay closed with 12Vdc on them?Why the 20Vac on rectifier legs 1&4?

author: perieanuo
author_rank: Frequent Contributor
timestamp: July 12, 2024, 08:56:55 am
content: I suggest stoping bypassing relays before You understand the fault.After rectifier You get DC normally If filter capacitors are fine. You said You get AC, maybe one rectifier diode îs dead.

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 12, 2024, 03:00:42 pm
content: Quote from: Crambone on July 12, 2024, 02:45:25 amThat's just ripple. It's normal to see some "AC" on a DMM set to AC at that point in the circuit.It looks like this power supply has PFC (Power Factor Correction) which is probably why there appears to be a lot of "AC" (Actually pulsating DC) after the bridge rectifier. (As compared to a traditional linear power supply)ie: There's no large filter capacitor immediately after the bridge rectifier in a PFC supply.

author: Crambone
author_rank: Frequent Contributor
timestamp: July 12, 2024, 08:33:01 pm
content: Copy of full schematic posted now.Andrew

author: Crambone
author_rank: Frequent Contributor
timestamp: July 12, 2024, 08:35:10 pm
content: Interconnect Diagram if this helps?

author: Crambone
author_rank: Frequent Contributor
timestamp: July 12, 2024, 08:44:33 pm
content: UPDATE, I was able to pull a HARD FAULT log from the unit and it shows LOW or NO 26V on Power SupplyAndrew

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 12, 2024, 10:16:09 pm
content: Well, you should have some high voltage DC across across C16/C17/C18. This HVDC voltage is what feeds U2 (TOP244), a small switching power supply, which creates the 26Vdc source for U7 (which creates 5Vdc) and 26Vdc for other circuitry. (via J1A-8, J1B-4, etc)Perhaps U2, or some of it's associated circuitry, is defective? Or there is a short on the 26V line?

author: Chris56000
author_rank: Frequent Contributor
timestamp: July 13, 2024, 03:45:05 pm
content: . . .Where did the schematic come from? There might be extra sheets covering the PFC and PWM sub–panels!Chris Williams

author: Crambone
author_rank: Frequent Contributor
timestamp: July 13, 2024, 10:03:51 pm
content: Here is the PFC schematic

author: Crambone
author_rank: Frequent Contributor
timestamp: July 13, 2024, 10:06:13 pm
content: Control Board schematic

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 14, 2024, 02:37:08 am
content: Quote from: Crambone on July 14, 2024, 01:10:13 amThis is normal. You can see pin9 (Down-arrow ground symbol) joined to ground at the "junction point" just under U4 on the DC1500 CNTRL BOARD Unit.QuoteNormal to see when capacitors are charged up by an ohm meter. Get a 470uf or more capacitor, discharge it, and then ohmmeter it to see a similar effect.QuoteNormal.QuoteIt is the non-isolated, rectified mains voltage, negative potential. (BD1 pin 4) Dangerous point to put a scope probe's ground clip. (Don't do it!)

author: Crambone
author_rank: Frequent Contributor
timestamp: July 14, 2024, 03:30:50 am
content: I wish I had a way to send or post the Tech bulletin with all the schematics.

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 14, 2024, 04:26:15 am
content: You could print the tech bulletin to PDF and post it. As long as the file is less than 8 MB.I notice you tried to post the PFC schematic, but I see nothing attached tothat comment.

author: Crambone
author_rank: Frequent Contributor
timestamp: July 14, 2024, 04:35:32 am
content: Yes it’s too bigOk I have the PSU out of the unit no interconnections, I applied 120Vac and took voltage readings. The red highlighted is where I am seeing 11.7 to 12VDC.I then decided to check the J204 interconnect cable that becomes J502 at the CPU board. When I checked the interconnect cable to chassis ground of the unit not connected to PSU I am getting a direct contact to ground on pins 1 & 4.

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 14, 2024, 05:03:33 am
content: Well that makes sense, since these pins (1 & 4) are grounded on J502 of the CPU unit.

author: Crambone
author_rank: Frequent Contributor
timestamp: July 14, 2024, 05:52:39 am
content: OK I see that now, man I’m learning a ton thank you. So what am I missing? Why won’t the 120vac relay close?Andrew

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 14, 2024, 03:44:26 pm
content: Refering to the "ACOM 1200S PS Unit" PCB.When the user is pushing the "on button", pins 5 & 6 of J205 are shorted together & grounding pin 6 of J205. This turns on the relays via D208 and starts the power supply. This also lights the LED inside U201 which turns it's transistor on. That makes J204 pin 3 go low which signals, to the now powered up CPU Unit, that the button has been pushed. If the CPU decides that all is well to stay powered up, then it'll output a voltage on J204 pin 2 (J502-2 ON_HOLD# on CPU unit) which will light the LED in U202 and turn on it's transistor. This will put a voltage (6V) on the gate of Q200 causing it to turn on and keep the relays turned on.

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 14, 2024, 06:21:53 pm
content: Quote from: Crambone on July 12, 2024, 08:44:33 pmDid you ever check to see if the 26V was there while the power button was pressed? That could be why the CPU shuts the power supply down.(J1A pin 8 on ACOM 1200S PS Unit)

author: Crambone
author_rank: Frequent Contributor
timestamp: July 15, 2024, 02:13:20 am
content: Kim, I just tested with CPU back in unit and J1A pin 8 has no voltage even when power button is pushed multiple quick times.Andrew

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 15, 2024, 03:05:12 am
content: Press and hold the power button for a couple of seconds while measuring J1A pin 8.If there's no voltage at all, then press and hold the power button for a second or two while carefully measuring the high-voltage rectified DC at a convenient point. (+ of C16,17,18 or 21; cathode of D4 or D3; etc, referenced to FCOM) If F1 (fuse) is OK, you could also measure between pins 1&3 of J101 instead. Should be something like 160V or more.

author: Crambone
author_rank: Frequent Contributor
timestamp: July 15, 2024, 04:34:55 pm
content: Kim thanks I will try on Thursday when I return home, I really appreciate all the help!.Andrew

author: Jill Kent
author_rank: Newbie
timestamp: July 16, 2024, 02:03:00 am
content: Let's carefully analyze your issues and provide some potential solutions.Issue 1: Relays Not Closing1. **Check 12VDC Voltage**: Ensure that the 12VDC across the relay coils is stable. Use an oscilloscope to check for any voltage fluctuations or noise that might prevent the relays from closing reliably.2. **Current Measurement**: While you haven’t measured the current, it’s an important factor. Relays require a certain amount of current to close. If the current is insufficient, the relays might not function correctly. Use an ammeter to measure the current through the relay coils and ensure it’s within the specified range.3. **Relay Drive Circuit**: Check if the relay drive circuit is functioning correctly. This includes checking any driving transistors or other control elements to ensure they provide the correct voltage and current to the relay coils.4. **Relays Themselves**: Although you have tested the relays outside the circuit, their behavior might differ in the actual working environment. Try replacing the relays to rule out any issues with the relays themselves.Issue 2: Bridge Rectifier Output1. **Bridge Rectifier Outputting Both DC and AC**: Normally, the output of a bridge rectifier should be pulsating DC, not pure DC. This is because the rectified voltage varies with the input AC voltage. However, you should not see significant AC components at the output.- **Presence of 20Vac**: If you see 20Vac at the DC output of the bridge rectifier, it could indicate a filtering problem or a fault in the rectifier diodes. Filter capacitors (typically large electrolytic capacitors) are used to smooth the pulsating DC output. Check these capacitors for issues.- **Frequency Check**: You mentioned checking the frequency, which is a good idea. Use an oscilloscope to view the waveform at the bridge rectifier output and confirm whether there are abnormal AC components or noise.Troubleshooting Steps1. **Check Filter Capacitors**: Ensure the filter capacitors following the bridge rectifier are not damaged or failing. Damaged filter capacitors can cause AC components to appear in the DC output.2. **Test Bridge Rectifier**: Use a multimeter to test each diode in the bridge rectifier to ensure none are shorted or open.3. **Inspect Circuit Connections**: Verify all connection points for any loose or poor contacts that might cause unstable voltage or prevent the relays from functioning correctly.4. **Replace Faulty Components**: If you find any damaged components (such as filter capacitors or rectifier diodes), replace them promptly.I hope these suggestions help you identify the root cause of your issues and resolve them. If you have further details or test results, feel free to share, and we can discuss and analyze further.