Power Supply Question

[Crambone]

I know this is limited in information but manufacturer won’t release a schematic. Unit is an ACOM 1200S linear amplifier for 1.6-50MHZ
To 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.

Questions
1) 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

[Vovk_Z]

You test diodes and bridges with a diode test feature.
A (cheap) multimeter may show everything at DC/AC modes.

[Crambone]

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 120vac
K202 & 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?

[perieanuo]

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.

[Kim Christensen]

Why the 20Vac on rectifier legs 1&4?

That'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.

[Crambone]

Copy of full schematic posted now.

Andrew

[Crambone]

Interconnect Diagram if this helps?

[Crambone]

UPDATE, I was able to pull a HARD FAULT log from the unit and it shows LOW or NO 26V on Power Supply

Andrew

[Kim Christensen]

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?

[Chris56000]

. . .Where did the schematic come from? There might be extra sheets covering the PFC and PWM sub–panels!

Chris Williams

[Crambone]

Here is the PFC schematic

[Crambone]

Control Board schematic

[Kim Christensen]

PCB1 9. Dead short 0.03 ohm   XXXXXXXXX

This 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.

J1A & J1B connection  All 26V ohms go from 0 to 1900 then OPEN XXXXXXXXXX

Normal 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.

J101  Pin 1 shows open (J101) is normally Not Connected XXXXXXXXX

Normal.

Also what is FCOM ? I see it all over the schematic?

It 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!)

[Crambone]

I wish I had a way to send or post the Tech bulletin with all the schematics.

[Kim Christensen]

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 to that comment.

[Crambone]

Yes it’s too big


Ok 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.

[Kim Christensen]

Well that makes sense, since these pins (1 & 4) are grounded on J502 of the CPU unit.

[Crambone]

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