Switching PSU drops 5v.

[Simon_m74]

I am repairing one of my virtuality machines, as it has a voltage drop problem .

The psu outputs all voltages fine (+/- 5v and 15v and +25v.)
Thing is, when I plug the psu into the pcb it's used for, the +5v drops to almost 0v.

I tried another known working psu, so I assume its a short or bad component on the pcb.
Nothing looks blown, nothing gets warm.

Any suggestions how to find the short?

Thanks

Simon

 

[kellys_eye]

On most SMPS the output regulation is set by the monitoring of only ONE of the output supply rails so you need to check all the power rails (load them up) to see which one is actually pulling the 5V down.

Equally, the SMPS has (probably) current monitoring which, if the sensing component (resistor?) has changed value, may be limiting the output by thinking it has a short/too heavy load.

Got a make/model and/or schematic for the PSU?

 

[Simon_m74]

Hi
The system is about 25 years old. The company who made the psu has been bought out by antex, and the pcb is simple, but custom for the machine.
I will upload schematics of both the psu and pcb tomorrow when I get near my pc.

 

[Simon_m74]

Are Kicad files ok? or just a screenshot?
Or save a PDF and upload?

 

[kellys_eye]

Screenshot will do. Not Kicad but most others will also work.

 

[Simon_m74]

These are the schematics.
Drawn up by myself.

The PSU is a copy of what was apparently is the original schematic - although not checked.
The PCB - I had a partial schematic, but I have cross referenced the partial one I have to the PCB itself, and also finished (well almost) the schematic by reverse engineering the PCB.

 

[73's de Edd]

Sir Simon_m74 . . . . .


Looking at your second schematic supplied.
With it using design waaaay back when no IC's were need to make a switch mode power supply !
Long live the discretes.

If being power supply related . . . . .
Look at the derived supply that is being one section up from the bottom of the page and being fed from the first transformer up and ends up as the +5VDC output at the far right side..
There is a current sensing / ripple detection circuit being assosciated with that supply using its associated SCR.
Shunt another 2200 ufd cap across the existing C24 , also the C25 10 ufd reservior cap, and the final C35 2200 ufd cap all of those are all being critical for the operation of this particular stage.
TIME may have taken its toll on those units capacitances, plus encroached excess ESR.

Next you might insert a DC ammeter in series with the + 5VDC supply line at the end of that +5VDC supply line and see how close . . .OR EXCESS . . .of that specified 2.8 amp is being drawn.



73's de Edd

 

[Simon_m74]

I am sure that the PSU is ok. Last time I checked it - about 6 months ago, to be honest, I think I remember testing the PSU on another machine without a problem, and/or testing another PSU on this system with the same results.

Hence my original question - if it is a short on the PCB, whats the best way to find it?

 

[kellys_eye]

Ah.... confusion, but understood now.

the fault is on the board the PSU plugs in to..

Show us pics of that PCB and any schematic you have for it.

 

[Simon_m74]

The schematic is the VR_PCB pdf file.

These are the best current photos of the PCB I have .

 

[kellys_eye]

The (lower) section of board where the PSU plugs into seems to be isolated from any outside interference by the opto-couplers so it's very unlikely there is anything off-board causing a problem - which leaves...... whatever might be under that big heatsink.......

The top half of the board seems to be powered from the toroidal transformer, rectifiers and smoothing associated with it. Without a closer look I can't see the PSU 'module' having any outputs going to the top part of the board..... am I on the right lines here?

So, what's under the heatsink?

 

[73's de Edd]

Sir Simon_m74 . . . . .

A ceramic bypass or like electrolytic / tantalum problem, wouldn't be "smart" enough to provide that specific time delayed voltage drop.

The only chips that I see on the other board that are using the +5VDC supply is the one 555 and (5) AD7111's and multi sets of LM324 quad op amps.

Just as per the big heat sinks . . .I can't see that sizing . . . . being related to the pipsqueak TDA2003 audio power IC's that the unit is incorporating. Plus, they are not solo 5VDC operation.

In a stone cold dead power condition can you low ohm out the +5VDC single supply wire out to see if any low ohmage reads to ground, relating to the plug in board

If you do get the unit operating at the sub voltage level of the +5VDC supply . . . could you have already premarked the different +5vdc supply points around the board . . . so that you can THEN make flying measurements with DC metering in using . . . its 3 sub decimal . . .(or greater) voltage resolution capability , to see which point has the lowest voltage reading.


73's de Edd
.....

 

[Simon_m74]

The Toroidal powers the audio amps, that are under the heatsinks.
Thats all that does.
The rest of the pcb is powered by the PSU.
If you look closer - you can see the `dark lines` that are the underside of the PCB, power lines.
You can see on the top photo, where these connect to the PSU connection.

A description of the layout...

Top left:
Power input, turned on by a solid-state relay - thats controlled from the parallel port on the Commodore Amiga 3000 computer thats the `heart` of the VR machine. (Thats another story - I have a few non-working Amigas!)
The ribbon cable that is connected to the edge, is only for LED`s to show power is on - one for each +/-5, +/-15 from the PSU, and +/-25 thats from the toroidal for the amps. The LED`s also show Amiga power and HDD activity. - 8 LEDs in total.

The long ribbon under the toroidal is for the VR tracking PCB serial connection - PCB not installed on this photo.

Orange connector in the middle - is a power connection for the tracker PCB.

2 x 15 pin D sockets - for L/R joystick input on the machine.

Bottom Left:
Video circuit.
Video is controlled, switched and `protected` with the MAX453/452 ics. The two 25pin dplugs are for the headset
The IDC connector is for the `control console` - flashing buttons for option selection.

And of course, the external audio amps.

Top Right.
The line of ICs, are simple logic, Shift register, Or gates, And gates, 555 timer - This is to `dispense` the 8-bit data from the Amiga` parallel port:
Console light on/off/flashing
Video switching (External display - GFX card or Amiga output)
AD7111 control - system volumes - CD, Amiga audio, MicNet (player to player communication network)

A group of Op Amps to control the audio.
And in the top right corner - the bass & treble tuning.

Bottom Right.
Other than the PSU - Joystick trimming circuit.

The 4 pots at the front (right on photo) are left/right headset screen brightness, and external / headset audio volume.

The connections on the back (left on photo) from top to bottom.
Power - 240/120V
Coin-Op - credit module.
Parallel port in from Amiga
Left Joystick to Amiga
Right Joystick to Amiga
Micnet connection in/out
CD audio in
Amiga audio in
Amiga Video connection
Right graphics card input
Left graphics card input.


As for testing.
Low ohm out - I assume you mean check ohm between 5v+ and Gnd? at various points?

When I try to power it up - it seems that I get 0v out of the 5v connection.
I cant check closer with a higher resolution to see if I get any better readings over the board.

I will double check it works or not, with another PSU though... to fully decide if the fault is the PSU for the PCB - as I cant honestly remember!

 

[Simon_m74]

A few more photos.

Another of the PCB. All the photos of the PCB`s are not the EXACT one I have the issue with - but are all the same anyway.
The LEDs at the front - on a working system

The VR machine I`m repairing.

Some of the collection of machines I have.

 

[kellys_eye]

I tried another known working psu, so I assume its a short or bad component on the pcb.

So did this other working PSU actually work? The confusion arises over whether the fault is on the PSU board or the board it plugs into - that statement isn't clear.

Start by disconnecting the 5V output and ensure the other supplies remain as they should be. You might have to add a small resistive load to the disconnected terminals to ensure the regulation circuitry keeps going. Something that causes a 20mA drain would do (i.e. 220 ohm 1/2W resistor will do on the 5V line)

Do this for each supply voltage individually and note which supply(ies) deflect from their designated outputs.

For the supply line that is noted to cause the issue, check/change the decoupling/smoothing capacitors immediately adjacent to the supply output.

If this doesn't fix it we'll need a close up of the main board area around the supply output - both sides - with any 'interfering' items removed (i.e. heatsinks).

 

[Simon_m74]

Hi all,

I finally got round to checking if the fault is the PSU or the PCB.
As I have a few PSU`s, and PCB`s.. and its been a while since I dismantled the machine - I could not be sure which one were the potential issue.

I came to the conclusion of when PCB I was using, as I had done an audio mod on it.
I only have 3 PSU`s that are fully assembled... so I tested all three - on another PCB I have.

2 PSU`s seemed to work fine - one looses 5v.

So I am now sure that it is the PSU thats at fault.

For now, I`ll rebuild the machine with the replacement PSU, but I hope that soon I`ll get back onto the problem with the PSU.

Thanks all

Simon - Retro VR
https://www.facebook.com/RetroVR2014

 

[Simon_m74]

Hi all.
Sorry for confusion...
Its NOT the PSU..
I tested a fully working PSU on the board that lost the 5v, and again it lost it.
The same PSU was tested on another board, and 5v is fine...

So any ideas now that it is sure the PCB?

 

[kellys_eye]

Post clear pictures of the PCB without any interfering boxes/heatsinks in the way. Both sides.

 

[Simon_m74]

I will try to get some good photos over the weekend . I can't remove the heatsinks though as they are the actual audio power amps (hy60's), but I do have another board that's had the amps removed, so I'll get a photo of that too.
The faulty board has had some moss by myself, back a good few years ago.
The digital control system was not working, which affected the audio, so I removed some op-amps, and bypassed them.
Also, before I got it, someone had scraped off some pcb traces that are the digital lines. I reinstated them
Just a heads up, that this board is not the best condition.