Laney Ah210 PA mixer power supply

[ziggy007]

G'day all,
I try and stay away from the 'modern' stuff like this but said I'd do this for a mate......
This post I guess is to more understand the circuit as much as fix it. Initial diagnosis , nil power up.
I found the Pwr supply chip U1 melted and the zenner D9 (ZD101) on the VCC input open cct. Replaced both and have checked all components in the primary side of the circuit.
Now have 13vdc at the U1 VCC (although it seems to modulate around a volt+/-.)

Still no good. There is 48V phantom power, which comes off TXFR tap 10 but nothing else.

I'll admit i'm at a loss.
How do you experienced fellas troubleshoot these things? im not 100% what I am actually looking for to see whats going on.

There is 110vdc at the o/p of all the Secondary Diodes D6 to D8 (D202 - 205 off datasheet) which is obviously not right. Them all being the same to me points to a common issue upstream???

Hope someone can shine a light at my incompetence!!!!

Trying in vain to upload the schematic but it is not co operating.

Here is the Data sheet link which is essentially the same although a bit simplified on the secondary side.

TIA
Gav

[Enzo]

Not sure what you have there, but first thing I check is the secondary side rectifiers for shorts.

Remember this is an SMPS - switchmode power supply - and it does not run at 50/60Hz. SO any "AC" over there will not register on your meter well.

Note pin 4 of the IC is the feedback from the secondary, and it can shut things down. Make sure we are not in shutdown.

Note the primary and secondary sides have different grounds. SO you can't use the same ground as a reference to take readings on both sides. I suspect the you were using the primary side ground - where you got the 13v DC - to read the secondary voltages. That would explain the extra 100v everywhere.

[ziggy007]

Thanks Enzo.
5vdc on the 30v rail and 3vdc on the 15v rail using the right ground reference this time. (Cheers)
There is less than 100mv on the pin 4 feedback.
The data sheet says there needs to be 15v Vcc for the chip start up to function so my guess is this is the issue.
There is a waveform on the primary using my old cro. Ballpark 200v (300Dc) so something is trying to happen.

[J M Fahey]

First and foremost, the primary side INCLUDING GROUND is LIVE MAINS.

Everything to the left of the transformer core (including primary winding) is deadly.

Keep both grounds separate, do not join them with a piece of wire or your own body.

So do NOT touch it or at least get a 220V-220V 1:1 isolating transformer of at least 100VA just for initial testing.

You may try to disconnect all secondary loads, check (unplugged) that everything at left is fine, including new chio and other semiconductiors, plug it in (you might use a lamp bulb limiter but with a largish bulb, say 100W) and turn it on.

Then measure secondary voltages referred to the secondary ground.
Post results.

PS: in any case, supply includes a Stby/Normal switch, try it both ways and see if supply starts.

PS2: check you donīt have fried secondary diodes or shorted capacitors, *something* killed that supply.

Some SMPS do not like a bulb limiter, but we start using it anyway; worst case we find some other way later.

[ziggy007]

Thanks Juan.
Appreciate the reply mate.

The Txfr is PCB mounted so not sure how I can isolate it? Definitely aware of those primary voltages! The resovoir cap holds 300v well!

The primary side powers up “ok” now. Nothing is burning however there’s is only 13 volts Dc on the chips Vcc input.

5v and 3v on the 2 secondary rails. I’ll try and isolate components on the secondary side tomorrow.

48 v phantom power is coming through ok which again I don’t get.

Not sure by what you mean about the stby/normal switch? It’s just a standard power on 240v in switch.

I still feel like it’s definitely primary side related. I’m doing a lot of reading tomorrow on SMPS theory. I want to know why this is doing what it’s doing.

I think my old analog Cro struggles to see the primary pulses. Not sure. Very inexperienced at troubleshooting this SMPS thing.

I’ll try and get the PDF schematic to upload as it wouldn’t today. (Maybe too big at 3meg)

Again, appreciate the replies.
Cheers

[J M Fahey]

Not sure by what you mean about the stby/normal switch? It’s just a standard power on 240v in switch.

Iīm referring to the schematic you posted, look at the bottom right showing such a switch.

If the full manual is too large for MEF, host it somewhere else and post download link here.

[ziggy007]

http://https://www.dropbox.com/s/t9e...anual.pdf?dl=0

Hopefully this works.

[Dave H]

Try it without the http://

AH210 Service Manual

What Juan said is worth repeating.

"First and foremost, the primary side INCLUDING GROUND is LIVE MAINS.

Everything to the left of the transformer core (including primary winding) is deadly."

[J M Fahey]

Can somebody else try to download it?

The post link sends me to a home improvement site ????????????????????

Mistakes I found so far:
1) your link starts with

http://https://

which isnīt possible, itīs either one or the other.
2) what firefox shows on top of the home improvement page is:
http://www.https.com//www.dropbox.co...anual.pdf?dl=0
which again looks funny, since we have been sent to https.com ... ?????????

Please check and repost

[Dave H]

The Txfr is PCB mounted so not sure how I can isolate it?

The isolation transformer is used to isolate the incoming mains supply to the unit then the "ground" of the switchmode transformer primary side can be connected to true ground to make it safe(r) to work on.

[Dave H]

Can somebody else try to download it?

The post link sends me to a home improvement site ????????????????????

Could have been worse

Check the link in #8 above. I think I've fixed it now.

[dmeek]

Here it is as an attachment

https://music-electronics-forum.com/...1&d=1564520933

[J M Fahey]

Thanks, dave hīs modded link works.

[ziggy007]

The isolation transformer is used to isolate the incoming mains supply to the unit then the "ground" of the switchmode transformer primary side can be connected to true ground to make it safe(r) to work on.

Firstly thx for fixing the link Dave.

My query was more that I was not sure how to isolate the secondary loads in the way that Juan mentioned as the Txfr is PCB mounted.

[J M Fahey]

My query was more that I was not sure how to isolate the secondary loads in the way that Juan mentioned as the Txfr is PCB mounted.

The idea is to have (just for testing purposes) the various supplies on the secondary side separated from their respective loads.

So if connectors or wiring are in the path, just unplug or unsolder whateverīs needed to disconnect them.
Remember to label them properly to avoid reconnection errors.

IF only copper tracks are involved, standard procedure is to cut them open with an X Acto knife, simply slitting them away, so thereīs no continuity between both sides.

You measure your now unloaded supply and hopefully you have full expected voltage.
Often a little high because itīs unloaded.

IF you have a problem, you know itīs a supply problem (diode or cap, not much else there) and solve it, knowing itīs not being causedn byb the load.

Once you are happy with that, you scratch a little of the epoxy solder mask on both sides of your testing cut and you apply a drop of solder on the now shiny copper exposed, period.

If now some voltage is funky (or straight shorted to 0) you know itīs some load fault and not a supply one.
The old "divide and conquer" method.

EDIT:
IF this were your supply, the points to cut and measure would be: