Hi all,

I'm looking for some help regarding replacing the transformer of the power supply of my aging but beloved Event Electronics PS8 monitor plate amps.

One is buzzing, it's driving me up the wall. Delaminating transformer.

Nothing is written on them, Event itself doesn't exist anymore and I can't find anything spec-wise.

What some research has taught me is that if I know the voltage it is outputting, it's input voltage (230) and how many amps it should be able to deliver (0.5A@230 says the badge on the amp) I should be able to determine the transformer I need to replace them with, is that correct?

So I should open it up, power it up and measure output voltage, leg to earth and leg to leg? 

If for example its 2x 30v to ground and 60v leg to leg, I am looking for a 30V, 150 VA torroidal transformer?

Thanks for any help.

Yup.

Open, measure AC on secondary & replace with something similar - that fits.  

A fairly simple job. The real problem is if you're faced with a blown transformer & no way of knowing the exact voltages needed. The actual physical size of the transformer will be more a clue to the VA than the consumption figure on the badge, but more or less, yes.

Thanks. 

Luckily the transformer still works, and I have a second to compare.

Yes the VA rating is a bit mystifying to me. Also I would like to go from this steel core vibrating junk to https://www.tme.eu/en/details/55154-p1s2/toroidal-transformers/talema/" rel="nofollow - something like this . 

0.5A @ 230 is 115Watt. If it turns out to be 30V that works out to roughly 4 amps, so then I spec it slightly beefier it works out to 150VA.

But I'm unclear if this calculation stays the same if the transformer has 2x 30volt secondary. I'll open up the amp this afternoon.

The VA is calculated using the total voltage, so 150VA, 2 x 30v would give 2,5A. That should be enough for the monitor, but like I say, take a look at the original to judge the size needed. If something bigger than a 150VA will fit, use a bigger transformer. That can't do any harm & will run cooler. And yes, a toroidal is better. More efficient & less likely to make noises.

A couple of things, Double check that it is only two windings and that there isn't a smaller pair that would supply the 15v for the input stage. Secondly you mentioned you are looking for a toroidial transformer, it is very rare that toroids buzz without outside intervention, loose clamps or covers. Check those before you go too far.

Don't worry too much about the current because if you get the correct voltage and correct physical size you can't go wrong, you can however go wrong if you select just current and voltage because it could be way too tall or look like a dinner plate and still be correct. You should be able to measure the ac voltage at the transformer or as it joins the board but you can also measure the dc voltage and divide the number by 1.414 to get the lower ac voltage.

Thanks guys. I have some free time now so I'll open them up and poke around with my meter.

I wish to replace the current transformer, a delaminating iron core thing with a torroidal.

If I'm totally confused I might post some pics.

Thanks again.

It measures at 2x 27.4V at idle, so that would make it a 25V transformer I guess?

There is quite a bit of room so I think I will order two of https://www.tme.eu/nl/en/details/55153-p1s2/toroidal-transformers/talema/" rel="nofollow - these .

A pic of the transformer:

Ok, cool. 

So just one set of secondaries? It must generate any lower voltages needed via dropper resistors, or something similar. Simple. Good luck!!   

The schematic shows that the 15v are generated by a pair of zener diodes and dropper resistors from the supply to the lm3886s so only the two windings required in this instance.

Ok, that is common, especially if the amp is only using relatively low voltage rails. When you have to drop high rail voltages, the resistors invariably end up black & eventually failing!  

I ordered the transformers and I found the schematic.

In the manual it says the amp has turn on/off transient protection, but mine do like to emit a slight BANG when powering on or off.

Do you electronics wizards have an idea if I could fix that myself? It's not a huge problem because I rarely turn the off, but whilst I'm in there...

uploads/1919/PS8_Schematic.pdf" rel="nofollow - uploads/1919/PS8_Schematic.pdf

Received the transformers and I have a newbie question.

I ground together 2 ends of the secondaries I presume but does it matter which side of the secondary?

Orange and black to ground, yellow and red are the two 25v legs?

Before you guys think "dude bring them to a pro", a sentiment which I share, trying to find someone that is willing to fix my aging stuff A: In a timely manner B: Doesn't bullshit me and C: I trust to do a good job is very hard, even though I work in the industry.

Plus now I get to learn something and scare my girlfriend.

Thanks again.

Yes, try one pair and measure the voltages. If you don't get summation, swap one of the ends!

service dept Steve wrote: you can also measure the dc voltage and divide the number by 1.414 to get the lower ac voltage.

 

If the 27.4V was the DC rail voltage. The calculation would be.

Firstly add the Diode voltage drop for the rectifier 

27.4 V DC + 0.7V = 28.1V. This would be the Peak AC Voltage

then divide that by 1.41 (square root of 2) to get the RMS AC voltage

28.1V /1.41 = 19.91V AC

If the transformer you have bought is a 25V AC

Are you sure that is correct or should it have 20V secondaries?

(25V AC *1.41)-0.7 = 34.55V DC rails

This is significanly more than 27.4V if that was the DC voltage you measured.

It was AC straight off the transformer secondary. You're making me doubt myself now so I'll measure again but I'm 99% sure I was measuring AC.

Ok yellow and black it is, thanks all.

Sorted, thanks everyone!

They sound great still, and no more buzz.

Now I can enjoy my tinnitus in silence.

Damn, now I hear a very very faint buzz coming from one of the 8 inch drivers that I didn't hear before because of the transformer (no input).

Could that be something simple?

The old transformer secondaries were twisted around each other before the pcb, is that an interference thing?

The large filter caps are "leaky"? Or try put a couple of 100nF caps across the rails. See if that helps.

kedwardsleisure wrote: where is that earth wire going near the transformer?

I knew someone was going to comment on that.

It's just jammed under that large washer. It used to run to the base of the old transformer because the old one was mounted in rubber, and therefore it needed it's own ground. The new one has a bolt going through the grounded aluminium plate so it is now unnecessary, so I just jammed it under there.

Both the earth wires, that one and the one connected to the plate go to the earth pole on the power connector. 

If the capacitors are dying it could cause a faint hum?

Thanks.

kedwardsleisure wrote: take the earth wire out, if it touches the washer it will damage the transformer.

Ok. Between the washer and the transformer and on the base of the transformer are pretty thick rubbery insulating discs, so it's not metal to metal.

That earth wire could be creating a shorted turn, a loop around the transformer core if there is metal to metal connection with the top plate, bolt and amp chassis.

I removed the pointless wire, and replaced the big capacitors (same spec, nice brand).

I'm afraid the slight hum is still present.

What I noticed while checking the voltage on the capacitors prior to soldering them off the board is that one of them read a negative voltage. Only slightly, 0.6V. But the 'good' amp didn't have that.

Could that be a clue as to why that one is humming?

Is the bridge rectifier dying?

I have no experience with this, that's why I'm asking. It has become a bit of a mission to get these b@stards 100% silent. Thanks for any help.

It is 100% coming from the 8 inch driver, not the newly replaced transformer.

No input present or even connected. The potmeter has no effect on hum level. 

I just measured between the earth on the power connector and a radiator on the central heating, 0.6V AC.

Just recap everything?

Also I was searching for a new bridge rectifier, and I could find similar components, but they were labeled PC and not CP 604. Does that matter?

Thanks.

While researching this subject I stumbled upon a few forum threads of people building their own amps and I learned a few things: 

Contrary to al lot of information(wikipedia), the jury is still out on whether torroidal transformers emit more or less interference that a iron core one. That got me thinking, maybe I assumed the hum was present before I changed the transformers. Maybe the transformer is actually causing the humming.

So I cracked open the cabs again.

Another thing I read that seemed to be backed up by measurements, is that even the orientation of the torroidal transformer can make a difference in interference, so I started with rotating the transformer and powering up the amp. No difference.

So then I grabbed the shielding that was around the old transformers and modified them to fit the new ones. Seems to be galvanized sheet steel. I wrapped it in tape to insulate. And presto, the speakers are quite a bit quieter. Not completely silent, but the hum is pretty much inaudible at listening distance.

Maybe I'll bend up a chunky piece of metal as a cage around the pcb to get it even quieter. Steel or aluminium do you think? Connected to ground or no?

What a faf, but I learned a lot, thanks again for all the advise!

Ahaaa, good info, thanks.

I'll order new rectifiers and report back.