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pangea
11-04-2005, 07:10 AM
Could anyone please help me, by suggesting what might be wrong.

I've got a BGW 250D amp, where the transformer is humming slightly and after an hour or two, the transformer is becoming quite hot, even at idle.

Other than that everything is working OK and those weird multiplugs in the rear, are in place.

All feedback is highly appreciated.

BR
Roland

EDIT: Perhaps I should add that I'm connecting unbalanced with 1/4" phono plugs.

Chas
11-04-2005, 07:25 AM
Roland, an amp that old might have leaky filter capacitors, although I had a 250 once and it had very good quality caps. However, it is still possible for them to die.

Can you check the DC current flowing from the rectifier bridge diodes to the caps, then from the caps to the amplifier boards? If I remember, this should be easy to do. This will help to isolate where the excessive current draw is originating from.

Be careful, there are relatively high voltages (around +/-50V) inside there and make sure that you do not allow rail power to be removed from either side or both sides of the circuit while it is operating. If you are inexperienced, I would recommend you find a good tech. The early BGW's were textbook circuit designs and would be fairly easy for a qualified tech to work on.

First though, I would check the bias of the output stages if you have a manual with the specs. Are the heatsinks getting hot too? This could be a clue to possible excessive bias current. These amps should be quite cool at idle.

morbo!
11-04-2005, 07:26 AM
maybe a really good cleaning
i submerged my old technics(i used metho)
smelled funny for the 1st few hours use!
but fixed alot of the smaller probs with it

p.s read my thread putting trimpots in a amp and click on the link

jpb_dk
11-04-2005, 07:45 AM
Hi there,
I dont know that specific amp, but if it has never happend before, somehting is wrong..!
Some transformers are not well-made and production of electronic is just so.

In a tupe amp it happens when the power-tubes are "running", but the trafo newer get that hot as you describe.

In transistor-amps and Fet's it should never happen.

It can also be a matter of noice. If the amp is surfing on a noice-layer. Try
the XLR instead of the phono-plug.

You say the trafo is "humming" the cores inside are "singing" and they can only do that if its not correct wind. Its an indication that its a defective trafo and You better get it checkked at professional. The Defective trafos should not be repaired but changed and stored for door stop or missiles if someone try to brake into your house unless they give you a "replasement" price.

It can also be defective Capasitors. In that case they will be hot also and You should be carefull. A Cap. of 100.00uF are not "funny" when it explows.
I would say go for a professional check m8

Good luck, Jens

GordonW
11-04-2005, 08:06 AM
It can also be a matter of noice. If the amp is surfing on a noice-layer. Try
the XLR instead of the phono-plug.



Expanding on this, it's also possible that something in the pre-driver or output stage is oscillating, causing a steady-state current draw. Anything from a bad compensation cap (on one of the transistors) to a bad resistor, to a bad transistor, could cause this. I'd put the amp on an oscilloscope, to make sure there aren't some parasitic oscillations on the outputs...

Regards,
Gordon.

Chas
11-04-2005, 08:42 AM
Expanding on this, it's also possible that something in the pre-driver or output stage is oscillating, causing a steady-state current draw. Anything from a bad compensation cap (on one of the transistors) to a bad resistor, to a bad transistor, could cause this. I'd put the amp on an oscilloscope, to make sure there aren't some parasitic oscillations on the outputs...

Regards,
Gordon.

Good call Gordon, I thought of this right after I hit the send button!:p Could be upstream oscillation from a preamp, etc. too.

jpb_dk
11-04-2005, 09:57 AM
did not even get the idea op-amps oscillating in yea 2005. When will audio designers get quality-standards *lol*
Yea scope it if you got one, or direct to the repair-shop.. All those ideas whats wrong it should not work at all :blah:
Good luck

Regis
11-04-2005, 07:21 PM
First though, I would check the bias of the output stages if you have a manual with the specs. Are the heatsinks getting hot too? This could be a clue to possible excessive bias current. These amps should be quite cool at idle.

Good idea Chas, that'd be one of the first things I do. As far as the transformer hum, I eliminated just about all of it on my vintage reciever by replacing a bunch of the 25 year old leaky caps. You can hardly hear it now. Also check the 'can' surrounding the transformer and make sure it's tight.

Flodstroem
11-05-2005, 12:02 PM
Hi Roland

Have you discovered this recently or has the transformer always been so hot?

Is this transformer for 220 or 230VAC, 50 or....60Hz

Transformers in use generally get hot due to the fact that the iron core has power losses if using standard iron core stacks. So, this is normal.

If this particular transformer didn get hot earlier, then something could be wrong.

Questions: are the heatsinks getting hot to?

If they are then it could be due to a to high idle current in the power stage output transistors (is this a AB-amp ore a class A-amp). Do you have a schematics for this amp? If not, then you could do nothing to this issue if you dont are a skilled (hobby) electro engineer.

If the idle current is low then it could be the caps as mention earlier in this treads (could be indicated with a DVM: low power rails DC voltage/high ripple.

Gordon W suggested oscillating problems and this is right and you could have a faulty oscillating module some where in the power stage.

If you fail to solve the problem (if there is one) I recommend you to contact Sontec Audio in Stockhiolm(Mr Max Weber). They are skilled and could check the amp for all this faulty thing pointing at here in this thread (and more)

Let us know what happens with your amp.

Good luck

Regards

Chas
11-05-2005, 07:00 PM
[QUOTE=Flodstroem]If they are then it could be due to a to high idle current in the power stage output transistors (is this a AB-amp ore a class A-amp). Do you have a schematics for this amp? If not, then you could do nothing to this issue if you dont are a skilled (hobby) electro engineer.

Hi Flod, the amp is B, basically a textbook quasi-complementary design. The heatsinking was very conservative, it SHOULD run VERY cool unless you are really punishing it! :p

Ian Mackenzie
11-05-2005, 08:39 PM
Sounds like the iron laminate transformer is breaking down causing eddie currents to circulate in the core.

Refer it to a qualified technician or buy yourself a new amp (it won't be cheap)..its used by date is up.

You will easily do better sonically with current technology over that vintage

Ian

pangea
11-09-2005, 02:38 PM
Wow, so many replies and I didn't know. THANKS!
I was waiting for a reply notification, which never came.

Could it be, because the thread was moved?

Anyway, the amp is clean inside, the heatsinks don't get hot at all, nor does the transistors. It also takes maybee an hour, before the transformer gets really hot, regardless if anything is attached to it. The big caps stay cool. It's a class AB amp and I've got the schematics. I'm not sure, but I don't think I've seen any OP-Amps in there.

I have noticed though, that when I turn the amp off, the right channel clip-led lights up for a few seconds. I have also noticed that there is some humming around a relay near the transformer.

I don't know if it's always been like this, since I got it as payment for a Hypex amp I built a while back and now is really the first time I've hooked it up with a preamp and speakers.

Unfortunately I don't have a scope, only a small multimeter with the usuals.

BR
Roland

Zilch
11-09-2005, 03:09 PM
Relay should not hum. Take it to a tech with a 'scope to look at the supply voltages for AC ripple, and particularly what's driving that relay....