quad 405-3 power amplifier making weird noises

Hi and thanks for reading this post.

Not sure why but my quad 405-3 is making weird noises.

When switched on it works ok for the first five minutes then it makes a loud crackling noise for a second, a minute later it crackles again and this is loud, i had the volume low but it nearly blew my tweeters then it started to hum as well.

Not sure where these noises are coming from but I just had the amp boards serviced by the manufacturer so theoretically these should be okay, also i fitted a new 35 amp 200 volt Vishay bridge rectifier replacing the stock rectifier bridge, a 7amp PM7A2Q, and installed new smoothing capacitors, the stock ones were 10k uf each, due to tolerances the new ones measure in at 9k uf, the only component left untouched is the transformer.

Does the noise i am experiencing indicate a transformer problem, Is there any way you can advise.

By the way, i am measuring infinite resistance between the primary and secondary and 0.5 ohms across the secondary.

The secondary is actually two secondaries parallel connected and should be approximately 35 volts each with my voltmeter i am reading a fluctuating voltage which varies between 72 and 76 volts.

Many thanks in advance. John.
DC on output would be my first guess having the fact of recent service that replaced filter caps.
Did they check/replace output transisors?
Hi. Thanks for the reply, much appreciated.

Yes, they went through the amp boards so should have done the transistors as well.

Forgot to mention the voltage across the transformer secondary is fluctuating between 72 - 76 volts ac, i know this can happen due to fluctuations in mains supply voltage but it does seem like a lot. i did measure the secondary a few months ago and the voltage was going up and down by fractions of a volt but not this much.

Do you think this may be suspicious.

Many thanks. John.
check your wall power and try to connect thru power conditioner - voltage stabilizer.

Hi Marakanetz.

Thanks for the reply.

I tried that but there does not appear to be a problem with the mains, i have another 2 amps and they are both working fine.

I disconnected the quad transformer from the rest of the amp and left it plugged in for an hour and it kept blowing the mains fuse so think it has had it.

I have found a replacement here: http://uk.farnell.com/multicomp/mcfe300-35/transformer-300va-2-x-35v/dp/9532013

but this does not have a centre tap. The old quad transformer is 35-0-35, this replacement is 35-0

Is is possible to use this by rewiring in some way.

Many thanks. John.
try to contact mercury magnetics and they will create the same copy of your power transformer.
seems like your original one had a three phase ac supply while this one is 2x one phase. rewireing may be tricky.