Vintage McIntosh Transformer Hum...

[EchoWars]

Convinced a buddy to take a leap into the vintage gear craze...he's always wanted a McIntosh anyway, and went and bought a Mc C28 preamp. He bought it knowing the phono section was bOrked, but a new zener in the power supply, and a new power transistor for the +75V supply, plus one small-signal transistor in the phono section, and he's up and running.

However...
He emailed me saying the transformer is buzzing. This is a standard type transformer in a rather old piece of gear, and I suggested that perhaps it has seen better days...maybe laminations are loosening up.

I've read all the threads about DC traps and such, but seems like about all the guys here who had such problems were using toroidal transformers.

Should I tell him to see if the transformer case can be opened up, and see if the laminations can be tightened? McIntosh charges insane prices for their parts...I can only imagine what they want for a transformer...

[li_gangyi]

if he has the time...he might wanna try re-lacquering the transformer layer by layer..for me..I would just give it a cleaning...get rid of the old wax...and the dip the whole thing in lacquer and letting it dry...

[PRR]

> the transformer is buzzing. This is a standard type transformer in a rather old piece of gear

It is an excellent transformer and is unlikely to have aged. Remember Mac made his reputation with very high-tech output transformers. His small power transformers were not sexy, but they are far from junk.

How sure are you about the repairs? Were all bad parts found? Were they replaced with reasonable parts?

Buzz could be overload. The tranny in a C28 will probably get hot if overloaded for an hour. If you need a comparison, I have a similar C-something I could fire-up and feel.

Buzz could be a sick rectifier. If shorted it would probably cause big trouble. But if one of the two rectifiers is open, the preamp will work with insignificantly higher ripple and lower raw supply voltage, but unbalanced transformer current.

Also be sure all screws are tight, even ones at the far end of the chassis.

[EchoWars]

The 75V regulator was toast. The parts were ordered from Mc....you mention 'replaced with reasonable parts'. Heh. Mc sent a NTE transistor as a replacement for the original Moto power transistor. When I was told this I was suprised.

Yeah, it's not like I'm unfamiliar with McIntosh quality. Still, this piece is 30+ years old. I don't think it's drawing too much current, as the transformer isn't hot... Plus, both the 75V supply and the 18V supply have dropping resistors...if too much current was being drawn then I'd think that the regulaor would be putting out way low voltages.

if he has the time...he might wanna try re-lacquering the transformer layer by layer..for me..I would just give it a cleaning...get rid of the old wax...and the dip the whole thing in lacquer and letting it dry...

Is that possible? Can you really disassemble the transformer to that degree?

Gonna have to try some obvious stuff, such as tightening screws and such first.

[li_gangyi]

yeah...I tried disesembly of a small 20W bogen PA amp...the whole thing was lose and peeling away...no chpoice but to undo everything and use thinner to dissolve all the lacquer away...then I got it re-lacquered...stopped all my hum problems right away...

[EchoWars]

Is it possible that I could get you to describe the process in a little detail? Disassemble, strip, clean, reassemble, then lacquer? Or lacquer each lamination, assemble, and then dip the whole thing? Or do I have even that wrong?

Thanks...

[li_gangyi]

disassemble strip thin and then clean it off...
then reapply with a little lacquer in between...dip and then dry it out...put it's end bells back and then sand and re-paint it...

[EchoWars]

I'll have him give it a shot. Advice appreciated...

[li_gangyi]

he should probably check out his rectifiers first~!!!...reduce the work needed..just to find out it's not the trafo...

[EchoWars]

OK...dumb question time then...how would you go about checking a bridge rectifier for problems? I mean, if the regulator output voltages look OK. Scope on the output of the rectifier, look for 120Hz ripple (US)? On supplies that are not split (neg side of bridge connected to ground) seems that you wouldn't see anything even if one diode was bad.

So what's the proper way to check a sealed bridge rectifier?