UREI EQ repair! Any ideas?

I begin to repair my 537 EQ.
At the output i have constant buzz only and after two minutes power transformer starts being really hot...
Disconnected transformer has proper AC voltage. I swapped rectifier diodes, voltage regulators.
Checked all diodes and transistors in the circuit, swapped all opamps except LF356 which seems to be fine.

Proper schematic attached.

Any ideas or tips what's going on?

Only thing that really can draw current in there without letting out smoke is probably the output driver transistors. Try lifting power to those, and see if heating-problem goes away. And/or measure DC offset at the output of driver stage - even a small DC offset will draw a lot of current through the output transformer primary (Why isn't there an output coupling cap there, like in 1176?).

And then consider replacing C15, 100u/25V. It may have had a hard life.

Jakob E.

Thank you Jakob!
I didn't expect so quick response
My first suspected were output transistors, i've checked them in the circuit with multimeter but it good to be make it again. I will disconnect them and check transformer heat.
Definitely i will replace all electrolytics There's only few.
I will back soon with the report

Small DC offset at the output:

between terminals "+/-"  and "com" 0.3mV DC
between terminals "+/-"  and "grd" (same with "com" and "grd") 1.5mV DC

At the full gain setup AC readings:

between terminals "+/-"  and "com" 0.165mV AC
between terminals "+/-"  and "grd" (same with "com" and "grd") 0.1mV AC

Desoldered transistors seems to be fine. NPN and PNP passed the tests - HFE and BE, BC, EB, CB, CE.
Transformer, without transistors in the circuit, still getting hot! Any ideas?

gyraf said:

(Why isn't there an output coupling cap there, like in 1176?).

Click to expand...

Why? I don't know, but definitely there's no coupling cap at the output

I hope you measured mV not mA

We won't see DC on the output/secondary because of the transformer - we're interested in what DC the output stage sends through the primary side.

DC Voltage at the joint R33/R34 (referred to ground off course)?

We need to know if it's the + or the - line that's drawing too much. Power drain should be more or less symmetric. Does one of the 7818/7918 regulators get significantly hotter than the other?

Have you measured AC ripple on the two +/-18V power rails?

Jakob E.

Of course mV, i don't know why i wrote mA Corrected already
Both regulators doesn't seems to be overheat...
They have typical (i think) temperature for regulators on heatsinks, both similar.
I'll check everything and i'm back in the circa 20 minutes with report

OK - if power transformer gets really hot, but regulators not, the only place to look for trouble is before regulators: Diode bridge CR1 to CR4, and reservoir electrolytics C17-C18.

Could be a shorted diode and/or dried-out electrolytic

Jakob E.

Diodes are new, but you are right - C17 and C18 are damaged!
I don't know how i checked them before...  :-[
Shame on me...
I need to order caps, after replacing i'll be back with the results!

Thank you very much Jakob for your time and help!