api 312 weirdness

[pucho812]

my buddy's 312 is acting up. If we feed it 1k  at -40dBu which is about .0774 volts we can measure 1 volt at the output.  He tried to swap the opamp but same results. Recapped the unit and same results.  He is bringing it by this week. They seem straight forward anything to look for out of the usual?

 

[CJ]

check supply rails,
then feedback path,
measure transformer ratios out of circuit, pray to the electronic gods,

 

[pucho812]

a known good/working one  has an output at around 12 volts.

so lets see what we can make it do.

 

[gridcurrent]

Abused 312's often have a shorted primary windings.

 

[pucho812]

O.k. the unit came in.

Testing against a known good unit we have the following 1k at around 7 millivolts

yields 12V on the output of the good one
and a max of 2 volts on the bad one with getting a squared waveform on the bad one. If I lower the level to a under 2 volts, I get a nice sine wave

The 2520 tested good in my opamp test jig
the rest of the circuit seems o.k.

About the only thing left to do is alligator clip around the transformers and see what the circuit does...