[damage]

I just acquired an mint Mosfet 120B from a garage sale. However there is about 100mv DC in each channel. Since I am assuming this is still a cap-coupled design, after replacing the electrolycs, how do I set DC offset and bias? Or is there something else I should be doing?

Thanks!

[avahifi]

Unlike our newer solid state designs, the old (1970 -1980s) Mos-Fet 120B amp did use the original Dyna ST-120 single ended power transformer, providing 90V DC B+ but no B-, so the outputs had to be capacitively coupled.  Thus, in a properly working unit, there is no DC offset at the speaker terminals.  You should make this measurement with an eight ohm dummy load attached, as there may be a bit of DC measured with the outputs open as the very large (5000 uF 100V) output coupling capacitors form up.

There are two adjustment per channel.  A signal generator, 8 ohm load bank, and a dual trace scope are required.  The amp is run to full power (both channels driven) on a 1 KHz sine wave into the load and the DC centerline pot (near the string of 914 diodes) is adjusted for symmetrical clipping.  Then at idle the bias current is adjusted for 115 mA.  The adjustments interact so they must be repeated a couple of times.  Let the amplifier idle for an hour for full warmup and repeat the adjustments. This is not an "amateur by ear" adjustment process.  We can do it for your for our minimum bench charge ($80) plus return shipping.

Note that if the amplifier is very old, it will use the original Dynaco output coupling capacitors and if they are  replaced, then the ouput inductors wound around them must be replaced too.  On newer units the output couplers would have been replaced with a new AVA PC card, which includes modern PC card mount output couplers and audio board power supply capacitors.  These likely will never require service.

If you have any further questions, please call me at 651-330-9871.

Frank Van Alstinhe

[damage]

I did check with a dummy load and the reading was .3 mV on my B&K. So I assume that is good. I will let it warm up again and then actually try playing it. I was just concerned that there might be damaging DC voltage present.

The capacitors are very shiny blue and gold. Don't know if this gives you any idea as to the date.

Thanks for all the help!