UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Leak Stereo 30 Plus

orbanp1 · 16th Feb 2018, 3:09 pm

Hi Jim,

Just which end of the pot are running up against?

There are a couple of things you could check.
First, do check the DC voltage at the output, before the output coupling capacitor. It should be roughly half of the supply voltage. If the "half-voltage" is off, do check other voltages in the circuit as well.
I enclose a picture of the simulation of the circuit with a number of voltages and currents.

Do measure the voltage drop on those two combined biasing diodes, D1R, D2R, at 5mA current, for both channels and compare them.

The other thing that the simulation has shown that the existence of R49, 22kOhm (R22 in the simulation), is questionable, with that resistor in position, the value of P5 (R5 in the simulation), that 200 Ohm trimpot is nearly zero for the 30mA idle current.
Having simulated quite a number of audio amps with quasi-complemeter final stages this is the first amp where I see such additional biasing, such a resistor. You could try removing it.

If you feel uncomfortable removing that resistor then my bet would be on the different forward bias voltage on those two diodes.

Regards, Peter

16th Feb 2018, 3:09 pm	#10
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Leak Stereo 30 Plus - Setting Quiescent Currents Hi Jim, Just which end of the pot are running up against? There are a couple of things you could check. First, do check the DC voltage at the output, before the output coupling capacitor. It should be roughly half of the supply voltage. If the "half-voltage" is off, do check other voltages in the circuit as well. I enclose a picture of the simulation of the circuit with a number of voltages and currents. Do measure the voltage drop on those two combined biasing diodes, D1R, D2R, at 5mA current, for both channels and compare them. The other thing that the simulation has shown that the existence of R49, 22kOhm (R22 in the simulation), is questionable, with that resistor in position, the value of P5 (R5 in the simulation), that 200 Ohm trimpot is nearly zero for the 30mA idle current. Having simulated quite a number of audio amps with quasi-complemeter final stages this is the first amp where I see such additional biasing, such a resistor. You could try removing it. If you feel uncomfortable removing that resistor then my bet would be on the different forward bias voltage on those two diodes. Regards, Peter Attached Thumbnails