Help with Direct coupled amp

with

Lon ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

My Variac has a bypassable lamp in series, best of both worlds.

Ron(UK)

Ah yes James, gives a nice visual indication of what you just did when the meter probe slipped ...!! Been there, got the T-shirt, as they say ... d;~}

Arfa

Does the T-shirt have scorch marks, too? :-)

You betcha !

Arfa

Only 25 years? I was working on DC coupled amps from the late 60's That Wurlitzer looks pretty straight forward with 2 constant current sources and a long tail pair. The only thing different from the 70's is the Darlington output devices rather than separate drivers. When I worked on those critters way back when I would remove the output devices and power it up with only the drivers. It would deliver full amplitude without a load but the scope was sufficient to verify it's mostly OK. The remove sidnal and verify that the base drive voltage to the outputs is around 0.6 V and that varying the bias will change it higher/lower and leave it a little low. Then install the new power transistors and power it up. Run 30KHz at around a Watt into a dummy load. Observe the crossover distortion on a scope and advance the bias until the crossover goes away.

Obviously verify the power supplies are functioning correctly, voltage around +/- 30, filter caps OK - no ripple. To test this Wurlitzer, pull the power transistors and connect the emitter of TR204 to TR205 to make the channel into a giant Opamp by completing the feedback loop and temporarily remove R228 so there is no load. With no load except a scope on the collector or TR210, the amp should deliver 50Vp-p or more (Rails- 2-3 volts) I would expect the voltage between TR203 collector and TR210 collector to be somewhere around 2 volts. Adjust the bias for less than 2 volts. If this is all OK, it should be safe to install the power transistor and R228 and fire it up, run a Watt into 8 ohms with 30KHz and bring up the bias to reduce crossover notch. Verify 11-12 mV across R224 or R236 with no load and no signal. DC level

Only 25 years? I was working on DC coupled amps from the late 60's That Wurlitzer looks pretty straight forward with 2 constant current sources and a long tail pair. The only thing different from the 70's is the Darlington output devices rather than separate drivers. When I worked on those critters way back when I would remove the output devices and power it up with only the drivers. It would deliver full amplitude without a load but the scope was sufficient to verify it's mostly OK. The remove sidnal and verify that the base drive voltage to the outputs is around 0.6 V and that varying the bias will change it higher/lower and leave it a little low. Then install the new power transistors and power it up. Run 30KHz at around a Watt into a dummy load. Observe the crossover distortion on a scope and advance the bias until the crossover goes away.

Obviously verify the power supplies are functioning correctly, voltage around +/- 30, filter caps OK - no ripple. To test this Wurlitzer, pull the power transistors and connect the emitter of TR204 to TR205 to make the channel into a giant Opamp by completing the feedback loop and temporarily remove R228 so there is no load. With no load except a scope on the collector or TR210, the amp should deliver 50Vp-p or more (Rails- 2-3 volts) I would expect the voltage between TR203 collector and TR210 collector to be somewhere around 2 volts. Adjust the bias for less than 2 volts. If this is all OK, it should be safe to install the power transistor and R228 and fire it up, run a Watt into 8 ohms with 30KHz and bring up the bias to reduce crossover notch. Verify 11-12 mV across R224 or R236 with no load and no signal. DC level