Biasing a 1594b

[spiders]

I was wondering if anyone had any info on biasing the transistors in a 1594b? I looked at the owners/service manual, but it recommends a milliamp clamp meter, which looks expensive... I was able to bias it the other way, with a scope, a dummy load, and a signal generator, but it still sounds off ? like it is clipping the audio sometimes. Oh, and if I touch the power transistors while it is running, it shocks me... not sure if it is supposed to do that. The scope said it was putting out up to 25v peak to peak between the power transistors case and the chassis ground when there is audio running through it, and 800-1200mv when it was idle.

[aditya]

Welcome to the forum.

From your description it seems that you have not done any biasing adjustment at all. What you have done is the 'driver balancing', which adjusts the swing balancing between the two input poles of the output trafo. This brings symmetry between the output signal's upper & lower halves.

You can do the biasing adjustment by measuring the voltage (DC) across the emitter resistors (R7 & R8). The process is mentioned there in the manual after the clamp-meter method. Pls remember to zero the volume control while doing this. After the biasing adj is done, repeat the balancing adj again which you have done already.

And yes, pls don't touch the output transistors while in operation, they can get destroyed instantly or become partially leaky etc. Do not even do any voltage measurements with a meter on the collector. This is a collector-loaded-transformer o/p stage, and by touching the collector by hand or a meter probe etc may make the system to jump into oscillation. The later designs used the emitter loading method, which is not prone to this malady, but the collector loaded method is more suitable for battery backup-ed designs, as more power can be produced for the same supply voltage.

Hope this helps.