Hiraga 20 watt, output transistor biasing

[legarem]

I began to rebuild two homemade Hiraga Class A homemade amplifiers. They are using the schematic below. There's only .035 volts across the .47 ohm collector resistors.

I tried to change the value of the 12K resistors but it didn't help a lot.

Naturally, there's a lot of distorsion and the amplifier doesn't have power.

What could be the problem ?

Thanks

Here are the voltages I got

 

[Bigun]

I assume you decreased the value of the 12k resistors ? - as you expected, this should increase the bias current to the output driver and hence the idle current.

Did the amplifier work in the past ?

 

[legarem]

Yes it worked long time ago

It was tampered by someone and I decided to refurbish it with Takman resistors. All the transistors were checked and are ok.

Do the 1.8K at the base of output transistors have something to do with bias ?

 

[wahab]

The two 12K resistors only set the input pair current,and reducing them act only very marginaly by the resulting current increase in the 200/240R resistors and the slight increase of voltage at the bases of the second stage transistors.

To increase the quiescent current, you must increase the two 1.1K resistors that are connected from bases of 2SA634/2SC1096 to supply rails.
Try first 1.2K , if not enough , 1.3K and so on, by little steps..

 

[legarem]

I tried 1.5 K and i got near the same value

 

[legarem]

I checked the other amp and it seems to work. The resistors values appear to be not as original schematic. The 12 K are changed for 28K. The two 680 ohms near the pot are 220 ohms. NFB resistor is 300 ohms instead of 200. Collector output transistors have .33 ohm instead of .47 ohm. When I take voltage on this amplifier, there's near .650 volt at the base of the output transistors. The faulty amp has 14V and 18V at output bases.

It seems strange that the one which has not the recommended resistors values is working ok and the one with recommended values is not working.

Here is the schematic with voltage values I got with the working amp. This one has +20 V and -20 V unregulated at the first stage.

 

[toprepairman]

Nice circuit.
I doubt this is simply a problem of insufficient bias. If that was the only problem you'd still get full output and little distortion and it'd be running class B ish.

 

[toprepairman]

If you really have +18V on the base of the 2SD188 and -14v on the base of the 2SA627, then either one or both of those transistors are faulty.
What is the voltage reading on the output?